Internet Engineering Task Force (IETF) B. Haberman, Ed.
Request for Comments: 5906 JHU/APL
Category: Informational D. Mills
ISSN: 2070-1721 U. Delaware
June 2010
Network Time Protocol Version 4: Autokey Specification
Abstract
This memo describes the Autokey security model for authenticating
servers to clients using the Network Time Protocol (NTP) and public
key cryptography. Its design is based on the premise that IPsec
schemes cannot be adopted intact, since that would preclude stateless
servers and severely compromise timekeeping accuracy. In addition,
Public Key Infrastructure (PKI) schemes presume authenticated time
values are always available to enforce certificate lifetimes;
however, cryptographically verified timestamps require interaction
between the timekeeping and authentication functions.
This memo includes the Autokey requirements analysis, design
principles, and protocol specification. A detailed description of
the protocol states, events, and transition functions is included. A
prototype of the Autokey design based on this memo has been
implemented, tested, and documented in the NTP version 4 (NTPv4)
software distribution for the Unix, Windows, and Virtual Memory
System (VMS) operating systems at http://www.ntp.org.
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/rfc5906.
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Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
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Table of Contents
1. Introduction ....................................................4
2. NTP Security Model ..............................................4
3. Approach ........................................................7
4. Autokey Cryptography ............................................8
5. Autokey Protocol Overview ......................................12
6. NTP Secure Groups ..............................................14
7. Identity Schemes ...............................................19
8. Timestamps and Filestamps ......................................20
9. Autokey Operations .............................................22
10. Autokey Protocol Messages .....................................23
10.1. No-Operation .............................................26
10.2. Association Message (ASSOC) ..............................26
10.3. Certificate Message (CERT) ...............................26
10.4. Cookie Message (COOKIE) ..................................27
10.5. Autokey Message (AUTO) ...................................27
10.6. Leapseconds Values Message (LEAP) ........................27
10.7. Sign Message (SIGN) ......................................27
10.8. Identity Messages (IFF, GQ, MV) ..........................27
11. Autokey State Machine .........................................28
11.1. Status Word ..............................................28
11.2. Host State Variables .....................................30
11.3. Client State Variables (all modes) .......................33
11.4. Protocol State Transitions ...............................34
11.4.1. Server Dance ......................................34
11.4.2. Broadcast Dance ...................................35
11.4.3. Symmetric Dance ...................................36
11.5. Error Recovery ...........................................37
12. Security Considerations .......................................39
12.1. Protocol Vulnerability ...................................39
12.2. Clogging Vulnerability ...................................40
13. IANA Considerations ...........................................42
13. References ....................................................42
13.1. Normative References .....................................42
13.2. Informative References ...................................43
Appendix A. Timestamps, Filestamps, and Partial Ordering .........45
Appendix B. Identity Schemes .....................................46
Appendix C. Private Certificate (PC) Scheme ......................47
Appendix D. Trusted Certificate (TC) Scheme ......................47
Appendix E. Schnorr (IFF) Identity Scheme ........................48
Appendix F. Guillard-Quisquater (GQ) Identity Scheme .............49
Appendix G. Mu-Varadharajan (MV) Identity Scheme .................51
Appendix H. ASN.1 Encoding Rules .................................54
Appendix I. COOKIE Request, IFF Response, GQ Response, MV
Response .............................................54
Appendix J. Certificates .........................................55
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1. Introduction
A distributed network service requires reliable, ubiquitous, and
survivable provisions to prevent accidental or malicious attacks on
the servers and clients in the network or the values they exchange.
Reliability requires that clients can determine that received packets
are authentic; that is, were actually sent by the intended server and
not manufactured or modified by an intruder. Ubiquity requires that
a client can verify the authenticity of a server using only public
information. Survivability requires protection from faulty
implementations, improper operation, and possibly malicious clogging
and replay attacks.
This memo describes a cryptographically sound and efficient
methodology for use in the Network Time Protocol (NTP) [RFC5905].
The various key agreement schemes [RFC4306][RFC2412][RFC2522]
proposed require per-association state variables, which contradicts
the principles of the remote procedure call (RPC) paradigm in which
servers keep no state for a possibly large client population. An
evaluation of the PKI model and algorithms, e.g., as implemented in
the OpenSSL library, leads to the conclusion that any scheme
requiring every NTP packet to carry a PKI digital signature would
result in unacceptably poor timekeeping performance.
The Autokey protocol is based on a combination of PKI and a pseudo-
random sequence generated by repeated hashes of a cryptographic value
involving both public and private components. This scheme has been
implemented, tested, and deployed in the Internet of today. A
detailed description of the security model, design principles, and
implementation is presented in this memo.
This informational document describes the NTP extensions for Autokey
as implemented in an NTPv4 software distribution available from
http://www.ntp.org. This description is provided to offer a basis
for future work and a reference for the software release. This
document also describes the motivation for the extensions within the
protocol.
2. NTP Security Model
NTP security requirements are even more stringent than most other
distributed services. First, the operation of the authentication
mechanism and the time synchronization mechanism are inextricably
intertwined. Reliable time synchronization requires cryptographic
keys that are valid only over designated time intervals; but, time
intervals can be enforced only when participating servers and clients
are reliably synchronized to UTC. In addition, the NTP subnet is
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hierarchical by nature, so time and trust flow from the primary
servers at the root through secondary servers to the clients at the
leaves.
A client can claim authentic to dependent applications only if all
servers on the path to the primary servers are bona fide authentic.
In order to emphasize this requirement, in this memo, the notion of
"authentic" is replaced by "proventic", an adjective new to English
and derived from "provenance", as in the provenance of a painting.
Having abused the language this far, the suffixes fixable to the
various derivatives of authentic will be adopted for proventic as
well. In NTP, each server authenticates the next-lower stratum
servers and proventicates (authenticates by induction) the lowest
stratum (primary) servers. Serious computer linguists would
correctly interpret the proventic relation as the transitive closure
of the authentic relation.
It is important to note that the notion of proventic does not
necessarily imply the time is correct. An NTP client mobilizes a
number of concurrent associations with different servers and uses a
crafted agreement algorithm to pluck truechimers from the population
possibly including falsetickers. A particular association is
proventic if the server certificate and identity have been verified
by the means described in this memo. However, the statement "the
client is synchronized to proventic sources" means that the system
clock has been set using the time values of one or more proventic
associations and according to the NTP mitigation algorithms.
Over the last several years, the IETF has defined and evolved the
IPsec infrastructure for privacy protection and source authentication
in the Internet. The infrastructure includes the Encapsulating
Security Payload (ESP) [RFC4303] and Authentication Header (AH)
[RFC4302] for IPv4 and IPv6. Cryptographic algorithms that use these
headers for various purposes include those developed for the PKI,
including various message digest, digital signature, and key
agreement algorithms. This memo takes no position on which message
digest or digital signature algorithm is used. This is established
by a profile for each community of users.
It will facilitate the discussion in this memo to refer to the
reference implementation available at http://www.ntp.org. It
includes Autokey as described in this memo and is available to the
general public; however, it is not part of the specification itself.
The cryptographic means used by the reference implementation and its
user community are based on the OpenSSL cryptographic software
library available at http://www.openssl.org, but other libraries with
equivalent functionality could be used as well. It is important for
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distribution and export purposes that the way in which these
algorithms are used precludes encryption of any data other than
incidental to the construction of digital signatures.
The fundamental assumption in NTP about the security model is that
packets transmitted over the Internet can be intercepted by those
other than the intended recipient, remanufactured in various ways,
and replayed in whole or part. These packets can cause the client to
believe or produce incorrect information, cause protocol operations
to fail, interrupt network service, or consume precious network and
processor resources.
In the case of NTP, the assumed goal of the intruder is to inject
false time values, disrupt the protocol or clog the network, servers,
or clients with spurious packets that exhaust resources and deny
service to legitimate applications. The mission of the algorithms
and protocols described in this memo is to detect and discard
spurious packets sent by someone other than the intended sender or
sent by the intended sender, but modified or replayed by an intruder.
There are a number of defense mechanisms already built in the NTP
architecture, protocol, and algorithms. The on-wire timestamp
exchange scheme is inherently resistant to spoofing, packet-loss, and
replay attacks. The engineered clock filter, selection, and
clustering algorithms are designed to defend against evil cliques of
Byzantine traitors. While not necessarily designed to defeat
determined intruders, these algorithms and accompanying sanity checks
have functioned well over the years to deflect improperly operating
but presumably friendly scenarios. However, these mechanisms do not
securely identify and authenticate servers to clients. Without
specific further protection, an intruder can inject any or all of the
following attacks.
1. An intruder can intercept and archive packets forever, as well as
all the public values ever generated and transmitted over the
net.
2. An intruder can generate packets faster than the server, network,
or client can process them, especially if they require expensive
cryptographic computations.
3. In a wiretap attack, the intruder can intercept, modify, and
replay a packet. However, it cannot permanently prevent onward
transmission of the original packet; that is, it cannot break the
wire, only tell lies and congest it. Except in the unlikely
cases considered in Section 12, the modified packet cannot arrive
at the victim before the original packet, nor does it have the
server private keys or identity parameters.
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4. In a man-in-the-middle or masquerade attack, the intruder is
positioned between the server and client, so it can intercept,
modify, and replay a packet and prevent onward transmission of
the original packet. Except in unlikely cases considered in
Section 12, the middleman does not have the server private keys.
The NTP security model assumes the following possible limitations.
1. The running times for public key algorithms are relatively long
and highly variable. In general, the performance of the time
synchronization function is badly degraded if these algorithms
must be used for every NTP packet.
2. In some modes of operation, it is not feasible for a server to
retain state variables for every client. It is however feasible
to regenerated them for a client upon arrival of a packet from
that client.
3. The lifetime of cryptographic values must be enforced, which
requires a reliable system clock. However, the sources that
synchronize the system clock must be cryptographically
proventicated. This circular interdependence of the timekeeping
and proventication functions requires special handling.
4. Client security functions must involve only public values
transmitted over the net. Private values must never be disclosed
beyond the machine on which they were created, except in the case
of a special trusted agent (TA) assigned for this purpose.
Unlike the Secure Shell (SSH) security model, where the client must
be securely authenticated to the server, in NTP, the server must be
securely authenticated to the client. In SSH, each different
interface address can be bound to a different name, as returned by a
reverse-DNS query. In this design, separate public/private key pairs
may be required for each interface address with a distinct name. A
perceived advantage of this design is that the security compartment
can be different for each interface. This allows a firewall, for
instance, to require some interfaces to authenticate the client and
others not.
3. Approach
The Autokey protocol described in this memo is designed to meet the
following objectives. In-depth discussions on these objectives is in
the web briefings and will not be elaborated in this memo. Note that
here, and elsewhere in this memo, mention of broadcast mode means
multicast mode as well, with exceptions as noted in the NTP software
documentation [RFC5905].
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1. It must interoperate with the existing NTP architecture model and
protocol design. In particular, it must support the symmetric
key scheme described in [RFC1305]. As a practical matter, the
reference implementation must use the same internal key
management system, including the use of 32-bit key IDs and
existing mechanisms to store, activate, and revoke keys.
