Internet Engineering Task Force (IETF) K. Hoeper
Request for Comments: 6678 Motorola Solutions, Inc.
Category: Informational S. Hanna
ISSN: 2070-1721 Juniper Networks
H. Zhou
J. Salowey, Ed.
Cisco Systems, Inc.
July 2012
Requirements for a
Tunnel-Based Extensible Authentication Protocol (EAP) Method
Abstract
This memo defines the requirements for a tunnel-based Extensible
Authentication Protocol (EAP) Method. This tunnel method will use
Transport Layer Security (TLS) to establish a secure tunnel. The
tunnel will provide support for password authentication, EAP
authentication, and the transport of additional data for other
purposes.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6678.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Provisions Relating to IETF Documents
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publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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RFC 6678 EAP Tunnel Method Requirements July 2012
to this document. Code Components extracted from this document must
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Conventions Used in This Document . . . . . . . . . . . . . . 4
3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Password Authentication . . . . . . . . . . . . . . . . . 5
3.2. Protection of Weak EAP Methods . . . . . . . . . . . . . . 5
3.3. Chained EAP Methods . . . . . . . . . . . . . . . . . . . 6
3.4. Identity Protection . . . . . . . . . . . . . . . . . . . 6
3.5. Anonymous Service Access . . . . . . . . . . . . . . . . . 7
3.6. Network Endpoint Assessment . . . . . . . . . . . . . . . 7
3.7. Client Authentication during Tunnel Establishment . . . . 7
3.8. Extensibility . . . . . . . . . . . . . . . . . . . . . . 8
3.9. Certificate-Less Authentication and Generic EAP Method
Extension . . . . . . . . . . . . . . . . . . . . . . . . 8
4. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.1. General Requirements . . . . . . . . . . . . . . . . . . . 9
4.1.1. RFC Compliance . . . . . . . . . . . . . . . . . . . . 9
4.2. Tunnel Requirements . . . . . . . . . . . . . . . . . . . 10
4.2.1. TLS Requirements . . . . . . . . . . . . . . . . . . . 10
4.2.1.1. Cipher Suites . . . . . . . . . . . . . . . . . . 10
4.2.1.1.1. Cipher Suite Negotiation . . . . . . . . . . . 10
4.2.1.1.2. Tunnel Data Protection Algorithms . . . . . . 10
4.2.1.1.3. Tunnel Authentication and Key Establishment . 11
4.2.1.2. Tunnel Replay Protection . . . . . . . . . . . . . 11
4.2.1.3. TLS Extensions . . . . . . . . . . . . . . . . . . 11
4.2.1.4. Peer Identity Privacy . . . . . . . . . . . . . . 11
4.2.1.5. Session Resumption . . . . . . . . . . . . . . . . 12
4.2.2. Fragmentation . . . . . . . . . . . . . . . . . . . . 12
4.2.3. Protection of Data External to Tunnel . . . . . . . . 12
4.3. Tunnel Payload Requirements . . . . . . . . . . . . . . . 12
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4.3.1. Extensible Attribute Types . . . . . . . . . . . . . . 12
4.3.2. Request/Challenge Response Operation . . . . . . . . . 13
4.3.3. Indicating Criticality of Attributes . . . . . . . . . 13
4.3.4. Vendor-Specific Support . . . . . . . . . . . . . . . 13
4.3.5. Result Indication . . . . . . . . . . . . . . . . . . 13
4.3.6. Internationalization of Display Strings . . . . . . . 13
4.4. EAP Channel Binding Requirements . . . . . . . . . . . . . 14
4.5. Requirements Associated with Carrying Username and
Passwords . . . . . . . . . . . . . . . . . . . . . . . . 14
4.5.1. Security . . . . . . . . . . . . . . . . . . . . . . . 14
4.5.1.1. Confidentiality and Integrity . . . . . . . . . . 14
4.5.1.2. Authentication of Server . . . . . . . . . . . . . 14
4.5.1.3. Server Certificate Revocation Checking . . . . . . 14
4.5.2. Internationalization . . . . . . . . . . . . . . . . . 15
4.5.3. Metadata . . . . . . . . . . . . . . . . . . . . . . . 15
4.5.4. Password Change . . . . . . . . . . . . . . . . . . . 15
4.6. Requirements Associated with Carrying EAP Methods . . . . 15
4.6.1. Method Negotiation . . . . . . . . . . . . . . . . . . 16
4.6.2. Chained Methods . . . . . . . . . . . . . . . . . . . 16
4.6.3. Cryptographic Binding with the TLS Tunnel . . . . . . 16
4.6.4. Peer-Initiated EAP Authentication . . . . . . . . . . 17
4.6.5. Method Metadata . . . . . . . . . . . . . . . . . . . 17
5. Security Considerations . . . . . . . . . . . . . . . . . . . 18
5.1. Cipher Suite Selection . . . . . . . . . . . . . . . . . . 18
5.2. Tunneled Authentication . . . . . . . . . . . . . . . . . 19
5.3. Data External to Tunnel . . . . . . . . . . . . . . . . . 19
5.4. Separation of TLS Tunnel and Inner Authentication
Termination . . . . . . . . . . . . . . . . . . . . . . . 19
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
6.1. Normative References . . . . . . . . . . . . . . . . . . . 20
6.2. Informative References . . . . . . . . . . . . . . . . . . 21
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1. Introduction
An Extensible Authentication Protocol (EAP) tunnel method is an EAP
method that establishes a secure tunnel and executes other EAP
methods under the protection of that secure tunnel. An EAP tunnel
method can be used in any lower-layer protocol that supports EAP
authentication. There are several existing EAP tunnel methods that
use Transport Layer Security (TLS) to establish the secure tunnel.
