Internet Engineering Task Force David L. Mills
Internet Draft University of Delaware
Standards Track February 2002
Public key Cryptography for the Network Time Protocol
Version 2
< draft-ietf-stime-ntpauth-03.txt >
Status of this Memorandum
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Task
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The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html. This document is an Internet-
Draft.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [1].
1. Abstract
This memorandum describes a scheme for authenticating servers to clients
using the Network Time Protocol. It extends prior schemes based on
symmetric key cryptography to a new scheme based on public key
cryptography. The new scheme, called Autokey, is based on the premiss
that the IPSEC schemes proposed by the IETF cannot be adopted intact,
since that would preclude stateless servers and severely compromise
timekeeping accuracy. In addition, the IPSEC model presumes
authenticated timestamps are always available; however,
cryptographically verified timestamps require interaction between the
timekeeping function and authentication function in ways not yet
considered in the IPSEC model.
The main body of this memorandum contains a description of the security
model, approach rationale, protocol design and vulnerability analysis.
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It obsoletes a previous report [11] primarily in the schemes for
distributing public keys and related values. A detailed description of
the protocol states, events and transition functions is included.
Detailed packet formats and field descriptions are given in the
appendix. A prototype of the Autokey design based on this memorandum has
been implemented, tested and documented in the NTP Version 4 software
distribution for Unix, Windows and VMS at www.ntp.org.
While not strictly a security function, the Autokey protocol also
provides means to securely retrieve a table of historic leap seconds
necessary to convert ordinary civil time (UTC) to atomic time (TAI)
where needed. The tables can be retrieved either directly from national
time servers operated by NIST or indirectly through NTP and intervening
servers.
Changes Since the Preceding Draft
This is a major rewrite of the previous draft. There are numerous
changes scattered through this memorandum to clarify the presentation
and add a few new features. Among the most important:
1. The reference implementation now uses the OpenSSL cryptographic
software library. Besides being somewhat faster than the older RSAref2.0
library, it supports several different message digest and signature
encryption schemes.
2. The Autokey protocol and reference implementation support the Public
Key Infrastructure (PKI), including X.500 certificates.
3. The Autokey protocol has been redesigned to be simpler, more uniform
and more robust. There is only one generic message format and all
requests can carry signed parameters.
4. Strong assertions are now possible about the authentication of
timestamps and filestamps. This makes correctness modeling more robust
and simplifies vulnerability assessment.
5. Certain security potholes have been filled in, in particular the
cookie in client/server and symmetric modes is now encrypted.
6. The description of the protocol, its state variables, transition
function, inputs and outputs are simpler, less wordy and more amenable
to correctness modelling.
7. Provisions have been made to handle cases when the endpoint addresses
are changed, as in mobile IP.
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
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are authentic; that is, were actually sent by the intended server and
not manufactured or modified by an intruder. Ubiquity requires that any
client can verify the authenticity of any server using only public
information. Survivability requires protection from faulty
implementations, improper operation and possibly malicious clogging and
replay attacks with or without data modification. These requirements are
especially stringent with widely distributed network services, since
damage due to failures can propagate quickly throughout the network,
devastating archives, routing databases and monitoring systems and even
bring down major portions of the network.
The Network Time Protocol (NTP) contains provisions to cryptographically
authenticate individual servers as described in the most recent protocol
specification RFC-1305 [7]; however, that specification does not provide
a scheme for the distribution of cryptographic keys, nor does it provide
for the retrieval of cryptographic media that reliably bind the server
identification credentials with the associated private keys and related
public values. However, conventional key agreement and digital
signatures with large client populations can cause significant
performance degradations, especially in time critical applications such
as NTP. In addition, there are problems unique to NTP in the interaction
between the authentication and synchronization functions, since each
requires the other.
This memorandum describes a cryptographically sound and efficient
methodology for use in NTP and similar distributed protocols. As
demonstrated in the reports and briefings cited in the references at the
end of this memorandum, there is a place for PKI and related schemes,
but none of these schemes alone satisfies the requirements of the NTP
security model. The various key agreement schemes [2, 5, 12] proposed by
the IETF 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 as implemented in the RSAref2.0 package
formerly distributed by RSA Laboratories leads to the conclusion that
any scheme requiring every NTP packet to carry a PKI digital signature
would result in unacceptably poor timekeeping performance.
A revised security model and authentication scheme called Autokey was
proposed in earlier reports [5, 6, 8]. It has been evolved and refined
since then and implemented in NTP Version 4 for Unix, Windows and VMS
[11]. It 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 tested and evaluated
in a local environment and in the CAIRN experiment network funded by
DARPA. A detailed description of the security model, design principles
and implementation experience is presented in this memorandum.
Additional information about NTP, including executive summaries,
software documentation, briefings and bibliography can be found at
www.eecis.udel.edu/~mills/ntp.htm. Additional information about the
reference implementation can be found at
www.eecis.udel.edu/~ntp/ntp_spool/html/authopt.htm.
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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
which are valid only over designated time intervals; but, time intervals
can be enforced only when participating servers and clients are reliably
synchronized to UTC. Second, the NTP subnet is 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 bone-fide authentic. In
order to emphasize this requirement, in this memorandum the notion of
"authentic" is replaced by "proventic", a noun 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 noun
and verb derivatives of authentic will be adopted for proventic as well.
In NTP each server authenticates the next lower stratum servers and
proventicates 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. A client considers a server
proventic if it can validate its certificate and its apparent time is
within the valid interval specified on the certificate. 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 client
associations and according to the NTP mitigation algorithms. While a
certificate authority must satisfy this requirement when signing a
certificate request, the certificate itself can be stored in public
directories and retrieved over unsecured networks.
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) [4] and Authentication Header (AH) [3] for IPv4 and IPv6.
Cryptographic algorithms that use these headers for various purposes
include those developed for the PKI, including MD5 message digests, RSA
digital signatures and several variations of Diffie-Hellman key
agreements. The fundamental assumption in the security model is that
packets transmitted over the Internet can be intercepted by other than
the intended receiver, 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 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 or servers or
clients with spurious packets that exhaust resources and deny service to
legitimate applications. The mission of the algorithms and protocols
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described in this memorandum is to detect and discard spurious packets
sent by other than the intended sender or sent by the intended sender,
but modified or replayed by an intruder. The cryptographic means of the
reference implementation are based on the OpenSSL cryptographic software
library available at www.openssl.org, but other libraries with
equivalent functionality could be used as well. It is important for
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.
There are a number of defense mechanisms already built in the NTP
architecture, protocol and algorithms. The fundamental timestamp
exchange scheme is inherently resistant to 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 mischiefs. Further discussion on the assumed
intruder model is given in [9], but beyond the scope of this memorandum.
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 or client can
process them, especially if they require expensive cryptographic
computations.
3. An intruder can originate, intercept, modify and replay a packet.
However, it cannot permanently prevent packet transmission over the net;
that is, it cannot break the wire, only tell lies and congest it. In
this memorandum a distinction is made between a middleman attack, where
the intruder can modify and replace an intercepted packet, and a wiretap
attack, where the intruder can modify and replay the packet only after
the original packet has been received.
