Network Working Group David L. Mills
Internet Draft University of Delaware
Document: < draft-ietf-stime-ntpauth-01.txt > April 2001
Category: Standards Track
Public-Key Cryptography for the Network Time Protocol
Version 1
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
in 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.
It obsoletes a previous report [11] primarily in the schemes for
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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 intervening servers.
Changes Since the Preceeding Draft
There are a number of changes scattered through this memorandum to
clarify the presentation and add a few new features. Among the most
important:
1. An optional parameter negotiation message has been added to the
protocol state machine. The values it may carry and the interpretation
of these values are not defined in this memorandum.
2. A preliminary value exchange has been added to begin the protocol
dance. This is necessary to avoid a vulnerability where unsolicited
public key responses could clog the victim with needless signature
cycles.
3. The value exchange, which is piggybacked on the association ID
message, supports a timestamp-based agreement scheme which floods the
latest version of the agreement parameters and leapseconds table. Using
this scheme any one of a clique of trusted primary servers running
symmetric modes with each other and broadcast or client/server modes
with the secondary server population can refresh these data at any time
and the refreshed data will update all older data everywhere in the NTP
subnet within one day.
4. An optional certificate retrieval operation has been added to the
protocol state machine. While the operation has been implemented and
tested, the contents of the certificate itself have not been determined.
5. A couple of subtle livelock problems with symmetric mode and
broadcast mode were found and fixed. The problem with source addresses
not yet bound has been fixed in the reference implementation.
6. The protocol descriptions and state diagrams have been updated. Some
packet formats have been changed in minor ways.
7. Provisions for the use of IPv6 addresses in calculating the autokey
have been added.
8. Provisions for the use of arbitrary identification values to be used
in lieu or IP addresses in calculating the autokey have been added.
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9. A simplified version of the protocol appropriate for SNTP clients is
proposed; details to follow.
Introduction
A distributed network service requires reliable, ubiquitous and
survivable provisions to prevent accidental or malicious attacks on the
servers and clients in the network or the values they exchange.
Reliability requires that clients can determine that received packets
are authentic; that is, were actually sent by the intended server and
not manufactured or modified by an intruder. Ubiquity requires that 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 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 Public-Key Infrastructure
(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
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public and private components. This scheme has been tested and evaluated
in a local environment and is being deployed now 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.
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 all 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 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.
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 processor resources with needless
cryptographic calculations.
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 processes. The mission of the algorithms and protocols
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 rsaref2.0 algorithms, but
other algorithms 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.
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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 Byzantine traitors and evil cliques. 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 and can archive
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 PKI operations.
3. An intruder can intercept, modify and replay a packet. However, it
cannot permanently prevent the original packet transmission over the
net; that is, it cannot break the wire, only congest it.
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 discussed 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
cryptographic state variables for every client. It is however feasible
to regenerated them for a client upon arrival of a packet from that
client.
3. The lifetime of cryptographic values must be enforced, which requires
a reliable system clock. However, the sources that synchronize the
system clock must be cryptographically proventicated. This circular
interdependence of the timekeeping and proventication functions requires
special handling.
4. 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 keys and agreement parameters, where necessary, must be
retrievable directly from servers without requiring secured channels;
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however, the fundamental security of identification credentials and
public values bound to those credentials must eventually be a function
of 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
cryptoraphic values are public, so there is no need to associate each
interface with different cryptoraphic values. In the NTP design the host
name, as returned by the gethostname() 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 as noted.
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.
2. It must provide for the independent collection of cryptographic
values and time values. A client is proventicated only when the all
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, including
replay/modification and clogging attacks. In particular, it must be
tolerant of operation or implementation variances, such as packet loss
or misorder, or suboptimal configuration.
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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
and verification functions.
6. It must function in all the modes supported by NTP, including
client/server, broadcast and symmetric active/passive modes.
7. It must not require intricate per-client or per-server configuration
other than the availability of public/private key files and agreement
parameter files, as required.