EAP methods supporting this include Protected EAP [PEAP], Tunneled
Transport Layer Security EAP (TTLS) [RFC5281] and EAP Flexible
Authentication via Secure Tunneling (EAP-FAST) [RFC4851]. In
general, this has worked well so there is consensus to continue to
use TLS as the basis for a tunnel method. There have been various
reasons for employing a protected tunnel for EAP processes. They
include protecting weak authentication exchanges, such as username
and password. In addition, a protected tunnel can provide means to
provide peer identity protection and EAP method chaining. Finally,
systems have found it useful to transport additional types of data
within the protected tunnel.
This document describes the requirements for a EAP tunnel method as
well as for a password protocol supporting legacy password
verification within the tunnel method.
2. Conventions Used in This Document
Use of each capitalized word within a sentence or phrase carries the
following meaning during the EAP Method Update (EMU) WG's method
selection process:
MUST - indicates an absolute requirement
MUST NOT - indicates something absolutely prohibited
SHOULD - indicates a strong recommendation of a desired result
SHOULD NOT - indicates a strong recommendation against a result
MAY - indicates a willingness to allow an optional outcome
Lowercase uses of "MUST", "MUST NOT", "SHOULD", "SHOULD NOT" and
"MAY" carry their normal meaning and are not subject to these
definitions.
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3. Use Cases
To motivate and explain the requirements in this document, a
representative set of use cases for the EAP tunnel method are
supplied here. It is mandatory for a candidate tunnel method to
support all of the use cases that are marked below as "MUST".
3.1. Password Authentication
Many legacy systems only support user authentication with passwords.
Some of these systems require transport of the actual username and
password to the authentication server. This is true for systems
where the authentication server does not have access to the cleartext
password or a consistent transform of the cleartext password.
Examples of such systems are some one-time password (OTP) systems and
other systems where the username and password are submitted to an
external party for validation. The tunnel method MUST support
transporting cleartext username and password to the EAP server. It
MUST NOT reveal information about the username and password to
parties in the communication path between the peer and the EAP
server. The advantage any attacker gains against the tunnel method
when employing a username and password for authentication MUST be
through interaction and not computation. The tunnel MUST support
protection from man-in-the-middle attacks. The combination of the
tunnel authentication and password authentication MUST enable mutual
authentication.
Since EAP authentication occurs before network access is granted the
tunnel method SHOULD enable an inner exchange to provide support for
minimal password management tasks including password change, "new PIN
mode", and "next token mode" required by some systems.
3.2. Protection of Weak EAP Methods
Some existing EAP methods have vulnerabilities that could be
eliminated or reduced by running them inside a protected tunnel. For
example, an EAP-MD5 method does not provide mutual authentication or
protection from dictionary attacks. Without extra protection, EAP
tunnel methods are vulnerable to a special type of tunnel man-in-the-
middle (MitM) attack [TUNNEL-MITM]. This attack is referred to as
"tunnel MitM attack" in the remainder of this document. The
additional protection needed to thwart tunnel MitM attacks depends on
the inner method executed within the tunnel. When weak methods are
used, these attacks can be mitigated via security policies that
require the method to be used only within a tunnel. On the other
hand, a technical solution (so-called cryptographic bindings) can be
used whenever the inner method derives key material and is not
susceptible to attacks outside a tunnel. Only the latter mitigation
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technique can be made an actual requirement for EAP tunnel methods
(see Section 4.6.3), while security policies are outside the scope of
this requirement document. Please refer to the NIST "Recommendation
for EAP Methods Used in Wireless Network Access Authentication"
[NIST-SP-800-120] and [LCN-2010] for a discussion on security
policies and complete solutions for thwarting tunnel MitM attacks.
The tunnel method MUST support protection of weak EAP methods.
Cryptographic protection from tunnel MitM attacks MUST be provided
for all key-generating methods. In combination with an appropriate
security policy this will thwart MitM attacks against inner methods.
3.3. Chained EAP Methods
Several circumstances are best addressed by using chained EAP
methods. For example, it may be desirable to authenticate the user
and also authenticate the device being used. However, chained EAP
methods from different conversations can be redirected into the same
conversation by an attacker giving the authenticator the impression
that both conversations terminate at the same endpoint.
Cryptographic binding can be used to bind the results of chained key-
generating methods together or to an encompassing tunnel.
