RFC4793: The EAP Protected One-Time Password Protocol (EAP-POTP)
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Related keywords:
(extensible authentication protocol)
(OTP)
Network Working Group M. Nystroem
Request for Comments: 4793 RSA Security
Category: Informational February 2007
The EAP Protected One-Time Password Protocol (EAP-POTP)
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This document describes a general Extensible Authentication Protocol
(EAP) method suitable for use with One-Time Password (OTP) tokens,
and offers particular advantages for tokens with direct electronic
interfaces to their associated clients. The method can be used to
provide unilateral or mutual authentication, and key material, in
protocols utilizing EAP, such as PPP, IEEE 802.1X, and Internet Key
Exchange Protocol Version 2 (IKEv2).
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Table of Contents
1. Introduction ....................................................4
1.1. Scope ......................................................4
1.2. Background .................................................4
1.3. Rationale behind the Design ................................4
1.4. Relationship with EAP Methods in RFC 3748 ..................5
2. Conventions Used in This Document ...............................5
3. Authentication Model ............................................5
4. Description of the EAP-POTP Method ..............................6
4.1. Overview ...................................................6
4.2. Version Negotiation ........................................9
4.3. Cryptographic Algorithm Negotiation .......................10
4.4. Session Resumption ........................................11
4.5. Key Derivation and Session Identifiers ....................13
4.6. Error Handling and Result Indications .....................13
4.7. Use of the EAP Notification Method ........................14
4.8. Protection against Brute-Force Attacks ....................14
4.9. MAC Calculations in EAP-POTP ..............................16
4.9.1. Introduction .......................................16
4.9.2. MAC Calculation ....................................16
4.9.3. Message Hash Algorithm .............................16
4.9.4. Design Rationale ...................................17
4.9.5. Implementation Considerations ......................17
4.10. EAP-POTP Packet Format ...................................17
4.11. EAP-POTP TLV Objects .....................................20
4.11.1. Version TLV .......................................20
4.11.2. Server-Info TLV ...................................21
4.11.3. OTP TLV ...........................................23
4.11.4. NAK TLV ...........................................33
4.11.5. New PIN TLV .......................................35
4.11.6. Confirm TLV .......................................38
4.11.7. Vendor-Specific TLV ...............................41
4.11.8. Resume TLV ........................................43
4.11.9. User Identifier TLV ...............................46
4.11.10. Token Key Identifier TLV .........................47
4.11.11. Time Stamp TLV ...................................48
4.11.12. Counter TLV ......................................49
4.11.13. Challenge TLV ....................................50
4.11.14. Keep-Alive TLV ...................................51
4.11.15. Protected TLV ....................................52
4.11.16. Crypto Algorithm TLV .............................54
5. EAP Key Management Framework Considerations ....................57
6. Security Considerations ........................................57
6.1. Security Claims ...........................................57
6.2. Passive and Active Attacks ................................58
6.3. Denial-of-Service Attacks .................................59
6.4. The Use of Pepper .........................................59
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6.5. The Race Attack ...........................................60
7. IANA Considerations ............................................60
7.1. General ...................................................60
7.2. Cryptographic Algorithm Identifier Octets .................61
8. Intellectual Property Considerations ...........................61
9. Acknowledgments ................................................61
10. References ....................................................62
10.1. Normative References .....................................62
10.2. Informative References ...................................62
Appendix A. Profile of EAP-POTP for RSA SecurID ...................64
Appendix B. Examples of EAP-POTP Exchanges ........................65
B.1. Basic Mode, Unilateral Authentication .....................65
B.2. Basic Mode, Session Resumption ............................66
B.3. Mutual Authentication without Session Resumption ..........67
B.4. Mutual Authentication with Transfer of Pepper .............69
B.5. Failed Mutual Authentication ..............................70
B.6. Session Resumption ........................................71
B.7. Failed Session Resumption .................................73
B.8. Mutual Authentication, and New PIN Requested ..............75
B.9. Use of Next OTP Mode ......................................78
Appendix C. Use of the MPPE-Send/Receive-Key RADIUS Attributes ....80
C.1. Introduction ..............................................80
C.2. MPPE Key Attribute Population .............................80
Appendix D. Key Strength Considerations ...........................80
D.1. Introduction ..............................................80
D.2. Example 1: 6-Digit One-Time Passwords .....................81
D.3. Example 2: 8-Digit One-Time Passwords .....................81
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1. Introduction
1.1. Scope
This document describes an Extensible Authentication Protocol (EAP)
[1] method suitable for use with One-Time Password (OTP) tokens, and
offers particular advantages for tokens that are electronically
connected to a user's computer, e.g., through a USB interface. The
method can be used to provide unilateral or mutual authentication,
and key material, in protocols utilizing EAP, such as PPP [10], IEEE
802.1X [11], and IKEv2 [12].
1.2. Background
A One-Time Password (OTP) token may be a handheld hardware device, a
hardware device connected to a personal computer through an
electronic interface such as USB, or a software module resident on a
personal computer, which generates one-time passwords that may be
used to authenticate a user towards some service. This document
describes an EAP method intended to meet the needs of organizations
wishing to use OTP tokens in an interoperable manner to authenticate
users over EAP. The method is designed to be independent of
particular OTP algorithms and to meet the requirements on modern EAP
methods (see [13]).
The basic variant of this method provides client authentication only.
This mode is only to be used within a secured tunnel. A more
advanced variant provides mutual authentication, integrity protection
of the exchange, protection against eavesdroppers, and establishment
of authenticated keying material. Both variants allow for fast
session resumption.
While this document also includes a profile of the general method for
the RSA SecurID(TM) mechanism, it is described in terms of general
constructions. It is therefore intended that the document will also
serve as a framework for use with other OTP algorithms.
Note: The term "OTP" as used herein shall not be confused with the
EAP OTP method defined in [1].
1.3. Rationale behind the Design
EAP-POTP has been designed with the intent that its messages and data
elements be easily parsed by EAP implementations. This makes it
easier to programmatically use the EAP method in the peer and the
authenticator, reducing the need for user interactions and allowing
for local generation of user prompts, when needed. In contrast, the
Generic Token Card (GTC) method from [1], which uses text strings
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generated by the EAP server, is intended to be interpreted and acted
upon by humans. Furthermore, EAP-POTP allows for mutual
authentication and establishment of keying material, which GTC does
not. To retain the generic nature of GTC, the EAP-POTP method has
been designed to support a wide range of OTP algorithms, with
profiling expected for specific such algorithms. This document
provides a profile of EAP-POTP for RSA SecurID tokens.
1.4. Relationship with EAP Methods in RFC 3748
The EAP OTP method defined in [1], which builds on [14], is an
example of a particular OTP algorithm and is not related to the EAP
method defined in this document, other than that a profile of EAP-
POTP may be created for the OTP algorithm from [14].
The Generic Token Card EAP method defined in [1] is intended to work
with a variety of OTP algorithms. The same is true for EAP-POTP, the
EAP method defined herein. Advantages of profiling a particular OTP
algorithm for use with EAP-POTP, compared to using EAP GTC, are
described in Section 1.3.
2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "SHALL", "SHALL NOT", "SHOULD",
"SHOULD NOT", "RECOMMENDED", and "MAY", in this document are to be
interpreted as described in RFC 2119 [2].
3. Authentication Model
The EAP-POTP method provides user authentication as defined below.
Additionally, it may provide mutual authentication (authenticating
the EAP server to the EAP client) and establish keying material.
There are basically three entities in the authentication method
described here:
o A client, or "peer", using EAP terminology, acting on behalf of a
user possessing an OTP token;
o A server, or "authenticator", using EAP terminology, to which the
user needs to authenticate; and
o A backend authentication server, providing an authentication
service to the authenticator.
The term "EAP server" is used here with the same meaning as in [1].
Any protocol used between the authenticator and the backend
authentication server is outside the scope of this document, although
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RADIUS [15] is a typical choice. It is assumed that the EAP client
and the peer are located on the same host, and hence only the term
"peer" is used in the following for these entities.
The EAP-POTP method assumes the use of a shared secret key, or
"seed", which is known both by the user and the backend
authentication server. The secret seed is stored on an OTP token
that the user possesses, as well as on the authentication server.
In its most basic variant, the EAP-POTP method provides only one
Service (namely, user authentication) where the user provides
information to the authentication server so that the server can
authenticate the user. A more advanced variant provides mutual
authentication, protection against eavesdropping, and establishment
of authenticated keying material.
4. Description of the EAP-POTP Method
4.1. Overview
Note: Since the EAP-POTP method is general in nature, the term
"POTP-X" is used below as a placeholder for an EAP method type
identifier, identifying the use of a particular OTP algorithm with
EAP-POTP. As an example, in the case of using RSA SecurID tokens
within EAP-POTP, the EAP method type shall be 32 (see Appendix A).
A typical EAP-POTP authentication is performed as follows (Appendix B
provides more detailed examples):
a. The optional EAP Identity Request/Response is exchanged, as per
RFC 3748 [1]. An identity provided here may alleviate the need
for a "User Identifier" or a "Token Key Identifier" triplet
(TLV), defined below, later in the exchange.
b. The EAP server sends an EAP-Request of type POTP-X with a Version
TLV. The Version TLV indicates the highest and lowest version of
this method supported by the server. The EAP server typically
also includes an OTP TLV in the EAP-Request. The OTP TLV
instructs the peer to respond with the current OTP (possibly in
protected form), and may contain a challenge and some other
information, like server policies. The EAP server should also
include a Server-Info TLV in the request, and must do so if it
supports session resumption. The Server-Info TLV identifies the
authentication server, contains an identifier for this (new)
session, and may be used by the peer to find an already existing
session with the EAP server.
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c. The peer responds with an EAP-Response of type Nak (3) if it does
not support POTP-X or if it does not support a version of this
method that is also supported by the server, as indicated in the
server's Version TLV.
If the peer supports a version of this method that is also
supported by the EAP server, the peer generates an EAP-Response
of type POTP-X as follows:
* First, it generates a Version TLV, which indicates the peer's
highest supported version within the range of versions offered
by the server. This Version TLV will be part of the EAP-
Response to the EAP server.
* Next, if the peer's highest supported version equals that of
the EAP server, and the EAP server sent a Server-Info TLV, the
peer checks if it has a saved session with the EAP server. If
an existing session with the server is found, and session
resumption is possible (the Server-Info TLV may explicitly
disallow it), the peer calculates new session keys (if the
session is a protected-mode session) and responds with a
Resume TLV and the Version TLV.
* Otherwise, if the peer's highest supported version equals that
of the EAP server, and the received EAP-Request message
contains an OTP TLV, the peer requests (possibly through user
interaction) the OTP token to calculate a one-time password
based on the information in the received EAP-Request message
(which could, for example, carry a challenge), the current
token state (e.g., token time), a shared secret (the "seed"),
and a user-provided PIN (note that, depending on the OTP token
type, some of the information in the EAP-Request may not be
used in the OTP calculation, and the PIN may be optional too).
If the received OTP TLV has the P bit set (see below), the
peer then combines the token-provided OTP with other
information, and provides the combined data to a key
derivation function. The key derivation function generates
several keys, of which one is used to calculate a Message
Authentication Code (MAC) on the received message, together
with some other information. The resulting MAC, together with
some additional information, is then placed in an OTP TLV
(with the P bit set) that is sent in a response to the EAP
server, together with the Version TLV. If the P bit is not
set in the received OTP TLV, the peer instead inserts the
calculated OTP value directly in an OTP TLV, which then is
sent to the EAP server together with the Version TLV.
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* Finally, if the peer's highest supported version differs from
the server's, or if the server did not provide any TLVs
besides the Version TLV in its initial request, the peer just
sends back the generated Version TLV as an EAP-Response to the
EAP server.
d. If the EAP server receives an EAP-Response of type Nak (3), the
session negotiation failed and the EAP server may try with
another EAP method. Otherwise, the EAP server checks the peer's
supported version. If the peer did not support the highest
version supported by the server, the server will send a new EAP-
Request with TLVs adjusted for that version. Otherwise, assuming
the EAP server did send additional TLVs in its initial EAP-
Request, the EAP server will attempt to authenticate the peer
based on the response provided in c). Depending on the result of
this authentication, the EAP server may do one of the following:
* send a new EAP-Request of type POTP-X to the peer indicating
that session resumption was not possible, and ask for a new
OTP (this would be the case when the peer responded with a
Resume TLV, and the session indicated in the Resume TLV was
not valid),
* send a new EAP-Request of type POTP-X to the peer (e.g., to
ask for the next OTP),
* accept the authentication (and send an EAP-Request message
containing a Confirm TLV to the peer if the received response
has the P bit set or was a successful attempt at a protected-
mode session resumption; otherwise, send an EAP-Success
message to the peer), or
* fail the authentication (and send an EAP-Failure message --
possibly preceded by an EAP-Request message of type
Notification (2) -- to the peer).
e. If the peer receives an EAP-Success or an EAP-Failure message the
protocol run is finished. If the peer receives an EAP-Request of
type Notification, it responds as specified by RFC 3748 [1]. If
the peer receives an EAP-Request of type POTP-X with a Confirm
TLV, it attempts to authenticate the EAP server using the
provided data. If the authentication is successful, the peer
responds with an EAP-Response of type POTP-X with a Confirm TLV.
If it is unsuccessful, the peer responds with an empty EAP-
Response of type POTP-X. If the peer receives an EAP-Request of
type POTP-X containing some other TLVs, it continues as specified
in c) above (though no version negotiation will take place in
this case) or as described for those TLVs.
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f. When an EAP server, which has sent an EAP-Request of type POTP-X
with a Confirm TLV, receives an EAP-Response of type POTP-X with
a Confirm TLV present, it can proceed in one of two ways: If it
has detected that there is a need to send additional EAP-Requests
of type POTP-X, it shall enter a "protected state", where, from
then on, all POTP-X TLVs must be encrypted and integrity-
protected before being sent (at this point, the parties shall
have calculated a master session key as described in Section
4.5). One reason to continue the POTP-X conversation after
exchange of the Confirm TLV could be that the user needs to
update her OTP PIN; hence, the EAP server needs to send a New PIN
TLV. At that point, the handshake is back at step c) above
(except for the version negotiation and the protection of all
TLVs). If there is no need to send additional EAP-Request
packets, the EAP server shall instead send an EAP-Success method
to the peer to indicate successful protocol completion. The EAP
server may not continue the conversation unless it indicates its
intent to do so in the Confirm TLV.