2. It must provide for the independent collection of cryptographic
values and time values. An NTP packet is accepted for processing
only when the required cryptographic values have been obtained
and verified and the packet has passed all header sanity checks.
3. It must not significantly degrade the potential accuracy of the
NTP synchronization algorithms. In particular, it must not make
unreasonable demands on the network or host processor and memory
resources.
4. It must be resistant to cryptographic attacks, specifically those
identified in the security model above. In particular, it must
be tolerant of operational or implementation variances, such as
packet loss or disorder, or suboptimal configurations.
5. It must build on a widely available suite of cryptographic
algorithms, yet be independent of the particular choice. In
particular, it must not require data encryption other than that
which is incidental to signature and cookie encryption
operations.
6. It must function in all the modes supported by NTP, including
server, symmetric, and broadcast modes.
4. Autokey Cryptography
Autokey cryptography is based on the PKI algorithms commonly used in
the Secure Shell and Secure Sockets Layer (SSL) applications. As in
these applications, Autokey uses message digests to detect packet
modification, digital signatures to verify credentials, and public
certificates to provide traceable authority. What makes Autokey
cryptography unique is the way in which these algorithms are used to
deflect intruder attacks while maintaining the integrity and accuracy
of the time synchronization function.
Autokey, like many other remote procedure call (RPC) protocols,
depends on message digests for basic authentication; however, it is
important to understand that message digests are also used by NTP
when Autokey is not available or not configured. Selection of the
digest algorithm is a function of NTP configuration and is
transparent to Autokey.
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The protocol design and reference implementation support both 128-bit
and 160-bit message digest algorithms, each with a 32-bit key ID. In
order to retain backwards compatibility with NTPv3, the NTPv4 key ID
space is partitioned in two subspaces at a pivot point of 65536.
Symmetric key IDs have values less than the pivot and indefinite
lifetime. Autokey key IDs have pseudo-random values equal to or
greater than the pivot and are expunged immediately after use.
Both symmetric key and public key cryptography authenticate as shown
in Figure 1. The server looks up the key associated with the key ID
and calculates the message digest from the NTP header and extension
fields together with the key value. The key ID and digest form the
message authentication code (MAC) included with the message. The
client does the same computation using its local copy of the key and
compares the result with the digest in the MAC. If the values agree,
the message is assumed authentic.
+------------------+
| NTP Header and |
| Extension Fields |
+------------------+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | | Message Authentication Code |
\|/ \|/ + (MAC) +
******************** | +-------------------------+ |
* Compute Hash *<----| Key ID | Message Digest | +
******************** | +-------------------------+ |
| +-+-+-+-+-+-+-|-+-+-+-+-+-+-+-+-+
\|/ \|/
+------------------+ +-------------+
| Message Digest |------>| Compare |
+------------------+ +-------------+
Figure 1: Message Authentication
Autokey uses specially contrived session keys, called autokeys, and a
precomputed pseudo-random sequence of autokeys that are saved in the
autokey list. The Autokey protocol operates separately for each
association, so there may be several autokey sequences operating
independently at the same time.
+-------------+-------------+--------+--------+
| Src Address | Dst Address | Key ID | Cookie |
+-------------+-------------+--------+--------+
Figure 2: NTPv4 Autokey
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An autokey is computed from four fields in network byte order as
shown in Figure 2. The four values are hashed using the MD5
algorithm to produce the 128-bit autokey value, which in the
reference implementation is stored along with the key ID in a cache
used for symmetric keys as well as autokeys. Keys are retrieved from
the cache by key ID using hash tables and a fast lookup algorithm.
For use with IPv4, the Src Address and Dst Address fields contain 32
bits; for use with IPv6, these fields contain 128 bits. In either
case, the Key ID and Cookie fields contain 32 bits. Thus, an IPv4
autokey has four 32-bit words, while an IPv6 autokey has ten 32-bit
words. The source and destination addresses and key ID are public
values visible in the packet, while the cookie can be a public value
or shared private value, depending on the NTP mode.
The NTP packet format has been augmented to include one or more
extension fields piggybacked between the original NTP header and the
MAC. For packets without extension fields, the cookie is a shared
private value. For packets with extension fields, the cookie has a
default public value of zero, since these packets are validated
independently using digital signatures.
There are some scenarios where the use of endpoint IP addresses may
be difficult or impossible. These include configurations where
network address translation (NAT) devices are in use or when
addresses are changed during an association lifetime due to mobility
constraints. For Autokey, the only restriction is that the address
fields that are visible in the transmitted packet must be the same as
those used to construct the autokey list and that these fields be the
same as those visible in the received packet. (The use of
alternative means, such as Autokey host names (discussed later) or
hashes of these names may be a topic for future study.)
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+-----------+-----------+------+------+ +---------+ +-----+------+
|Src Address|Dst Address|Key ID|Cookie|-->| | |Final|Final |
+-----------+-----------+------+------+ | Session | |Index|Key ID|
| | | | | Key ID | +-----+------+
\|/ \|/ \|/ \|/ | List | | |
************************************* +---------+ \|/ \|/
* COMPUTE HASH * *******************
************************************* *COMPUTE SIGNATURE*
| Index n *******************
\|/ |
+--------+ |
| Next | \|/
| Key ID | +-----------+
+--------+ | Signature |
Index n+1 +-----------+
Figure 3: Constructing the Key List
Figure 3 shows how the autokey list and autokey values are computed.
The key IDs used in the autokey list consist of a sequence starting
with a random 32-bit nonce (autokey seed) greater than or equal to
the pivot as the first key ID. The first autokey is computed as
above using the given cookie and autokey seed and assigned index 0.
The first 32 bits of the result in network byte order become the next
key ID. The MD5 hash of the autokey is the key value saved in the
key cache along with the key ID. The first 32 bits of the key become
the key ID for the next autokey assigned index 1.
Operations continue to generate the entire list. It may happen that
a newly generated key ID is less than the pivot or collides with
another one already generated (birthday event). When this happens,
which occurs only rarely, the key list is terminated at that point.
The lifetime of each key is set to expire one poll interval after its
scheduled use. In the reference implementation, the list is
terminated when the maximum key lifetime is about one hour, so for
poll intervals above one hour, a new key list containing only a
single entry is regenerated for every poll.
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+------------------+
| NTP Header and |
| Extension Fields |
+------------------+
| |
\|/ \|/ +---------+
**************** +--------+ | Session |
* COMPUTE HASH *<---| Key ID |<---| Key ID |
**************** +--------+ | List |
| | +---------+
\|/ \|/
+-----------------------------------+
| Message Authentication Code (MAC) |
+-----------------------------------+
Figure 4: Transmitting Messages
The index of the last autokey in the list is saved along with the key
ID for that entry, collectively called the autokey values. The
autokey values are then signed for use later. The list is used in
reverse order as shown in Figure 4, so that the first autokey used is
the last one generated.
The Autokey protocol includes a message to retrieve the autokey
values and verify the signature, so that subsequent packets can be
validated using one or more hashes that eventually match the last key
ID (valid) or exceed the index (invalid). This is called the autokey
test in the following and is done for every packet, including those
with and without extension fields. In the reference implementation,
the most recent key ID received is saved for comparison with the
first 32 bits in network byte order of the next following key value.
This minimizes the number of hash operations in case a single packet
is lost.
5. Autokey Protocol Overview
The Autokey protocol includes a number of request/response exchanges
that must be completed in order. In each exchange, a client sends a
request message with data and expects a server response message with
data. Requests and responses are contained in extension fields, one
request or response in each field, as described later. An NTP packet
can contain one request message and one or more response messages.
The following is a list of these messages.
o Parameter exchange. The request includes the client host name and
status word; the response includes the server host name and status
word. The status word specifies the digest/signature scheme to
use and the identity schemes supported.
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o Certificate exchange. The request includes the subject name of a
certificate; the response consists of a signed certificate with
that subject name. If the issuer name is not the same as the
subject name, it has been signed by a host one step closer to a
trusted host, so certificate retrieval continues for the issuer
name. If it is trusted and self-signed, the trail concludes at
the trusted host. If nontrusted and self-signed, the host
certificate has not yet been signed, so the trail temporarily
loops. Completion of this exchange lights the VAL bit as
described below.
o Identity exchange. The certificate trail is generally not
considered sufficient protection against man-in-the-middle attacks
unless additional protection such as the proof-of-possession
scheme described in [RFC2875] is available, but this is expensive
and requires servers to retain state. Autokey can use one of the
challenge/response identity schemes described in Appendix B.
Completion of this exchange lights the IFF bit as described below.
o Cookie exchange. The request includes the public key of the
server. The response includes the server cookie encrypted with
this key. The client uses this value when constructing the key
list. Completion of this exchange lights the COOK bit as
described below.
o Autokey exchange. The request includes either no data or the
autokey values in symmetric modes. The response includes the
autokey values of the server. These values are used to verify the
autokey sequence. Completion of this exchange lights the AUT bit
as described below.
o Sign exchange. This exchange is executed only when the client has
synchronized to a proventic source. The request includes the
self-signed client certificate. The server acting as
certification authority (CA) interprets the certificate as a
X.509v3 certificate request. It extracts the subject, issuer, and
extension fields, builds a new certificate with these data along
with its own serial number and expiration time, then signs it
using its own private key and includes it in the response. The
client uses the signed certificate in its own role as server for
dependent clients. Completion of this exchange lights the SIGN
bit as described below.
o Leapseconds exchange. This exchange is executed only when the
client has synchronized to a proventic source. This exchange
occurs when the server has the leapseconds values, as indicated in
the host status word. If so, the client requests the values and
compares them with its own values, if available. If the server
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values are newer than the client values, the client replaces its
own with the server values. The client, acting as server, can now
provide the most recent values to its dependent clients. In
symmetric mode, this results in both peers having the newest
values. Completion of this exchange lights the LPT bit as
described below.
Once the certificates and identity have been validated, subsequent
packets are validated by digital signatures and the autokey sequence.
The association is now proventic with respect to the downstratum
trusted host, but is not yet selectable to discipline the system
clock. The associations accumulate time values, and the mitigation
algorithms continue in the usual way. When these algorithms have
culled the falsetickers and cluster outliers and at least three
survivors remain, the system clock has been synchronized to a
proventic source.
The time values for truechimer sources form a proventic partial
ordering relative to the applicable signature timestamps. This
raises the interesting issue of how to differentiate between the
timestamps of different associations. It might happen, for instance,
that the timestamp of some Autokey message is ahead of the system
clock by some presumably small amount. For this reason, timestamp
comparisons between different associations and between associations
and the system clock are avoided, except in the NTP intersection and
clustering algorithms and when determining whether a certificate has
expired.
6. NTP Secure Groups
NTP secure groups are used to define cryptographic compartments and
security hierarchies. A secure group consists of a number of hosts
dynamically assembled as a forest with roots the trusted hosts (THs)
at the lowest stratum of the group. The THs do not have to be, but
often are, primary (stratum 1) servers. A trusted authority (TA),
not necessarily a group host, generates private identity keys for
servers and public identity keys for clients at the leaves of the
forest. The TA deploys the server keys to the THs and other
designated servers using secure means and posts the client keys on a
public web site.