The tunnel method MUST support chained EAP methods while including
protection against attacks on method chaining.
3.4. Identity Protection
When performing an EAP authentication, the peer may want to protect
its identity and only disclose it to a trusted EAP server. This
helps to maintain peer privacy.
The tunnel method MUST support identity protection, therefore the
identity of the peer used for authentication purposes MUST NOT be
obtainable by any entity other than the EAP server terminating the
tunnel method. Peer identity protection provided by the tunnel
method applies to the identities that are specific to the tunnel
method and inner method being used. In a roaming scenario, note that
the peer may need to expose the realm portion of the EAP outer
identity in the Network Access Identifier (NAI) [RFC4282] in order to
reach the appropriate authentication server.
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3.5. Anonymous Service Access
When network service is provided, it is sometimes desirable for a
user to gain network access in order to access limited services for
emergency communication or troubleshooting information. To avoid
eavesdropping, it's best to negotiate link-layer security as with any
other authentication.
Therefore, the tunnel method SHOULD allow anonymous peers or server-
only authentication, while still deriving keys that can be used for
link-layer security. The tunnel method MAY also allow for the bypass
of server authentication processing on the client.
Foregoing user or server authentication increases the chance of man-
in-the-middle and other types of attacks that can compromise the
derived keys used for link-layer security. Therefore, passwords and
other sensitive information MUST NOT be disclosed to an
unauthenticated server, or to a server that is not authorized to
authenticate the user.
3.6. Network Endpoint Assessment
The Network Endpoint Assessment (NEA) protocols and reference model
described in [RFC5209] provide a standard way to check the health
("posture") of a device at or after the time it connects to a
network. If the device does not comply with the network's
requirements, it can be denied access to the network or granted
limited access to remediate itself. EAP is a convenient place for
conducting an NEA exchange.
The tunnel method SHOULD support carrying NEA protocols such as a
Posture Broker protocol compatible with Trusted Network Connect
(PB-TNC) [RFC5793]. Depending on the specifics of the tunnel method,
these protocols may be required to be carried in an EAP method.
3.7. Client Authentication during Tunnel Establishment
In some cases, the peer will have credentials that allow it to
authenticate during tunnel establishment. These credentials may only
partially authenticate the identity of the peer and additional
authentication may be required inside the tunnel. For example, a
communication device may be authenticated during tunnel
establishment; in addition, user authentication may be required to
satisfy authentication policy. The tunnel method MUST be capable of
providing client-side authentication during tunnel establishment.
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3.8. Extensibility
The tunnel method MUST provide extensibility so that additional data
related to authentication, authorization, and network access can be
carried inside the tunnel in the future. This removes the need to
develop new tunneling methods for specific purposes.
An application for extensibility is credential provisioning. When a
peer has authenticated with EAP, this is a convenient time to
distribute credentials to that peer that may be used for later
authentication exchanges. For example, the authentication server can
provide a private key or shared key to the peer that can be used by
the peer to perform rapid re-authentication or roaming. In addition,
there have been proposals to perform enrollment within EAP, such as
[EAP-ENROLL]. Another use for extensibility is support for alternate
authentication frameworks within the tunnel.
3.9. Certificate-Less Authentication and Generic EAP Method Extension
In some cases, the peer will not have a way to verify a server
certificate and, in some cases, a server might not have a certificate
to verify. Therefore, it is desirable to support certificate-less
authentication. An application for this is credential provisioning
where the peer and server authenticate each other with a shared
password and credentials for subsequent authentication (e.g., a key
pair and certificate, or a shared key) can be passed inside the
tunnel. Another application is to extend existing EAP methods with
new features such as EAP channel bindings.
Great care must be taken when using tunnels with no server
authentication for the protection of an inner method. For example,
the client may lack the appropriate trust roots to fully authenticate
the server, but may still establish the tunnel to execute an inner
EAP method to perform mutual authentication and key derivation. In
these cases, the inner EAP method MUST provide resistance to
dictionary attack and a cryptographic binding between the inner
method and the tunnel method MUST be established. Furthermore, the
cipher suite used to establish the tunnel MUST derive the master key
using contributions from both client and server, as in ephemeral
Diffie-Hellman cipher suites.
The tunnel method MAY allow for certificate-less authentication.
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4. Requirements
4.1. General Requirements
4.1.1. RFC Compliance
The tunnel method MUST include a Security Claims section with all
security claims specified in Section 7.2 in RFC 3748 [RFC3748]. In
addition, it MUST meet the requirement in Sections 2.1 and 7.4 of RFC
3748 that tunnel methods MUST support protection against man-in-the-
middle attacks. Furthermore, the tunnel method MUST support identity
protection as specified in Section 7.3 of RFC 3748.
The tunnel method MUST be unconditionally compliant with RFC 4017
[RFC4017] (using the definition of "unconditionally compliant"
contained in Section 1.1 of RFC 4017). This means that the method
MUST satisfy all the "MUST", "MUST NOT", "SHOULD", and "SHOULD NOT"
requirements in RFC 4017.