An EAP server, which has sent an EAP-Request of type POTP-X with
a Confirm TLV and receives an EAP-Response of type POTP-X, which
is empty (i.e., does not contain any TLVs), shall respond with an
EAP-Failure and terminate the handshake.
As implied by the description, steps c) through f) may be carried out
a number of times before completion of the exchange. One example of
this is when the authentication server initially requests an OTP,
accepts the response from the peer, performs an (intermediary)
Confirm TLV exchange, requests the peer to select a new PIN, and
finally asks the peer to authenticate with an OTP based on the new
PIN (which again will be followed with a final Confirm TLV exchange).
4.2. Version Negotiation
The EAP-POTP method provides a version negotiation mechanism that
enables implementations to be backward compatible with previous
versions of the protocol. This specification documents the EAP-POTP
protocol version 1. Version negotiation proceeds as follows:
a. In the first EAP-Request of type POTP-X, the EAP server MUST send
a Version TLV in which it sets the "Highest" field to its highest
supported version number, and the "Lowest" field to its lowest
supported version number. The EAP server MAY include other TLV
triplets, as described below, that are compatible with the
"Highest" supported version number to optimize the number of
round-trips in the case of a peer supporting the server's
"Highest" version number.
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b. If the peer supports a version of the protocol that falls within
the range of versions indicated by the EAP server, it MUST
respond with an EAP-Response of type POTP-X that contains a
Version TLV with the "Highest" field set to the highest version
supported by the peer. The peer MUST also respond to any TLV
triplets included in the EAP-Request, if it supported the
"Highest" supported version indicated in the server's Version
TLV.
The EAP peer MUST respond with an EAP-Response of type Nak (3) if
it does not support a version that falls within the range of
versions indicated by the EAP server. This will allow the EAP
server to use another EAP method for peer authentication.
c. When the EAP server receives an EAP-Response containing a Version
TLV from the peer, but the "Highest" supported version field in
the TLV differs from the "Highest" supported version field sent
by the EAP server, or when the version is the same as the one
originally proposed by the EAP server, but the EAP server did not
include any TLV triplets in the initial request, the EAP server
sends a new EAP-Request of type POTP-X with the negotiated
version and TLV triplets as desired and described herein.
The version negotiation procedure guarantees that the EAP peer and
server will agree to the highest version supported by both parties.
If version negotiation fails, use of EAP-POTP will not be possible,
and another mutually acceptable EAP method will need to be negotiated
if authentication is to proceed.
The EAP-POTP version field may be modified in transit by an attacker.
It is therefore important that EAP entities only accept EAP-POTP
versions according to an explicit policy.
4.3. Cryptographic Algorithm Negotiation
Cryptographic algorithms are negotiated through the use of the Crypto
Algorithm TLV. EAP-POTP provides a default digest algorithm
(SHA-256) [3], a default encryption algorithm (AES-CBC) [4] , and a
default MAC algorithm (HMAC) [5], and these algorithms MUST be
supported by all EAP-POTP implementations. An EAP server that does
not want to make use of any other algorithms than the default ones
need not send a Crypto Algorithm TLV. An EAP server that does want
to negotiate use of some other algorithms MUST send the Crypto
Algorithm TLV in the initial EAP-Request of type POTP-X that also
contains an OTP TLV with the P bit set. The TLV MUST NOT be present
in any other EAP-Request in the session. (The two exceptions to this
are 1) if the client attempted a session resumption that failed and
therefore did not evaluate a sent Crypto Algorithm TLV, or 2) if the
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Crypto Algorithm TLV was part of the initial message from the EAP
server, and the client negotiated another EAP-POTP version than the
highest one supported by the EAP server. When either of these cases
apply, the server MUST include the Crypto Algorithm TLV in the first
EAP-Request that also contains an OTP TLV with the P bit set
subsequent to the failed session resumption / protocol version
negotiation.) In the Crypto Algorithm TLV, the EAP server suggests
some combination of digest, encryption, and MAC algorithms. (If the
server only wants to negotiate a particular class of algorithms, then
suggestions for the other classes need not be present, since the
default applies.)
The peer MUST include a Crypto Algorithm TLV in an EAP-Response if
and only if an EAP-Request of type POTP-X has been received
containing a Crypto Algorithm TLV, it was legal for that EAP-Request
to contain a Crypto Algorithm TLV, the peer does not try to resume an
existing session, and the peer and the EAP server agree on at least
one algorithm not being the default one. If the peer does not supply
a value for a particular class of algorithms in a responding Crypto
Algorithm TLV, then the default algorithm applies for that class.
When resuming an existing session (see the next section), there is no
need for the peer to negotiate since the session already is
associated with a set of algorithms. Servers MUST fail a session
(i.e., send an EAP-Failure) if they receive an EAP-Response TLV
containing both a Resume TLV and a Crypto Algorithm TLV.
Clearly, EAP servers and peers MUST NOT suggest any other algorithms
than the ones their policy allows them to use. Policies may also
restrict what combinations of cryptographic algorithms are
acceptable.
4.4. Session Resumption
This method makes use of session identifiers and server identifiers
to allow for improved efficiency in the case where a peer repeatedly
attempts to authenticate to an EAP server within a short period of
time. This capability is particularly useful for support of wireless
roaming.
In order to help the peer find a session associated with the EAP
server, an EAP server that supports session resumption MUST send a
Server-Info TLV containing a server identifier in its initial EAP-
Request of type POTP-X that also contains an OTP TLV. The identifier
may then be used by the peer for lookup purposes.
It is left to the peer whether or not to attempt to continue a
previous session, thus shortening the negotiation. Typically, the
peer's decision will be made based on the time elapsed since the
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previous authentication attempt to that EAP server. If the peer
decides to attempt to resume a session with the EAP server, it sends
a Resume TLV identifying the chosen session and other contents, as
described below, to the EAP server.
Based on the session identifier chosen by the peer, and the time
elapsed since the previous authentication, the EAP server will decide
whether to allow the session resumption, or continue with a new
session.
o If the EAP server is willing to resume a previously established
session, it MUST authenticate the peer based on the contents of
the Resume TLV. If the authentication succeeds, the handshake
will continue in one of two ways:
* If the session is a protected-mode session, then the server
MUST respond with a request containing a Confirm TLV. If the
Confirm TLV authenticates the EAP server, then the peer
responds with an empty Confirm TLV, to which the EAP server
responds with an EAP-Success message. If the Confirm TLV does
not authenticate the EAP server, the peer responds with an
empty EAP-Response of type POTP-X.
* If the session is not a protected-mode session, i.e., it is a
session created from a basic-mode peer authentication, then the
server MUST respond with an EAP-Success message.
If the authentication of the peer fails, the EAP server SHOULD
send another EAP-Request containing an OTP TLV and a Server-Info
TLV with the N bit set to indicate that no session resumption is
possible. The EAP server MAY also send an EAP-Failure message,
possibly preceded by an EAP-Request of type Notification (2), in
which case, the EAP run will terminate.
o If the EAP server is not willing or able to resume a previously
established session, it will respond with another EAP-Request
containing an OTP TLV and a Server-Info TLV with the N bit set
(indicating no session resumption).
Sessions SHOULD NOT be maintained longer than the security of the
exchange which created the session permits. For example, if it is
estimated that an attacker could be successful in brute-force
searching for the OTP in 24 hours, then EAP-POTP session lifetimes
should be clearly less than this value.
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4.5. Key Derivation and Session Identifiers
The EAP-POTP method described herein makes use of a key derivation
function denoted "PBKDF2". PBKDF2 is described in [6], Section 5.2.
The PBKDF2 PRF SHALL be set to the negotiated MAC algorithm. The
default MAC algorithm, which MUST be supported, is HMAC-SHA256. HMAC
is defined in [5], and SHA-256 is defined in [3]. HMAC-SHA256 is the
HMAC construct from [5] with SHA-256 as the hash function H. The
output length of HMAC-SHA256, when used as a PRF for PBKDF2, shall be
32 octets (i.e., the full output length).
The output from PBKDF2 as described here will consist of five keys
(see Section 4.11.3 for details on how to calculate these keys):
o K_MAC, a MAC key used for mutual authentication and integrity
protection,
o K_ENC, an encryption key used to protect certain data during the
authentication,
o SRK, a session resumption key only used for session resumption
purposes,
o MSK, a Master Session Key, as defined in [1], and
o EMSK, an Extended Master Session Key, also as defined in [1].
For the default algorithms, K_MAC, K_ENC, and SRK SHALL be 16
octets. For other cases, the key lengths will be as determined by
the negotiated algorithms. The MSK and the EMSK SHALL each be 64
octets, in conformance with [1]. Therefore, in the case of
default algorithms, the "dkLen" parameter from Section 5.2 of [6]
SHALL be set to 176 (the combined length of K_MAC, K_ENC, SRK,
MSK, and EMSK).
[1] and [16] define usage of the MSK and the EMSK . For a particular
use case, see also Appendix C.
4.6. Error Handling and Result Indications
EAP does not allow for the sending of an EAP-Response of type Nak (3)
within a method after the initial EAP-Request and EAP-Response pair
of that particular method has been exchanged (see [1], Section 2.1).
Instead, when a peer is unable to continue an EAP-POTP session, the
peer MAY respond to an outstanding EAP-Request by sending an empty
EAP-Response of type POTP-X rather than immediately terminating the
conversation. This allows the EAP server to log the cause of the
error.
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To ensure that the EAP server receives the empty EAP-Response, the
peer SHOULD wait for the EAP server to reply before terminating the
conversation. The EAP server MUST reply with an EAP-Failure.
When EAP-POTP is run in protected mode, the exchange of the Confirm
TLV (Section 4.11.6) serves as a success result indication; when the
peer receives a Confirm TLV, it knows that the EAP server has
successfully authenticated it. Similarly, when the EAP server
receives the Confirm TLV response from the peer, it knows that the
peer has authenticated it. In protected mode, the peer will not
accept an EAP-Success packet unless it has received and validated a
Confirm TLV. The Confirm TLV sent from the EAP server to the peer is
a "protected result indication" as defined in [1], as it is integrity
protected and cannot be replayed. The Confirm TLV sent from the peer
to the EAP server is, however, not a protected result indication. An
empty EAP-POTP response sent from the peer to the EAP server serves
as a failure result indication.
4.7. Use of the EAP Notification Method
Except where explicitly allowed in the following, the EAP
Notification method MUST NOT be used within an EAP-POTP session. The
EAP Notification method MAY be used within an EAP-POTP session in the
following situations:
o The EAP server MAY send an EAP-Request of type Notification (2)
when it has received an EAP-Response containing an OTP TLV and is
unable to authenticate the user. In this case, once the EAP-
Response of type Notification is received, the EAP server MAY
retry the authentication and send a new EAP-Request containing an
OTP TLV, or it MAY fail the session and send an EAP-Failure
message.
o The EAP server MAY send an EAP-Request of type Notification (2)
when it has received an unacceptable New PIN TLV. In this case,
once the EAP-Response of type Notification is received, the EAP
server MAY retry the PIN update and send a new EAP-Request with a
New PIN TLV, or it MAY fail the session and send an EAP-Failure
message.
4.8. Protection against Brute-Force Attacks
Since OTPs may be relatively short, it is important to slow down an
attacker sufficiently so that it is economically unattractive to
brute-force search for an OTP, given an observed EAP-POTP handshake
in protected mode. One way to do this is to do a high number of
iterated hashes in the PBKDF2 function. Another is for the client to
include a value ("pepper") unknown to the attacker in the hash
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computation. Whereas a traditional "salt" value normally is sent in
the clear, this "pepper" value will not be sent in the clear, but may
instead be transferred to the EAP server in encrypted form. In
practice, the procedure is as follows:
a. The EAP server indicates in its OTP TLV whether it supports
pepper searching. Additionally, it may indicate to the peer that
a new pepper shall be chosen.
b. If the peer supports the use of pepper, the peer checks whether
it already has established a shared pepper with this server:
If it does have a pepper stored for this server, and the server
did not indicate that a new pepper shall be generated, then it
uses the existing pepper value, as specified in Section 4.11.3
below, to calculate an OTP TLV response. In this case, the
iteration count shall be kept to a minimum, as the security of
the scheme is provided through the pepper, and efficiency
otherwise is lost.
If the peer does not have a pepper stored for this server, but
the server indicated support for pepper searching, or the server
indicated that a new pepper shall be generated, then the peer
generates a random and uniformly distributed pepper of sufficient
length (the maximum length supported by the server is provided in
the server's OTP TLV), and includes the new pepper in the PBKDF2
computation.
If the peer does not have a pepper stored for this server, and
the server did not indicate support for pepper searching, then a
pepper will not be used in the response computation.
Clearly, if the peer itself does not support the use of pepper,
then a pepper will not be used in the response computation.
c. The EAP server may, in its subsequent Confirm TLV, provide a
pepper to the peer for later use. In this case, the pepper will
be substantially longer than a peer-chosen pepper, and encrypted
with a key derived from the PBKDF2 computation.
The above procedure allows for pepper updates to be initiated by
either side, e.g., based on policy. Since the pepper can be seen as
a MAC key, its lifetime should be limited.
An EAP server that is not capable of storing pepper values for each
user it is authenticating may still support the use of pepper; the
cost for this will be the extra computation time to do pepper
searches. This cost is still substantially lower than the cost for
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an attacker, however, since the server already knows the underlying
OTP.
4.9. MAC Calculations in EAP-POTP
4.9.1. Introduction
In protected mode, EAP-POTP uses MACs for authentication purposes, as
well as to ensure the integrity of protocol sessions. This section
defines how the MACs are calculated and the rationale for the design.
4.9.2. MAC Calculation
In protected mode, and when resuming a previous session, rather than
sending authenticating credentials (such as one-time passwords or
shared keys) directly, evidence of knowledge of the credentials is
sent. This evidence is a MAC on the hash of (certain parts of) EAP-
POTP messages exchanged so far in a session using a key K_MAC:
mac = MAC(K_MAC, msg_hash(msg_1, msg_2, ..., msg_n))
where
"MAC" is the negotiated MAC algorithm, "K_MAC" is a key derived as
specified in Section 4.5, and "msg_hash(msg_1, msg_2, ..., msg_n)" is
the message hash defined below of messages msg_1, msg_2, ..., msg_n.
4.9.3. Message Hash Algorithm
To compute a message hash for the MAC, given a sequence of EAP
messages msg_1, msg_2, ..., msg_n, the following operations shall be
carried out:
a. Re-transmitted messages are removed from the sequence of
messages.