For Autokey purposes, all hosts belonging to a secure group have the
same group name but different host names, not necessarily related to
the DNS names. The group name is used in the subject and issuer
fields of the TH certificates; the host name is used in these fields
for other hosts. Thus, all host certificates are self-signed.
During the use of the Autokey protocol, a client requests that the
server sign its certificate and caches the result. A certificate
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trail is constructed by each host, possibly via intermediate hosts
and ending at a TH. Thus, each host along the trail retrieves the
entire trail from its server(s) and provides this plus its own signed
certificates to its clients.
Secure groups can be configured as hierarchies where a TH of one
group can be a client of one or more other groups operating at a
lower stratum. In one scenario, THs for groups RED and GREEN can be
cryptographically distinct, but both be clients of group BLUE
operating at a lower stratum. In another scenario, THs for group
CYAN can be clients of multiple groups YELLOW and MAGENTA, both
operating at a lower stratum. There are many other scenarios, but
all must be configured to include only acyclic certificate trails.
In Figure 5, the Alice group consists of THs Alice, which is also the
TA, and Carol. Dependent servers Brenda and Denise have configured
Alice and Carol, respectively, as their time sources. Stratum 3
server Eileen has configured both Brenda and Denise as her time
sources. Public certificates are identified by the subject and
signed by the issuer. Note that the server group keys have been
previously installed on Brenda and Denise and the client group keys
installed on all machines.
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+-------------+ +-------------+ +-------------+
| Alice Group | | Brenda | | Denise |
| Alice | | | | |
| +-+-+-+-+ | | +-+-+-+-+ | | +-+-+-+-+ |
Certificate | | Alice | | | | Brenda| | | | Denise| |
+-+-+-+-+-+ | +-+-+-+-+ | | +-+-+-+-+ | | +-+-+-+-+ |
| Subject | | | Alice*| 1 | | | Alice | 4 | | | Carol | 4 |
+-+-+-+-+-+ | +-+-+-+-+ | | +-+-+-+-+ | | +-+-+-+-+ |
| Issuer | S | | | | | |
+-+-+-+-+-+ | +=======+ | | +-+-+-+-+ | | +-+-+-+-+ |
| ||Alice|| 3 | | | Alice | | | | Carol | |
Group Key | +=======+ | | +-+-+-+-+ | | +-+-+-+-+ |
+=========+ +-------------+ | | Alice*| 2 | | | Carol*| 2 |
|| Group || S | Alice Group | | +-+-+-+-+ | | +-+-+-+-+ |
+=========+ | Carol | | | | |
| +-+-+-+-+ | | +-+-+-+-+ | | +-+-+-+-+ |
S = step | | Carol | | | | Brenda| | | | Denise| |
* = trusted | +-+-+-+-+ | | +-+-+-+-+ | | +-+-+-+-+ |
| | Carol*| 1 | | | Brenda| 1 | | | Denise| 1 |
| +-+-+-+-+ | | +-+-+-+-+ | | +-+-+-+-+ |
| | | | | |
| +=======+ | | +=======+ | | +=======+ |
| ||Alice|| 3 | | ||Alice|| 3 | | ||Alice|| 3 |
| +=======+ | | +=======+ | | +=======+ |
+-------------+ +-------------+ +-------------+
Stratum 1 Stratum 2
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+---------------------------------------------+
| Eileen |
| |
| +-+-+-+-+ +-+-+-+-+ |
| | Eileen| | Eileen| |
| +-+-+-+-+ +-+-+-+-+ |
| | Brenda| 4 | Carol | 4 |
| +-+-+-+-+ +-+-+-+-+ |
| |
| +-+-+-+-+ +-+-+-+-+ |
| | Alice | | Carol | |
| +-+-+-+-+ +-+-+-+-+ |
| | Alice*| 2 | Carol*| 2 |
| +-+-+-+-+ +-+-+-+-+ |
| |
| +-+-+-+-+ +-+-+-+-+ |
| | Brenda| | Denise| |
| +-+-+-+-+ +-+-+-+-+ |
| | Alice | 2 | Carol | 2 |
| +-+-+-+-+ +-+-+-+-+ |
| |
| +-+-+-+-+ |
| | Eileen| |
| +-+-+-+-+ |
| | Eileen| 1 |
| +-+-+-+-+ |
| |
| +=======+ |
| ||Alice|| 3 |
| +=======+ |
+---------------------------------------------+
Stratum 3
Figure 5: NTP Secure Groups
The steps in hiking the certificate trails and verifying identity are
as follows. Note the step number in the description matches the step
number in the figure.
1. The girls start by loading the host key, sign key, self-signed
certificate, and group key. Each client and server acting as a
client starts the Autokey protocol by retrieving the server host
name and digest/signature. This is done using the ASSOC exchange
described later.
2. They continue to load certificates recursively until a self-
signed trusted certificate is found. Brenda and Denise
immediately find trusted certificates for Alice and Carol,
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respectively, but Eileen will loop because neither Brenda nor
Denise have their own certificates signed by either Alice or
Carol. This is done using the CERT exchange described later.
3. Brenda and Denise continue with the selected identity schemes to
verify that Alice and Carol have the correct group key previously
generated by Alice. This is done using one of the identity
schemes IFF, GQ, or MV, described later. If this succeeds, each
continues in step 4.
4. Brenda and Denise present their certificates for signature using
the SIGN exchange described later. If this succeeds, either one
of or both Brenda and Denise can now provide these signed
certificates to Eileen, which may be looping in step 2. Eileen
can now verify the trail via either Brenda or Denise to the
trusted certificates for Alice and Carol. Once this is done,
Eileen can complete the protocol just as Brenda and Denise did.
For various reasons, it may be convenient for a server to have client
keys for more than one group. For example, Figure 6 shows three
secure groups Alice, Helen, and Carol arranged in a hierarchy. Hosts
A, B, C, and D belong to Alice with A and B as her THs. Hosts R and
S belong to Helen with R as her TH. Hosts X and Y belong to Carol
with X as her TH. Note that the TH for a group is always the lowest
stratum and that the hosts of the combined groups form an acyclic
graph. Note also that the certificate trail for each group
terminates on a TH for that group.
***** ***** @@@@@
Stratum 1 * A * * B * @ R @
***** ***** @@@@@
\ / /
\ / /
***** @@@@@ *********
2 * C * @ S @ * Alice *
***** @@@@@ *********
/ \ /
/ \ / @@@@@@@@@
***** ##### @ Helen @
3 * D * # X # @@@@@@@@@
***** #####
/ \ #########
/ \ # Carol #
##### ##### #########
4 # Y # # Z #
##### #####
Figure 6: Hierarchical Overlapping Groups
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The intent of the scenario is to provide security separation, so that
servers cannot masquerade as clients in other groups and clients
cannot masquerade as servers. Assume, for example, that Alice and
Helen belong to national standards laboratories and their server keys
are used to confirm identity between members of each group. Carol is
a prominent corporation receiving standards products and requiring
cryptographic authentication. Perhaps under contract, host X
belonging to Carol has client keys for both Alice and Helen and
server keys for Carol. The Autokey protocol operates for each group
separately while preserving security separation. Host X can prove
identity in Carol to clients Y and Z, but cannot prove to anybody
that it belongs to either Alice or Helen.
7. Identity Schemes
A digital signature scheme provides secure server authentication, but
it does not provide protection against masquerade, unless the server
identity is verified by other means. The PKI model requires a server
to prove identity to the client by a certificate trail, but
independent means such as a driver's license are required for a CA to
sign the server certificate. While Autokey supports this model by
default, in a hierarchical ad hoc network, especially with server
discovery schemes like NTP manycast, proving identity at each rest
stop on the trail must be an intrinsic capability of Autokey itself.
While the identity scheme described in [RFC2875] is based on a
ubiquitous Diffie-Hellman infrastructure, it is expensive to generate
and use when compared to others described in Appendix B. In
principle, an ordinary public key scheme could be devised for this
purpose, but the most stringent Autokey design requires that every
challenge, even if duplicated, results in a different acceptable
response.
1. The scheme must have a relatively long lifetime, certainly longer
than a typical certificate, and have no specific lifetime or
expiration date. At the time the scheme is used, the host has
not yet synchronized to a proventic source, so the scheme cannot
depend on time.
2. As the scheme can be used many times where the data might be
exposed to potential intruders, the data must be either nonces or
encrypted nonces.
3. The scheme should allow designated servers to prove identity to
designated clients, but not allow clients acting as servers to
prove identity to dependent clients.
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4. To the greatest extent possible, the scheme should represent a
zero-knowledge proof; that is, the client should be able to
verify that the server has the correct group key, but without
knowing the key itself.
There are five schemes now implemented in the NTPv4 reference
implementation to prove identity: (1) private certificate (PC), (2)
trusted certificate (TC), (3) a modified Schnorr algorithm (IFF aka
Identify Friendly or Foe), (4) a modified Guillou-Quisquater (GQ)
algorithm, and (5) a modified Mu-Varadharajan (MV) algorithm. Not
all of these provide the same level of protection and one, TC,
provides no protection but is included for comparison. The following
is a brief summary description of each; details are given in
Appendix B.
The PC scheme involves a private certificate as group key. The
certificate is distributed to all other group members by secure means
and is never revealed outside the group. In effect, the private
certificate is used as a symmetric key. This scheme is used
primarily for testing and development and is not recommended for
regular use and is not considered further in this memo.
All other schemes involve a conventional certificate trail as
described in [RFC5280]. This is the default scheme when an identity
scheme is not required. While the remaining identity schemes
incorporate TC, it is not by itself considered further in this memo.
The three remaining schemes IFF, GQ, and MV involve a
cryptographically strong challenge-response exchange where an
intruder cannot deduce the server key, even after repeated
observations of multiple exchanges. In addition, the MV scheme is
properly described as a zero-knowledge proof, because the client can
verify the server has the correct group key without either the server
or client knowing its value. These schemes start when the client
sends a nonce to the server, which then rolls its own nonce, performs
a mathematical operation and sends the results to the client. The
client performs another mathematical operation and verifies the
results are correct.
8. Timestamps and Filestamps
While public key signatures provide strong protection against
misrepresentation of source, computing them is expensive. This
invites the opportunity for an intruder to clog the client or server
by replaying old messages or originating bogus messages. A client
receiving such messages might be forced to verify what turns out to
be an invalid signature and consume significant processor resources.
In order to foil such attacks, every Autokey message carries a
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timestamp in the form of the NTP seconds when it was created. If the
system clock is synchronized to a proventic source, a signature is
produced with a valid (nonzero) timestamp. Otherwise, there is no
signature and the timestamp is invalid (zero). The protocol detects
and discards extension fields with old or duplicate timestamps,
before any values are used or signatures are verified.
Signatures are computed only when cryptographic values are created or
modified, which is by design not very often. Extension fields
carrying these signatures are copied to messages as needed, but the
signatures are not recomputed. There are three signature types:
1. Cookie signature/timestamp. The cookie is signed when created by
the server and sent to the client.
2. Autokey signature/timestamp. The autokey values are signed when
the key list is created.
3. Public values signature/timestamp. The public key, certificate,
and leapsecond values are signed at the time of generation, which
occurs when the system clock is first synchronized to a proventic
source, when the values have changed and about once per day after
that, even if these values have not changed.