The tunnel method MUST meet all the "MUST" and "SHOULD" requirements
relevant to EAP methods contained in the EAP key management framework
[RFC5247] or any successor. This includes the generation of the
Master Session Key (MSK), Extended Master Session Key (EMSK),
Peer-Id, Server-Id, and Session-Id. These requirements will enable
the tunnel method to properly fit into the EAP key management
framework, maintaining all of the security properties and guarantees
of that framework.
The tunnel method MUST NOT be tied to any single cryptographic
algorithm. Instead, it MUST support run-time negotiation to select
among an extensible set of cryptographic algorithms, such as
algorithms used with certificates presented during tunnel
establishment. This "cryptographic algorithm agility" provides
several advantages. Most important, when a weakness in an algorithm
is discovered or increased processing power overtakes an algorithm,
users can easily transition to a new algorithm. Also, users can
choose the algorithm that best meets their needs.
The tunnel method MUST meet the SHOULD and MUST requirements
pertinent to EAP method contained in Section 3 of RFC 4962 [RFC4962].
This includes: cryptographic algorithm independence; strong, fresh
session keys; replay detection; keying material confidentiality and
integrity; and confirmation of cipher suite selection.
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4.2. Tunnel Requirements
The following section discusses requirements for TLS tunnel
establishment.
4.2.1. TLS Requirements
The tunnel-based method MUST support TLS version 1.2 [RFC5246] and
may support earlier versions greater than SSL 2.0 in order to enable
the possibility of backwards compatibility.
4.2.1.1. Cipher Suites
4.2.1.1.1. Cipher Suite Negotiation
Cipher suite negotiations always suffer from downgrading attacks when
they are not secured by any kind of integrity protection. A common
practice is a post-negotiation integrity check in which, as soon as
available, the established keys (here, the tunnel key) are used to
derive integrity keys. These integrity keys are then used by the
peer and authentication server to verify whether the cipher suite
negotiation has been maliciously altered by another party.
Integrity checks prevent downgrading attacks only if the derived
integrity keys and the employed integrity algorithms cannot be broken
in real-time. See Section 5.1 or [HC07] for more information on
this. Hence, the tunnel method MUST provide integrity-protected
cipher suite negotiation with secure integrity algorithms and
integrity keys.
TLS provides protected cipher suite negotiation as long as all the
cipher suites supported provide authentication, key establishment,
and data integrity protection as discussed in Section 5.1.
4.2.1.1.2. Tunnel Data Protection Algorithms
In order to prevent attacks on the cryptographic algorithms employed
by inner authentication methods, a tunnel protocol's protection needs
to provide a basic level of algorithm strength. The tunnel method
MUST provide at least one mandatory-to-implement cipher suite that
provides the equivalent security of 128-bit AES for encryption and
message authentication. See Part 1 of the NIST "Recommendation for
Key Management" [NIST-SP-800-57] for a discussion of the relative
strengths of common algorithms.
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4.2.1.1.3. Tunnel Authentication and Key Establishment
A tunnel method MUST provide unidirectional authentication from
authentication server to EAP peer and mutual authentication between
authentication server and EAP peer. The tunnel method MUST provide
at least one mandatory-to-implement cipher suite that provides
certificate-based authentication of the server and provides optional
certificate-based authentication of the client. Other types of
authentication MAY be supported.
At least one mandatory-to-implement cipher suite MUST be approved by
the NIST "Draft Recommendation for Key Management", Part 3
[NIST-SP-800-57p3], i.e., the cipher suite MUST be listed in Table
4-1, 4-2, or 4-3 in that document.
The mandatory-to-implement cipher suites MUST only include cipher
suites that use strong cryptographic algorithms. They MUST NOT
include cipher suites providing mutually anonymous authentication or
static Diffie-Hellman cipher suites.
Other cipher suites MAY be selected following the security
requirements for tunnel protocols in the NIST "Recommendation for EAP
Methods Used in Wireless Network Access Authentication"
[NIST-SP-800-120].
4.2.1.2. Tunnel Replay Protection
In order to prevent replay attacks on a tunnel protocol, the message
authentication MUST be generated using a time-variant input such as
timestamps, sequence numbers, nonces, or a combination of these, so
that any reuse of the authentication data can be detected as invalid.
TLS provides sufficient replay protection to meet this requirement as
long as weak cipher suites discussed in Section 5.1 are avoided.
4.2.1.3. TLS Extensions
In order to meet the requirements in this document, TLS extensions
MAY be used. For example, TLS extensions may be useful in providing
certificate revocation information via the TLS Online Certificate
Status Protocol (OCSP) extension [RFC6066] (thus meeting the
requirement in Section 4.5.1.3).
4.2.1.4. Peer Identity Privacy
A tunnel protocol MUST support peer privacy. This requires that the
username and other attributes associated with the peer are not
transmitted in the clear or to an unauthenticated, unauthorized
party. Peer identity protection provided by the tunnel method
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applies to establishment of the tunnel and protection of inner method
specific identities. If applicable, the peer certificate is sent
confidentially (i.e., encrypted).