Note: The resulting sequence of messages must be an alternating
sequence of EAP Request and EAP Response messages.
b. The contents (i.e., starting with the EAP "Type" field and
excluding the EAP "Code", "Identifier", and "Length" fields) of
each message, msg_1, msg_2, ..., msg_n, is concatenated together.
c. User identifier TLVs MUST NOT be included in the hash (this is to
allow for a backend service that does not know about individual
user names), i.e., any such TLV is removed from the message in
which it appeared.
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d. The resulting string is hashed using the negotiated hash
algorithm.
4.9.4. Design Rationale
The reason for excluding the "Identifier" field is that the actual,
transmitted "Identifier" field is not always known to the EAP method
layer. The reason for excluding the "Length" field is to allow the
possibility for an intermediary to remove or replace a Username TLV
(e.g., for anonymity or service reasons) before passing a received
response on to an authentication server. While this on the surface
may appear as bad security practice, it may in practice only result
in denial of service, something which always may be achieved by an
attacker able to modify messages in transit. By excluding the "Code"
field, the hash is simply calculated on applicable sent and received
message contents. Excluding the "Code" field is regarded as harmless
since the hash is to be made on the sequence of POTP-X messages, all
having alternating (known) Code values, namely 1 (Request) and 2
(Response).
4.9.5. Implementation Considerations
To save on storage space, each EAP entity may partially hash messages
as they are sent and received (e.g., HashInit(); HashUpdate(message
1); ...; HashUpdate(message n-1); HashFinal(message n)). This
reduces the amount of state needed for this purpose to the internal
state required for the negotiated hash algorithm.
4.10. EAP-POTP Packet Format
A summary of the EAP-POTP packet format is shown below. The fields
are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Reserved | TLV-based EAP-POTP message ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
1 - Request
2 - Response
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Identifier
The Identifier field is 1 octet and aids in matching responses
with requests. For a more detailed description of this field and
how to use it, see [1].
Length
The Length field is 2 octets and indicates the length of the EAP
packet including the Code, Identifier, Length, Type, Version,
Flags, and TLV-based EAP-POTP message fields.
Type
Identifies use of a particular OTP algorithm with EAP-POTP.
Reserved
This octet is reserved for future use. It SHALL be set to zero
for this version. Recipients SHALL ignore this octet for this
version of EAP-POTP.
TLV-based EAP-POTP message
This field will contain 0, 1, or more Type-Length-Value triplets
defined as follows (this is similar to the EAP-TLV TLVs defined in
PEAPv2 [17], and the explanation of the generic fields is borrowed
from that document).
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
0 - Non-mandatory TLV
1 - Mandatory TLV
The TLVs within EAP POTP-X are used to carry parameters between
the EAP peer and the EAP server. An EAP peer may not necessarily
implement all the TLVs supported by an EAP server, and to allow
for interoperability, a special TLV allows an EAP server to
discover if a TLV is supported by the EAP peer.
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The mandatory bit in a TLV indicates that if the peer or server
does not support the TLV, it MUST send a NAK TLV in response; all
other TLVs in the message MUST be ignored. If an EAP peer or
server finds an unsupported TLV that is marked as non-mandatory
(i.e., optional), it MUST NOT send a NAK TLV on this ground only.
The mandatory bit does not imply that the peer or server is
required to understand the contents of the TLV. The appropriate
response to a supported TLV with content that is not understood is
defined by the specification of the particular TLV.
R
Reserved for future use. This bit SHALL be set to zero (0) for
this version. Recipients SHALL ignore this bit for this version
of the EAP-POTP.
TLV Type
The following TLV types are defined for use with EAP-POTP:
0 - Reserved for future use
1 - Version
2 - Server-Info
3 - OTP
4 - NAK
5 - New PIN
6 - Confirm
7 - Vendor-Specific
8 - Resume
9 - User Identifier
10 - Token Key Identifier
11 - Time Stamp
12 - Counter
13 - Keep-Alive
14 - Protected
15 - Crypto Algorithm
16 - Challenge
These TLVs are defined in the following. With the exception of
the NAK TLV, a particular TLV type MUST NOT appear more than once
in a message of type POTP-X.
Length
The length of the Value field in octets.
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Value
The value of the TLV.
4.11. EAP-POTP TLV Objects
4.11.1. Version TLV
The Version TLV carries information about the supported EAP-POTP
method version.
This TLV MUST be present in the initial EAP-Request of type POTP-X
from the EAP server and in the initial response of type POTP-X from
the peer. It MUST NOT be present in any subsequent EAP-Request or
EAP-Response in the session. The Version TLV MUST be supported by
all peers, and all EAP servers conforming to this specification and
MUST NOT be responded to with a NAK TLV. The version negotiation
procedure is described in detail in Section 4.2.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Highest | Lowest |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
1 - Mandatory TLV
R
Reserved for future use. This bit SHALL be set to zero (0) for
this version. Recipients SHALL ignore this bit for this version
of EAP-POTP.
TLV Type
1
Length
3 in EAP-Requests, 2 in EAP-Responses
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Reserved
Reserved for future use. This octet MUST be set to zero for this
version. Recipients SHALL ignore this octet for this version of
EAP-POTP.
Highest
This field contains an unsigned integer representing the highest
protocol version supported by the sender. If a value provided by
a peer to an EAP server falls between the server's "Highest" and
"Lowest" supported version (inclusive), then that value will be
the negotiated version for the authentication session.
Lowest
This field contains an unsigned integer representing the lowest
version acceptable by the EAP server. The field MUST be present
in an EAP-Request. The field MUST NOT be present in an EAP-
Response. A peer SHALL respond to an EAP-Request of type POTP-X
with an EAP-Response of type Nak (3) if the peer's highest
supported version is lower than the value of this field.
This document defines version 1 of the protocol. Therefore, EAP
server implementations conforming to this document SHALL set the
"Highest" field to 1. Peer implementations conforming to this
document SHALL set the "Highest" field to 1.
4.11.2. Server-Info TLV
The Server-Info TLV carries information about the EAP server and the
session (when applicable). It provides one piece in the framework
for fast session resumption.
This TLV SHOULD always be present in an EAP-Request of type POTP-X
that also carries an OTP TLV, as long as the peer has not been
authenticated, and MUST be present in such a request if the server
supports session resumption. It MUST NOT be present in any other
EAP-Request of type POTP-X or in any EAP-Response packets. This TLV
type MUST be supported by all peers conforming to this specification
and MUST NOT be responded to with a NAK TLV (this is not to say that
all peers need to support session resumption, only that they cannot
respond to this TLV with a NAK TLV).
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |N| Session Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Session Identifier (continued) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Sess.Id (cont.)| Nonce ... (16 octets)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Server Identifier ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
1 - Mandatory TLV
R
Reserved for future use. This bit SHALL be set to zero (0) for
this version. Recipients SHALL ignore this bit for this version
of EAP-POTP.
TLV Type
2
Length
25 + length of Server Identifier field
Reserved
Reserved for future use. All 7 bits MUST be set to zero for this
version. Recipients SHALL ignore this bit for this version of
EAP-POTP.
N
The N bit signals that the peer MUST NOT attempt to resume any
session it has stored associated with this server.
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Session Identifier
An 8-octet identifier for the session about to be negotiated.
Note that, in the case of session resumption, this session
identifier will not be used (the session identifier for the
resumed session will continue to be used).
Nonce
A 16-octet nonce chosen by the server. During session resumption,
this nonce is used when calculating new K_ENC, K_MAC, SRK, MSK,
and EMSK keys as specified below.
Server Identifier
An identifier for the authentication server. The peer MAY use
this identifier to search for a stored session associated with
this server, or to associate the session to be negotiated with the
server. The value of the identifier SHOULD be chosen so as to
reduce the risk of collisions with other EAP server identifiers as
much as possible. One possibility is to use the DNS name of the
EAP server. The identifier MAY also be used by the peer to select
a suitable key on the OTP token (when there are multiple keys
available).
The identifier MUST NOT be longer than 128 octets. The identifier
SHALL be a UTF-8 [7] encoded string of printable characters
(without any terminating NULL character).
4.11.3. OTP TLV
In an EAP-Request, the OTP TLV is used to request an OTP (or a value
derived from an OTP) from the peer. In an EAP-Response, the OTP TLV
carries an OTP or a value derived from an OTP.
This TLV type MUST be supported by all peers and all EAP servers
conforming to this specification and MUST NOT be responded to with a
NAK TLV. The OTP TLV MUST NOT be present in an EAP-Request of type
POTP-X that contains a New PIN TLV. Further, the OTP TLV MUST NOT be
present in an EAP-Response of type POTP-X unless the preceding EAP-
Request of type POTP-X contained an OTP TLV and it was valid for it
to do so. Finally, an OTP TLV MUST NOT be present in an EAP-
Response of type POTP-X that also contains a Resume TLV. The OTP TLV
is defined as follows:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |A|P|C|N|T|E|S| Pepper Length |Iteration Count|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Iteration Count (cont.) | Auth. Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication Data (cont.) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
1 - Mandatory TLV
R
Reserved for future use. This bit SHALL be set to zero (0) for
this version. Recipients SHALL ignore this bit for this version
of EAP-POTP.
TLV Type
3
Length
7 + length of Authentication Data field
Reserved
Reserved for future use. All 9 bits SHALL be set to zero (0) for
this version. Recipients SHALL ignore these bits for this version
of EAP-POTP.
A
The A bit MUST be set in an EAP-Request if and only if the request
immediately follows an EAP-Response of type POTP-X containing a
New PIN TLV (see Section 4.11.5), and the new PIN in the response
was accepted by the EAP server. In this case, the A bit signals
that the EAP-server has accepted the PIN, and that the peer SHALL
use the newly established PIN when calculating the response (when
applicable). The A bit MUST NOT be set if the S bit is set. If a
request has both the S bit and the A bit set, the peer SHALL
regard the request as invalid, and return an empty POTP-X EAP-
Response message.
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In an EAP-Response, the A bit, when set, indicates that the OTP
was calculated with the use of the newly selected user PIN. The A
bit MUST be set in a response if and only if the EAP-Request which
triggered the response contained an OTP TLV with the A bit set.
P
In an EAP-Request, the P bit indicates that the OTP in the
response MUST be protected. Use of this bit also indicates that
mutual authentication will take place, as well as generation of
keying material. It is RECOMMENDED to always set the P bit. If a
peer receives an EAP-Request with an OTP TLV that does not have
the P bit set, and the peer's policy dictates protected mode, the
peer MUST respond with an empty POTP-X EAP-Response message. All
peers MUST support protected mode.
In an EAP-Response, this bit indicates that the provided OTP has
been protected (see below). The P bit MUST be set in a response
(and hence the OTP MUST be protected) if and only if the EAP-
Request that triggered the response contained an OTP TLV with the
P bit set.
In an 802.1x EAP over LAN (EAPOL) environment (this includes
wireless LAN environments), the P bit MUST be set, or,
alternatively, the EAP-POTP method MUST be carried out inside an
authenticated tunnel that provides a cryptographic binding with
inner EAP methods such as the one provided by PEAPv2 [17].
C
The C bit carries meaning only when the OTP algorithm in question
makes use of server challenges. For other OTP algorithms, the C
bit SHALL always be set to zero.
In an EAP-Request, the C bit ("Combine") indicates that the OTP
SHALL be calculated using both the provided challenge and internal
state (e.g., current token time). The OTP SHALL be calculated
based only on the provided challenge (and the shared secret) if
the C bit is not set, and a challenge is present. The returned
OTP SHALL always be calculated based on the peer's current state
(and the shared secret) if no challenge is present. If the C bit
is set but no challenge is provided, the peer SHALL regard the
request as invalid, and return an empty POTP-X EAP-Response
message.
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In an EAP response, this bit indicates that the provided OTP has
been calculated using a provided challenge and the token state.
The C bit MUST be set in a response if and only if the EAP-Request
that triggered the response contained an OTP TLV with the C bit
set and a challenge.
N
In an EAP-Request, the N bit, when set, indicates that the OTP to
calculate SHALL be based on the next token "state", and not the
current one. As an example, for a time-based token, this means
the next time slot. For an event-based token, this could mean the
next counter value, if counter values are used. This bit will
normally not be set in initial EAP-Request messages, but may be
set in subsequent ones. Further, the N bit carries no meaning in
an EAP-Request if a challenge is present and the C bit is not set,
and SHALL be set to 0, in this case. If a request that has the N
bit set also contains a challenge, but does not have the C bit
set, the peer SHALL regard the request as invalid, and return an
empty POTP-X EAP-Response message. Note that setting the N bit in
an EAP-Request will normally advance the internal state of the
token.
In an EAP-Response, the N bit, when set, indicates that the OTP
was calculated based on the next token "state" (as explained
above), and not the current one. The N bit MUST be set in a
response if and only if the EAP-Request that triggered the
response contained an OTP TLV with the N bit set.
T
The T bit only carries meaning for OTP methods normally
incorporating a user PIN in the OTP computation.
In an EAP-Request, the T bit, when set, indicates that the OTP to
calculate MUST NOT include a user PIN.
In an EAP-Response, the T bit, when set, indicates that the OTP
was calculated without the use of a user PIN. The T bit MUST be
set in a response if and only if the EAP-Request that triggered
the response contained an OTP TLV with the T bit set. Note that
client policy may prohibit PIN-less calculations; in these cases,
the client MAY respond with an empty POTP-X EAP response message.
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E
In an EAP-Request, the E bit, when set, indicates that the peer
MUST NOT use any stored pepper value associated with this server
in the PBKDF2 computation. Rather, it MUST generate a new pepper
(if supported by the peer) and/or use the iteration count
parameter to protect the OTP (if the server's Max Pepper Length is
0, then the peer MUST rely on the iteration count only to protect
the OTP). This bit will usually not be set in initial EAP-Request
messages, but may be set in subsequent ones, e.g., if the server,
upon receipt of an OTP TLV with a pepper identifier, detects that
it does not have a pepper with that identifier in storage. This
bit carries no meaning, and MUST be set to zero, when the P bit is
not set. If a request has the E bit set but not the P bit, a peer
SHALL regard the request as invalid, and return an empty POTP-X
EAP-Response message.
In an EAP-Response, the E bit indicates that the response has been
calculated without use of any stored pepper value.
S
In an EAP-Request, the S bit ("Same"), when set, indicates that
the peer SHOULD calculate its response based on the same OTP value
as was used for the preceding response. This bit MAY be set when
the EAP server has received an OTP TLV from the peer protected
with a pepper, of which the server is no longer in possession.