The most recent timestamp received of each type is saved for
comparison. Once a signature with a valid timestamp has been
received, messages with invalid timestamps or earlier valid
timestamps of the same type are discarded before the signature is
verified. This is most important in broadcast mode, which could be
vulnerable to a clogging attack without this test.
All cryptographic values used by the protocol are time sensitive and
are regularly refreshed. In particular, files containing
cryptographic values used by signature and encryption algorithms are
regenerated from time to time. It is the intent that file
regenerations occur without specific advance warning and without
requiring prior distribution of the file contents. While
cryptographic data files are not specifically signed, every file is
associated with a filestamp showing the NTP seconds at the creation
epoch.
Filestamps and timestamps can be compared in any combination and use
the same conventions. It is necessary to compare them from time to
time to determine which are earlier or later. Since these quantities
have a granularity only to the second, such comparisons are ambiguous
if the values are in the same second.
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It is important that filestamps be proventic data; thus, they cannot
be produced unless the producer has been synchronized to a proventic
source. As such, the filestamps throughout the NTP subnet represent
a partial ordering of all creation epochs and serve as means to
expunge old data and ensure new data are consistent. As the data are
forwarded from server to client, the filestamps are preserved,
including those for certificate and leapseconds values. Packets with
older filestamps are discarded before spending cycles to verify the
signature.
9. Autokey Operations
The NTP protocol has three principal modes of operation: client/
server, symmetric, and broadcast and each has its own Autokey
program, or dance. Autokey choreography is designed to be non-
intrusive and to require no additional packets other than for regular
NTP operations. The NTP and Autokey protocols operate simultaneously
and independently. When the dance is complete, subsequent packets
are validated by the autokey sequence and thus considered proventic
as well. Autokey assumes NTP clients poll servers at a relatively
low rate, such as once per minute or slower. In particular, it
assumes that a request sent at one poll opportunity will normally
result in a response before the next poll opportunity; however, the
protocol is robust against a missed or duplicate response.
The server dance was suggested by Steve Kent over lunch some time
ago, but considerably modified since that meal. The server keeps no
state for each client, but uses a fast algorithm and a 32-bit random
private value (server seed) to regenerate the cookie upon arrival of
a client packet. The cookie is calculated as the first 32 bits of
the autokey computed from the client and server addresses, key ID
zero, and the server seed as cookie. The cookie is used for the
actual autokey calculation by both the client and server and is thus
specific to each client separately.
In the server dance, the client uses the cookie and each key ID on
the key list in turn to retrieve the autokey and generate the MAC.
The server uses the same values to generate the message digest and
verifies it matches the MAC. It then generates the MAC for the
response using the same values, but with the client and server
addresses interchanged. The client generates the message digest and
verifies it matches the MAC. In order to deflect old replays, the
client verifies that the key ID matches the last one sent. In this
dance, the sequential structure of the key list is not exploited, but
doing it this way simplifies and regularizes the implementation while
making it nearly impossible for an intruder to guess the next key ID.
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In the broadcast dance, clients normally do not send packets to the
server, except when first starting up. At that time, the client runs
the server dance to verify the server credentials and calibrate the
propagation delay. The dance requires the association ID of the
particular server association, since there can be more than one
operating in the same server. For this purpose, the server packet
includes the association ID in every response message sent and, when
sending the first packet after generating a new key list, it sends
the autokey values as well. After obtaining and verifying the
autokey values, no extension fields are necessary and the client
verifies further server packets using the autokey sequence.
The symmetric dance is similar to the server dance and requires only
a small amount of state between the arrival of a request and
departure of the response. The key list for each direction is
generated separately by each peer and used independently, but each is
generated with the same cookie. The cookie is conveyed in a way
similar to the server dance, except that the cookie is a simple
nonce. There exists a possible race condition where each peer sends
a cookie request before receiving the cookie response from the other
peer. In this case, each peer winds up with two values, one it
generated and one the other peer generated. The ambiguity is
resolved simply by computing the working cookie as the EXOR of the
two values.
Once the Autokey dance has completed, it is normally dormant. In all
except the broadcast dance, packets are normally sent without
extension fields, unless the packet is the first one sent after
generating a new key list or unless the client has requested the
cookie or autokey values. If for some reason the client clock is
stepped, rather than slewed, all cryptographic and time values for
all associations are purged and the dances in all associations
restarted from scratch. This ensures that stale values never
propagate beyond a clock step.
10. Autokey Protocol Messages
The Autokey protocol data unit is the extension field, one or more of
which can be piggybacked in the NTP packet. An extension field
contains either a request with optional data or a response with
optional data. To avoid deadlocks, any number of responses can be
included in a packet, but only one request can be. A response is
generated for every request, even if the requestor is not
synchronized to a proventic source, but most contain meaningful data
only if the responder is synchronized to a proventic source. Some
requests and most responses carry timestamped signatures. The
signature covers the entire extension field, including the timestamp
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and filestamp, where applicable. Only if the packet has correct
format, length, and message digest are cycles spent to verify the
signature.
There are currently eight Autokey requests and eight corresponding
responses. The NTP packet format is described in [RFC5905] and the
extension field format used for these messages is illustrated in
Figure 7.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R|E| Code | Field Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Association ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Filestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ /
/ Value \
\ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Signature Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ /
/ Signature \
\ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ /
/ Padding (if needed) \
\ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: NTPv4 Extension Field Format
While each extension field is zero-padded to a 4-octet (word)
boundary, the entire extension is not word-aligned. The Length field
covers the entire extension field, including the Length and Padding
fields. While the minimum field length is 8 octets, a maximum field
length remains to be established. The reference implementation
discards any packet with a field length more than 1024 octets.
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One or more extension fields follow the NTP packet header and the
last followed by the MAC. The extension field parser initializes a
pointer to the first octet beyond the NTP packet header and
calculates the number of octets remaining to the end of the packet.
If the remaining length is 20 (128-bit digest plus 4-octet key ID) or
22 (160-bit digest plus 4-octet key ID), the remaining data are the
MAC and parsing is complete. If the remaining length is greater than
22, an extension field is present. If the remaining length is less
than 8 or not a multiple of 4, a format error has occurred and the
packet is discarded; otherwise, the parser increments the pointer by
the extension field length and then uses the same rules as above to
determine whether a MAC is present or another extension field.
In Autokey the 8-bit Field Type field is interpreted as the version
number, currently 2. For future versions, values 1-7 have been
reserved for Autokey; other values may be assigned for other
applications. The 6-bit Code field specifies the request or response
operation. There are two flag bits: bit 0 is the Response Flag (R)
and bit 1 is the Error Flag (E); the Reserved field is unused and
should be set to 0. The remaining fields will be described later.
In the most common protocol operations, a client sends a request to a
server with an operation code specified in the Code field and both
the R bit and E bit dim. The server returns a response with the same
operation code in the Code field and lights the R bit. The server
can also light the E bit in case of error. Note that it is not
necessarily a protocol error to send an unsolicited response with no
matching request. If the R bit is dim, the client sets the
Association ID field to the client association ID, which the server
returns for verification. If the two values do not match, the
response is discarded as if never sent. If the R bit is lit, the
Association ID field is set to the server association ID obtained in
the initial protocol exchange. If the Association ID field does not
match any mobilized association ID, the request is discarded as if
never sent.
In some cases, not all fields may be present. For requests, until a
client has synchronized to a proventic source, signatures are not
valid. In such cases, the Timestamp field and Signature Length field
(which specifies the length of the Signature) are zero and the
Signature field is absent. Some request and error response messages
carry no value or signature fields, so in these messages only the
first two words (8 octets) are present.
The Timestamp and Filestamp words carry the seconds field of an NTP
timestamp. The timestamp establishes the signature epoch of the data
field in the message, while the filestamp establishes the generation
epoch of the file that ultimately produced the data that is signed.
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A signature and timestamp are valid only when the signing host is
synchronized to a proventic source; otherwise, the timestamp is zero.
A cryptographic data file can only be generated if a signature is
possible; otherwise, the filestamp is zero, except in the ASSOC
response message, where it contains the server status word.
As in all other TCP/IP protocol designs, all data are sent in network
byte order. Unless specified otherwise in the descriptions to
follow, the data referred to are stored in the Value field. The
Value Length field specifies the length of the data in the Value
field.
10.1. No-Operation
A No-operation request (Code 0) does nothing except return an empty
response, which can be used as a crypto-ping.
10.2. Association Message (ASSOC)
An Association Message (Code 1) is used in the parameter exchange to
obtain the host name and status word. The request contains the
client status word in the Filestamp field and the Autokey host name
in the Value field. The response contains the server status word in
the Filestamp field and the Autokey host name in the Value field.
The Autokey host name is not necessarily the DNS host name. A valid
response lights the ENAB bit and possibly others in the association
status word.
When multiple identity schemes are supported, the host status word
determines which ones are available. In server and symmetric modes,
the response status word contains bits corresponding to the supported
schemes. In all modes, the scheme is selected based on the client
identity parameters that are loaded at startup.
10.3. Certificate Message (CERT)
A Certificate Message (Code 2) is used in the certificate exchange to
obtain a certificate by subject name. The request contains the
subject name; the response contains the certificate encoded in X.509
format with ASN.1 syntax as described in Appendix H.
If the subject name in the response does not match the issuer name,
the exchange continues with the issuer name replacing the subject
name in the request. The exchange continues until a trusted, self-
signed certificate is found and lights the CERT bit in the
association status word.
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10.4. Cookie Message (COOKIE)
The Cookie Message (Code 3) is used in server and symmetric modes to
obtain the server cookie. The request contains the host public key
encoded with ASN.1 syntax as described in Appendix H. The response
contains the cookie encrypted by the public key in the request. A
valid response lights the COOKIE bit in the association status word.
10.5. Autokey Message (AUTO)
The Autokey Message (Code 4) is used to obtain the autokey values.
The request contains no value for a client or the autokey values for
a symmetric peer. The response contains two 32-bit words, the first
is the final key ID, while the second is the index of the final key
ID. A valid response lights the AUTO bit in the association status
word.
10.6. Leapseconds Values Message (LEAP)
The Leapseconds Values Message (Code 5) is used to obtain the
leapseconds values as parsed from the leapseconds table from the
National Institute of Standards and Technology (NIST). The request
contains no values. The response contains three 32-bit integers:
first the NTP seconds of the latest leap event followed by the NTP
seconds when the latest NIST table expires and then the TAI offset
following the leap event. A valid response lights the LEAP bit in
the association status word.
10.7. Sign Message (SIGN)
The Sign Message (Code 6) requests that the server sign and return a
certificate presented in the request. The request contains the
client certificate encoded in X.509 format with ASN.1 syntax as
described in Appendix H. The response contains the client
certificate signed by the server private key. A valid response
lights the SIGN bit in the association status word.
10.8. Identity Messages (IFF, GQ, MV)
The Identity Messages (Code 7 (IFF), 8 (GQ), or 9 (MV)) contains the
client challenge, usually a 160- or 512-bit nonce. The response
contains the result of the mathematical operation defined in
Appendix B. The Response is encoded in ASN.1 syntax as described in
Appendix H. A valid response lights the VRFY bit in the association
status word.