4.2.1.5. Session Resumption
The tunnel method MUST support TLS session resumption as defined in
[RFC5246]. The tunnel method MAY support other methods of session
resumption such as those defined in [RFC5077].
4.2.2. Fragmentation
Tunnel establishment sometimes requires the exchange of information
that exceeds what can be carried in a single EAP message. In
addition, information carried within the tunnel may also exceed this
limit. Therefore, a tunnel method MUST support fragmentation and
reassembly.
4.2.3. Protection of Data External to Tunnel
A man-in-the-middle attacker can modify cleartext values such as
protocol version and type code information communicated outside the
TLS tunnel. The tunnel method MUST provide implicit or explicit
protection of the protocol version and type code. If modification of
other information external to the tunnel can cause exploitable
vulnerabilities, the tunnel method MUST provide protection against
modification of this additional data.
4.3. Tunnel Payload Requirements
This section describes the payload requirements inside the tunnel.
These requirements frequently express features that a candidate
protocol must be capable of offering so that a deployer can decide
whether to make use of that feature. This section does not state
requirements about what features of each protocol must be used during
a deployment.
4.3.1. Extensible Attribute Types
The payload MUST be extensible. Some standard payload attribute
types will be defined to meet known requirements listed below, such
as password authentication, inner EAP method, vendor-specific
attributes, and result indication. Additional payload attributes MAY
be defined in the future to support additional features and data
types.
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4.3.2. Request/Challenge Response Operation
The payload MUST support the request and response type of half-duplex
operation typical of EAP. Multiple attributes may be sent in a
single payload. The payload MAY support transporting multiple
authentications in a single payload packet.
4.3.3. Indicating Criticality of Attributes
It is expected that new attributes will be defined to be carried
within the tunnel method. In some cases, it is necessary for the
sender to know if the receiver did not understand the attribute. To
support this, there MUST be a way for the sender to mark attributes
such that the receiver will indicate if an attribute is not
understood.
4.3.4. Vendor-Specific Support
The payload MUST support communication of an extensible set of
vendor-specific attributes. These attributes will be segmented into
uniquely identified vendor-specific namespaces. They can be used for
experiments or vendor-specific features.
4.3.5. Result Indication
The payload MUST support result indication and its acknowledgement,
so both the EAP peer and server will end up with a synchronized
state. The result indication is needed after each chained inner
authentication method and at the end of the authentication, so
separate result indications for intermediate and final results MUST
be supported.
4.3.6. Internationalization of Display Strings
The payload MAY provide a standard attribute format that supports
international strings. This attribute format MUST support encoding
strings in UTF-8 [RFC3629] format. Any strings sent by the server
intended for display to the user MUST be sent in UTF-8 format and
SHOULD be able to be marked with language information and adapted to
the user's language preference as indicated by RFC 5646 [RFC5646].
Note that in some cases, such as when transmitting error codes, it is
acceptable to exchange numeric codes that can be translated by the
client to support the particular local language. These numeric codes
are not subject to internationalization during transmission.
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4.4. EAP Channel Binding Requirements
The tunnel method MUST be capable of meeting EAP channel binding
requirements described in [RFC6677]. As discussed in [RFC5056], EAP
channel bindings differ from channel bindings discussed in other
contexts. Cryptographic binding between the TLS tunnel and the inner
method discussed in Section 4.6.3 relates directly to the non-EAP
channel binding concepts discussed in RFC 5056.
4.5. Requirements Associated with Carrying Username and Passwords
This section describes the requirements associated with tunneled
password authentication. The password authentication mentioned here
refers to user or machine authentication using a legacy password
database or verifier, such as the Lightweight Directory Access
Protocol (LDAP) [RFC4511], OTP, etc. These typically require the
password in its original text form in order to authenticate the peer;
hence, they require the peer to send the cleartext username and
password to the EAP server.
4.5.1. Security
Many internal EAP methods have the peer send its password in the
clear to the EAP server. Other methods (e.g., challenge-response
methods) are vulnerable to attacks if an eavesdropper can intercept
the traffic. For any such methods, the security measures in the
following sections MUST be met.
4.5.1.1. Confidentiality and Integrity
The cleartext password exchange MUST be integrity and confidentiality
protected. As long as the password exchange occurs inside an
authenticated and encrypted tunnel, this requirement is met.
4.5.1.2. Authentication of Server
The EAP server MUST be authenticated before the peer sends the
cleartext password to the server.
4.5.1.3. Server Certificate Revocation Checking
When certificate authentication is used during tunnel establishment,
the EAP peer may need to present its password to the server before it
has network access to check the revocation status of the server's
credentials. Therefore, the tunnel method MUST support mechanisms to
check the revocation status of a credential. The tunnel method
SHOULD make use of Online Certificate Status Protocol (OCSP)
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[RFC2560] or Server-based Certificate Validation Protocol (SCVP)
[RFC5055] to obtain the revocation status of the EAP server
certificate.