Since the server has not attempted validation of the provided
data, there is no need for the EAP peer to retrieve a new OTP
value. This bit carries no meaning, and MUST be set to zero, when
the E bit is not set. A peer SHALL regard a request where the S
bit is set, but not the E bit, as invalid, and return an empty
POTP-X EAP-Response message. Further, the S bit MUST NOT be set
when the A bit also is set; see above.
In an EAP-Response, the S bit is never set.
Pepper Length
This octet SHALL be present if and only if the P bit is set. When
present, it contains an unsigned integer, having a value between 0
and 255 (inclusive). In an EAP-Request, the integer represents
the maximum length (in bits) of a client-generated pepper the
server is prepared to search for. Peers MUST NOT generate peppers
longer than this value. If the value is set to zero, it means the
peer MUST NOT generate a pepper for the PBKDF2 calculation. In an
EAP-Response, it indicates the length of the used pepper.
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Iteration Count
These 4 octets SHALL be present if and only if the P bit is set.
When present, they contain an unsigned, 4-octet integer in network
byte order. In an EAP-Request, the integer represents the maximum
iteration count the peer may use in the PBKDF2 computation. Peers
MUST NOT use iteration counts higher than this value. In an EAP-
Response, it indicates the actual iteration count used.
Note regarding the Pepper Length and Iteration Count parameters: A
peer MUST compare these policy parameters provided by the EAP server
with local policy and MUST NOT continue the handshake if use of the
EAP server's suggested parameters would result in a lower security
than the client's acceptable policy. If the security given by the
EAP server's provided policy parameters surpasses the security level
given by the peer's local policy, the client SHOULD use the server's
parameters (subject to reason - active attackers could otherwise
mount simple denial-of-service attacks against peers or servers,
e.g., by providing unreasonably high values for the iteration count).
Note that the server-provided parameters only apply to the case where
the peer cannot use or does not have a previously provided server-
provided pepper. If a peer cannot continue the handshake due to the
server's policy being unacceptable, it MUST return an empty POTP-X
EAP-Response message.
Authentication Data
EAP-Request: In an EAP-Request, the Authentication Data field, when
present, contains an optional "challenge". The challenge is an
octet string that SHOULD be uniquely generated for each request in
which it is present (i.e., it is a "nonce"), and SHOULD be 8
octets or longer. To avoid fragmentation (i.e., EAP messages
longer than the minimum EAP MTU size; see [1]), the challenge MUST
NOT be longer than 64 octets. When the challenge is not present,
the OTP will be calculated on the current token state only. The
peer MAY ignore a provided challenge if and only if the OTP token
the peer is interacting with is not capable of including a
challenge in the OTP calculation. In this case, EAP server
policies will determine whether or not to accept a provided OTP
value.
EAP-Response: The following applies to the Authentication Data field
in an EAP-Response:
* When the P bit is not set, the peer SHALL directly place the
OTP value calculated by the token in the Authentication Data
field. In this case, the EAP server MUST NOT send a Confirm
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TLV upon successful authentication of the peer (instead, it
sends an EAP-Success message).
* When the P bit is set, the peer SHALL populate this field as
follows. After the token has calculated the OTP value, the
peer SHALL compute:
K_MAC | K_ENC | MSK | EMSK | SRK = PBKDF2(otp, salt | pepper
| auth_id, iteration_count, key_length)
where
"|" denotes concatenation,
"otp" is the already computed OTP value,
"salt" is a 16-octet nonce,
"pepper" is an optional nonce (at most, 255 bits long, and,
if necessary, padded to be a multiple of 8 bits long; see
below) included to complicate the task of finding a matching
"otp" value for an attacker,
"auth_id" is an identifier (at most, 255 octets in length)
for the authenticator (i.e., the network access server) as
reported by lower layers and as specified below,
"iteration_count" is an iteration count chosen such that the
computation time on the peer is acceptable (based on the
server's indicated policy and the peer's local policy),
while an attacker, having observed the response and
initiating a search for a matching OTP, will be sufficiently
slowed down. The "iteration_count" value MUST be chosen to
provide a suitable level of protection (e.g., at least
100,000) unless a server-provided pepper is being used, in
which case, it SHOULD be 1.
"key_length" is the combined length of the desired key
material, in octets. When the default algorithms are used,
key_length is 176.
The "pepper" values are only included in PBKDF2 calculations
and are never sent to EAP servers (though the peers do send
their length, in bits). The purpose of the pepper values
are, as mentioned above, to slow down an attacker's search
for a matching OTP, while not slowing down the peer (which
iterated hashes do). If the pepper has been generated by
the peer, and the chosen pepper length in bits is not a
Nystroem Informational [Page 29]
RFC 4793 EAP-POTP February 2007
multiple of 8, then the pepper value SHALL be padded to the
left, with '0' bits to the nearest multiple of 8 before
being used in the PBKDF2 calculation. This is to ensure the
input to the calculation consists only of whole octets. As
an example, if the chosen pepper length is 4, the pepper
value will be padded to the left, with 4 '0' bits to form an
octet before being used in the PBKDF2 calculation.
When pepper is used, it is RECOMMENDED that the length of
the pepper and the iteration count are chosen in such a way
that it is computationally infeasible/unattractive for an
attacker to brute-force search for the given OTP within the
lifetime of that OTP.
As mentioned previously, a peer MUST NOT include a newly
generated pepper value in the PBKDF2 computation if the
server did not indicate its support for pepper searching in
this session. If the server did not indicate support for
pepper searching, then the PBKDF2 computation MUST be
carried out with a sufficiently higher number of iterations
so as to compensate for the lack of pepper (see further
Appendix D).
A server may, in an earlier session, have transferred a
pepper value to the peer in a Confirm TLV (see below). When
this is the case, and the peer still has that pepper value
stored for this server, the peer MUST NOT generate a new
pepper but MUST, instead, use this transferred pepper value
in the PBKDF2 calculations. The only exception to this is
when a local policy (e.g., timer) dictates that the peer
must switch to a new pepper (and the server indicated
support for pepper searching).
The following applies to the auth_id component:
- For dial-up, "auth_id" SHALL be either the empty string
or the phone number called by the peer. The phone number
SHALL be specified in the form of a URL conformant with
RFC 3966 [8], e.g., "tel:+16175550101". Processing of
received phone numbers SHALL be conformant with RFC 3966
(this assumes that "tel" URIs will be shorter than 256
octets, which would normally be the case).
- For use with IEEE 802.1X, "auth_id" SHALL be either the
empty string or the MAC address of the authenticator in
canonical binary format (6 octets).
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RFC 4793 EAP-POTP February 2007
- For IP-based EAP, "auth_id" SHALL be either the empty
string or the IPv4 or IPv6 address of the authenticator
as seen by the peer and in binary format (4 or 16 octets,
respectively). As an example, the IPv4 address
"192.0.2.5" would be represented as (in hex) C0 00 02 05,
whereas the IPv6 address "2001:DB8::101" would be
represented as (in hex) 20 01 0D B8 00 00 00 00 00 00 00
00 00 00 01 01.
Note: Use of the authenticator's identifying information
within the computation aids in protection against man-in-
the-middle attacks, where a rogue authenticator seeks to
intercept and forward the Authentication Data in order to
impersonate the peer at a legitimate authenticator (but see
also the discussion around spoofed authenticator addresses
in Section 6). For these reasons, a peer SHOULD NOT set the
auth_id component to the empty string unless it is unable to
learn the identifying information of the authenticator. In
these cases, the EAP server's policy will determine whether
or not the session may continue.
As an example, when otp = "12345678", salt =
0x54434534543445435465768789099880, pepper is not used,
auth_id = "192.0.2.5", iteration_count = 2000 (decimal), and
key_length = 176 (decimal), the input to the PBKDF2
calculation will be (first two parameters in hex, line wrap
for readability):
(3132333435363738, 54434534543445435465768789099880 |
c0000205, 2000, 176)
As described, when the default algorithms are used, K_MAC is
the first 16 octets of the output from PBKDF2, K_ENC the
next 16 octets, MSK the following 64 octets, EMSK the next
64 octets, and SRK the final 16 octets. Using K_MAC, the
peer calculates:
mac = MAC(K_MAC, msg_hash(msg_1, msg_2, ..., msg_n))
as specified in Section 4.9 and where msg_1, msg_2, ...,
msg_n is a sequence of all EAP messages of type POTP-X
exchanged so far in this session, as sent and received by
the peer (for the peer's initial MAC, it will typically be
just one message: the EAP server's initial EAP-Request of
type POTP-X).
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The peer then places the first 16 octets of "mac" in the
Authentication Data field, followed by the "salt" value,
followed by one octet representing the length of the
"auth_id" value in octets, followed by the actual "auth_id"
value in binary form, and optionally followed by a pepper
identifier (only when the peer made use of a pepper value
previously provided by the EAP server). Pepper identifiers,
when present, are always 4 octets. All variables SHALL be
present in the form they were input to the PBKDF2 algorithm.
This will result in the Authentication Data field being 33 +
(length of auth_id in octets) + (4, for pepper identifier,
when present) octets in length.
Continuing the previous example, the Authentication Data
field will be populated with (in hex, line wrap for
readability):
< 16 octets of mac > | 54434534543445435465768789099880 |
04 | c0000205
Note: Since in this case (i.e., when the P bit is set)
successful authentication of the peer by the EAP server will
be followed by the transmission of an EAP-Request of type
POTP-X containing a Confirm TLV for mutual authentication,
the peer MUST save either all the input parameters to the
PBKDF2 computation or the keys K_MAC, K_ENC, SRK, MSK, and
EMSK (recommended, since they will be used later). This is
because the peer cannot be guaranteed to be able to generate
the same OTP value again. For the same reason (the Confirm-
TLV from the EAP server), the peer MUST also store either
the hash of the contents of the sent EAP-Response or the
EAP-Response itself (but see the note above about not
including any User Identifier TLVs in the hash computation).
Given a set of possible OTP values, the authentication
server verifies an authentication request from the peer by
computing
K_MAC' | K_ENC' | MSK' | EMSK' | SRK' = PBKDF2 (otp',
salt | pepper' | auth_id, iteration_count, key_length)
for each possible OTP value otp' and each possible pepper
value pepper' , and the provided values for salt,
authenticator identity, and iteration count, as well as the
applicable key length (default: 176). Note: Doing the
computation for each possible pepper value implements the
pepper search mentioned elsewhere in this document. Note
also that the EAP server may accept more than one OTP value
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at a given time, e.g., due to clock drift in the token. If
the given pepper length is not a multiple of 8, each tested
pepper value will be padded to the left to the nearest
multiple of 8, in the same manner as was done by the peer.
If the server already shares a secret pepper value with this
peer, then obviously there will only be one possible pepper
value, and the server will find it based on the
pepper_identifier provided by the peer. The server SHALL
send a new EAP-Request of type POTP-X with an OTP TLV with
the E bit set if the peer provided a pepper identifier
unknown to the server.
For each K_MAC', the EAP server computes
mac' = MAC(K_MAC', msg_hash(msg_1', msg_2', ..., msg_n'))
where MAC is the negotiated MAC algorithm, msg_hash is the
message hash algorithm defined in Section 4.9, and msg_1',
msg_2', ... msg_n' are the same messages on which the peer
calculated its message hash, but this time, as sent and
received by the EAP server. If the first 16 octets of mac'
matches the first 16 octets in the Authentication Data field
of the EAP-Response in question, and the provided
authenticator identity is acceptable (e.g., matches the EAP
server's view of the authenticator's identity), then the
peer is authenticated.
If the authentication is successful, the authentication
server then attempts to authenticate itself to the peer by
use of the Confirm TLV (see below). If the authentication
fails, the EAP server MAY send another EAP-Request of type
POTP-X containing an OTP TLV to the peer, or it MAY send an
EAP-Failure message (in both cases, possibly preceded by an
EAP-Request of type Notification).
4.11.4. NAK TLV
Presence of this TLV indicates that the peer did not support a
received TLV with the M bit set. This TLV may occur 0, 1, or more
times in an EAP-Response of type POTP-X. Each occurrence flags the
non-support of a particular received TLV.
The NAK TLV MUST be supported by all peers and all EAP servers
conforming to this specification and MUST NOT be responded to with a
NAK TLV. Receipt of a NAK TLV by an EAP server MAY cause an
authentication to fail, and the EAP server to send an EAP-Failure
message to the peer.
Nystroem Informational [Page 33]
RFC 4793 EAP-POTP February 2007
Note: The definition of the NAK TLV herein matches the definition
made in [17], and has the same type number. Field descriptions are
copied from that document, with some minor modifications.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vendor-Id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NAK-Type | TLVs ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
1 - Mandatory TLV
R
Reserved for future use. This bit SHALL be set to zero (0) for
this version. Recipients SHALL ignore this bit for this version
of EAP-POTP.
TLV Type
4
Length
6 + cumulative total length of embedded TLVs
Vendor-Id
The Vendor-Id field is 4 octets, and contains the Vendor-Id of the
TLV that was not supported. The high-order octet is 0 and the
low-order 3 octets are the Structure of Management Information
(SMI) Network Management Private Enterprise Code of the Vendor in
network byte order. The Vendor-Id field MUST be zero for TLVs
that are not Vendor-Specific TLVs. For Vendor-Specific TLVs, the
Vendor-ID MUST be set to the SMI code.
NAK-Type
The type of the unsupported TLV. The TLV MUST have been included in
the most recently received EAP message.
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RFC 4793 EAP-POTP February 2007
TLVs
This field contains a list of TLVs, each of which MUST NOT have the
mandatory bit set. These optional TLVs can be used in the future to
communicate why the offending TLV was determined to be unsupported.
4.11.5. New PIN TLV
In an EAP-Request, the New PIN TLV is used to request a new user PIN
from the peer. The EAP server MAY provide a new PIN, as described
below. In an EAP-Response, the New PIN TLV carries a chosen new user
PIN. This TLV may be used by an EAP server when policy dictates that
the peer (user) needs to change a PIN associated with the OTP Token.
This TLV type SHOULD be supported by peers and EAP servers conforming
to this specification. The New PIN TLV MUST NOT be sent by an EAP
server unless the peer has been authenticated. If the peer was
authenticated in protected mode, then the New PIN TLV MUST NOT be
present in an EAP-Request until after the exchange of the Confirm TLV
(i.e., until after mutual authentication has occurred and keys are in
place to protect the TLV). The New PIN TLV MUST be sent by a peer if
and only if the EAP-Request that triggered the response contained a
New PIN TLV, it was valid for the EAP server to send such a TLV in
that request, and the TLV is supported by the peer.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |Q|A| PIN Length | PIN ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Min. PIN Length|Max. PIN Length|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
1 - Mandatory TLV
R
Reserved for future use. This bit SHALL be set to zero (0) for
this version. Recipients SHALL ignore this bit for this version
of EAP-POTP.