The following assumptions are fundamental to the Autokey design. They
are discussed at some length in the briefing slides and links at
www.eecis.udel.edu/~mills/ntp.htm and will not be further elaborated in
this memorandum.
1. The running times for public key algorithms are relatively long and
highly variable. In general, the performance of the 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.
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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. All proventication functions must involve only public values
transmitted over the net. Private values must never be disclosed beyond
the machine on which they were created.
5. Public encryption keys and certificates must be retrievable directly
from servers without requiring secured channels; however, the
fundamental security of identification credentials and public values
bound to those credentials must be a function of external certificate
authorities and/or webs of trust.
Unlike the ssh security model, where the client must be securely
identified to the server, in NTP the server must be securely identified
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
proventicate the client and others not.
However, the NTP security model specifically assumes all time values and
cryptographic values are public, so there is no need to associate each
interface with different cryptographic values. In other words, there is
one set of private secrets for the host, not one for each interface. In
the NTP design the host name, as returned by the gethostname() Unix
library function, represents all interface addresses. Since at least in
some host configurations the host name may not be identifiable in a DNS
query, the name must be either configured in advance or obtained
directly from the server using the Autokey protocol.
Approach
The Autokey protocol described in this memorandum is designed to meet
the following objectives. Again, in-depth discussions on these
objectives is in the web briefings and will not be elaborated in this
memorandum. Note that here and elsewhere in this memorandum mention of
broadcast mode means multicast mode as well, with exceptions noted in
the web page at www.eecis.udel.edu/~ntp/ntp_spool/html/assoc.htm.
1. It must interoperate with the existing NTP architecture model and
protocol design. In particular, it must support the symmetric key scheme
described in RFC-1305. 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.
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2. It must provide for the independent collection of cryptographic
values and time values. A client is synchronized to a proventic source
only when the required cryptographic values have been obtained and
verified and the NTP timestamps have passed all 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 misorder, 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 incidental to signature
encryption and cookie encryption operations.
6. It must function in all the modes supported by NTP, including
client/server, broadcast and symmetric modes.
7. It must not require intricate per-client or per-server configuration
other than the availability of the required cryptographic keys and
certificates.
8. The reference implementation must contain provisions to generate
cryptographic key files specific to each client and server. Eventually,
it must contain provisions to validate public values using certificate
authorities and/or webs of trust.
Autokey Proventication Scheme
Autokey public key cryptography is based on the PKI algorithms commonly
used in the Secure Shell and Secure Sockets Layer applications. As in
these applications Autokey uses keyed message digests to detect packet
modification, digital signatures to verify the source and public key
algorithms to encrypt session keys or cookies. 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.
The NTP Version 3 symmetric key cryptography uses keyed-MD5 message
digests with a 128-bit private key and 32-bit key ID. In order to retain
backward compatibility, the 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.
There are three Autokey protocol variants corresponding to each of the
three NTP modes: client/server, broadcast and symmetric. All three
variants make use of specially contrived session keys, called autokeys,
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and a precomputed pseudo-random sequence of autokeys with the key IDs
saved in a key list. As in the original NTP Version 3 authentication
scheme, the Autokey protocol operates separately for each association,
so there may be several autokey sequences operating independently at the
same time.
An autokey is computed from four fields in network byte order as shown
below:
+-----------+-----------+-----------+-----------+
| Source IP | Dest IP | Key ID | Cookie |
+-----------+-----------+-----------+-----------+
The four values are hashed by the MD5 message digest algorithm to
produce the 128-bit key 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.
The NTP packet format has been augmented to include one or more
extension fields piggybacked between the original NTP header and the
message authenticator code (MAC) at the end of the packet. For packets
without extension fields, the cookie is a shared private value conveyed
in encrypted form. For packets with extension fields, the cookie has a
default public value of zero, since these packets can be validated
independently using digital signatures.
For use with IPv4, the Source IP and Dest IP 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 IP 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 mode.
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 addresses visible in the
transmitted packet must be the same as those used to construct the
autokey sequence and key list and that these addresses be the same as
those visible in the received packet. Provisions are included in the
reference implementation to handle cases when these addresses change, as
possible in mobile IP. For scenarios where the endpoint IP addresses are
not available, an optional public identification value could be used
instead of the addresses. Examples include the Interplanetary Internet,
where bundles are identified by name rather than address. Specific
provisions are for further study.
The key list consists of a sequence of key IDs starting with a 32-bit
random private value called the autokey seed. The associated autokey is
computed as above using the specified cookie and the first 32 bits in
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network byte order of this value become the next key ID. Operations
continue in this way to generate the entire list, which may have up to
100 entries. 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 should occur 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.
The index of the last key ID in the list is saved along with the next
key ID for that entry, collectively called the autokey values. The list
is used in reverse order, so that the first autokey used is the last one
generated. The Autokey protocol includes a message to retrieve the
autokey values and signature, so that subsequent packets can be
validated using one or more hashes that eventually match the first 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 packet is lost.
Autokey Operations
Autokey works differently in the various NTP modes. The scheme used in
client/server mode 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 private random value called
the server seed to regenerate the cookie upon arrival of a client
packet. The cookie is calculated in a manner similar to the autokey, but
the key ID field is zero and the cookie field is the server seed. The
first 32 bits of the hash is the cookie used for the actual autokey
calculation by both the client and server. It is thus specific to each
client separately and of no use to other clients or an intruder.
In previous versions of the Autokey protocol the cookie was transmitted
in clear on the assumption it was not useful to a wiretapper other than
to launch an ineffective replay attack. However, an middleman could
intercept the cookie and manufacture bogus messages acceptable to the
client. In order to reduce the vulnerability to such an attack, the
Autokey Version 2 server encrypts the cookie using a public key supplied
by the client. While requiring additional processor resources for the
encryption, this makes it effectively impossible to spoof a cookie.
[Note in passing. In an attempt to avoid the use of overt encryption
operations, an experimental scheme used a Diffie-Hellman agreed key as a
stream cipher to encrypt the cookie. However, not only was the protocol
extremely awkward, but the processing time to execute the agreement,
encrypt the key and sign the result was horrifically expensive - 15
seconds(!) in a vintage Sun IPC. This scheme was quickly dropped in
favor of generic public key encryption.]
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In client/server mode the client uses the cookie and each key ID on the
key list in turn to retrieve the autokey and generate the MAC in the NTP
packet. The server uses the same values to generate the message digest
and verifies it matches the MAC in the packet. It then generates the MAC
for the response using the same values, but with the IP source and
destination addresses exchanged. The client generates the message digest
and verifies it matches the MAC in the packet. In order to deflect old
replays, the client verifies the key ID matches the last one sent. In
this mode 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.