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11. Autokey State Machine
This section describes the formal model of the Autokey state machine,
its state variables and the state transition functions.
11.1. Status Word
The server implements a host status word, while each client
implements an association status word. These words have the format
and content shown in Figure 8. The low-order 16 bits of the status
word define the state of the Autokey dance, while the high-order 16
bits specify the Numerical Identifier (NID) as generated by the
OpenSSL library of the OID for one of the message digest/signature
encryption schemes defined in [RFC3279]. The NID values for the
digest/signature algorithms defined in RFC 3279 are as follows:
+------------------------+----------------------+-----+
| Algorithm | OID | NID |
+------------------------+----------------------+-----+
| pkcs-1 | 1.2.840.113549.1.1 | 2 |
| md2 | 1.2.840.113549.2.2 | 3 |
| md5 | 1.2.840.113549.2.5 | 4 |
| rsaEncryption | 1.2.840.113549.1.1.1 | 6 |
| md2WithRSAEncryption | 1.2.840.113549.1.1.2 | 7 |
| md5WithRSAEncryption | 1.2.840.113549.1.1.4 | 8 |
| id-sha1 | 1.3.14.3.2.26 | 64 |
| sha-1WithRSAEncryption | 1.2.840.113549.1.1.5 | 65 |
| id-dsa-wth-sha1 | 1.2.840.10040.4.3 | 113 |
| id-dsa | 1.2.840.10040.4.1 | 116 |
+------------------------+----------------------+-----+
Bits 24-31 are reserved for server use, while bits 16-23 are reserved
for client use. In the host portion, bits 24-27 specify the
available identity schemes, while bits 28-31 specify the server
capabilities. There are two additional bits implemented separately.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Digest / Signature NID | Client | Ident | Host |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: Status Word
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The host status word is included in the ASSOC request and response
messages. The client copies this word to the association status word
and then lights additional bits as the dance proceeds. Once enabled,
these bits ordinarily never become dark unless a general reset occurs
and the protocol is restarted from the beginning.
The host status bits are defined as follows:
o ENAB (31) is lit if the server implements the Autokey protocol.
o LVAL (30) is lit if the server has installed leapseconds values,
either from the NIST leapseconds file or from another server.
o Bits (28-29) are reserved - always dark.
o Bits 24-27 select which server identity schemes are available.
While specific coding for various schemes is yet to be determined,
the schemes available in the reference implementation and
described in Appendix B include the following:
* none - Trusted Certificate (TC) Scheme (default).
* PC (27) Private Certificate Scheme.
* IFF (26) Schnorr aka Identify-Friendly-or-Foe Scheme.
* GQ (25) Guillard-Quisquater Scheme.
* MV (24) Mu-Varadharajan Scheme.
o The PC scheme is exclusive of any other scheme. Otherwise, the
IFF, GQ, and MV bits can be enabled in any combination.
The association status bits are defined as follows:
o CERT (23): Lit when the trusted host certificate and public key
are validated.
o VRFY (22): Lit when the trusted host identity credentials are
confirmed.
o PROV (21): Lit when the server signature is verified using its
public key and identity credentials. Also called the proventic
bit elsewhere in this memo. When enabled, signed values in
subsequent messages are presumed proventic.
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RFC 5906 NTPv4 Autokey June 2010
o COOK (20): Lit when the cookie is received and validated. When
lit, key lists with nonzero cookies are generated; when dim, the
cookie is zero.
o AUTO (19): Lit when the autokey values are received and validated.
When lit, clients can validate packets without extension fields
according to the autokey sequence.
o SIGN (18): Lit when the host certificate is signed by the server.
o LEAP (17): Lit when the leapseconds values are received and
validated.
o Bit 16: Reserved - always dark.
There are three additional bits: LIST, SYNC, and PEER not included in
the association status word. LIST is lit when the key list is
regenerated and dim when the autokey values have been transmitted.
This is necessary to avoid livelock under some conditions. SYNC is
lit when the client has synchronized to a proventic source and never
dim after that. PEER is lit when the server has synchronized, as
indicated in the NTP header, and never dim after that.
11.2. Host State Variables
The following is a list of host state variables.
Host Name: The name of the host, by default the string
returned by the Unix gethostname() library
function. In the reference implementation, this
is a configurable value.
Host Status Word: This word is initialized when the host first
starts up. The format is described above.
Host Key: The RSA public/private key pair used to encrypt/
decrypt cookies. This is also the default sign
key.
Sign Key: The RSA or Digital Signature Algorithm (DSA)
public/private key pair used to encrypt/decrypt
signatures when the host key is not used for
this purpose.
Sign Digest: The message digest algorithm used to compute the
message digest before encryption.
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IFF Parameters: The parameters used in the optional IFF identity
scheme described in Appendix B.
GQ Parameters: The parameters used in the optional GQ identity
scheme described in Appendix B.
MV Parameters: The parameters used in the optional MV identity
scheme described in Appendix B.
Server Seed: The private value hashed with the IP addresses
and key identifier to construct the cookie.
CIS: Certificate Information Structure. This
structure includes certain information fields
from an X.509v3 certificate, together with the
certificate itself. The fields extracted
include the subject and issuer names, subject
public key and message digest algorithm
(pointers), and the beginning and end of the
valid period in NTP seconds.
The certificate itself is stored as an extension
field in network byte order so it can be copied
intact to the message. The structure is signed
using the sign key and carries the public values
timestamp at signature time and the filestamp of
the original certificate file. The structure is
used by the CERT response message and SIGN
request and response messages.
A flags field in the CIS determines the status
of the certificate. The field is encoded as
follows:
* TRUST (0x01) - The certificate has been
signed by a trusted issuer. If the
certificate is self-signed and contains
"trustRoot" in the Extended Key Usage field,
this bit is lit when the CIS is constructed.
* SIGN (0x02) - The certificate signature has
been verified. If the certificate is self-
signed and verified using the contained
public key, this bit is lit when the CIS is
constructed.
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* VALID (0x04) - The certificate is valid and
can be used to verify signatures. This bit
is lit when a trusted certificate has been
found on a valid certificate trail.
* PRIV (0x08) - The certificate is private and
not to be revealed. If the certificate is
self-signed and contains "Private" in the
Extended Key Usage field, this bit is lit
when the CIS is constructed.
* ERROR (0x80) - The certificate is defective
and not to be used in any way.
Certificate List: CIS structures are stored on the certificate
list in order of arrival, with the most recently
received CIS placed first on the list. The list
is initialized with the CIS for the host
certificate, which is read from the host
certificate file. Additional CIS entries are
added to the list as certificates are obtained
from the servers during the certificate
exchange. CIS entries are discarded if
overtaken by newer ones.
The following values are stored as an extension
field structure in network byte order so they
can be copied intact to the message. They are
used to send some Autokey requests and
responses. All but the Host Name Values
structure are signed using the sign key and all
carry the public values timestamp at signature
time.
Host Name Values: This is used to send ASSOC request and response
messages. It contains the host status word and
host name.
Public Key Values: This is used to send the COOKIE request message.
It contains the public encryption key used for
the COOKIE response message.
Leapseconds Values: This is used to send the LEAP response message.
It contains the leapseconds values in the LEAP
message description.
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11.3. Client State Variables (all modes)
The following is a list of state variables used by the various dances
in all modes.
Association ID: The association ID used in responses. It
is assigned when the association is
mobilized.
Association Status Word: The status word copied from the ASSOC
response; subsequently modified by the
state machine.
Subject Name: The server host name copied from the ASSOC
response.
Issuer Name: The host name signing the certificate. It
is extracted from the current server
certificate upon arrival and used to
request the next host on the certificate
trail.
Server Public Key: The public key used to decrypt signatures.
It is extracted from the server host
certificate.
Server Message Digest: The digest/signature scheme determined in
the parameter exchange.
Group Key: A set of values used by the identity
exchange. It identifies the cryptographic
compartment shared by the server and
client.
Receive Cookie Values: The cookie returned in a COOKIE response,
together with its timestamp and filestamp.
Receive Autokey Values: The autokey values returned in an AUTO
response, together with its timestamp and
filestamp.
Send Autokey Values: The autokey values with signature and
timestamps.
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Key List: A sequence of key IDs starting with the
autokey seed and each pointing to the next.
It is computed, timestamped, and signed at
the next poll opportunity when the key list
becomes empty.
Current Key Number: The index of the entry on the Key List to
be used at the next poll opportunity.
11.4. Protocol State Transitions
The protocol state machine is very simple but robust. The state is
determined by the client status word bits defined above. The state
transitions of the three dances are shown below. The capitalized
truth values represent the client status bits. All bits are
initialized as dark and are lit upon the arrival of a specific
response message as detailed above.
11.4.1. Server Dance
The server dance begins when the client sends an ASSOC request to the
server. The clock is updated when PREV is lit and the dance ends
when LEAP is lit. In this dance, the autokey values are not used, so
an autokey exchange is not necessary. Note that the SIGN and LEAP
requests are not issued until the client has synchronized to a
proventic source. Subsequent packets without extension fields are
validated by the autokey sequence. This example and others assumes
the IFF identity scheme has been selected in the parameter exchange.
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1 while (1) {
2 wait_for_next_poll;
3 make_NTP_header;
4 if (response_ready)
5 send_response;
6 if (!ENB) /* parameter exchange */
7 ASSOC_request;
8 else if (!CERT) /* certificate exchange */
9 CERT_request(Host_Name);
10 else if (!IFF) /* identity exchange */
11 IFF_challenge;
12 else if (!COOK) /* cookie exchange */
13 COOKIE_request;
14 else if (!SYNC) /* wait for synchronization */
15 continue;
16 else if (!SIGN) /* sign exchange */
17 SIGN_request(Host_Certificate);
18 else if (!LEAP) /* leapsecond values exchange */
19 LEAP_request;
20 send packet;
21 }
Figure 9: Server Dance
If the server refreshes the private seed, the cookie becomes invalid.
The server responds to an invalid cookie with a crypto-NAK message,
which causes the client to restart the protocol from the beginning.
11.4.2. Broadcast Dance
The broadcast dance is similar to the server dance with the cookie
exchange replaced by the autokey values exchange. The broadcast
dance begins when the client receives a broadcast packet including an
ASSOC response with the server association ID. This mobilizes a
client association in order to proventicate the source and calibrate
the propagation delay. The dance ends when the LEAP bit is lit,
after which the client sends no further packets. Normally, the
broadcast server includes an ASSOC response in each transmitted
packet. However, when the server generates a new key list, it
includes an AUTO response instead.
In the broadcast dance, extension fields are used with every packet,
so the cookie is always zero and no cookie exchange is necessary. As
in the server dance, the clock is updated when PREV is lit and the
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dance ends when LEAP is lit. Note that the SIGN and LEAP requests
are not issued until the client has synchronized to a proventic
source. Subsequent packets without extension fields are validated by
the autokey sequence.