TLV Type
5
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RFC 4793 EAP-POTP February 2007
Length
2 + length of the PIN field (as specified in the PIN Length field)
+ (0, 1, or 2)
Note: The final term above is
- 0 if none of the optional Min. / Max. PIN Length fields is
present in the TLV,
- 1 if only the Min. PIN Length field is present in the TLV,
- 2 if both of these optional fields are present in the TLV.
Reserved
Reserved for future use. All six bits SHALL be set to zero for
this version. Recipients SHALL ignore these bits for this version
of EAP-POTP.
Q
The Q bit, when set in an EAP-Request, indicates that an
accompanying PIN is required, i.e., the peer (user) is not free to
choose another PIN. When the Q bit is set, there MUST be an
accompanying PIN and the provided PIN MUST be used in subsequent
OTP generations. A peer SHALL respond with an empty POTP-X EAP-
Response message if the Q bit is set but there is not any
accompanying PIN. When the Q bit is not set, any provided PIN is
suggested only, and the peer is free to choose another PIN,
subject to local policy.
The Q bit carries no meaning, and SHALL be set to zero, in an EAP-
Response.
A
This bit allows methods that distinguish between two different PIN
types (e.g., decimal vs. alphanumeric) to designate whether the
augmented set is to be used (when set) or not (when not set). The
A bit carries no meaning, and SHALL be set to zero, in an EAP-
Response.
PIN Length
This field contains an unsigned integer representing the length of
the provided PIN (this implies that the maximum length of a PIN
will be 255 octets).
Nystroem Informational [Page 36]
RFC 4793 EAP-POTP February 2007
PIN
In an EAP-Request, subject to the setting of the Q bit, the PIN
field MAY be empty. If empty, the peer (user) will need to choose
a PIN subject to local and (any) provided policy. When the PIN
field is not empty, it MUST consist of UTF-8 encoded printable
characters without a terminating NULL character.
In an EAP-Response, the PIN value SHALL consist of a UTF-8 encoded
string of printable characters without a terminating NULL
character.
The peer accepts a PIN suggested by the EAP server by replying
with the same PIN, but MAY replace it with another one, depending
on the server's setting of the Q bit. The length of the PIN is
application-dependent, as are any other requirements for the PIN,
e.g., allowed characters. The peer MUST be prepared to receive a
repeated request for a new PIN, as described above, if the EAP
server, for some reason does not accept the received PIN. Such a
request MAY be preceded by an EAP-Request of type Notification (2)
providing information to the user about the reason for the
rejection. Mechanisms for transferring knowledge about PIN
requirements from the EAP server to the peer (beyond those
specified for this TLV, such as maximal and minimal PIN length)
are outside the scope of this document. However, some information
MAY be provided in notification messages transferred from the EAP
server to the peer, as per above.
Min. PIN Length
This field MAY be present in an EAP-Request. This field MUST NOT
be present in an EAP-Response. It SHALL be interpreted as an
unsigned integer in network byte order representing the minimum
length allowed for a new PIN.
Max. PIN Length
This field MUST NOT be present in an EAP-Request unless the Min.
PIN Length field is present, in which case it MAY be present. The
field MUST NOT be present in an EAP-Response. It SHALL be
interpreted as an unsigned integer in network byte order
representing the maximum length allowed for a new PIN. The value
of this field, when present, MUST be equal to, or larger than, the
value of the Min. PIN Length field.
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RFC 4793 EAP-POTP February 2007
4.11.6. Confirm TLV
Presence of this TLV in a request indicates that the EAP server has
successfully authenticated the peer and now attempts to authenticate
itself to the peer. Presence of this TLV in a response indicates
that the peer successfully authenticated the EAP server, and that
calculated keys (K_MAC, K_ENC, MSK, EMSK, and SRK) now become
available for use.
The Confirm TLV MUST NOT appear together with any other TLV in an
EAP-Request message of type POTP-X and MUST NOT be sent unless the
peer has been authenticated through an OTP TLV with the P bit set or
through a Resume TLV for which the underlying session was established
in protected mode. The Confirm TLV MUST be present in an EAP-
Response if and only if the request that triggered the response
contained a Confirm TLV, it was legal for it to do so, and the
Confirm TLV authenticated the EAP server to the peer. If the peer
was not able to authenticate the server, then it MUST send an empty
(i.e., no TLVs present) EAP-Response of type POTP-X.
An EAP server MUST send an EAP-Success message after receiving an
EAP-Response of type POTP-X containing a valid Confirm TLV, sent in
response to an EAP-Request containing a Confirm TLV where the C bit
was not set. A peer MUST NOT accept an EAP-Success message when it
has sent an OTP TLV with the P bit set unless it has received an
acceptable Confirm TLV from the EAP server.
This TLV type MUST be supported by all peers and EAP servers
conforming to this specification and MUST NOT be responded to with a
NAK TLV.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |C| Authentication Data ... (16 octets)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pepper Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IV ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encrypted Pepper ... (16 octets)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
1 - Mandatory TLV
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RFC 4793 EAP-POTP February 2007
R
Reserved for future use. This bit SHALL be set to zero (0) for
this version. Recipients SHALL ignore this bit for this version
of EAP-POTP.
TLV Type
6
Length
17 or 37 + length of IV in requests, 1 in responses.
Reserved
Reserved for future use. These 7 bits SHALL be set to zero (0)
for this version. Recipients SHALL ignore these bits for this
version of EAP-POTP.
C
The C bit, when set in an EAP-Request, indicates that the EAP
server intends to send more EAP-Requests of type POTP-X in this
session, after receipt of a Confirm TLV from the peer.
The C bit carries no meaning in EAP-Responses, and MUST NOT be set
within them.
Note: An EAP-Response containing a Confirm TLV, sent in response
to an EAP-Request containing a Confirm TLV that did not have the C
bit set, MUST be followed by an EAP-Success message from the EAP
server concluding the handshake. However, when the C bit was set
in an EAP-Request, the EAP server MAY send another EAP-Request
(containing, for example, a New PIN TLV wrapped in a Protected
TLV) rather than an EAP-Success message. Therefore, peers MUST
NOT assume that the only EAP message following an EAP-Response of
type POTP-X containing a Confirm TLV is EAP-Success. The C bit
gives EAP servers a way to indicate their intent to follow the
Confirm TLV with more requests, and allows the peer's state
machine to adapt to this.
Authentication Data
EAP-Request:
In a request, this field consists of the first 16 octets of
(see also Section 4.11.3):
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RFC 4793 EAP-POTP February 2007
mac_a = MAC(K_MAC', msg_hash(trig_msg))
where
MAC is the negotiated MAC algorithm,
"K_MAC'" has been calculated as described in Section 4.11.3 or
(in the case of session resumption) Section 4.11.8, and
"msg_hash" is the message hash algorithm defined in Section
4.9, and "trig_msg" the latest EAP-Response of type POTP-X
received from the peer (the one which triggered this request).
Given a saved or recomputed value for K_MAC, the peer
authenticates the EAP server by computing
mac'' = MAC(K_MAC, msg_hash(trig_msg'))
where "msg_hash(trig_msg')" is the peer's hash of the EAP-
Response message that it sent to the server (and that the
server calculated its message hash on). If the first 16 octets
of mac'' matches the first 16 octets in the Authentication Data
field of the EAP-Request in question, then the EAP server is
authenticated.
EAP-Response:
Not used in this version, and SHALL NOT be present in EAP-
Responses.
Pepper Identifier
In an EAP-Request, the truncated MAC MAY optionally be followed by
an encrypted pepper and its identifier. This initial, 4-octet
field identifies a pepper generated by the server.
For this version of EAP-POTP, this field SHALL NOT be present in
EAP-Responses.
IV (Initialization Vector)
An initialization vector for the encryption. The length of the
vector is dependent on the negotiated encryption algorithm. For
example, for AES-CBC, it SHALL be 16 octets. The IV is only
present if a pepper is present, and the negotiated encryption
algorithm makes use of an IV. This field SHALL NOT be present in
EAP-Response messages for this version of EAP-POTP.
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RFC 4793 EAP-POTP February 2007
Encrypted Pepper
When present in an EAP-Request, this will be a uniformly
distributed and randomly chosen 16-octet pepper generated by the
EAP server and encrypted with the negotiated encryption algorithm,
using K_ENC as the encryption key and possibly (depending on the
encryption algorithm) using an IV (stored in the IV field). This
field MUST be present if and only if the Pepper Identifier field
is present.
EAP servers are RECOMMENDED to include a freshly generated
encrypted pepper (and a corresponding Pepper Identifier) in every
Confirm TLV.
This field SHALL NOT be present in EAP-Response messages for this
version of EAP-POTP.
When a new pepper is generated by the server and transferred in
encrypted form to the peer, then this new pepper value will be stored
in the EAP server upon receipt of the Confirm TLV from the peer, and
SHOULD be stored with its identifier and associated with the EAP
server and the current user in the peer upon receipt of the EAP-
Success message. If the peer already had a pepper stored for the EAP
server, it SHALL replace it with the newly received one.
4.11.7. Vendor-Specific TLV
The Vendor-Specific TLV is available to allow vendors to support
their own extended attributes not suitable for general usage. A
Vendor-Specific TLV can contain one or more inner TLVs, referred to
as Vendor TLVs. The TLV-type of a Vendor TLV will be defined by the
vendor. All the Vendor TLVs inside a single Vendor-Specific TLV
SHALL belong to the same vendor.
This TLV type MAY be sent by EAP servers, as well as by peers, and
MUST be supported by all entities conforming to this specification.
Conforming implementations may not support specific Vendor TLVs
inside a Vendor-Specific TLV, however. They MAY, in this case,
respond to the Vendor TLVs with a NAK TLV containing the appropriate
Vendor-ID and Vendor TLV type.
The presence of a Vendor-Specific TLV in an EAP-Request or EAP-
Response of type POTP-X MUST NOT violate any existing rules for
coexistence of TLVs in such requests or responses. If it does, then
it will result in an EAP-Failure (when the peer made the violation)
or an empty EAP-POTP response (when the EAP-server made the
violation). It is left to the definition of specific Vendor-Specific
TLVs to further constrain when they are allowed to appear. In
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RFC 4793 EAP-POTP February 2007
particular, EAP-POTP implementations may have policies that
completely disallow use of the Vendor-Specific TLV before protected
mode mutual authentication has occurred (since the Protected TLV,
Section 4.11.15, then can be used to protect all TLVs).
Note: This TLV type has the same definition and TLV type number as
the Vendor-Specific TLV in [17], and the description of it is largely
borrowed from that document.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vendor-Id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vendor TLVs ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
1 - Mandatory TLV
R
Reserved for future use. This bit SHALL be set to zero (0) for
this version. Recipients SHALL ignore this bit for this version
of EAP-POTP.
TLV Type
7
Length
4 + cumulative total length of inner Vendor TLVs
Vendor-ID
The Vendor-Id field is 4 octets. The high-order octet SHALL be
set to 0, and the low-order 3 octets SHALL be set to the SMI
Network Management Private Enterprise Code (see [18]) of the
Vendor in network byte order.
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RFC 4793 EAP-POTP February 2007
Vendor TLVs
This field shall contain vendor-specific TLVs, in a format defined
by the vendor. To avoid fragmentation (i.e., EAP messages longer
than the minimum EAP MTU size), the field SHOULD NOT be longer
than 256 octets.
To ensure interoperability when an EAP entity (peer or server) from
vendor A sends a vendor-specific TLV that is not understood by the
recipient EAP entity from vendor B, the vendor A entity SHALL, upon
receipt of the NAK TLV from the recipient, refrain from usage of the
vendor-specific TLV in question for the rest of the handshake, and
MUST NOT fail the session due to the receipt of the NAK TLV for the
Vendor TLV (i.e., it SHALL continue as if the vendor-specific TLV had
not been sent). Additionally, all implementations conformant with
this document SHOULD allow use of vendor-specific extensions to be
turned off via configuration.
4.11.8. Resume TLV
The Resume TLV MAY be sent by a peer to an authentication server to
attempt session resumption.
This TLV type MUST only be sent in response to an EAP-Request of type
POTP-X containing a Server-Info TLV allowing session resumption. The
Resume TLV MUST be supported by all EAP servers that send a Server-
Info TLV allowing session resumption.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Session Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Session Identifier (continued) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Sess.Id (cont.)| Authentication Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication Data (cont.) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
0 - Non-mandatory TLV
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RFC 4793 EAP-POTP February 2007
R
Reserved for future use. This bit SHALL be set to zero (0) for
this version. Recipients SHALL ignore this bit for this version
of EAP-POTP.
TLV Type
8
Length
45
Reserved
Reserved for future use. This octet SHALL be set to zero (0) for
this version. Recipients SHALL ignore this octet for this version
of EAP-POTP.
Session Identifier
An 8-octet identifier for the session the peer is trying to
resume.
Authentication Data
Upon receipt of the Server-Info TLV, and if the N bit is not set,
the peer searches for any stored sessions associated with the
server identified by the Server Name field. If a stored session
is found, the peer generates a random, 16-octet nonce, "c_nonce",
and calculates:
K_MAC | K_ENC | MSK | EMSK | SRK = PBKDF2(base_key, c_nonce |
s_nonce, iteration_count, key_length)
where
"|" denotes concatenation,
"base_key" is either the current SRK for the session (if the
session was created in protected mode) or the OTP used when the
session was created (if the session was created in basic mode),
"c_nonce" is the generated 16-octet nonce,
"s_nonce" is the server nonce from the Server-Info TLV,
Nystroem Informational [Page 44]
RFC 4793 EAP-POTP February 2007
"iteration_count" is the iteration count as determined by local
policy, and
"key_length" is the combined length of the desired key material,
in octets. When the default algorithms are used, key_length is
176.
The iteration count need only be 1 (one) when resuming a session
established in protected mode, but MUST be chosen to provide a
suitable level of protection when resuming a session established
in basic mode (see also Section 4.11.3).
Note: Session resumption for basic mode MUST only be carried out
in a server-authenticated and protected tunnel that also provides
a cryptographic binding for inner EAP methods.
The peer then calculates:
mac = MAC(K_MAC, msg_hash(resume_req))
where
"MAC" is the negotiated MAC algorithm, and
"msg_hash(resume_req) is the message hash algorithm defined in
Section 4.9 applied on resume_req, the EAP server's EAP-Request of
type POTP-X containing the Server-Info TLV that allowed session
resumption.