In broadcast mode clients normally do not send packets to the server,
except when first starting up to calibrate the propagation delay in
client/server mode. At the same time the client runs the Autokey
protocol as in that mode. After obtaining and verifying the cookie, the
client continues to obtain and verify the autokey values. To obtain
these values, the client must provide the ID of the particular server
association, since there can be more than one operating in the same
server. For this purpose, the NTP broadcast packet includes the
association ID in every packet sent, except when sending the first
packet after generating a new key list, when it sends the autokey values
instead.
In symmetric mode each peer keeps state variables related to the other.
A shared private cookie is conveyed using the same scheme as in
client/server mode, except that the cookie is a random value. The key
list for each direction is generated separately by each peer and used
independently, but each is generated with the same cookie. There exists
a possible race condition where each peer sends a cookie request message
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 exclusive-OR of the two values.
Once the client receives and validates the certificate, subsequent
packets containing valid signed extension fields are presumed to contain
valid time values, unless these values fall outside the valid interval
specified on the certificate. However, unless the system clock has
already been set by some other proventic means, it is not known whether
these values actually represent a truechime or falsetick source. As the
protocol evolves, the NTP associations continue to accumulated time
values until a majority clique is available to synchronize the system
clock. At this point the NTP intersection algorithm culls the
falsetickers from the population and the remaining truechimers are
allowed to discipline the clock.
The time values for even falsetick sources form a proventic total
ordering relative to the applicable signature timestamps. This raises
the interesting issue of how to mitigate 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
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different associations and between associations and the system clock are
avoided, except in the NTP intersection and clustering algorithms.
Once the Autokey values have been instantiated, the protocol is normally
dormant. In all modes except broadcast, 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 Autokey protocol restarted from scratch
in all associations. This insures that stale values never propagate
beyond a clock step.
Public Key Signatures and Timestamps
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 to originate 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 signed extension field carries a
timestamp in the form of the NTP seconds at the signature epoch. The
signature span includes the timestamp itself together with optional
additional data. If the Autokey protocol has verified a proventic source
and the NTP algorithms have validated the time values, the system clock
can be synchronized and signatures will then carry a nonzero (valid)
timestamp. Otherwise the system clock is unsynchronized and signatures
carry a zero (invalid) timestamp. Extension fields with invalid
timestamps are discarded before any values are used or signatures
verified.
There are three signature types currently defined:
1. Cookie signature/timestamp: Each association has a cookie for use
when generating a key list. The cookie value is determined along with
the cookie signature and timestamp upon arrival of a cookie request
message. The values are returned in a a cookie response message.
2. Autokey signature/timestamp: Each association has a key list for
generating the autokey sequence. The autokey values are determined along
with the autokey signature and timestamp when a new key list is
generated, which occurs about once per hour in the reference
implementation. The values are returned in a autokey response message.
3. Public values signature/timestamp: The public key, certificate and
leapsecond table 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. During protocol operations, each
of these values and associated signatures and timestamps are returned in
the associated request or response message. While there are in fact
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three public value signatures, the values are all signed at the same
time, so there is only one public value timestamp.
The most recent timestamp of each type is saved for comparison. Once a
valid signature with valid timestamp has been received, messages with
invalid timestamps or earlier valid timestamps of the same type are
discarded before the signature is verified. For signed messages this
deflects replays that otherwise might consume significant processor
resources; for other messages the Autokey protocol deflects message
modification or replay by a wiretapper, but not necessarily by a
middleman. In addition, the NTP protocol itself is inherently resistant
to replays and consumes only minimal processor resources.
All cryptographic values used by the protocol are time sensitive and are
regularly refreshed. In particular, files containing cryptographic basis
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 name includes an extension called the filestamp,
which is a string of decimal digits representing the NTP seconds at the
generation 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 the same. Thus, the ambiguity must be resolved for each
comparison operation as described below.
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 represent a total ordering of creation
epoches and serve as means to expunge old data and insure new data are
consistent. As the data are forwarded from server to client, the
filestamps are preserved, including those for certificate and
leapseconds files. Packets with older filestamps are discarded before
spending cycles to verify the signature.
Autokey Dances
This section presents an overview of the three Autokey protocols, called
dances, corresponding to the NTP client/server, broadcast and symmetric
active/passive modes. Each dance is designed to be nonintrusive and to
require no additional packets other than for regular NTP operations. The
NTP protocol and Autokey dance operate independently and simultaneously
and use the same packets. When the Autokey dance is over, subsequent
packets are authenticated by the autokey sequence and thus considered
proventic as well. Autokey assumes clients poll servers at a relatively
low rate, such as once per minute. In particular, it is assumed that a
request sent at one poll opportunity will normally result in a response
before the next poll opportunity.
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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 data. To avoid
deadlocks, any number of responses can be included in a packet, but only
one request. A response is generated for every request, even if the
requestor is not synchronized or proventicated. Some requests and most
responses carry timestamped signatures. The signature covers the data,
timestamp and filestamp, where applicable. Only if the packet passes all
extension field tests is the signature verified.
Dance Steps
The protocol state machine is very simple. The state is determined by
nine bits, four provided by the server, five determined by the client
association operations. The nine bits are stored along with the
digest/signature scheme identifier in the host status word of the server
and in the association status word of the client. In all dances the
client first sends an Association request message and receives the
Association response specifying which cryptographic values the server is
prepared to offer and the digest/signature scheme it will use.
If compatible, the client installs the server status word as the
association status word and sends a Certificate request message to the
server. The server returns a Certificate response including the
certificate and signature. The reference implementation requires the
certificate to be self-signed, which serves as an additional consistency
check. This check may be removed in future and replaced with a
certificate trail mechanism. If the certificate contents and signature
are valid, NTP timestamps in this and subsequent messages with valid
signatures are considered proventic.
In client/server mode the client sends a Cookie request message
including the public key of the host key. The server constructs the
cookie as described above and encrypts it using this key. It sends a
Cookie response including the encrypted cookie to the client and
expunges all values resulting from the calculations in order to remain
stateless. The client verifies the signature and decrypts the cookie. A
similar dance is used in symmetric modes, but the cookie is generated as
a random value.
The cookie is used to construct the key list and autokey values in all
modes. In client/server mode there is no need to provide these values to
the server, so once the cookie has been obtained the client can generate
the key list and validate succeeding packets directly. In other modes
the client retrieves the autokey values from the server using an Autokey
message exchange. Once these values have been received, the client
validates succeeding packets using the autokey sequence as described
previously.
A final exchange occurs when the server has the leapseconds table, as
indicated in the host status word. If so, the client obtains the table
and compares the filestamp with its own leapseconds table filestamp, if
available. If the server table is newer than the client table, the
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client replaces its table with the server table. The client, acting as
server, can now provide the most recent table to any of its own
dependent clients. In symmetric modes, this results in both peers having
the newest table.
Status Words
Each sever and client operating as a server implements a host status
word and an association status word with the format and content shown
below. The low order four host status bits are lit during host
initialization, depending on whether cryptographic data files are
present or not; the next four association bits are dark. There are two
additional bits implemented separately. The high order 16 bits specify
the message digest/signature encryption scheme.