1 while (1) {
2 wait_for_next_poll;
3 make_NTP_header;
4 if (response_ready)
5 send_response;
6 if (!ENB) /* parameters exchange */
7 ASSOC_request;
8 else if (!CERT) /* certificate exchange */
9 CERT_request(Host_Name);
10 else if (!IFF) /* identity exchange */
11 IFF_challenge;
12 else if (!AUT) /* autokey values exchange */
13 AUTO_request;
14 else if (!SYNC) /* wait for synchronization */
15 continue;
16 else if (!SIGN) /* sign exchange */
17 SIGN_request(Host_Certificate);
18 else if (!LEAP) /* leapsecond values exchange */
19 LEAP_request;
20 send NTP_packet;
21 }
Figure 10: Broadcast Dance
If a packet is lost and the autokey sequence is broken, the client
hashes the current autokey until either it matches the previous
autokey or the number of hashes exceeds the count given in the
autokey values. If the latter, the client sends an AUTO request to
retrieve the autokey values. If the client receives a crypto-NAK
during the dance, or if the association ID changes, the client
restarts the protocol from the beginning.
11.4.3. Symmetric Dance
The symmetric dance is intricately choreographed. It begins when the
active peer sends an ASSOC request to the passive peer. The passive
peer mobilizes an association and both peers step a three-way dance
where each peer completes a parameter exchange with the other. Until
one of the peers has synchronized to a proventic source (which could
be the other peer) and can sign messages, the other peer loops
waiting for a valid timestamp in the ensuing CERT response.
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1 while (1) {
2 wait_for_next_poll;
3 make_NTP_header;
4 if (!ENB) /* parameters exchange */
5 ASSOC_request;
6 else if (!CERT) /* certificate exchange */
7 CERT_request(Host_Name);
8 else if (!IFF) /* identity exchange */
9 IFF_challenge;
10 else if (!COOK && PEER) /* cookie exchange */
11 COOKIE_request);
12 else if (!AUTO) /* autokey values exchange */
13 AUTO_request;
14 else if (LIST) /* autokey values response */
15 AUTO_response;
16 else if (!SYNC) /* wait for synchronization */
17 continue;
18 else if (!SIGN) /* sign exchange */
19 SIGN_request;
20 else if (!LEAP) /* leapsecond values exchange */
21 LEAP_request;
22 send NTP_packet;
23 }
Figure 11: Symmetric Dance
Once a peer has synchronized to a proventic source, it includes
timestamped signatures in its messages. The other peer, which has
been stalled waiting for valid timestamps, now mates the dance. It
retrieves the now nonzero cookie using a cookie exchange and then the
updated autokey values using an autokey exchange.
As in the broadcast dance, if a packet is lost and the autokey
sequence broken, the peer hashes the current autokey until either it
matches the previous autokey or the number of hashes exceeds the
count given in the autokey values. If the latter, the client sends
an AUTO request to retrieve the autokey values. If the peer receives
a crypto-NAK during the dance, or if the association ID changes, the
peer restarts the protocol from the beginning.
11.5. Error Recovery
The Autokey protocol state machine includes provisions for various
kinds of error conditions that can arise due to missing files,
corrupted data, protocol violations, and packet loss or misorder, not
to mention hostile intrusion. This section describes how the
protocol responds to reachability and timeout events that can occur
due to such errors.
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A persistent NTP association is mobilized by an entry in the
configuration file, while an ephemeral association is mobilized upon
the arrival of a broadcast or symmetric active packet with no
matching association. Subsequently, a general reset reinitializes
all association variables to the initial state when first mobilized.
In addition, if the association is ephemeral, the association is
demobilized and all resources acquired are returned to the system.
Every NTP association has two variables that maintain the liveness
state of the protocol, the 8-bit reach register and the unreach
counter defined in [RFC5905]. At every poll interval, the reach
register is shifted left, the low order bit is dimmed and the high
order bit is lost. At the same time, the unreach counter is
incremented by one. If an arriving packet passes all authentication
and sanity checks, the rightmost bit of the reach register is lit and
the unreach counter is set to zero. If any bit in the reach register
is lit, the server is reachable; otherwise, it is unreachable.
When the first poll is sent from an association, the reach register
and unreach counter are set to zero. If the unreach counter reaches
16, the poll interval is doubled. In addition, if association is
persistent, it is demobilized. This reduces the network load for
packets that are unlikely to elicit a response.
At each state in the protocol, the client expects a particular
response from the server. A request is included in the NTP packet
sent at each poll interval until a valid response is received or a
general reset occurs, in which case the protocol restarts from the
beginning. A general reset also occurs for an association when an
unrecoverable protocol error occurs. A general reset occurs for all
associations when the system clock is first synchronized or the clock
is stepped or when the server seed is refreshed.
There are special cases designed to quickly respond to broken
associations, such as when a server restarts or refreshes keys.
Since the client cookie is invalidated, the server rejects the next
client request and returns a crypto-NAK packet. Since the crypto-NAK
has no MAC, the problem for the client is to determine whether it is
legitimate or the result of intruder mischief. In order to reduce
the vulnerability in such cases, the crypto-NAK, as well as all
responses, is believed only if the result of a previous packet sent
by the client and not a replay, as confirmed by the NTP on-wire
protocol. While this defense can be easily circumvented by a man-in-
the-middle, it does deflect other kinds of intruder warfare.
There are a number of situations where some event happens that causes
the remaining autokeys on the key list to become invalid. When one
of these situations happens, the key list and associated autokeys in
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the key cache are purged. A new key list, signature, and timestamp
are generated when the next NTP message is sent, assuming there is
one. The following is a list of these situations:
1. When the cookie value changes for any reason.
2. When the poll interval is changed. In this case, the calculated
expiration times for the keys become invalid.
3. If a problem is detected when an entry is fetched from the key
list. This could happen if the key was marked non-trusted or
timed out, either of which implies a software bug.
12. Security Considerations
This section discusses the most obvious security vulnerabilities in
the various Autokey dances. In the following discussion, the
cryptographic algorithms and private values themselves are assumed
secure; that is, a brute force cryptanalytic attack will not reveal
the host private key, sign private key, cookie value, identity
parameters, server seed or autokey seed. In addition, an intruder
will not be able to predict random generator values.
12.1. Protocol Vulnerability
While the protocol has not been subjected to a formal analysis, a few
preliminary assertions can be made. In the client/server and
symmetric dances, the underlying NTP on-wire protocol is resistant to
lost, duplicate, and bogus packets, even if the clock is not
synchronized, so the protocol is not vulnerable to a wiretapper
attack. The on-wire protocol is resistant to replays of both the
client request packet and the server reply packet. A man-in-the-
middle attack, even if it could simulate a valid cookie, could not
prove identity.
In the broadcast dance, the client begins with a volley in client/
server mode to obtain the autokey values and signature, so has the
same protection as in that mode. When continuing in receive-only
mode, a wiretapper cannot produce a key list with valid signed
autokey values. If it replays an old packet, the client will reject
it by the timestamp check. The most it can do is manufacture a
future packet causing clients to repeat the autokey hash operations
until exceeding the maximum key number. If this happens the
broadcast client temporarily reverts to client mode to refresh the
autokey values.
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By assumption, a man-in-the-middle attacker that intercepts a packet
cannot break the wire or delay an intercepted packet. If this
assumption is removed, the middleman could intercept a broadcast
packet and replace the data and message digest without detection by
the clients.
As mentioned previously in this memo, the TC identity scheme is
vulnerable to a man-in-the-middle attack where an intruder could
create a bogus certificate trail. To foil this kind of attack,
either the PC, IFF, GQ, or MV identity schemes must be used.
A client instantiates cryptographic variables only if the server is
synchronized to a proventic source. A server does not sign values or
generate cryptographic data files unless synchronized to a proventic
source. This raises an interesting issue: how does a client generate
proventic cryptographic files before it has ever been synchronized to
a proventic source? (Who shaves the barber if the barber shaves
everybody in town who does not shave himself?) In principle, this
paradox is resolved by assuming the primary (stratum 1) servers are
proventicated by external phenomenological means.
12.2. Clogging Vulnerability
A self-induced clogging incident cannot happen, since signatures are
computed only when the data have changed and the data do not change
very often. For instance, the autokey values are signed only when
the key list is regenerated, which happens about once an hour, while
the public values are signed only when one of them is updated during
a dance or the server seed is refreshed, which happens about once per
day.
There are two clogging vulnerabilities exposed in the protocol
design: an encryption attack where the intruder hopes to clog the
victim server with needless cryptographic calculations, and a
decryption attack where the intruder attempts to clog the victim
client with needless cryptographic calculations. Autokey uses public
key cryptography and the algorithms that perform these functions
consume significant resources.
In client/server and peer dances, an encryption hazard exists when a
wiretapper replays prior cookie request messages at speed. There is
no obvious way to deflect such attacks, as the server retains no
state between requests. Replays of cookie request or response
messages are detected and discarded by the client on-wire protocol.
In broadcast mode, a decryption hazard exists when a wiretapper
replays autokey response messages at speed. Once synchronized to a
proventic source, a legitimate extension field with timestamp the
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same as or earlier than the most recently received of that type is
immediately discarded. This foils a man-in-the-middle cut-and-paste
attack using an earlier response, for example. A legitimate
extension field with timestamp in the future is unlikely, as that
would require predicting the autokey sequence. However, this causes
the client to refresh and verify the autokey values and signature.
A determined attacker can destabilize the on-wire protocol or an
Autokey dance in various ways by replaying old messages before the
client or peer has synchronized for the first time. For instance,
replaying an old symmetric mode message before the peers have
synchronize will prevent the peers from ever synchronizing.
Replaying out of order Autokey messages in any mode during a dance
could prevent the dance from ever completing. There is nothing new
in these kinds of attack; a similar vulnerability even exists in TCP.
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13. IANA Consideration
The IANA has added the following entries to the NTP Extensions Field
Types registry:
+------------+------------------------------------------+
| Field Type | Meaning |
+------------+------------------------------------------+
| 0x0002 | No-Operation Request |
| 0x8002 | No-Operation Response |
| 0xC002 | No-Operation Error Response |
| 0x0102 | Association Message Request |
| 0x8102 | Association Message Response |
| 0xC102 | Association Message Error Response |
| 0x0202 | Certificate Message Request |
| 0x8202 | Certificate Message Response |
| 0xC202 | Certificate Message Error Response |
| 0x0302 | Cookie Message Request |
| 0x8302 | Cookie Message Response |
| 0xC302 | Cookie Message Error Response |
| 0x0402 | Autokey Message Request |
| 0x8402 | Autokey Message Response |
| 0xC402 | Autokey Message Error Response |
| 0x0502 | Leapseconds Value Message Request |
| 0x8502 | Leapseconds Value Message Response |
| 0xC502 | Leapseconds Value Message Error Response |
| 0x0602 | Sign Message Request |
| 0x8602 | Sign Message Response |
| 0xC602 | Sign Message Error Response |
| 0x0702 | IFF Identity Message Request |
| 0x8702 | IFF Identity Message Response |
| 0xC702 | IFF Identity Message Error Response |
| 0x0802 | GQ Identity Message Request |
| 0x8802 | GQ Identity Message Response |
| 0xC802 | GQ Identity Message Error Response |
| 0x0902 | MV Identity Message Request |
| 0x8902 | MV Identity Message Response |
| 0xC902 | MV Identity Message Error Response |
+------------+------------------------------------------+
14. References
14.1. Normative References
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, June 2010.
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14.2. Informative References
[DASBUCH] Mills, D., "Computer Network Time Synchronization - the
Network Time Protocol", 2006.