8. The reference implementation must contain provisions to generate
cryptographic key values, including private/public keys and agreement
parameters 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 of the
rsaref2.0 library, although other libraries with a compatible interface
could be used as well. The reference implementation uses keyed-MD5
message digests to detect packet modification, timestamped RSA digital
signatures to verify the source, and Diffie-Hellman key agreements to
construct a private shared key from public values. However, there is no
reason why alternative signature schemes and agreement algorithms could
be supported. 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 given values less than
65536 and indefinite lifetime. Autokey key IDs have pseudo-random values
equal to or greater than 65536 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 active/passive.
All three variants make use of a specially contrived session key called
an autokey and a pseudo-random sequence of key IDs called the key list.
As in the original NTP Version 3 authentication scheme, the Autokey
protocol operates separately for each association, so there may be
several key lists operating independently at the same time and with
distinct associated values and signatures.
An autokey consists of four fields in network byte order as shown below:
+-----------+-----------+-----------+-----------+
| Source IP | Dest IP | Key ID | Cookie |
+-----------+-----------+-----------+-----------+
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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 a private value,
depending on the mode.
There are some scenarios where the use of endpoint IP addresses when
calculating the autokey 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. As described below, NTP associations are
identified by the endpoint IP addresses, so the natural approach is to
authenticate associations using these values. For scenarios where this
is not possible, an optional identification value can be used instead of
the endpoint IP addresses. The Parameter Negotiation message contains an
option to specify these data; however, the format, encoding and use of
this option are not specified in this memorandum. For the purposes of
this memorandum, the endpoint IP addresses will be assumed.
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 private value computed by an
agreement algorithm. For packets with extension fields, the cookie has a
default public value of zero, since these packets can be validated
independently using signed data in the extension fields. The four values
are hashed by the message digest algorithm to produce the actual key
value, which 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 algorithm.
The key list consists of a sequence of key IDs starting with a random
value and each pointing to the next. To generate the next autokey on the
key list, the next key ID is the first 32 bits in network byte order of
the previous key value. It may happen that a newly generated key ID is
less than 65536 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 of that entry, collectively called the autokey values. The list
is used in reverse order, so that the first key ID 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
authenticated 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
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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.
The scheme used in client/server mode was suggested by Steve Kent over
lunch. The server keeps no state for each client, but uses a fast
algorithm and a private value 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
private value. The first 32 bits of the hash is the cookie used for the
actual autokey calculation and is returned to the client on request. It
is thus specific to each client separately and of no use to other
clients or an intruder. A client obtains the cookie and signature using
the Autokey protocol and saves it for later use.
In client/server mode the cookie is a relatively weak function of the IP
addresses and a server private value. The client uses the cookie and
each key ID on the key list in turn to calculate the MAC for the next
NTP packet. The server calculates these values and checks the MAC, then
generates the MAC for the response using the same values, but with the
IP source and destination addresses exchanged. The client calculates and
checks the MAC and verifies the key ID matches the 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.
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 temporarily
authenticates 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
machine. For this purpose, the broadcast server 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,
so that a private cookie can be computed by a strong agreement
algorithm. The cookie itself is the first 32 bits of the agreed key. The
key list for each direction is generated separately by each peer and
used independently, but each is generated with the same cookie.
The server proventic bit is set only when the cookie or autokey values,
depending on mode, and the associated timestamp and signature are all
valid. If the bit is set, the client processes NTP time values; if the
bit is not set, extension field messages are processed in order to run
the Autokey protocol, but the NTP time values are ignored. Packets with
old timestamps are discarded immediately while avoiding expensive
cryptographic algorithms. Bogus packets with newer timestamps must pass
the MAC and autokey tests, which is highly unlikely.
Once the proventic bit has been set, the Autokey protocol is normally
dormant. In all modes except broadcast server, packets are normally sent
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without an extension field, 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.
This insures that stale values never propagate beyond a clock step.