The host status word is provided to clients in the Association response
message. The client initializes the association status word and then
lights and dims the association bits as the dance proceeds.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |L|K|C|A|L|S|E|E|
| Digest/Signature NID | Reserved |P|E|K|U|P|I|N|N|
| | |T|Y|Y|T|F|G|C|B|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The host status bits are defined as follows:
ENB - Lit if the server implements the Autokey protocol and is prepared
to dance.
ENC - Lit if the server has loaded a valid encryption key file. This bit
is normally lit, but can dim if an error occurs.
SIG - Lit if the server has loaded a valid signature key file. This bit
is included primarily for error supervision and can be either lit or
dim.
LPF - Lit if the server has loaded a valid leapseconds file. This bit
can be either lit or dim.
The client association status bits are defined as follows:
AUT - Lit when the certificate is present and validated. When lit,
signed values in subsequent messages are presumed proventic.
CKY - Lit when the cookie is first received and validated.
KEY - Lit when the autokey values are first received and validated. When
lit, clients can validate packets without extension fields according to
the autokey sequence.
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LPT - Lit when the leapseconds table is received and validated.
An additional bit LST not part of the association status word lights
when the key list is regenerated and signed and dims when the autokey
values are transmitted. This is necessary to avoid livelock under some
conditions.
An additional bit LBK not part of the association status word lights
when the association transmit timestamp matches the packet originate
timestamp and dims otherwise. If lit, this confirms the packet was
received in response to one previously sent by this association.
Host State Variables
Host Name
The name of the host returned by the Unix gethostname() library
function. It must agree with the subject and issuer name in the
certificate.
Host Key
The RSA key from the host key file and used to encrypt/decrypt cookies.
It carries the public value timestamp and the filestamp at the host key
file creation epoch. This is also the signature key, unless a signature
key is specified.
Public Key
The public encryption key for the Cookie request message and derived
from the host key. It carries the public value timestamp and the
filestamp at the host key file creation epoch.
Sign Key
The RSA or DSA key from the sign key file and used to encrypt
signatures. It carries the public value timestamp and the filestamp at
the sign key file creation epoch.
Certificate
The X.509 certificate from the certificate file. It carries the public
value timestamp and the filestamp at the certificate file creation
epoch.
Leapseconds Table, Leapseconds Table Filestamp
The NIST leapseconds table from the NIST leapseconds file. It carries
the public value timestamp and the filestamp at the leapseconds file
creation epoch.
Digest/signature NID
The identifier of the message digest/signature encryption scheme derived
from the sign key. It must agree with the NID on the certificate.
Client Association State Variables
Peer Association ID
The association ID of the peer as received in a response message.
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Host Name
The name of the host returned by the Association response. It must agree
with the subject name in the certificate.
Digest/Signature NID
The identifier of the message digest/signature encryption scheme
returned in the Association response message. It must agree with the
value encoded in the certificate.
Public Values Timestamp
The timestamp returned by the latest Certificate response, Cookie
request or Leapseconds message.
Certificate
The X.509 certificate returned in the certificate response message,
together with its timestamp and filestamp.
Cookie
The cookie returned in a Cookie response message, together with its
timestamp and filestamp.
Receive Autokey values
The autokey values returned in an Autokey response message, together
with its timestamp and filestamp.
Server Association State Variables (broadcast and symmetric modes)
Association ID
The association ID of the server for use in client request messages.
Send Autokey Values
The autokey values, signature and timestamp.
Key List
A sequence of key IDs starting with a random autokey seed and each
pointing to the next. It is computed timestamped and signed at the next
poll opportunity when the key list is empty.
Autokey Seed
The private value used to initialize the key list. It is randomized for
each new key list.
Current Key Number
The index of the entry on the Key List to be used at the next poll
opportunity.
Send Encrypt Values (symmetric modes only)
The encrypted cookie, signature and timestamp computed upon arrival of
the Cookie request message. These data are held until the next poll
opportunity.
Server seed
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The private value hashed with the IP addresses to construct the cookie
used in client/server mode. It is randomized when the public value
signatures are refreshed.
Autokey Messages
There are currently five Autokey request types and five corresponding
responses. An abbreviated description of these messages is given below;
the detailed formats are described in Appendix A.
Association Message (1)
The client sends the request to retrieve the host status word and host
name. The server responds with these values.
Certificate Message (2)
The client sends the request to retrieve the server certificate. The
server responds with the certificate.
Cookie Message (3)
The client sends the request, including the public member of the host
key, to retrieve the cookie. The server responds with the cookie
encrypted with the public key.
Autokey Message (4)
The client sends the request to retrieve the autokey values, if
available. The server responds with these values.
Leapseconds Message (5)
The client sends the request including its leapseconds table, if
available. The server responds with its own leapseconds table. Both the
client and server agree to use the version with the latest filestamp.
State Transitions
The state transitions of the three dances are shown below. The
capitalized truth values represent the association status word bits,
except for the SYNC value, which is true when the host is synchronized
to a proventic source and false otherwise. All truth values are
initialized false and become true upon the arrival of a specific
response messages, as detailed in the above status bits description.
Client/Server Dance
The client/server dance begins when the client sends an Association
request message to the server. It ends upon arrival of the Cookie
response, which lights the CKY and KEY bits. Subsequent packets received
without extension fields are validated by the autokey sequence. An
optional final exchange is possible to retrieve the leapseconds table.
while (1) {
wait_for_next_packet;
make_NTP_header;
if (response_ready)
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send_response;
if (!ENB)
send_Association_request;
else if (!CRF)
send_Certificate_request;
else if (!CKY)
send_Cookie_request;
else if (LPF & !LPT)
send_Leapseconds_request;
}
Broadcast Client Dance
The broadcast client dance begins when the client receives the first
broadcast packet, which includes an Association response with the
association ID. The broadcast client uses the association ID to initiate
a client/server dance in order to calibrate the propagation delay. The
dance ends upon arrival of the Autokey response, which lights the KEY
bit. Subsequent packets received without extension fields are validated
by the autokey sequence. An optional final exchange is possible to
retrieve the leapseconds table. When the server generates a new key
list, the server replaces the Association response with an Autokey
response in the first packet sent.
while (1) {
wait_for_next_packet;
make_NTP_header;
if (response_ready)
send_response;
if (!ENB)
send_Association_request;
else if (!CRF)
send_Certificate_request;
else if (!CKY)
send_Cookie_request;
else if (!KEY)
send_Autokey_request;
else if (LPF & !LPT)
send_Leapseconds_request;
}
Symmetric Dance
The symmetric active dance begins when the active peer sends an
Association request to the passive peer. The passive peer mobilizes an
association and steps the same dance from the beginning. Until the
active peer is synchronized to a proventic source (which could be the
passive peer) and can sign messages, the passive peer will loop waiting
to light the CRF bit and the active peer will skip the cookie exchange.