[GUILLOU] Guillou, L. and J. Quisquatar, "A "paradoxical" identity-
based signature scheme resulting from zero-knowledge",
1990.
[MV] Mu, Y. and V. Varadharajan, "Robust and secure
broadcasting", 2001.
[RFC1305] Mills, D., "Network Time Protocol (Version 3)
Specification, Implementation", RFC 1305, March 1992.
[RFC2412] Orman, H., "The OAKLEY Key Determination Protocol",
RFC 2412, November 1998.
[RFC2522] Karn, P. and W. Simpson, "Photuris: Session-Key Management
Protocol", RFC 2522, March 1999.
[RFC2875] Prafullchandra, H. and J. Schaad, "Diffie-Hellman Proof-
of-Possession Algorithms", RFC 2875, July 2000.
[RFC3279] Bassham, L., Polk, W., and R. Housley, "Algorithms and
Identifiers for the Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 3279, April 2002.
[RFC4210] Adams, C., Farrell, S., Kause, T., and T. Mononen,
"Internet X.509 Public Key Infrastructure Certificate
Management Protocol (CMP)", RFC 4210, September 2005.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
December 2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
[RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
RFC 4306, December 2005.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.
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RFC 5906 NTPv4 Autokey June 2010
[SCHNORR] Schnorr, C., "Efficient signature generation for smart
cards", 1991.
[STINSON] Stinson, D., "Cryptography - Theory and Practice", 1995.
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Appendix A. Timestamps, Filestamps, and Partial Ordering
When the host starts, it reads the host key and host certificate
files, which are required for continued operation. It also reads the
sign key and leapseconds values, when available. When reading these
files, the host checks the file formats and filestamps for validity;
for instance, all filestamps must be later than the time the UTC
timescale was established in 1972 and the certificate filestamp must
not be earlier than its associated sign key filestamp. At the time
the files are read, the host is not synchronized, so it cannot
determine whether the filestamps are bogus other than by using these
simple checks. It must not produce filestamps or timestamps until
synchronized to a proventic source.
In the following, the relation A --> B is Lamport's "happens before"
relation, which is true if event A happens before event B. When
timestamps are compared to timestamps, the relation is false if A
<--> B; that is, false if the events are simultaneous. For
timestamps compared to filestamps and filestamps compared to
filestamps, the relation is true if A <--> B. Note that the current
time plays no part in these assertions except in (6) below; however,
the NTP protocol itself ensures a correct partial ordering for all
current time values.
The following assertions apply to all relevant responses:
1. The client saves the most recent timestamp T0 and filestamp F0
for the respective signature type. For every received message
carrying timestamp T1 and filestamp F1, the message is discarded
unless T0 --> T1 and F0 --> F1. The requirement that T0 --> T1
is the primary defense against replays of old messages.
2. For timestamp T and filestamp F, F --> T; that is, the filestamp
must happen before the timestamp. If not, this could be due to a
file generation error or a significant error in the system clock
time.
3. For sign key filestamp S, certificate filestamp C, cookie
timestamp D and autokey timestamp A, S --> C --> D --> A; that
is, the autokey must be generated after the cookie, the cookie
after the certificate, and the certificate after the sign key.
4. For sign key filestamp S and certificate filestamp C specifying
begin time B and end time E, S --> C--> B --> E; that is, the
valid period must not be retroactive.
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5. A certificate for subject S signed by issuer I and with filestamp
C1 obsoletes, but does not necessarily invalidate, another
certificate with the same subject and issuer but with filestamp
C0, where C0 --> C1.
6. A certificate with begin time B and end time E is invalid and
cannot be used to verify signatures if t --> B or E --> t, where
t is the current proventic time. Note that the public key
previously extracted from the certificate continues to be valid
for an indefinite time. This raises the interesting possibility
where a truechimer server with expired certificate or a
falseticker with valid certificate are not detected until the
client has synchronized to a proventic source.
Appendix B. Identity Schemes
There are five identity schemes in the NTPv4 reference
implementation: (1) private certificate (PC), (2) trusted certificate
(TC), (3) a modified Schnorr algorithm (IFF - Identify Friend or
Foe), (4) a modified Guillou-Quisquater (GQ) algorithm, and (5) a
modified Mu-Varadharajan (MV) algorithm.
The PC scheme is intended for testing and development and not
recommended for general use. The TC scheme uses a certificate trail,
but not an identity scheme. The IFF, GQ, and MV identity schemes use
a cryptographically strong challenge-response exchange where an
intruder cannot learn the group key, even after repeated observations
of multiple exchanges. These schemes begin when the client sends a
nonce to the server, which then rolls its own nonce, performs a
mathematical operation and sends the results to the client. The
client performs a second mathematical operation to prove the server
has the same group key as the client.
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Appendix C. Private Certificate (PC) Scheme
The PC scheme shown in Figure 12 uses a private certificate as the
group key.
Trusted
Authority
Secure +-------------+ Secure
+--------------| Certificate |-------------+
| +-------------+ |
| |
\|/ \|/
+-------------+ +-------------+
| Certificate | | Certificate |
+-------------+ +-------------+
Server Client
Figure 12: Private Certificate (PC) Identity Scheme
A certificate is designated private when the X.509v3 Extended Key
Usage extension field is present and contains "Private". The private
certificate is distributed to all other group members by secret
means, so in fact becomes a symmetric key. Private certificates are
also trusted, so there is no need for a certificate trail or identity
scheme.
Appendix D. Trusted Certificate (TC) Scheme
All other schemes involve a conventional certificate trail as shown
in Figure 13.
Trusted
Host Host Host
+-----------+ +-----------+ +-----------+
+--->| Subject | +--->| Subject | +--->| Subject |
| +-----------+ | +-----------+ | +-----------+
...---+ | Issuer |---+ | Issuer |---+ | Issuer |
+-----------+ +-----------+ +-----------+
| Signature | | Signature | | Signature |
+-----------+ +-----------+ +-----------+
Figure 13: Trusted Certificate (TC) Identity Scheme
As described in RFC 4210 [RFC4210], each certificate is signed by an
issuer one step closer to the trusted host, which has a self-signed
trusted certificate. A certificate is designated trusted when an
X.509v3 Extended Key Usage extension field is present and contains
"trustRoot". If no identity scheme is specified in the parameter
exchange, this is the default scheme.
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Appendix E. Schnorr (IFF) Identity Scheme
The IFF scheme is useful when the group key is concealed, so that
client keys need not be protected. The primary disadvantage is that
when the server key is refreshed all hosts must update the client
key. The scheme shown in Figure 14 involves a set of public
parameters and a group key including both private and public
components. The public component is the client key.
Trusted
Authority
+------------+
| Parameters |
Secure +------------+ Insecure
+-------------| Group Key |-----------+
| +------------+ |
\|/ \|/
+------------+ Challenge +------------+
| Parameters |<------------------------| Parameters |
+------------+ +------------+
| Group Key |------------------------>| Client Key |
+------------+ Response +------------+
Server Client
Figure 14: Schnorr (IFF) Identity Scheme
By happy coincidence, the mathematical principles on which IFF is
based are similar to DSA. The scheme is a modification an algorithm
described in [SCHNORR] and [STINSON] (p. 285). The parameters are
generated by routines in the OpenSSL library, but only the moduli p,
q and generator g are used. The p is a 512-bit prime, g a generator
of the multiplicative group Z_p* and q a 160-bit prime that divides
(p-1) and is a qth root of 1 mod p; that is, g^q = 1 mod p. The TA
rolls a private random group key b (0 < b < q), then computes public
client key v = g^(q-b) mod p. The TA distributes (p, q, g, b) to all
servers using secure means and (p, q, g, v) to all clients not
necessarily using secure means.
The TA hides IFF parameters and keys in an OpenSSL DSA cuckoo
structure. The IFF parameters are identical to the DSA parameters,
so the OpenSSL library can be used directly. The structure shown in
Figure 15 is written to a file as a DSA private key encoded in PEM.
Unused structure members are set to one.
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+----------------------------------+-------------+
| IFF | DSA | Item | Include |
+=========+==========+=============+=============+
| p | p | modulus | all |
+---------+----------+-------------+-------------+
| q | q | modulus | all |
+---------+----------+-------------+-------------+
| g | g | generator | all |
+---------+----------+-------------+-------------+
| b | priv_key | group key | server |
+---------+----------+-------------+-------------+
| v | pub_key | client key | client |
+---------+----------+-------------+-------------+
Figure 15: IFF Identity Scheme Structure
Alice challenges Bob to confirm identity using the following protocol
exchange.
1. Alice rolls random r (0 < r < q) and sends to Bob.
2. Bob rolls random k (0 < k < q), computes y = k + br mod q and x =
g^k mod p, then sends (y, hash(x)) to Alice.
3. Alice computes z = g^y * v^r mod p and verifies hash(z) equals
hash(x).
If the hashes match, Alice knows that Bob has the group key b.
Besides making the response shorter, the hash makes it effectively
impossible for an intruder to solve for b by observing a number of
these messages. The signed response binds this knowledge to Bob's
private key and the public key previously received in his
certificate.
Appendix F. Guillard-Quisquater (GQ) Identity Scheme
The GQ scheme is useful when the server key must be refreshed from
time to time without changing the group key. The NTP utility
programs include the GQ client key in the X.509v3 Subject Key
Identifier extension field. The primary disadvantage of the scheme
is that the group key must be protected in both the server and
client. A secondary disadvantage is that when a server key is
refreshed, old extension fields no longer work. The scheme shown in
Figure 16 involves a set of public parameters and a group key used to
generate private server keys and client keys.
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Trusted
Authority
+------------+
| Parameters |
Secure +------------+ Secure
+-------------| Group Key |-----------+
| +------------+ |
\|/ \|/
+------------+ Challenge +------------+
| Parameters |<------------------------| Parameters |
+------------+ +------------+
| Group Key | | Group Key |
+------------+ Response +------------+
| Server Key |------------------------>| Client Key |
+------------+ +------------+
Server Client
Figure 16: Schnorr (IFF) Identity Scheme
By happy coincidence, the mathematical principles on which GQ is
based are similar to RSA. The scheme is a modification of an
algorithm described in [GUILLOU] and [STINSON] (p. 300) (with
errors). The parameters are generated by routines in the OpenSSL
library, but only the moduli p and q are used. The 512-bit public
modulus is n=pq, where p and q are secret large primes. The TA rolls
random large prime b (0 < b < n) and distributes (n, b) to all group
servers and clients using secure means, since an intruder in
possession of these values could impersonate a legitimate server.
The private server key and public client key are constructed later.
The TA hides GQ parameters and keys in an OpenSSL RSA cuckoo
structure. The GQ parameters are identical to the RSA parameters, so
the OpenSSL library can be used directly. When generating a
certificate, the server rolls random server key u (0 < u < n) and
client key its inverse obscured by the group key v = (u^-1)^b mod n.
These values replace the private and public keys normally generated
by the RSA scheme. The client key is conveyed in a X.509 certificate
extension. The updated GQ structure shown in Figure 17 is written as
an RSA private key encoded in PEM. Unused structure members are set
to one.