4.5.2. Internationalization
The password authentication exchange MUST support usernames and
passwords in international languages. It MUST support encoding of
username and password strings in UTF-8 [RFC3629] format. The method
MUST specify how username and password normalizations and/or
comparisons are performed in reference to SASLprep [RFC4013],
Net-UTF-8 [RFC5198], or their replacements.
Any strings sent by the server intended for display to the user MUST
be sent in UTF-8 format and SHOULD be able to be marked with language
information and adapted to the user's language preference as
indicated by RFC 5646 [RFC5646]. Note that, in some cases, such as
when transmitting error codes, it is acceptable to exchange numeric
codes that can be translated by the client to support the particular
local language. These numeric codes are not subject to
internationalization during transmission.
4.5.3. Metadata
The password authentication exchange SHOULD support additional
associated metadata that can be used to indicate whether the
authentication is for a user or a machine. This allows the EAP
server and peer to request and negotiate authentication specifically
for a user or machine. This is useful in the case of multiple inner
authentications where the user and machine both need to be
authenticated.
4.5.4. Password Change
The password authentication exchange MUST support password change.
The exchange SHOULD be extensible to support other "housekeeping"
functions, such as the management of PINs or other data, required by
some systems.
4.6. Requirements Associated with Carrying EAP Methods
The tunnel method MUST be able to carry inner EAP methods without
modifying them. EAP methods MUST NOT be redefined inside the tunnel.
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4.6.1. Method Negotiation
The tunnel method MUST support the protected negotiation of the inner
EAP method. It MUST NOT allow the inner EAP method negotiation to be
manipulated by intermediaries.
4.6.2. Chained Methods
The tunnel method SHOULD support the chaining of multiple EAP
methods. The tunnel method MUST allow for the communication of
intermediate results and for the verification of compound binding
between executed inner methods when chained methods are employed.
4.6.3. Cryptographic Binding with the TLS Tunnel
The tunnel method MUST provide a mechanism to bind the tunnel
protocol and the inner EAP method. This property is referred to as
cryptographic binding. Such bindings are an important tool for
mitigating the tunnel MitM attacks [TUNNEL-MITM]. Cryptographic
bindings enable the complete prevention of tunnel MitM attacks
without the need of additional security policies, as long as the
inner method derives keys and is not vulnerable to attacks outside a
protected tunnel [LCN-2010]. Even though weak or non-key-deriving
inner methods may be permitted. Thus, security policies preventing
tunnel MitM attacks are still necessary, and the tunnel method MUST
provide cryptographic bindings, because only this allows migrating to
more secure, policy-independent implementations.
Cryptographic bindings are typically achieved by securely mixing the
established keying material (say, tunnel key TK) from the tunnel
protocol with the established keying material (say, method key MK)
from the inner authentication method(s) in order to derive fresh
keying material. If chained EAP methods are executed in the tunnel,
all derived inner keys are combined with the tunnel key to create a
new compound tunnel key (CTK). In particular, CTK is used to derive
the EAP MSK, EMSK and other transient keys (shown as "TEK" below),
such as transient encryption keys and integrity protection keys. The
key hierarchy for tunnel method executions that derive compound keys
for the purpose of cryptographic binding is depicted in Figure 1.
In the case of the sequential executions of n inner methods, a
chained compound key CTK_i MUST be computed upon the completion of
each inner method i such that it contains the compound key of all
previous inner methods, i.e., CTK_i=f(CTK_i-1, MK_i) with 0 < i <= n
and CTK_0=TK, where f() is a key derivation function, such as one
that complies with the NIST "Recommendation for Key Derivation Using
Pseudorandom Functions" [NIST-SP-800-108]. CTK_n SHOULD serve as the
key to derive further keys. Figure 1 depicts the key hierarchy in
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the case of a single inner method. Transient keys derived from the
compound key CTK are used in a cryptographic protocol to verify the
integrity of the tunnel and the inner authentication method.
-----------
| TK | MK |
-----------
| |
v v
--------
| CTK |
--------
|
v
----------------
| | |
v v v
------- ------ -------
| TEK | | MSK | | EMSK |
------- ------- --------
Figure 1: Compound Keys
Furthermore, all compound keys CTK_i and all keys derived from it
SHOULD follow the recommendations for key derivations and key
hierarchies as specified in [NIST-SP-800-108]. In particular, all
derived keys MUST have a lifetime assigned that does not exceed the
lifetime of any key higher in the key hierarchy. The derivation MUST
prevent a compromise in one part of the system from leading to
compromises in other parts of the system that relay on keys at the
same or higher level in the hierarchy.
4.6.4. Peer-Initiated EAP Authentication
The tunnel method SHOULD allow for the peer to initiate an inner EAP
authentication in order to meet its policy requirements for
authenticating the server.
4.6.5. Method Metadata
The tunnel method SHOULD allow for the communication of additional
data associated with an EAP method. This can be used to indicate
whether the authentication is for a user or a machine. This allows
the EAP server and peer to request and negotiate authentication
specifically for a user or machine. This is useful in the case of
multiple inner EAP authentications where the user and machine both
need to be authenticated.