Public-Key Signatures
Since public-key signatures provide strong protection against
misrepresentation of sources, probably the most obvious intruder
strategy is to deny or restrict service by replaying old packets with
signed cryptographic values in a cut-and-paste attack. The basis values
on which the cryptographic operations depend are changed often to
deflect brute force cryptanalysis, so the client must be prepared to
abandon an old key in favor of a refreshed one. This invites the
opportunity for an intruder to clog the client or server by replaying
old Autokey messages or to invent bogus new ones. A client receiving
such messages might be forced to refresh the correct value from the
legitimate server and consume significant processor resources.
In order to foil such attacks, every extension field carries a timestamp
in the form of the NTP seconds at the signature time. The signature
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 is
synchronized and signatures carry a nonzero (valid) timestamp. Otherwise
the system clock is unsynchronized and signatures carry a zero (invalid)
timestamp. Extension fields with invalid or old timestamps are discarded
before any values are used or signatures verified.
There are three signature types and six values to be signed:
1. The public value is signed at the time of generation, which occurs
when the system clock is first synchronized and about once per day after
that in the reference implementation. Besides the public value, the
public key/host name, agreement parameters and leapseconds table are all
signed as well, even if their values have not changed. All four of these
values carry the same timestamp. On request, each of these values and
associated signatures and timestamps are returned in an extension field.
2. The cookie value is computed and signed upon arrival of a cookie
request message. The response message contains the cookie, signature and
timestamp in an extension field.
3. The autokey values are signed when a new key list is generated, which
occurs about once per hour in the reference implementation. On request,
the autokey values, signature and timestamp are returned in an extension
field.
The most recent timestamp for each of the six values is saved for
comparison. Once a signature with valid timestamp has been received,
packets carrying extension fields with invalid timestamps or older valid
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timestamps for the same value are discarded before the signature is
verified. For packets containing signed extension fields, the timestamp
deflects replays that otherwise might consume significant processor
resources; for other packets the Autokey protocol deflects message
modification and replay. 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 agreement algorithms are regenerated from
time to time. It is the intent that file regeneration and loading of
these values occur without specific warning and without requiring
distribution in advance. While files carrying cryptographic data are not
specifically signed, the file names have extensions called filestamps
which reliably determine the time of generation. The filestamp for a
file is a string of decimal digits representing the NTP seconds at the
time the file was created.
As the data are forwarded from server to client, the filestamps are
preserved, including those for the public key/host name, agreement
parameters and leapseconds table. Packets with older filestamps are
discarded befor the signature is verified. Filestamps can in principle
be used as a total ordering function to verify that the data are
consistent and represent the latest available generation. For this
reason, the files should always be generated on a machine when the
system clock is valid.
When a client or server initializes, it reads its own public and private
key files, which are required for continued operation. Optionally, it
reads the agreement parameters file and constructs the public and
private values to be used later in the agreement algorithm. Also
optionally, it reads the leapseconds table file. When reading these
files it checks the filestamps for validity; for instance, all
filestamps must be later than the time the UTC timescale was established
in 1972.
When the client first validates a proventic source and when the clock is
stepped and when new cryptographic values are loaded from a server, the
client recomputes all signatures and checks the filestamps for validity
and consistency with the signature timestmaps:
1. If the system clock if valid, all timestamps and filestamps must be
earlier than the current clock time.
2. All signature timestamps must be later than the public key timestamp.
3. In broadcast client mode, the cookie timestamp must be later than the
autokey timestamp.
4. In symmetric modes the autokey timestamp must be later than the
public value timestamp.
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5. Timestamps for each cryptographic data type must be later than the
filestamps for that type.
In the above constraints, note that timestamps and filestamps have a
granularity of one second, so that a difference of zero seconds is
ambiguous. Furthermore, timestamps and filestamps can be in error as
much as the value of the synchronization distance; that is, the sum of
the root dispersion plus one-half the root delay. However, the NTP
protocol normally operates with polling intervals much longer than one
second, so that successive timestamps for the same data type are
nonambiguous. On most machines, the processor time to generate a
complete set of key files is longer than one second, so it is not
possible to generate two generations in the same second.