Meanwhile, the active peer retrieves the certificate and autokey values
from the passive peer and lights the KEY bit. When for some reason
either peer generates a new key list, at the first opportunity the peer
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sends the autokey values; that is, it pushes the values rather than
pulls them. This is to prevent a possible deadlock where each peer is
waiting for values from the other one.
while (1) {
wait_for_next_packet;
make_NTP_header;
if (response_ready)
send_response;
if (!ENB)
send_Association_request;
else if (!CRF)
send_Certificate_request;
else if (!CKY & SYNC)
send_Cookie_request;
else if (LST)
send_Autokey_response;
else if (!KEY)
send_Autokey_request;
else if (LPF & !LPT & SYNC)
send_Leapseconds_request;
}
Once the active peer has synchronized to a proventic source, it includes
timestamped signatures with its messages. The passive peer, which has
been stalled waiting for the CRF bit to light and the active peer, which
now finds the SYNC bit lit, continues their respective dances. The next
message sent by either peer is a Cookie request. The recipient rolls a
random cookie, lights its CKY bit and returns the encrypted cookie in
the Cookie response. The recipient decrypts the cookie and lights its
CKY bit.
It is not a protocol error if both peers happen to send a cookie request
at the same time. In this case both peers will have two values, one
generated by one peer and the other received from the other peer. In
such cases the working cookie is constructed as the exclusive-OR of the
two values.
At the next packet transmission opportunity, either peer generates a new
key list and lights the LST bit; however, there may already be an
Autokey request queued for transmission and the rules say no more than
one request in a packet. When available, either peer sends an Autokey
response and clears the LST bit. The recipient initializes the autokey
values, clears the LST bit and lights the KEY bit. Subsequent packets
received without extension fields are validated by the autokey sequence.
The above description assumes the active peer synchronizes to the
passive peer, which itself is synchronized to some other source, such as
a radio clock or another NTP server. In this case, the active peer is
operating at a stratum level one greater than the passive peer and so
the passive peer will not synchronize to it unless it loses its own
sources and the active peer itself has another source.
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Key Refreshment
About once per day the server seed is randomized and the signatures
recomputed. The operations are:
while (1) {
wait_for_next_refresh;
crank_random_generator;
generate_autokey_private_value;
if (!SYNC)
continue;
update_public_value_timestamp;
compute_signatures;
}
Error Recovery
The protocol state machine which drives the various Autokey operations
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. There are two
mechanisms which maintain the liveness state of the protocol, the
reachability register defined in RFC-1305 and the watchdog timer, which
is new in NTP Version 4.
The reachability register is an 8-bit register that shifts left with 0
replacing the rightmost bit. A shift occurs for every poll interval,
whether or not a poll is actually sent. If an arriving packet passes all
authentication and sanity checks, the rightmost bit is set to 1. If any
bit in this register is a 1, the server is reachable, otherwise it is
unreachable. If the server was once reachable and then becomes
unreachable, a general reset is performed. A general reset reinitializes
all association variables to the state when first mobilized and returns
all acquired resources to the system. In addition, if the association is
not configured, it is demobilized until the next packet is received.
The watchdog timer increments for every poll interval, whether or not a
poll is actually sent and regardless of the reachability state. The
counter is set to zero upon arrival of a packet from a proventicated
source, as determined by the Autokey protocol. In the reference
implementation, if the counter reaches 16 a general reset is performed.
In addition, if the association is configured, the poll interval is
doubled. 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 message 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. In some
cases noted below, certain kinds of errors cause appropriate action
which avoids the somewhat lengthy timeout/restart cycle. While this
behavior might be considered rather conservative, the advantage is that
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old cryptographic and time values can never persist from one
mobilization to the next.
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 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.
Following is a list of these situations.
1. When the cookie value changes for any reason.
2. When a client switches from client/server mode to broadcast mode.
There is no further need for the key list, since the client will not
transmit again.
3. When the poll interval is changed. In this case the calculated
expiration times for the keys become invalid.
4. When a general reset is performed.
5. 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.
6. When the signatures are refreshed, the key lists for all associations
are purged.
7. When the client is first synchronized or the system clock is stepped,
the key lists for all associations are purged.
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
to such mischief, the crypto-NAK is believed only if the result of a
previous packet sent by the client, as confirmed by the LBK status bit.
This bit is lit in the NTP protocol if the packet originate timestamp
matches the association transmit timestamp. While this defense can be
easily circumvented by a middleman, it does deflect other kinds of
intruder warfare. The LBK bit is also used to validate most responses
and some requests as well.
Security Analysis
This section discusses the most obvious security vulnerabilities in the
various Autokey dances. Throughout the discussion the cryptographic
algorithms themselves are assumed secure; that is, a brute force
cryptanalytic attack will not reveal the host private key or sign
private key or cookie value or server seed or autokey seed or be able to
predict the random generator values.
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There are three tiers of defense against intruder attacks. The first is
a keyed message digest including a secret cookie conveyed in encrypted
form. A packet is discarded if the message digest does not match the
MAC. The second tier is the autokey sequence, which is generated by
repeated hashes starting from a secret server seed and used in reverse
order. While any receiver can authenticate a packet relative to the last
one received and by induction to a signed extension field, as a
practical matter a wiretapper cannot predict the next autokey and thus
cannot spoof a valid packet. The third tier is timestamped signatures
which reliably bind the autokey values to the private key of a trusted
server.
In addition to the three-tier defense strategy, all packets are
protected by the NTP sanity checks. Since NTP packets carry time values,
replays of old or bogus packets can be deflected once the client has
synchronized to proventic sources. Additional sanity checks involving
timestamps and filestamps are summarized in Appendix C.
During the Autokey dances when extension fields are in use, the cookie
is a public value (0) rather than a shared private value. Therefore, an
intruder can easily construct a packet with a valid MAC; however, once
the certificate is stored, extension fields carry timestamped signatures
and bogus packets are readily avoided. While most request messages are
unsigned, only the Association response message is unsigned. This
message is used in the first packet sent by a server or peer and in most
NTP broadcast packets.
A bogus Association response message can cause a client livelock or
deadlock condition. However, these packets do not affect NTP time values
and do not consume significant resources. To reduce the vulnerability to
bogus packets, the NTP transmit timestamp in the Association and
Certificate request messages is used as a nonce. The NTP server copies
this value to the originate timestamp in the NTP header, so that the
client can verify that the message is a response to the original
request. To minimize the possibility that an intruder can guess the
nonce, the client should fill in the low order unused bits in the
transmit timestamp with random values. In addition, replays of all
except Autokey response messages are discarded before the signatures are
verified.
In client/server and symmetric modes extension fields are no longer
needed after the Autokey dance has concluded. The client validates the
packet using the message digest and autokey sequence. A successful
middleman attack is unlikely, since without the server seed the intruder
cannot produce the cookie and without the cookie cannot produce a valid
MAC. In broadcast mode a wiretapper cannot synthesize a valid packet
without the autokey seed, so cannot manufacture an bogus packet
acceptable to the receiver. The most the intruder can do is replay an
old packet causing the client to repeat hash operations until exceeding
the maximum key number. On the other hand, a middleman could do real
harm by intercepting a packet, using the key ID to generate a correct
autokey and then synthesizing a bogus packet. There does not seem to be
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a suitable solution for this as long as the server has no per-client
state.