The peer then places the first 16 octets of the MAC value,
followed by the c_nonce value, followed by the iteration count
value (as a 4-byte unsigned integer in network byte order), in the
Authentication Data field. As an example, when c_nonce =
0x2b3b1b12babdebebfb43bd7bdfbeb8df and iteration_count = 1, the
Authentication Data field will be populated with (in hex):
< 16 octets of mac > | 2b3b1b12babdebebfb43bd7bdfbeb8df | 00000001
The server authenticates the peer by performing the corresponding
calculations. If the authentication is successful, the server
MUST send an EAP-Request of type POTP-X containing a Confirm TLV
to the peer. If the authentication fails, the server MUST either
send an EAP-Request of type POTP-X containing an OTP TLV and a
Server-Info TLV, where the Server-Info TLV indicates that session
resumption is not possible, or send an EAP-Failure.
Nystroem Informational [Page 45]
RFC 4793 EAP-POTP February 2007
When resuming in basic mode, all calculated keys SHALL be
discarded after the MAC has been calculated and verified. When
resuming in protected mode, the new SRK will replace the stored
SRK, and the new MSK and EMSK will be exported upon successful
completion of the method.
4.11.9. User Identifier TLV
The User Identifier TLV carries an identifier, typically the
username, for the holder of the OTP token used to generate the OTP.
At least one of the User Identifier TLV and the Token Key Identifier
TLV SHOULD be present in the session's first EAP-Response of type
POTP-X that also carries an OTP TLV unless a suitable identity has
been provided in a preceding EAP-Response of type Identity (1) or is
determined by some other means (see [1], Section 2). Use of the User
Identifier TLV and/or the Token Key Identifier TLV is RECOMMENDED
even when an EAP-Response of type Identity (1) has been sent. If a
peer sends both a User Identifier TLV and a Token Key Identifier TLV,
then the EAP server SHALL interpret the Token Key Identifier TLV as
specifying a particular token key for the given user. The EAP server
MUST respond with an EAP-Failure if it cannot find a token key for
the provided user.
This TLV type is sent by peers and MUST be supported by all EAP
servers conforming to this specification. The User Identifier TLV
MUST NOT be present in a response that does not also carry an OTP
TLV.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| User Identifier ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
1 - Mandatory TLV
R
Reserved for future use. This bit SHALL be set to zero (0) for
this version. Recipients SHALL ignore this bit for this version
of EAP-POTP.
Nystroem Informational [Page 46]
RFC 4793 EAP-POTP February 2007
TLV Type
9
Length
Length of User Identifier, >= 1
User Identifier
The value SHALL be an UTF-8 encoded string representing the holder
of the token (MUST NOT be NULL-terminated). The string MUST be
less than 128 octets in length.
4.11.10. Token Key Identifier TLV
The Token Key Identifier TLV carries an identifier for the token key
used to generate the OTP.
At least one of the User Identifier TLV and the Token Key Identifier
TLV SHOULD be present in the session's first EAP-Response of type
POTP-X, which also carries the OTP TLV unless a suitable identity has
been provided in a preceding EAP-Response of type Identity (1) or is
determined by some other means (see [1], Section 2). Use of the User
Identifier TLV and/or the Token Key Identifier TLV is RECOMMENDED
even when an EAP-Response of type Identity (1) has been sent. If a
peer sends both a User Identifier TLV and a Token Key Identifier TLV,
then the EAP server SHALL interpret the Token Key Identifier TLV as
specifying a particular token key for the given user. The EAP server
MUST respond with an EAP-Failure if it cannot find a token key
corresponding to the provided token key identifier.
This TLV type is sent by peers and MUST be supported by all EAP
servers conforming to this specification. The Token Key Identifier
TLV MUST NOT be present in a response that does not also carry an OTP
TLV.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Token Key Identifier ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
1 - Mandatory TLV
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RFC 4793 EAP-POTP February 2007
R
Reserved for future use. This bit SHALL be set to zero (0) for
this version. Recipients SHALL ignore this bit for this version
of EAP-POTP.
TLV Type
10
Length
Length of Token Key Identifier, >= 1
Token Key Identifier
An identifier for the OTP token key used to generate the OTP. The
field MUST be less than 128 octets in length.
4.11.11. Time Stamp TLV
The Time Stamp TLV MAY be sent by peers to simplify authentications.
When present, it carries the time as reported by the OTP Token.
An EAP server conformant with this specification SHOULD support
(i.e., recognize) this TLV, but need not be able to process or act on
it. An EAP server that does not support this TLV, but receives an
EAP-Response with the TLV present, MAY ignore the value. The Time
Stamp TLV MUST NOT be present in any EAP-Responses of type POTP-X
other than those that also carries an OTP TLV.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time Stamp ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
0 - Non-mandatory TLV
R
Reserved for future use. This bit SHALL be set to zero (0) for
this version. Recipients SHALL ignore this bit for this version
of EAP-POTP.
Nystroem Informational [Page 48]
RFC 4793 EAP-POTP February 2007
TLV Type
11
Length
Length of Time Stamp field, >= 20 (depending on precision)
Time Stamp
The time, as reported by the OTP token, at which the OTP used for
the accompanying OTP TLV was calculated. The field SHALL contain
a UTF-8 encoded value of the XML simple type "dateTime", with time
zone information and precision down to at least seconds, e.g.,
"2004-06-16T15:20:02Z".
4.11.12. Counter TLV
The Counter TLV MAY be sent by peers to simplify authentications.
When present, it carries the token counter value, as reported by the
OTP Token.
An EAP server conformant with this specification SHOULD support
(i.e., recognize) this TLV, but need not be able to process or act on
it. An EAP server that does not support this TLV, but receives an
EAP-Response with the TLV present, MAY ignore the value. The Counter
TLV MUST NOT be present in any EAP-Responses of type POTP-X other
than those that also carries an OTP TLV.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Counter ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
0 - Non-mandatory TLV
R
Reserved for future use. This bit SHALL be set to zero (0) for
this version. Recipients SHALL ignore this bit for this version
of EAP-POTP.
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RFC 4793 EAP-POTP February 2007
TLV Type
12
Length
Length of Counter field, >= 1 (depending on precision)
Counter
The counter value, as reported by the OTP token, at which the OTP
used for the accompanying OTP TLV was calculated. The counter
value SHALL be represented as an unsigned integer in network-byte
order, e.g., a counter value of 1030 may be sent as the 2 octets
(in hex) 04 06.
4.11.13. Challenge TLV
The Challenge TLV carries the challenge used by the token to
calculate the OTP, as reported by the token to the peer. The
Challenge TLV MUST be sent by a peer if and only if the challenge
otherwise would be unknown to the EAP server (e.g., the token or peer
modified a received challenge or generated its own challenge).
An EAP server conformant with this specification SHOULD support
(i.e., recognize) this TLV, but need not be able to process or act on
it. An EAP server that does not support this TLV, but receives an
EAP-Response with the TLV present, MAY ignore the value. The
Challenge TLV MUST NOT be present in any EAP-Responses of type POTP-X
other than those that also carry an OTP TLV.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Challenge ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
0 - Non-mandatory TLV
R
Reserved for future use. This bit SHALL be set to zero (0) for
this version. Recipients SHALL ignore this bit for this version
of EAP-POTP.
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RFC 4793 EAP-POTP February 2007
TLV Type
16
Length
Length of Challenge field, >= 1
Challenge
The challenge value that was used to calculate the OTP used for
the accompanying OTP TLV.
4.11.14. Keep-Alive TLV
The Keep-Alive is used to avoid EAP-POTP timeouts.
The Keep-Alive TLV MAY be sent by a peer to avoid timeouts when the
peer has received an EAP-Request containing an OTP TLV or a New PIN
TLV and is waiting for a response from the user.
An EAP-Request containing a Keep-Alive TLV MUST be sent by an EAP
server when the server receives an EAP-Response containing a Keep-
Alive TLV, and the server has an outstanding request that did not
contain a Keep-Alive TLV. In this situation, the server does not
need to re-transmit its latest outstanding request, but, due to the
synchronous nature of EAP, it needs to send another request. Re-
transmission of the latest outstanding request could be confusing for
the peer since the request would get a new Identifier value. The
Keep-Alive TLV MAY also be sent by an EAP server when the server
detects that its processing time will exceed some locally configured
threshold and may cause a network timeout. In this case, the peer
MUST respond with an EAP-Response containing a Keep-Alive TLV.
This TLV type MUST be supported by all peers and all EAP servers
conforming to this specification and MUST NOT be responded to with a
NAK TLV. The Keep-Alive TLV MUST NOT be sent in any other situations
than the ones described above. The Keep-Alive TLV MUST NOT be sent
together with any other TLVs defined herein. Implementations SHOULD
also follow recommendations made in Section 4.3 of [1].
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Nystroem Informational [Page 51]
RFC 4793 EAP-POTP February 2007
M
1 - Mandatory TLV
R
Reserved for future use. This bit SHALL be set to zero (0) for this
version. Recipients SHALL ignore this bit for this version of EAP-
POTP.
TLV Type
13
Length
0
4.11.15. Protected TLV
The Protected TLV SHALL be used to encrypt individual or multiple
TLVs after successful exchange of the Confirm TLV (i.e., as soon as
calculated keys have been confirmed). The Protected TLV therefore
wraps "ordinary" TLVs.
This TLV type may be sent by EAP servers as well as by peers and MUST
be supported by all peers conforming to this specification. It
SHOULD be supported by all EAP servers conforming to this
specification (it need not be supported if a server never will have a
need to continue a POTP-X conversation after exchange of the Confirm
TLV).
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Authentication Code ... (16 octets)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IV ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encrypted TLVs ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
1 - Mandatory TLV
Nystroem Informational [Page 52]
RFC 4793 EAP-POTP February 2007
R
Reserved for future use. This bit SHALL be set to zero (0) for
this version. Recipients SHALL ignore this bit for this version
of EAP-POTP.
TLV Type
14
Length
>32
Message Authentication Code (MAC)
This field integrity-protects the TLV. The MAC SHALL be
calculated over the IV and the Encrypted TLVs field in the
following manner:
mac = MAC(K_MAC, iv | encrypted_tlvs)
where
MAC is the negotiated MAC algorithm, "iv" is the IV field's value,
and "encrypted_tlvs" is the value of the Encrypted TLVs field.
The first 16 octets of the MAC is placed in the Message
Authentication Code field.
Recipients MUST verify the MAC. If the verification fails, the
conversation SHALL be terminated (i.e., peers send an empty POTP-X
EAP-Response message, and EAP servers send an EAP-Failure message
possibly preceded by an EAP-Request of type Notification).
IV
An initialization vector for the encryption; see below. The
length of the vector is dependent on the negotiated encryption
algorithm, e.g., for AES-CBC, it shall be 16 octets. For some
encryption algorithms, there may not be any initialization vector.
An IV, when present, shall be randomly chosen and non-predictable.
Encrypted TLVs
This field SHALL contain one or more encrypted POTP-X TLVs. The
encryption algorithm SHALL be as negotiated; use K_ENC as the
encryption key, and use the IV field as the initialization vector
Nystroem Informational [Page 53]
RFC 4793 EAP-POTP February 2007
(when applicable), to encrypt the concatenation of all the TLVs to
be protected.
4.11.16. Crypto Algorithm TLV
The Crypto Algorithm TLV allows for negotiation of cryptographic
algorithms. Cryptographic Algorithm negotiation is described in
detail in Section 4.3.
This TLV MUST be present in the initial EAP-Request of type POTP-X
that also carries an OTP TLV indicating protected mode, assuming the
EAP server wants to negotiate use of any other algorithms than the
default ones. It MAY also be present in an EAP-Request of type
POTP-X that carries an OTP TLV that is sent as a result of a failed
session resumption (in this case, the peer has not yet responded to
this TLV), or when the Crypto Algorithm TLV was part of the initial
message from the EAP server, and the client negotiated another EAP-
POTP version than the highest one supported by the EAP server. The
Crypto Algorithm TLV MUST NOT be present in any other EAP-Requests.
Further, the Crypto Algorithm TLV MUST NOT be present in an EAP-
Response of type POTP-X unless the preceding EAP-Request also
contained it, and it was legal for it to do so. This TLV MUST be
supported by all peers and all EAP servers conforming to this
specification and MUST NOT be responded to with a NAK TLV.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |Hash Alg.Length| Hash Algorithms ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Encr.Alg.Length| Encryption Algorithms ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|MAC Alg. Length| MAC Algorithms ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
1 - Mandatory TLV
R
Reserved for future use. This bit SHALL be set to zero (0) for
this version. Recipients SHALL ignore this bit for this version
of EAP-POTP.
Nystroem Informational [Page 54]
RFC 4793 EAP-POTP February 2007
TLV Type
15
Length
>=4 (at least one class of algorithms and one algorithm for that
class needs to be present)
Reserved
Reserved for future use. This octet MUST be set to zero for this
version. Recipients SHALL ignore this octet for this version of
EAP-POTP.
Hash Alg. Length
The length of the Hash Algorithms field in octets.
Hash Algorithms
Each octet pair of this field represents a hash algorithm as
follows. An EAP server MAY supply several suggestions for hash
algorithms. Each algorithm MUST appear only once. The algorithms
SHALL be supplied in order of priority. Peers MUST supply, at
most, one algorithm (if none is present, the default applies).
The defined values are:
Value
Octet 1 Octet 2 Hash algorithm
------- ------- ----------------------------------
0x00 0x00 Reserved
0x00 0x01 SHA-1
0x00 0x02 SHA-224
0x00 0x03 SHA-256 (default)
0x00 0x04 SHA-384
0x00 0x05 SHA-512
0x80 - Vendor-specific (or experimental)
As indicated, values 0x8000 and higher are for proprietary
vendor-specific algorithms. Values in the range 0x0006 - 0x7fff
are to be assigned through IANA; see Section 7.
Encr Alg. Length
The length of the Encryption Algorithms field in octets.
Nystroem Informational [Page 55]
RFC 4793 EAP-POTP February 2007
Encryption Algorithms
Each octet pair of this field represents an encryption algorithm
as follows. An EAP server MAY supply several suggestions for
encryption algorithms. Each algorithm MUST appear only once. The
algorithms SHALL be supplied in order of priority. Peers MUST
supply, at most, one algorithm (if none is present, the default
applies). The defined values are:
Value
Octet 1 Octet 2 Encryption algorithm
------- ------- ------------------------
0x00 0x00 Reserved
0x00 0x01 AES-CBC (default) with 128-bit keys and 16-octet IVs
0x00 0x02 3DES-CBC with 112-bit keys and 8-octet IVs
0x80 - Vendor-specific
As indicated, values 0x8000 and higher are for vendor-specific
proprietary algorithms. Values in the range 0x0003 - 0x7fff are
to be assigned through IANA; see Section 7.