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5. Security Considerations
A tunnel method is often deployed to provide mutual authentication
between EAP Peer and EAP Server and to generate key material for use
in protecting lower-layer protocols. In addition the tunnel is used
to protect the communication of additional data, including peer
identity between the EAP Peer and EAP Server from disclosure to or
modification by an attacker. These sections cover considerations
that affect the ability for a method to achieve these goals.
5.1. Cipher Suite Selection
TLS supports a wide range of cipher suites providing a variety of
security properties. The selection of cipher suites is critical to
the security of the tunnel method. Selection of a cipher suite with
weak or no authentication, such as an anonymous Diffie-Hellman-based
cipher suite, will greatly increase the risk of system compromise.
Since a tunnel method uses the TLS tunnel to transport data, the
selection of a cipher suite with weak data encryption and integrity
algorithms will also increase the vulnerability of the method to
attacks.
A tunnel protocol is prone to downgrading attacks if the tunnel
protocol supports any key establishment algorithm that can be broken
on-line. In a successful downgrading attack, an adversary breaks the
selected "weak" key establishment algorithm and optionally the "weak"
authentication algorithm without being detected. Here, "weak" refers
to a key establishment algorithm that can be broken in real-time, and
an authentication scheme that can be broken off-line, respectively.
See [HC07] for more details. The requirements in this document
disapprove the use of key establishment algorithms that can be broken
on-line.
Mutually anonymous tunnel protocols are prone to man-in-the-middle
attacks described in [HC07]. During such an attack, an adversary
establishes one tunnel with the peer and one with the authentication
server, while the peer and server believe that they established a
tunnel with each other. Once both tunnels have been established, the
adversary can eavesdrop on all communications within the tunnels,
i.e., the execution of the inner authentication method(s).
Consequently, the adversary can eavesdrop on the identifiers that are
exchanged as part of the EAP method, and thus the privacy of peer
and/or authentication server is compromised along with any other data
transmitted within the tunnels. This document requires server
authentication to avoid the risks associated with anonymous cipher
suites.
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5.2. Tunneled Authentication
In many cases, a tunnel method provides mutual authentication by
authenticating the server during tunnel establishment and
authenticating the peer within the tunnel using an EAP method. As
described in [TUNNEL-MITM], this mode of operation can allow tunnel
man-in-the-middle attackers to authenticate to the server as the peer
by tunneling the inner EAP protocol messages to and from a peer that
is executing the method outside a tunnel or with an untrustworthy
server. Cryptographic binding between the established keying
material from the inner authentication method(s) and the tunnel
protocol verifies that the endpoints of the tunnel and the inner
authentication method(s) are the same. This can thwart the attack if
the inner-method-derived keys are of sufficient strength that they
cannot be broken in real-time.
In cases where the inner authentication method does not generate any
key material or only weak key material, security policies MUST be
enforced such that the peer cannot execute the inner method with the
same credentials outside a protective tunnel or with an untrustworthy
server.
5.3. Data External to Tunnel
The tunnel method will use data that is outside the TLS tunnel such
as the EAP type code or version numbers. If an attacker can
compromise the protocol by modifying these values, the tunnel method
MUST protect this data from modification. In some cases, external
data may not need additional protection because it is implicitly
verified during the protocol operation.
5.4. Separation of TLS Tunnel and Inner Authentication Termination
Terminating the inner method at a different location than the outer
tunnel needs careful consideration. The inner method data may be
vulnerable to modification and eavesdropping between the server that
terminates the tunnel and the server that terminates the inner
method. For example, if a cleartext password is used, then it may be
sent to the inner method server in a RADIUS password attribute, which
uses weak encryption that may not be suitable protection for many
environments.
In some cases, terminating the tunnel at a different location may
make it difficult for a peer to authenticate the server and trust it
for further communication. For example, if the TLS tunnel is
terminated by a different organization, the peer needs to be able to
authenticate and authorize the tunnel server to handle secret
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credentials that the peer shares with the home server that terminates
the inner method. This may not meet the security policy of many
environments.
6. References
6.1. Normative References
[RFC2560] Myers, M., Ankney, R., Malpani, A., Galperin, S., and
C. Adams, "X.509 Internet Public Key Infrastructure
Online Certificate Status Protocol - OCSP", RFC 2560,
June 1999.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and
H. Levkowetz, "Extensible Authentication Protocol
(EAP)", RFC 3748, June 2004.
[RFC4017] Stanley, D., Walker, J., and B. Aboba, "Extensible
Authentication Protocol (EAP) Method Requirements for
Wireless LANs", RFC 4017, March 2005.
[RFC4962] Housley, R. and B. Aboba, "Guidance for Authentication,
Authorization, and Accounting (AAA) Key Management",
BCP 132, RFC 4962, July 2007.
[RFC5055] Freeman, T., Housley, R., Malpani, A., Cooper, D., and
W. Polk, "Server-Based Certificate Validation Protocol
(SCVP)", RFC 5055, December 2007.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer
Security (TLS) Protocol Version 1.2", RFC 5246, August
2008.