A client instantiates cryptographic variables only if the server is
synchronized to a proventic source. A host 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 phenomological means.
Cryptanalysis
Some observations on the particular engineering constraints of the
Autokey protocol are in order. First, the number of bits in some
cryptographic values are considerably smaller than would ordinarily be
expected for strong cryptography. One of the reasons for this is the
need for compatibility with previous NTP versions; another is the need
for small and constant latencies and minimal processing requirements.
Therefore, what the scheme gives up on the strength of these values must
be regained by agility in the rate of change of the cryptographic basis
values. Thus, autokeys are used only once and basis values are
regenerated frequently. However, in most cases even a successful
cryptanalysis of these values compromises only a particular
client/server association and does not represent a danger to the general
population.
While the protocol has not been subjected to a formal analysis, a few
preliminary assertions can be made. The protocol cannot loop forever in
any state, since the association timeout and general reset insure that
the association variables will eventually be purged and the protocol
restarted from the beginning. However, if something is seriously wrong,
the timeout/restart cycle could continue indefinitely until whatever is
wrong is fixed.
Clogging Attacks
There are two clogging vulnerabilities exposed in the protocol design: a
sign attack where the intruder hopes to clog the victim server with
needless signature computations, and a verify attack where the intruder
attempts to clog the victim client with needless verification
computations. Autokey uses public key encryption algorithms for both
signature and cookie encryption and these algorithms require significant
processor resources.
In order to reduce the exposure to a sign attack, signatures are
computed only when the data have changed. 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 the
values are refreshed, which happens about once per day. However, in
client/server mode the protocol precludes server state variables on
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behalf of an individual client, so the cookie must be computed,
encrypted and signed for every cookie response. Ordinarily, cookie
requests are seldom used, except when the server seed or public value
signatures are refreshed. However, a determined intruder could replay
cookie requests at high rate, which may very well clog the server. There
appears no easy countermeasure for this particular attack.
A verify attack attempts to clog the receiver by provoking spurious
signature verifications. The signature timestamp is designed to deflect
replays of packets with old or duplicate extension fields before
invoking expensive signature operations. A bogus signature with a
timestamp in the future could do this, but the autokey sequence would
detect this, since success would require cryptanalysis of both the
server seed and autokey seed.
Since the Certificate response is signed, a middleman attack will not
compromise the certificate data; however, a determined middleman could
hammer the client with intentionally defective Certificate responses
before a valid one could be received and force spurious signature
verifications, which of course would fail. An intruder could flood the
server with Certificate request messages, but the Certificate response
message is signed only once, so the result would be no worse than
flooding the network with spurious packets.
An interesting vulnerability in client/server mode is for an intruder to
replay a recent client packet with an intentional bit error. This could
cause the server to return a crypto-NAK packet, which would then cause
the client to request the cookie and result in a sign attack on the
server. This results in the server and client burning spurious machine
cycles and resulting in denial of service. As in other cases mentioned
previously, the NTP timestamp check greatly reduces the likelihood of a
successful attack.
In broadcast and symmetric modes the client must include the association
ID in the Autokey request. Since association ID values for different
invocations of the NTP daemon are randomized over the 16-bit space, it
is unlikely that a very old packet would contain a valid association ID
value. An intruder could save old server packets and replay them to the
client population with the hope that the values will be accepted and
cause general chaos. The conservative client will discard them on the
basis of invalid timestamp.
As mentioned previously, an intruder could pounce on the initial volley
between peers in symmetric mode before both peers have determined each
other reachable. In this volley the peers are vulnerable to an intruder
using fake timestamps. The result can be that the peers never
synchronize the timestamps and never completely mobilize their
associations. A clever intruder might notice the interval between public
value signatures and concentrate attack on the vulnerable intervals. An
obvious countermeasure is to randomize these intervals. A more
comprehensive countermeasure remains to be devised.
Appendix A. Packet Formats
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The NTP Version 4 packet consists of a number of fields made up of 32-
bit (4 octet) words in network byte order. The packet consists of three
components, the header, one or more optional extension fields and an
optional message authenticator code (MAC), consisting of the Key ID and
Message Digest fields. The format is shown below, where the size of some
multiple word fields is shown in words.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|LI | VN |Mode | Stratum | Poll | Precision |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Root Delay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Root Dispersion |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reference ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Reference Timestamp (2) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Originate Timestamp (2) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Receive Timestamp (2) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Transmit Timestamp (2) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
= Extension Field(s) =
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
| Message Digest (4) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The NTP header extends from the beginning of the packet to the end of
the Transmit Timestamp field. The format and interpretation of the
header fields are backwards compatible with the NTP Version 3 header
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fields as described in RFC-1305, except for a slightly modified
computation for the Root Dispersion field. In NTP Version 3, this field
includes an estimated jitter quantity based on weighted absolute
differences, while in NTP Version 4 this quantity is based on weighted
root-mean-square (RMS) differences.
An unauthenticated NTP packet includes only the NTP header, while an
authenticated one contains in addition a MAC. The format and
interpretation of the NTP Version 4 MAC is described in RFC-1305 when
using the Digital Encryption Standard (DES) algorithm operating in
Cipher-Block Chaining (CBC) node. This algorithm and mode of operation
is no longer supported in NTP Version 4. The preferred replacement in
both NTP Version 3 and 4 is the Message Digest 5 (MD5) algorithm, which
is included in the distribution. For MD5 the Message Digest field is 4
words (8 octets), but the Key ID field remains 1 word (4 octets).
Extension Field Format
In NTP Version 4 one or more extension fields can be inserted after the
NTP header and before the MAC, which is always present when an extension
field is present. The extension fields can occur in any order; however,
in some cases there is a preferred order which improves the protocol
efficiency. While previous versions of the Autokey protocol used several
different extension field formats, in version 2 of the protocol only a
single extension field format is used.
Each extension field contains a request or response message in the
following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R|E| Version | Code | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Association ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Filestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
= Value =
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Signature Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
= Signature =
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| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Each extension field except the last is padded to a word (4 octets)
boundary, while the last is padded to a doubleword (8 octets) boundary.
The Length field covers the entire field length, including the Length
field itself and padding. While the minimum field length is 8 octets, a
maximum field length remains to be established. The reference
implementation discards any packet with an extension field length over
1024 octets.
The presence of the MAC and extension fields in the packet is determined
from the length of the remaining area after the header to the end of the
packet. The parser initializes a pointer just after the header. If the
length is not a multiple of 4, a format error has occurred and the
packet is discarded. If the length is zero the packet is not
authenticated. If the length is 4 (1 word), the packet is an error
report or crypto-NAK resulting from a previous packet that failed the
message digest check. The 4 octets are presently unused and should be
set to 0. If the length is 8 (2 words), 12 (3 words) or 16 (4 words),
the packet is discarded with a format error. If the length is greater
than 20 (5 words), one or more extension fields are present.
If an extension field is present, the parser examines the length field.