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+---------------------------------+-------------+
| GQ | RSA | Item | Include |
+=========+==========+============+=============+
| n | n | modulus | all |
+---------+----------+------------+-------------+
| b | e | group key | all |
+---------+----------+------------+-------------+
| u | p | server key | server |
+---------+----------+------------+-------------+
| v | q | client key | client |
+---------+----------+------------+-------------+
Figure 17: GQ Identity Scheme Structure
Alice challenges Bob to confirm identity using the following
exchange.
1. Alice rolls random r (0 < r < n) and sends to Bob.
2. Bob rolls random k (0 < k < n) and computes y = ku^r mod n and x
= k^b mod n, then sends (y, hash(x)) to Alice.
3. Alice computes z = (v^r)*(y^b) mod n and verifies hash(z) equals
hash(x).
If the hashes match, Alice knows that Bob has the corresponding
server key u. Besides making the response shorter, the hash makes it
effectively impossible for an intruder to solve for u by observing a
number of these messages. The signed response binds this knowledge
to Bob's private key and the client key previously received in his
certificate.
Appendix G. Mu-Varadharajan (MV) Identity Scheme
The MV scheme is perhaps the most interesting and flexible of the
three challenge/response schemes, but is devilishly complicated. It
is most useful when a small number of servers provide synchronization
to a large client population where there might be considerable risk
of compromise between and among the servers and clients. The client
population can be partitioned into a modest number of subgroups, each
associated with an individual client key.
The TA generates an intricate cryptosystem involving encryption and
decryption keys, together with a number of activation keys and
associated client keys. The TA can activate and revoke individual
client keys without changing the client keys themselves. The TA
provides to the servers an encryption key E, and partial decryption
keys g-bar and g-hat which depend on the activated keys. The servers
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have no additional information and, in particular, cannot masquerade
as a TA. In addition, the TA provides to each client j individual
partial decryption keys x-bar_j and x-hat_j, which do not need to be
changed if the TA activates or deactivates any client key. The
clients have no further information and, in particular, cannot
masquerade as a server or TA.
The scheme uses an encryption algorithm similar to El Gamal
cryptography and a polynomial formed from the expansion of product
terms (x-x_1)(x-x_2)(x-x_3)...(x-x_n), as described in [MV]. The
paper has significant errors and serious omissions. The cryptosystem
is constructed so that, for every encryption key E its inverse is
(g-bar^x-hat_j)(g-hat^x-bar_j) mod p for every j. This remains true
if both quantities are raised to the power k mod p. The difficulty
in finding E is equivalent to the discrete log problem.
The scheme is shown in Figure 18. The TA generates the parameters,
group key, server keys, and client keys, one for each client, all of
which must be protected to prevent theft of service. Note that only
the TA has the group key, which is not known to either the servers or
clients. In this sense, the MV scheme is a zero-knowledge proof.
Trusted
Authority
+------------+
| Parameters |
+------------+
| Group Key |
+------------+
| Server Key |
Secure +------------+ Secure
+-------------| Client Key |-----------+
| +------------+ |
\|/ \|/
+------------+ Challenge +------------+
| Parameters |<------------------------| Parameters |
+------------+ +------------+
| Server Key |------------------------>| Client Key |
+------------+ Response +------------+
Server Client
Figure 18: Mu-Varadharajan (MV) Identity Scheme
The TA hides MV parameters and keys in OpenSSL DSA cuckoo structures.
The MV parameters are identical to the DSA parameters, so the OpenSSL
library can be used directly. The structure shown in the figures
below are written to files as a the fkey encoded in PEM. Unused
structure members are set to one. The Figure 19 shows the data
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structure used by the servers, while Figure 20 shows the client data
structure associated with each activation key.
+---------------------------------+-------------+
| MV | DSA | Item | Include |
+=========+==========+============+=============+
| p | p | modulus | all |
+---------+----------+------------+-------------+
| q | q | modulus | server |
+---------+----------+------------+-------------+
| E | g | private | server |
| | | encrypt | |
+---------+----------+------------+-------------+
| g-bar | priv_key | public | server |
| | | decrypt | |
+---------+----------+------------+-------------+
| g-hat | pub_key | public | server |
| | | decrypt | |
+---------+----------+------------+-------------+
Figure 19: MV Scheme Server Structure
+---------------------------------+-------------+
| MV | DSA | Item | Include |
+=========+==========+============+=============+
| p | p | modulus | all |
+---------+----------+------------+-------------+
| x-bar_j | priv_key | public | client |
| | | decrypt | |
+---------+----------+------------+-------------+
| x-hat_j | pub_key | public | client |
| | | decrypt | |
+---------+----------+------------+-------------+
Figure 20: MV Scheme Client Structure
The devil is in the details, which are beyond the scope of this memo.
The steps in generating the cryptosystem activating the keys and
generating the partial decryption keys are in [DASBUCH] (page 170
ff).
Alice challenges Bob to confirm identity using the following
exchange.
1. Alice rolls random r (0 < r < q) and sends to Bob.
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2. Bob rolls random k (0 < k < q) and computes the session
encryption key E-prime = E^k mod p and partial decryption keys
g-bar-prime = g-bar^k mod p and g-hat-prime = g-hat^k mod p. He
encrypts x = E-prime * r mod p and sends (x, g-bar-prime, g-hat-
prime) to Alice.
3. Alice computes the session decryption key E^-1 = (g-bar-prime)^x-
hat_j (g-hat-prime)^x-bar_j mod p and verifies that r = E^-1 x.
Appendix H. ASN.1 Encoding Rules
Certain value fields in request and response messages contain data
encoded in ASN.1 distinguished encoding rules (DER). The BNF grammar
for each encoding rule is given below along with the OpenSSL routine
used for the encoding in the reference implementation. The object
identifiers for the encryption algorithms and message digest/
signature encryption schemes are specified in [RFC3279]. The
particular algorithms required for conformance are not specified in
this memo.
Appendix I. COOKIE Request, IFF Response, GQ Response, MV Response
The value field of the COOKIE request message contains a sequence of
two integers (n, e) encoded by the i2d_RSAPublicKey() routine in the
OpenSSL distribution. In the request, n is the RSA modulus in bits
and e is the public exponent.
RSAPublicKey ::= SEQUENCE {
n ::= INTEGER,
e ::= INTEGER
}
The IFF and GQ responses contain a sequence of two integers (r, s)
encoded by the i2d_DSA_SIG() routine in the OpenSSL distribution. In
the responses, r is the challenge response and s is the hash of the
private value.
DSAPublicKey ::= SEQUENCE {
r ::= INTEGER,
s ::= INTEGER
}
The MV response contains a sequence of three integers (p, q, g)
encoded by the i2d_DSAparams() routine in the OpenSSL library. In
the response, p is the hash of the encrypted challenge value and (q,
g) is the client portion of the decryption key.
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DSAparameters ::= SEQUENCE {
p ::= INTEGER,
q ::= INTEGER,
g ::= INTEGER
}
Appendix J. Certificates
Certificate extension fields are used to convey information used by
the identity schemes. While the semantics of these fields generally
conform with conventional usage, there are subtle variations. The
fields used by Autokey version 2 include:
o Basic Constraints. This field defines the basic functions of the
certificate. It contains the string "critical,CA:TRUE", which
means the field must be interpreted and the associated private key
can be used to sign other certificates. While included for
compatibility, Autokey makes no use of this field.
o Key Usage. This field defines the intended use of the public key
contained in the certificate. It contains the string
"digitalSignature,keyCertSign", which means the contained public
key can be used to verify signatures on data and other
certificates. While included for compatibility, Autokey makes no
use of this field.
o Extended Key Usage. This field further refines the intended use
of the public key contained in the certificate and is present only
in self-signed certificates. It contains the string "Private" if
the certificate is designated private or the string "trustRoot" if
it is designated trusted. A private certificate is always
trusted.
o Subject Key Identifier. This field contains the client identity
key used in the GQ identity scheme. It is present only if the GQ
scheme is in use.
The value field contains an X.509v3 certificate encoded by the
i2d_X509() routine in the OpenSSL distribution. The encoding follows
the rules stated in [RFC5280], including the use of X.509v3 extension
fields.
Certificate ::= SEQUENCE {
tbsCertificate TBSCertificate,
signatureAlgorithm AlgorithmIdentifier,
signatureValue BIT STRING
}
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The signatureAlgorithm is the object identifier of the message
digest/signature encryption scheme used to sign the certificate. The
signatureValue is computed by the certificate issuer using this
algorithm and the issuer private key.
TBSCertificate ::= SEQUENCE {
version EXPLICIT v3(2),
serialNumber CertificateSerialNumber,
signature AlgorithmIdentifier,
issuer Name,
validity Validity,
subject Name,
subjectPublicKeyInfo SubjectPublicKeyInfo,
extensions EXPLICIT Extensions OPTIONAL
}
The serialNumber is an integer guaranteed to be unique for the
generating host. The reference implementation uses the NTP seconds
when the certificate was generated. The signature is the object
identifier of the message digest/signature encryption scheme used to
sign the certificate. It must be identical to the
signatureAlgorithm.
CertificateSerialNumber
SET { ::= INTEGER
Validity ::= SEQUENCE {
notBefore UTCTime,
notAfter UTCTime
}
}
The notBefore and notAfter define the period of validity as defined
in Appendix B.
SubjectPublicKeyInfo ::= SEQUENCE {
algorithm AlgorithmIdentifier,
subjectPublicKey BIT STRING
}
The AlgorithmIdentifier specifies the encryption algorithm for the
subject public key. The subjectPublicKey is the public key of the
subject.
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Extensions ::= SEQUENCE SIZE (1..MAX) OF Extension
Extension ::= SEQUENCE {
extnID OBJECT IDENTIFIER,
critical BOOLEAN DEFAULT FALSE,
extnValue OCTET STRING
}
SET {
Name ::= SEQUENCE {
OBJECT IDENTIFIER commonName
PrintableString HostName
}
}
For trusted host certificates, the subject and issuer HostName is the
NTP name of the group, while for all other host certificates the
subject and issuer HostName is the NTP name of the host. In the
reference implementation, if these names are not explicitly
specified, they default to the string returned by the Unix
gethostname() routine (trailing NUL removed). For other than self-
signed certificates, the issuer HostName is the unique DNS name of
the host signing the certificate.
It should be noted that the Autokey protocol itself has no provisions
to revoke certificates. The reference implementation is purposely
restarted about once a week, leading to the regeneration of the
certificate and a restart of the Autokey protocol. This restart is
not enforced for the Autokey protocol but rather for NTP
functionality reasons.
Each group host operates with only one certificate at a time and
constructs a trail by induction. Since the group configuration must
form an acyclic graph, with roots at the trusted hosts, it does not
matter which, of possibly several, signed certificates is used. The
reference implementation chooses a single certificate and operates
with only that certificate until the protocol is restarted.
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Authors' Addresses
Brian Haberman (editor)
The Johns Hopkins University Applied Physics Laboratory
11100 Johns Hopkins Road
Laurel, MD 20723-6099
US
Phone: +1 443 778 1319
EMail: brian@innovationslab.net
Dr. David L. Mills
University of Delaware
Newark, DE 19716
US
Phone: +1 302 831 8247
EMail: mills@udel.edu
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