MAC Alg. Length
The length of the MAC Algorithms field in octets.
MAC Algorithms
Each octet pair of this field represents a MAC algorithm as
follows. An EAP server MAY supply several suggestions for MAC
algorithms. Each algorithm MUST appear only once. The algorithms
SHALL be supplied in order of priority. Peers MUST supply, at
most, one algorithm (if none is present, the default applies).
The defined values are:
Value
Octet 1 Octet 2 MAC algorithm
------- ------- -----------------
0x00 0x00 Reserved
0x00 0x01 HMAC (default)
0x80 - Vendor-specific
As indicated, values 0x8000 and higher are for vendor-specific
proprietary algorithms. Values in the range 0x0002 - 0x7fff are
to be assigned through IANA; see Section 7.
When HMAC is negotiated, the hash algorithm used for HMAC SHALL be
the negotiated hash algorithm.
Nystroem Informational [Page 56]
RFC 4793 EAP-POTP February 2007
5. EAP Key Management Framework Considerations
In line with recommendations made in [16], EAP-POTP defines the
following identifiers to be associated with generated key material:
Peer-ID: The combined contents of the User Identifier TLV and the
Token Key Identifier TLV.
Server-ID: The contents of the Server Identifier field of the
Server-Info TLV.
Method-ID: The identifier of the established session (i.e., the
contents of the Session Identifier field of the Server-Info TLV
that defined the session).
6. Security Considerations
6.1. Security Claims
In conformance with RFC 3748 [1], the following security claims are
made for the EAP-POTP method:
Authentication mechanism: Generic OTP
Ciphersuite negotiation: Yes (No in basic variant)
Mutual authentication: Yes (No in basic variant)
Integrity protection: Yes (No in basic variant)
Replay protection: Yes (see below)
Confidentiality: Only in the OTP protection variant, and
then only OTP values and any information
sent after exchange of the Confirm TLV
Key derivation: Yes (No in basic variant)
Key strength: Depends on size of OTP value, strength of
underlying shared secret, strength and
characteristics of OTP algorithm, pepper
length, iteration count, and whether the
method is used within a tunnel such as
PEAPv2. For some illustrative examples,
and a further discussion of this, see
Appendix D.
Dictionary attack prot.: N/A (Human-selected passwords not used)
Fast reconnect: Yes
Crypt. binding: N/A (EAP-POTP is not a tunnel method)
Session independence: Yes
Fragmentation: N/A (Packets shall not exceed MTU of 1020)
Channel binding: Yes (No in basic variant)
Acknowledged S/F: Yes
State Synchronization: Yes (No in basic variant)
Nystroem Informational [Page 57]
RFC 4793 EAP-POTP February 2007
6.2. Passive and Active Attacks
The basic variant (i.e., when the protection of OTPs and mutual
authentication is not used) of this EAP method does not provide
session privacy, session integrity, server authentication, or
protection from active attacks. In particular, man-in-the-middle
attacks, where an attacker acts as an authenticator in order to
acquire a valid OTP, are possible.
Similarly, the basic variant of this EAP method does not protect
against session hijacking taking place after authentication. Nor
does it, in itself, protect against replay attacks, where the
attacker gains access by replaying a previous valid request, but see
also the next subsection. When PIN codes are transmitted, they are
sent without protection and are also subject to replay attacks.
In order to protect against these attacks, the peer MUST only use the
basic variant of this method over a server-authenticated and
confidentiality-protected connection. This can be achieved via use
of, PEAPv2 [17], for example.
When the OTP protection variant is used, however, the EAP method
provides privacy for OTPs and new PINs, negotiation of cryptographic
algorithms, mutual authentication, and protection against replay
attacks and protocol version downgrades. It also provides protection
against man-in-the-middle attacks, not due to the infeasibility for a
man-in-the-middle to solve for a valid OTP given an OTP TLV, but due
to the computational expense of finding the OTP in the limited time
period during which it is valid (this is mainly true for tokens,
including the current time in their OTP calculations, or when a sent
challenge has a certain lifetime). It should be noted, however, that
a retrieved OTP, even if "old" and invalid, still may divulge some
information about the user's PIN. Clearly, this is also true for the
basic variant. Implementations of this EAP method, where user PINs
are sent with OTPs, are therefore RECOMMENDED to ensure regular user
PIN changes, regardless of whether the protected variant or the basic
variant is employed.
It should also be noted that, while it is possible for a rogue access
point, e.g., to clone MAC addresses, and hence mount a man-in-the-
middle attack, such an access point will not be able to calculate the
session keys MSK and EMSK. This demonstrates the importance of using
the derived key material properly to protect a subsequent session.
Protected mode protects against version downgrade attacks due to the
HMAC both parties transmit in this mode. As described, each party
calculates the HMAC on sent and received EAP-POTP handshake messages.
If an attacker were to modify a Version TLV, this would be reflected
Nystroem Informational [Page 58]
RFC 4793 EAP-POTP February 2007
in a difference between the calculated MACs (since the recipient of
the Version TLV received a different value than the sender sent).
Unless the attacker knows K_MAC, he cannot calculate the correct MAC,
and hence the difference will be detected.
The OTP protection variant also protects against session hijacking,
if the derived key material is used (directly or indirectly) to
protect a subsequent session. For these reasons, use of the OTP
protection variant is RECOMMENDED.
However, it should be noted that not even the OTP protection variant
provides privacy for user names and/or token key identifiers. EAP-
POTP MUST be used within a secure tunnel such as the one provided by
PEAPv2 [17] if privacy for these parameters is required.
When resuming sessions created in the basic variant (which MUST only
take place within a protected tunnel), the peer is authenticated by
demonstrating knowledge of not just a valid session identifier, but
also the OTP used when the session was created. Server nonces
prevent replay attacks, but there still remains some likelihood of an
attacker guessing the correct combination of session identifier and
OTP value. Assuming OTPs with entropy about 32 bits, this means that
the likelihood of succeeding with such an attack is about 1/2^48 due
to the birthday paradox. Servers allowing session resumption for the
basic variant MUST protect against such attacks, e.g., by keeping
track of the rate of failed resumption attempts.
6.3. Denial-of-Service Attacks
An active attacker may replace the iteration count value in OTP TLVs
sent by the peer to slow down an authentication server.
Authentication servers SHOULD protect against this, e.g., by
disregarding OTP TLVs with an iteration count value higher than some
number that is preset or dynamically set (depending on load).
6.4. The Use of Pepper
As described in Section 4.8, the use of pepper will slow down an
attacker's search for a matching OTP. The ability to transfer a
pepper value in encrypted form from the EAP server to the peer means
that, even though there may be an initial computational cost for the
EAP server to authenticate the peer, subsequent authentications will
be efficient, while at the same time more secure, since a pre-shared,
128-bit-long pepper value will not be easily found by an attacker.
An attacker, observing an EAP-Request containing an OTP TLV
calculated using a pepper chosen by the peer, may, however, depending
on available resources, be able to successfully attack that
particular EAP-POTP session, since it most likely will be based on a
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relatively short pepper value or only an iteration count. Once the
correct OTP has been found, eavesdropping on the EAP server's Confirm
TLV will potentially give the attacker access to the longer, server-
provided pepper for the remaining lifetime of that pepper value. For
this reason, initial exchanges with EAP servers SHOULD occur in a
secure environment (e.g., in a PEAPv2 tunnel offering cryptographic
binding with inner EAP methods). If initial exchanges do not occur
in a secure environment, the iteration count MUST be significantly
higher than for messages where a pre-shared pepper is used. The
lifetime of the shared pepper must also be calculated with this in
mind. Finally, the peer and the EAP server MUST store the pepper
value securely and associated with the user.
6.5. The Race Attack
In the case of fragmentation of EAP messages, it is possible (in the
basic variant of this method) for an attacker to listen to most of an
OTP, guess the remainder, and then race the legitimate user to
complete the authentication. Conforming backend authentication
server implementations MUST protect against this race condition. One
defense against this attack is outlined below and borrowed from [14];
implementations MAY use this approach or MAY select an alternative
defense. Note that the described defense relies on the user
providing the identity in response to an initial Identity EAP-
Request.
One possible defense is to prevent a user from starting multiple
simultaneous authentication sessions. This means that once the
legitimate user has initiated authentication, an attacker would be
blocked until the first authentication process has completed. In
this approach, a timeout is necessary to thwart a denial-of-service
attack.
7. IANA Considerations
7.1. General
This document is a description of a general EAP method for OTP
tokens. It also defines EAP method 32 as a profile of the general
method. Extending the set of EAP-POTP TLVs or the set of EAP-POTP
cryptographic algorithms shall be seen as revisions of the protocol
and hence shall require an RFC that updates or obsoletes this
document.
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7.2. Cryptographic Algorithm Identifier Octets
A new registry for EAP-POTP cryptographic algorithm identifier octets
has been created. The initial contents of this registry are as
specified in Section 4.11.16.
Assignment of new values for hash algorithms, encryption algorithms,
and MAC algorithms in the Crypto Algorithm TLV MUST be done through
IANA with "Specification Required" and "IESG Approval" (see [9] for
the meaning of these terms).
8. Intellectual Property Considerations
RSA, RSA Security, and SecurID are either registered trademarks or
trademarks of RSA Security Inc. in the United States and/or other
countries. The names of other products and services mentioned may be
the trademarks of their respective owners.
9. Acknowledgments
This document was improved by comments from, and discussion with, a
number of RSA Security employees. Simon Josefsson drafted the
initial versions of an RSA SecurID EAP method while working for RSA
Laboratories. The inspiration for the TLV-type of information
exchange comes from [17]. Special thanks to Oliver Tavakoli of Funk
Software who provided numerous useful comments and suggestions, Randy
Chou of Aruba Networks for good suggestions in the session resumption
area, and Jim Burns of Meetinghouse who provided inspiration for the
Protected TLV. Thanks also to the IESG reviewers, Pasi Eronen, David
Black, and Uri Blumenthal, for insightful comments that helped to
improve the document, and to Alfred Hoenes for a thorough editorial
review.
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10. References
10.1. Normative References
[1] Blunk, L., Vollbrecht, J., Aboba, B., Carlson, J., and H.
Levkowetz, Ed., "Extensible Authentication Protocol (EAP)", RFC
3748, June 2004.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] National Institute of Standards and Technology, "Secure Hash
Standard", FIPS 180-2, February 2004.
[4] National Institute of Standards and Technology, "Specification
for the Advanced Encryption Standard (AES)", FIPS 197, November
2001.
[5] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing
for Message Authentication", RFC 2104, February 1997.
[6] Kaliski, B., "PKCS #5: Password-Based Cryptography Specification
Version 2.0", RFC 2898, September 2000.
[7] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD
63, RFC 3629, November 2003.
[8] Schulzrinne, H., "The tel URI for Telephone Numbers", RFC 3966,
December 2004.
[9] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", RFC 2434, October 1998.
10.2. Informative References
[10] Simpson, W., Ed., "The Point-to-Point Protocol (PPP)", STD 51,
RFC 1661, July 1994.
[11] The Institute of Electrical and Electronics Engineers, Inc.,
"IEEE Standard for Local and metropolitan area networks --
Port-Based Network Access Control", IEEE 802.1X-2001, July
2001.
[12] Kaufman, C., Ed., "Internet Key Exchange (IKEv2) Protocol", RFC
4306, December 2005.
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[13] Stanley, D., Walker, J., and B. Aboba, "Extensible
Authentication Protocol (EAP) Method Requirements for Wireless
LANs", RFC 4017, March 2005.
[14] Haller, N., Metz, C., Nesser, P., and M. Straw, "A One-Time
Password System", STD 61, RFC 2289, February 1998.
[15] Rigney, C., Willens, S., Rubens, A., and W. Simpson, "Remote
Authentication Dial In User Service (RADIUS)", RFC 2865, June
2000.
[16] Aboba, B., Simon, D., Eronen, P., and H. Levkowetz, Ed.,
"Extensible Authentication Protocol (EAP) Key Management
Framework", Work in Progress, October 2006.
[17] Palekar, A., Simon, D., Zorn, G., Salowey, J., Zhou, H., and S.
Josefsson, "Protected EAP Protocol (PEAP) Version 2", Work in
Progress, October 2004.
[18] Internet Assigned Numbers Authority, "Private Enterprise
Numbers", December 2006.
[19] Zorn, G., "Microsoft Vendor-specific RADIUS Attributes", RFC
2548, March 1999.
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Appendix A. Profile of EAP-POTP for RSA SecurID
Note: The RSA SecurID product is a hardware token card (or software
emulation thereof) produced by RSA Security Inc., which is used for
end-user authentication.
The EAP method type identifier for the RSA SecurID profile of EAP-
POTP is 32.
Peers and EAP servers implementing the SecurID profile of EAP-POTP
SHALL conform to all EAP-POTP normative requirements in this
Document. In addition, the New PIN TLV and the Protected TLV MUST be
supported by peers.
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Appendix B. Examples of EAP-POTP Exchanges
This appendix is non-normative. In the examples, "V1", "V2", "V3",
etc., stand for arbitrary values of the correct type.
B.1. Basic Mode, Unilateral Authentication
This mode should only be used within a secured tunnel. The peer
identifies itself with a User Identifier TLV.
Peer EAP server
<- EAP-Request
Type=Identity
EAP-Response ->
Type=Identity
<- EAP-Request
Type=OTP-X
Version TLV:
Highest=0,Lowest=0
OTP TLV:
P=0,C=0,N=0,T=0,E=0,R=0
EAP-Response ->
Type=OTP-X
Version TLV:
Highest=0
OTP TLV:
P=0,C=0,N=0,T=0,E=0,R=0
Authentication Data=V1
User Identifier TLV:
User Identifier=V2
<- EAP-Success
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B.2. Basic Mode, Session Resumption
This example illustrates successful resumption of a basic mode
session. It must be carried out only in a protected tunnel.