If the length is less than 4 or not a multiple of 4, a format error has
occurred and the packet is discarded; otherwise, the parser increments
the pointer by this value. The parser now uses the same rules as above
to determine whether a MAC is present and/or another extension field. An
additional implementation-dependent test is necessary to ensure the
pointer does not stray outside the buffer space occupied by the packet.
In the Autokey Version 2 format, the Code field specifies the request or
response operation, while the Version field is 2 identifying the current
protocol version. There are two flag bits defined. Bit 0 is the response
flag (R) and bit 1 is the error flag (E); the other six bits are
presently 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 the R bit
set to 0. Ordinarily, the client sets the E bit to 0 as well, but may in
future set it to 1 for some purpose. The Association ID field is set to
the value previously received from the server or 0 otherwise. The server
returns a response with the same operation code in the Code field and
the R bit set to 1. The server can also set the E bit to 1 in case of
error. The Association ID field is set to the association ID sending the
response as a handle for subsequent exchanges. If for some reason the
association ID value in a request does not match the association ID of
any mobilized association, the server returns the request with both the
R and E bits set to 1. Note that, it is not a protocol error to send an
unsolicited response with no matching request.
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In some cases not all fields may be present. For instance, when a client
has not synchronized to a proventic source, signatures are not valid. In
such cases the Timestamp and Signature Length fields are 0 and the
Signature field is empty. Some request and error response messages carry
no value or signature fields, so in these messages only the first two
words are present. The extension field parser verifies that the
extension field length is at least 8 if no value field is expected and
at least 24 if it is. The parser also verifies that the sum of the value
and signature lengths is equal to or less than the extension field
length.
The Timestamp and Filestamp words carry the seconds field of the NTP
timestamp. The Timestamp field 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 was
signed. Since a signature and timestamp are valid only when the signing
host is synchronized to a proventic source and a cryptographic data file
can only be generated if a signature is possible, the filestamp is
always nonzero, except in the Association Response message, where it
contains the server status word.
Autokey Version 2 Messages
Association Message
The Association message is used to obtain the host name and related
values. The request message has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0| 1 | 1 | 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The response message has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|E| 1 | 1 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Public Value Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Status Word |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Host Name Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
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= Host Name =
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This is the only response that is accepted if the association status
word is zero; otherwise, it is ignored. As this is the first request
sent and the response is not from an association, the Association ID
fields are 0. The Host Name field contains the unterminated string
returned by the Unix gethostname() library function. The Status Word is
defined in previously in this memorandum. While minimum and maximum host
name lengths remain to be established, the reference implementation uses
the values 4 and 256, respectively.
Certificate Message
The Certificate message is used to obtain the certificate and related
values. The request message has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0| 2 | 2 | 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Association ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The response message has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|E| 2 | 2 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Association ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Public Values Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Certificate Filestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Certificate Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
= Certificate =
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Certificate Signature Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
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| |
= Certificate Signature =
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The response is accepted only if the association status word is nonzero,
AUT = 0 and LBK = 1. The certificate is encoded in X.509 format using
ASN.1 syntax. If the certificate has expired or for some reason is no
longer available, the response includes only the first two words with
the E bit set. The remaining fields are defined previously in this
memorandum.
Cookie Message
The Cookie is used in client/server and symmetric modes to obtain the
server cookie. The request message has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0| 3 | 3 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Association ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Public Values Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Certificate Filestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Public Key Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
= Public Key =
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Public Key Signature Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
= Public Key Signature =
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The request is accepted only if AUT = 1, CKY = 0 and LBK = 1. The Public
Key field contains the server public key values to be used for cookie
encryption. The values are encoded in ASN.1 format. The remaining fields
are defined previously in this memorandum.
The response message has the following format:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|E| 3 | 3 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Association ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cookie Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Certificate Filestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encrypted Cookie Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
= Encrypted Cookie =
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cookie Signature Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
= Cookie Signature =
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The response is accepted only if AUT = 1 and LBK = 1. The Cookie
Timestamp, Encrypted Cookie and Cookie Signature fields are determined
upon arrival of the request message. The Encrypted Cookie field contains
the encrypted cookie value according to the public key provided in the
request. If CKY = 0, the decrypted cookie is used directly. If CKY = 1,
the decrypted cookie is exclusive-ORed with the existing cookie. If an
error occurs when decoding the public key or encrypting the cookie, the
response includes only the first two words with the E bit set. The
remaining fields are defined previously in this memorandum.
Autokey Message
The Autokey message is used to obtain the autokey values. The request
message has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0| 2 | 4 | 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Association ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The response message has the following format:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|E| 4 | 4 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Association ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Autokey Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Certificate Filestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Autokey Signature Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
= Autokey Signature =
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The response is accepted only if AUT = 1 and KEY = 0 in the association
status word; otherwise, it is ignored. The Autokey Timestamp, Key ID,
Key Number and Autokey Signature fields are determined when the most
recent key list was generated. If a key list has not been generated or
the association ID matches no mobilized association, the response
includes only the first two words with the E bit set. The remaining
fields are defined previously in this memorandum.
Leapseconds Table Message
The Leapseconds Table message is used to exchange leapseconds tables.
The request and response messages have the following format, except that
the R bit is set in the response:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0| 2 | 5 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Association ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Public Values Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Leapseconds Filestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Leapseconds Table Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| |
| |
= Leapseconds Table =
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Leapseconds Signature Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
= Leapseconds Signature =
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The response is accepted only if AUT = 1 and LPT = 0 in the association
status word; otherwise, it is ignored. The Leapseconds Table field
contains the leapseconds table as parsed from the leapseconds file
available from NIST. In client/server mode the client requests the table
from the server when the LPF bit is set in the host status word. If the
client already has a copy, it uses the one with the latest filestamp. In
symmetric modes the peers exchange tables and both use the one with the
latest filestamp. If the leapseconds table is requested but unavailable,
the response includes only the first two words with the E bit set. The
remaining fields are defined previously in this memorandum.
Appendix B. Key Generation and Management
The ntp-genkeys utility program in the NTP software distribution
generates public/private key, certificate request and certificate files.
A set of files is generated for every message digest and signature
encryption scheme supported by the OpenSSL software library. All files
are based on a pseudo-random number generator seeded in such a way that
random values are exceedingly unlikely to repeat. The files are PEM
encoded in printable ASCII format suitable for mailing as MIME objects.
The file names include the name of the generating host together with the
filestamp, as described previously in this memorandum.
The generated files are typically stored in a shared directory in NFS
mounted file systems, with files containing private keys obscured to all
but root. Links from default file names assumed by the NTP daemon are
installed to the selected files for the host key, sign key and host
certificate. Since the files of successive generations and different
hosts have unique names, there is no possibility of name collisions. An
extensive set of consistency checks avoids linking from a particular
host to the files of another host, for example.
The ntp-genkeys program generates public/private key files for both the
RSA and DSA encryption algorithms with a default modulus of 512 bits.
The host key used for cookie encryption must be RSA. By default, the
same key is used for signature encryption. However, a different RSA key
or a DSA key can be specified for signature encryption.