[RFC5247] Aboba, B., Simon, D., and P. Eronen, "Extensible
Authentication Protocol (EAP) Key Management
Framework", RFC 5247, August 2008.
[RFC6677] Hartman, S., Ed., Clancy, T., and K. Hoeper, "Channel
Binding Support for Extensible Authentication Protocol
(EAP) Methods", RFC 6677, July 2012.
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RFC 6678 EAP Tunnel Method Requirements July 2012
6.2. Informative References
[EAP-ENROLL] Mahy, R., "An Extensible Authentication Protocol (EAP)
Enrollment Method", Work in Progress, March 2006.
[HC07] Hoeper, K. and L. Chen, "Where EAP Security Claims
Fail", Institute for Computer Sciences, Social
Informatics and Telecommunications Engineering (ICST),
The Fourth International Conference on Heterogeneous
Networking for Quality, Reliability, Security and
Robustness (QShine 2007), August 2007.
[LCN-2010] Hoeper, K. and L. Chen, "An Inconvenient Truth about
Tunneled Authentications", Proceedings of 35th Annual
IEEE Conference on Local Computer Networks (LCN 2010),
September 2009.
[NIST-SP-800-108]
Chen, L., "Recommendation for Key Derivation Using
Pseudorandom Functions", Draft NIST Special Publication
800-108, April 2008.
[NIST-SP-800-120]
Hoeper, K. and L. Chen, "Recommendation for EAP Methods
Used in Wireless Network Access Authentication", NIST
Special Publication 800-120, September 2009.
[NIST-SP-800-57]
Barker, E., Barker, W., Burr, W., Polk, W., and M.
Smid, "Recommendation for Key Management - Part 1:
General (Revised)", NIST Special Publication 800-57,
part 1, March 2007.
[NIST-SP-800-57p3]
Barker, E., Burr, W., Jones, A., Polk, W., Rose, S.,
and M. Smid, "Recommendation for Key Management, Part 3
Application-Specific Key Management Guidance", Draft
NIST Special Publication 800-57, part 3, October 2008.
[PEAP] Microsoft Corporation, "[MS-PEAP]: Protected Extensible
Authentication Protocol (PEAP) Specification", August
2009, <http:// download.microsoft.com/download/9/5/E/
95EF66AF-9026-4BB0-A41D-A4F81802D92C/%5BMS-
PEAP%5D.pdf>.
[RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User
Names and Passwords", RFC 4013, February 2005.
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RFC 6678 EAP Tunnel Method Requirements July 2012
[RFC4282] Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
Network Access Identifier", RFC 4282, December 2005.
[RFC4511] Sermersheim, J., "Lightweight Directory Access Protocol
(LDAP): The Protocol", RFC 4511, June 2006.
[RFC4851] Cam-Winget, N., McGrew, D., Salowey, J., and H. Zhou,
"The Flexible Authentication via Secure Tunneling
Extensible Authentication Protocol Method (EAP-FAST)",
RFC 4851, May 2007.
[RFC5056] Williams, N., "On the Use of Channel Bindings to Secure
Channels", RFC 5056, November 2007.
[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption
without Server-Side State", RFC 5077, January 2008.
[RFC5198] Klensin, J. and M. Padlipsky, "Unicode Format for
Network Interchange", RFC 5198, March 2008.
[RFC5209] Sangster, P., Khosravi, H., Mani, M., Narayan, K., and
J. Tardo, "Network Endpoint Assessment (NEA): Overview
and Requirements", RFC 5209, June 2008.
[RFC5281] Funk, P. and S. Blake-Wilson, "Extensible
Authentication Protocol Tunneled Transport Layer
Security Authenticated Protocol Version 0 (EAP-
TTLSv0)", RFC 5281, August 2008.
[RFC5646] Phillips, A. and M. Davis, "Tags for Identifying
Languages", BCP 47, RFC 5646, September 2009.
[RFC5793] Sahita, R., Hanna, S., Hurst, R., and K. Narayan,
"PB-TNC: A Posture Broker (PB) Protocol Compatible with
Trusted Network Connect (TNC)", RFC 5793, March 2010.
[RFC6066] Eastlake, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066, January
2011.
[TUNNEL-MITM] Asokan, N., Niemi, V., and K. Nyberg, "Man-in-the-
Middle in Tunnelled Authentication Protocols",
Cryptology ePrint Archive: Report 2002/163, November
2002.
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Authors' Addresses
Katrin Hoeper
Motorola Solutions, Inc.
1301 E. Algonquin Road
Schaumburg, IL 60196
USA
EMail: khoeper@motorolasolutions.com
Stephen Hanna
Juniper Networks
3 Beverly Road
Bedford, MA 01730
USA
EMail: shanna@juniper.net
Hao Zhou
Cisco Systems, Inc.
4125 Highlander Parkway
Richfield, OH 44286
USA
EMail: hzhou@cisco.com
Joseph Salowey (editor)
Cisco Systems, Inc.
2901 3rd. Ave
Seattle, WA 98121
USA
EMail: jsalowey@cisco.com
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