Peer EAP server
<- EAP-Request
Type=Identity
EAP-Response ->
Type=Identity
<- EAP-Request
Type=OTP-X
Version TLV:
Highest=0,Lowest=0
OTP TLV:
P=0,C=0,N=0,T=0,E=0,R=0
Server-Info TLV:
N=0
Session Identifier=V1
Server Identifier=V2
Nonce=V3
EAP-Response ->
Type=OTP-X
Version TLV:
Highest=0
Resume TLV:
Session Identifier=V4 (indicating earlier, basic mode, session)
Authentication Data=V5
<- EAP-Success
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B.3. Mutual Authentication without Session Resumption
In this case, the peer uses the token key identifier, in addition to
the user identifier. The initial EAP-Identity exchange may also
provide user information, or may be restricted to only general domain
information. Pepper is not used, but will be used in a subsequent
session since the server provides the peer with an encrypted pepper
in its Confirm TLV. Absence of the Crypto Algorithm TLV indicates
use of default cryptographic algorithms.
Peer EAP server
<- EAP-Request
Type=Identity
EAP-Response ->
Type=Identity
<- EAP-Request
Type=OTP-X
Version TLV:
Highest=0,Lowest=0
Server-Info TLV:
N=0
Session Identifier=V1
Server Identifier=V2
Nonce=V3
OTP TLV:
P=1,C=0,N=0,T=0,E=0,R=0
Pepper Length=0
Iteration Count=V4
EAP-Response ->
Type=OTP-X
Version TLV:
Highest=0
OTP TLV:
P=1,C=0,N=0,T=0,E=0,R=0
Pepper Length=0
Iteration Count=V4
Authentication Data=V5
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User Identifier TLV:
User Identifier=V6
Token Key Identifier TLV:
Token Key Identifier=V7
<- EAP-Request
Type=OTP-X
Confirm TLV:
C=0
Authentication Data=V8
Pepper Identifier=V9
Encrypted Pepper=V10
EAP-Response ->
Type=OTP-X
Confirm TLV:
(no data)
<- EAP-Success
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B.4. Mutual Authentication with Transfer of Pepper
The difference between this example and the previous one is that the
peer makes use of an existing pepper in the PBKDF2 computation. The
EAP server provides a new pepper to the peer in the Confirm TLV.
Note that the peer had not been able to use a pepper in the response
calculation unless it had found the existing pepper, since the server
specified a maximum (new) pepper length of zero.
Peer EAP server
<- EAP-Request
Type=Identity
EAP-Response ->
Type=Identity
<- EAP-Request
Type=OTP-X
Version TLV:
Highest=0,Lowest=0
Server-Info TLV:
N=0
Session Identifier=V1
Server Identifier=V2
Nonce=V3
OTP TLV:
P=1,C=0,N=0,T=0,E=0,R=0
Pepper Length=0
Iteration Count=V4
EAP-Response ->
Type=OTP-X
Version TLV:
Highest=0
OTP TLV:
P=1,C=0,N=0,T=0,E=0,R=0
Pepper Length=V5
Iteration Count=V6
Authentication Data=V7
(includes a pepper identifier)
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User Identifier TLV:
User Identifier=V8
Token Key Identifier TLV:
Token Key Identifier=V9
<- EAP-Request
Type=OTP-X
Confirm TLV:
C=0
Authentication Data=V10
Pepper Identifier=V11
Encrypted Pepper=V12
EAP-Response ->
Type=OTP-X
Confirm TLV:
(no data)
<- EAP-Success
B.5. Failed Mutual Authentication
This example differs from the previous one in that the peer is not
able to authenticate the server. Therefore, it sends an empty EAP-
Response of type POTP-X, which the EAP server acknowledges by
responding with an EAP-Failure. Pepper is not used.
Peer EAP server
<- EAP-Request
Type=Identity
EAP-Response ->
Type=Identity
<- EAP-Request
Type=OTP-X
Version TLV:
Highest=0,Lowest=0
OTP TLV:
P=1,C=0,N=0,T=0,E=0,R=0
Pepper Length=V1
Iteration Count=V2
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Server-Info TLV:
N=0
Session Identifier=V3
Server Identifier=V4
Nonce=V5
EAP-Response ->
Type=OTP-X
Version TLV:
Highest=0
OTP TLV:
P=1,C=0,N=0,T=0,E=0,R=0
Pepper Length=V1
Iteration Count=V2
Authentication Data=V6
User Identifier TLV:
User Identifier=V7
Token Key Identifier TLV:
Token Key Identifier=V8
<- EAP-Request
Type=OTP-X
Confirm TLV:
C=0
Authentication Data=V9
EAP-Response ->
Type=OTP-X
(no data)
<- EAP-Failure
B.6. Session Resumption
This example illustrates successful session resumption.
Peer EAP server
<- EAP-Request
Type=Identity
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EAP-Response ->
Type=Identity
<- EAP-Request
Type=OTP-X
Version TLV:
Highest=0,Lowest=0
OTP TLV:
P=1,C=0,N=0,T=0,E=0,R=0
Pepper Length=V1
Iteration Count=V2
Server-Info TLV:
N=0
Session Identifier=V3
Server Identifier=V4
Nonce=V5
EAP-Response ->
Type=OTP-X
Version TLV:
Highest=0
Resume TLV:
Session Identifier=V6 (indicating earlier, protected mode, session)
Authentication Data=V7
<- EAP-Request
Type=OTP-X
Confirm TLV:
C=0
Authentication Data=V8
EAP-Response ->
Type=OTP-X
Confirm TLV:
(no data)
<- EAP-Success
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B.7. Failed Session Resumption
This example illustrates a failed session resumption, followed by a
complete mutual authentication. The user is identified through the
User Identifier TLV. The client is able to reuse an older pepper.
The server sends a new pepper for subsequent use in its Confirm TLV.
The server suggests some non-default cryptographic algorithms, but
the client only supports the default ones.
Peer EAP server
<- EAP-Request
Type=Identity
EAP-Response ->
Type=Identity
<- EAP-Request
Type=OTP-X
Version TLV:
Highest=0,Lowest=0
OTP TLV:
P=1,C=0,N=0,T=0,E=0,R=0
Pepper Length=V1
Iteration Count=V2
Server-Info TLV:
N=0
Session Identifier=V3
Server Identifier=V4
Nonce=V5
Crypto Algorithm TLV:
Hash Alg. Length=V6
Hash Algorithms=V7
Encr. Alg. Length=V8
Encr. Algorithms=V9
MAC Alg. Length=V10
MAC Algorithms=V11
EAP-Response ->
Type=OTP-X
Version TLV:
Highest=0
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Resume TLV:
Session Identifier=V12 (indicating earlier session)
Authentication Data=V13
<- EAP-Request
Type=OTP-X
OTP TLV:
P=1,C=0,N=0,T=0,E=0,R=0
Pepper Length=V14
Iteration Count=V15
Server-Info TLV:
N=1 (no resumption)
Session Identifier=V3
Server Identifier=V4
Nonce=V16
EAP-Response ->
Type=OTP-X
OTP TLV:
P=1,C=0,N=1,T=1,E=0,R=0
Pepper Length=V17
Iteration Count=V18
Authentication Data=V19 (with pepper identifier)
User Identifier TLV:
User Identifier=V20
<- EAP-Request
Type=OTP-X
Confirm TLV:
C=0
Authentication Data=V21
Pepper Identifier=V22
Encrypted Pepper=V23
EAP-Response ->
Type=OTP-X
Confirm TLV:
(no data)
<- EAP-Success
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B.8. Mutual Authentication, and New PIN Requested.
In this example, the user is also requested to select a new PIN. The
new PIN is allowed to be alphanumeric, and must be at least 6
characters long. The user selects another PIN than the one suggested
by the server. The token key is identified through a combination of
the user identifier and the token key identifier. While waiting for
the user input, to avoid network timeouts, the peer sends an EAP-
Response containing a Keep-Alive TLV to the EAP server. The EAP
server responds by sending an EAP-Request containing a Keep-Alive TLV
back to the peer. Note that all TLVs exchanged after the Confirm TLV
exchange are wrapped in the Protected TLV. Absence of the Crypto
Algorithm TLV indicates use of default cryptographic algorithms.
Peer EAP server
<- EAP-Request
Type=Identity
EAP-Response ->
Type=Identity
<- EAP-Request
Type=OTP-X
Version TLV:
Highest=0,Lowest=0
OTP TLV:
P=1,C=0,N=0,T=0,E=0,R=0
Pepper Length=V1
Iteration Count=V2
Server-Info TLV:
N=0
Session Identifier=V3
Server Identifier=V4
Nonce=V5
EAP-Response ->
Type=OTP-X
Version TLV:
Highest=0
OTP TLV:
P=1,C=0,N=0,T=0,E=0,R=0
Pepper Length=V6
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Iteration Count=V7
Authentication Data=V8 (with pepper identifier)
User Identifier TLV:
User Identifier=V9
Token Key Identifier TLV:
Token Key Identifier=V10
<- EAP-Request
Type=OTP-X
Confirm TLV:
C=1
Authentication Data=V11
EAP-Response ->
Type=OTP-X
Confirm TLV:
(no data)
<- EAP-Request
Type=OTP-X
Protected TLV:
MAC=V12
IV=V13
Encrypted TLVs=V14
(Contains:
New PIN TLV:
Q=0,A=1
PIN=V15
Min. PIN Length=6)
EAP-Response ->
Type=OTP-X
Protected TLV:
MAC=V16
IV=V17
Encrypted TLVs=V18
(Contains:
Keep-Alive TLV:
(no data))
<- EAP-Request
Type=OTP-X
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Protected TLV:
MAC=V19
IV=V20
Encrypted TLVs=V21
(Contains:
Keep-Alive TLV:
(no data))
EAP-Response ->
Type=OTP-X
Protected TLV:
MAC=V22
IV=V23
Encrypted TLVs=V24
(Contains:
New PIN TLV:
Q=0,A=0
PIN=V25)
<- EAP-Request
Type=OTP-X
Protected TLV:
MAC=V26
IV=V27
Encrypted TLVs=V28
(Contains:
OTP TLV:
P=1,C=0,N=0,T=0,E=0,R=0
Pepper Length=V1
Iteration Count=V2)
EAP-Response ->
Type=OTP-X
Protected TLV
MAC=V29
IV=V30
Encrypted TLVs=V31
(Contains:
OTP TLV:
P=1,C=0,N=0,T=0,E=0,R=0
Pepper Length=V6
Iteration Count=V7
Authentication Data=V31)
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<- EAP-Request
Type=OTP-X
Protected TLV
MAC=V32
IV=V33
Encrypted TLVs=V34
(Contains:
Confirm TLV:
C=0
Authentication Data=V35)
EAP-Response ->
Type=OTP-X
Protected TLV
MAC=V36
IV=V37
Encrypted TLVs=V38
(Contains:
Confirm TLV:
(no data))
<- EAP-Success
B.9. Use of Next OTP Mode
In this example, the peer is requested to provide a second OTP to the
EAP server.
Peer EAP server
<- EAP-Request
Type=Identity
EAP-Response ->
Type=Identity
<- EAP-Request
Type=OTP-X
Version TLV:
Highest=0,Lowest=0
OTP TLV:
P=1,C=0,N=0,T=0,E=0,R=0
Pepper Length=V1
Iteration Count=V2
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Server-Info TLV:
N=0
Session Identifier=V3
Server Identifier=V4
Nonce=V5
EAP-Response ->
Type=OTP-X
Version TLV:
Highest=0
OTP TLV:
P=1,C=0,N=0,T=0,E=0,R=0
Pepper Length=V6
Iteration Count=V7
Authentication Data=V8
User Identifier TLV:
User Identifier=V9
<- EAP-Request
Type=OTP-X
OTP TLV:
P=1,C=0,N=1,T=1,E=0,R=0
Pepper Length=V1
Iteration Count=V2
EAP-Response ->
Type=OTP-X
OTP TLV:
P=1,C=0,N=1,T=1,E=0,R=0
Pepper Length=V6
Iteration Count=V7
Authentication Data=V10
<- EAP-Request
Type=OTP-X
Confirm TLV:
C=0
Authentication Data=V11
EAP-Response ->
Type=OTP-X
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Confirm TLV:
(no data)
<- EAP-Success
Appendix C. Use of the MPPE-Send/Receive-Key RADIUS Attributes
C.1. Introduction
This section describes how to populate the MPPE-Send-Key and the
MPPE-Receive-Key RADIUS attributes defined in [19], using an MSK
established in EAP-POTP.
C.2. MPPE Key Attribute Population
Once the EAP-POTP MSK has been generated, it is used as follows to
populate the MPPE-Send-Key and the MPPE-Receive-Key attributes:
Use the initial 32 octets of the MSK as the value for the "Key" sub-
field in the plaintext "String" field of the MPPE-Send-Key attribute,
and use the final 32 octets of the MSK as the "Key" sub-field in the
plaintext "String" field of the MPPE-Receive-Key attribute (Note:
"Send" and "Receive" here refer to the Authenticator; for the peer,
they are reversed).
Appendix D. Key Strength Considerations
D.1. Introduction
As described in Section 6, the strength of keys generated in EAP-POTP
protected mode depends on a number of factors. This appendix
provides examples of actual key strengths achieved under various
assumptions.
It should be noted that, while some of the examples indicate that the
strength of generated keys is relatively weak, the strength applies
only to those EAP-POTP sessions between a peer and an EAP server that
do not share a pepper. Once a pepper, provided by an EAP server to a
peer, has been established, future sessions using this pepper will
provide full-strength keys.
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D.2. Example 1: 6-Digit One-Time Passwords
In this example we assume the following:
OTPs are six decimal digits long;
4-digit PINs are added to generated OTPs; and
OTP hardening (iteration count and pepper searching combined)
effectively adds 10 bits of entropy. One way of achieving this
without use of pepper searching is to have the iteration count in
PBKDF2 set to 1,000,000.
The effective key strength then becomes roughly:
log_2(10**6) + log_2(10**4) + log_2(2**10) = 43 bits
The above assumes that the entropy of the underlying shared secret is
>43 bits and that there are no other weaknesses in the OTP algorithm.
D.3. Example 2: 8-Digit One-Time Passwords
In this example we assume the following:
OTPs are eight decimal digits long;
4-character alphanumeric PINs are added to generated OTPs; and
OTP hardening (iteration count and pepper searching combined)
effectively adds 10 bits of entropy.
The effective key strength then becomes roughly:
log_2(10**8) + log_2(26**4) + log_2(2**10) = 55 bits
The above assumes that the entropy of the underlying shared secret is
>55 bits and that there are no other weaknesses in the OTP algorithm.
Author's Address
Magnus Nystroem
RSA Security
EMail: magnus@rsa.com
Nystroem Informational [Page 81]
RFC 4793 EAP-POTP February 2007
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