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The ntp-genkeys program also generates certificate request and self-
signed certificate files. The X.509 certificate request used by Autokey
includes at the minimum these values and possibly related information
needed by an external certificate authority. Autokey expects the subject
name and issuer name to be the same as the generating host name.
The program avoids the need for a serial number file by using the
filestamp as the certificate serial number. By default, certificates are
valid for one year following the time of generation, although these
conventions may change. Also, the program assumes X.509 version 1
formats, although this may change to version 3 in future. Other
implementations might have different conventions.
Appendix C. Packet Processing Rules
Exhaustive examination of possible vulnerabilities at the various
processing steps of the NTP protocol as specified in RFC-1305 have
resulted in a revised list of packet sanity tests. There are 12 tests,
called TEST1 through TEST12 in the reference implementation, which are
performed in a specific order designed to gain maximum diagnostic
information while protecting against an accidental or malicious clogging
attack. These tests are described in detail in the Flash Codes section
of the ntpq documentation page at
www.eecis.udel.edu/~ntp/ntp_spool/html/ntpq.htm.
The sanity tests are divided into three tiers as previously described.
The first tier deflects access control and packet message digest
violations. The second deflects packets from broken or unsynchronized
servers and replays. The third deflects packets with invalid header
fields or time values with excessive errors. However, the tests in this
last group do not directly affect cryptographic the protocol
vulnerability, so are beyond the scope of discussion here.
When a host initializes, it reads its own host key, sign key and
certificate files, which are required for continued operation.
Optionally, it reads the leapseconds file, when available. When reading
these files the host checks the 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 the sign key filestamp (or host key filestamp, if that is the
default sign key). In general, at the time the files are read, the host
is not synchronized, so it cannot determine whether the filestamps are
bogus other than these simple checks.
Once a client has synchronized to a proventic source, additional checks
are implemented as each message arrives. In the following the relation A
-> B is Lamport's "happens before" relation which is true if event A
happens before event B. Here the relation is assume to hold if event A
is simultaneous with event B, unless noted to the contrary. The
following assertions are required:
1. For timestamp T and filestamp F, F->T; that is, the timestamp must
not be earlier than the filestamp.
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2. In client and symmetric modes, for host key filestamp H, public key
timestamp P, cookie timestamp C and autokey timestamp A, H->P->C->A;
that is, once the cookie is generated an earlier cookie will not be
accepted, and once the key list and autokey values are generated,
earlier autokey values will not be accepted.
3. For sign file S and certificate filestamp C specifying begin time B
and end time E, S->C->B->E; that is, the valid period must be nonempty
and not retroactive.
4. For timestamp T, begin time B and end time E, B->T->E; that is, the
timestamp T is valid from the beginning if second B through the end of
second E. This raises the interesting possibilities where a truechimer
server with expired certificate or a falseticker with valid certificate
are not detected until the client has synchronized to a clique of
proventic truechimers.
5. For each of signatures, the client saves the most recent valid
timestamp T0 and filestamp F0. For every received message carrying
timestamp T1 and filestamp F1, the message is discarded unless T0->T1
and F0->F1; however, if the KEY bit of the association status word is
dim, the message is not discarded if T1 = T0; that is, old messages are
discarded and, in addition, if the server is proventic, the message is
discarded if an old duplicate.
An interesting question is what happens if during regular operation a
certificate becomes invalid. The behavior assumed is identical to the
case where an incorrect sign key were used. Thus, the next time a client
attempts to verify an autokey signature, for example, the operation
would fail and eventually cause a general client reset and restart.
Security Considerations
Security issues are the main topic of this memorandum.
References
Note: Internet Engineering Task Force documents can be obtained at
www.ietf.org. Other papers and reports can be obtained at
www.eecis.udel.edu/~mills. Additional briefings in PowerPoint,
PostScript and PDF are at that site in ./autokey.htm.
1. Bradner, S. Key words for use in RFCs to indicate requirement levels.
Request for Comments RFC-2119, BCP 14, Internet Engineering Task Force,
March 1997.
2. Karn, P., and W. Simpson. Photuris: session-key management protocol.
Request for Comments RFC-2522, Internet Engineering Task Force, March
1999.
3. Kent, S., R. Atkinson. IP Authentication Header. Request for Comments
RFC-2402, Internet Engineering Task Force, November 1998.
Mills Expires August, 2002 [page 35]
Internet Draft Public-Key Cryptography for the NTP February, 2002
4. Kent, S., and R. Atkinson. IP Encapsulating security payload (ESP).
Request for Comments RFC-2406, Internet Engineering Task Force, November
1998.
5. Maughan, D., M. Schertler, M. Schneider, and J. Turner. Internet
security association and key management protocol (ISAKMP). Request for
Comments RFC-2408, Internet Engineering Task Force, November 1998.
6. Mills, D.L. Authentication scheme for distributed, ubiquitous, real-
time protocols. Proc. Advanced Telecommunications/Information
Distribution Research Program (ATIRP) Conference (College Park MD,
January 1997), 293-298.
7. Mills, D.L. Cryptographic authentication for real-time network
protocols. In: AMS DIMACS Series in Discrete Mathematics and Theoretical
Computer Science, Vol. 45 (1999), 135-144.
8. Mills, D.L. Network Time Protocol (Version 3) specification,
implementation and analysis. Network Working Group Report RFC-1305,
University of Delaware, March 1992, 113 pp.
9. Mills, D.L. Proposed authentication enhancements for the Network Time
Protocol version 4. Electrical Engineering Report 96-10-3, University of
Delaware, October 1996, 36 pp.
10. Mills, D.L, and A. Thyagarajan. Network time protocol version 4
proposed changes. Electrical Engineering Department Report 94-10-2,
University of Delaware, October 1994, 32 pp.
11. Mills, D.L. Public key cryptography for the Network Time Protocol.
Electrical Engineering Report 00-5-1, University of Delaware, May 2000.
23 pp.
12. Orman, H. The OAKLEY key determination protocol. Request for
Comments RFC-2412, Internet Engineering Task Force, November 1998.
Author's Address
David L. Mills
Electrical and Computer Engineering Department
University of Delaware
Newark, DE 19716
mail mills@udel.edu, phone 302 831 8247, fax 302 831 4316
web www.eecis.udel.edu/~mills
Full Copyright Statement
"Copyright (C) The Internet Society, 2001. All Rights Reserved. This
document and translations of it may be copied and furnished to others,
and derivative works that comment on or otherwise explain it or assist
in its implmentation may be prepared, copied, published and distributed,
in whole or in part, without restriction of any kind, provided that the
Mills Expires August, 2002 [page 36]
Internet Draft Public-Key Cryptography for the NTP February, 2002
above copyright notice and this paragraph are included on all such
copies and derivative works. However, this document itself may not be
modified in any way, such as by removing the copyright notice or
references to the Internet Society or other Internet organizations,
except as needed for the purpose of developing Internet standards in
which case the procedures for copyrights defined in the Internet
Standards process must be followed, or as required to translate it into.
Mills Expires August, 2002 [page 37]