However, it may happen that agreement parameters files may be generated
on two machines with the same filestamps, which creates an ordering
ambiguity. The filestamps for leapseconds files should also be
nonambiguous, since these files are created by NIST not more often than
twice per year. While filestamp collisions should be so rare as to be
safely ignored, a good management approach might require that these
files be generated only on a schedule that guarantees that no more than
one client or server generates a new key file set on any one day.
Certificates
PKI principles call for the use of certificates to reliably bind the
server distinguished name (host name), public key and related values to
each other. The certificate includes these values together with the
distinguished name of the certificate atuthority (CA) and other values
such as serial number and valid lifetime. These values are then signed
by the CA using its private key. The Autokey protocol includes
provisions to obtain the certificate, but at the present time does
nothing with the values. A future version of the protocol is to include
provisions to validate the binding using procedures established by the
IETF.
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. These tests have been
formulated to harden the protocol against defective header and data
values. These are summarized below, since they are an integral component
of the NTP cryptograhic defense mechanism. There are eleven tests,
called TEST1 through TEST11 in the reference implementation, which are
performed in a specific order designed to gain maximum diagnostic
information while protecting against accidental or malicious errors.
The tests are divided into three groups. The first group is designed to
deflect access control and authentication violations. While access
control and message digest violations always result immediate discard,
it is necessary when first mobilizing an association to disable the
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autokey test and certain timestamp tests. However, after the proventic
bit is set, all tests are enforced.
The second group of tests is designed to deflect packets from broken or
unsynchronized servers and replays. In order to synchronize an
association in symmetric modes, it is necessary to save the originate
and receive timestamps in order to send them at a later time. This
happens for the first packet that arrives, even if it violates the
autokey test. In the normal case, the second packet to arrive will be
accepted and the association marked reachable. However, an agressive
intruder could replay old packets that could disrupt the saved
timestamps. This could not result in incorrect time values, but could
prevent a legitimate client from synchronizing the association.
The third group of tests is designed to deflect packets with invalid
header fields or time values with excessive errors. However, these tests
do not directly affect cryptographic source proventication or
vulnerability, so are beyond the scope of discussion in this document.
For packets containing signed extension fields additional tests apply,
depending on request type. There are the usual tests for valid extension
field format, length and values. An instantiated variable, such as the
public key/host name, agreement paramaters, public value, cookie or
autokey values, is valid when the accompaning timestamp and filestamp
are valid. The public key must be instantiated before any signatures can
be verified. In symmetric modes the agreement parameters must be
instantiated before the public and private agreement values can be
determined; the public agreement value must be instantiated before the
agreement algorithm can be run to determine the cookie. In all modes the
cookie value must be determined before the key list can be generated.
The object of the Autokey dances described below is to set the proventic
bit. In client/server mode this bit is set when the cookie is validated.
In other modes this bit is set when the autokey values are validated.
The bit is cleared initially and when the autokey test fails. If once
the bit is set and then cleared, the protocol will send an autokey
request message at the next poll opportunity and continue to send this
message until receiving valid autokey values or a general reset occurs.
This behavior is a compromise between protocol responsiveness, where the
current association can be maintained without interruption, and protocol
vulnerability, where an intruder can repeatedly clog the receiver with
replays that cause the client to needlessly poll the server and refresh
the values.
Error Recovery
The protocol state machine which drives the various Autokey functions
includes provisions for various kinds of error conditions that can arise
due to missing files, corrupted data, protocol violation and packet loss
or misorder, not to mention hostile intrusion. There are two mechanisms
which maintain the liveness state of the protocol, the reachability
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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
zero 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
one. If any bit in this register is one, 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 every
poll interval until the authentic response is received or a general
reset occurs, in which case the protocol restarts from the beginning.
While this behavior might be considered rather conservative, the
advantage is that old cryptographic and time values can never persist
from one mobilization to the next.
There are a number of situations where some action on an association
causes the remaining autokeys on the key list to become invalid. When
one of these situations happens, the key list and associated keys 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 client
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.
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6. When the cryptographic values are refreshed, the key lists for all
associations are regenerated.
7. When the client is first proventicated or the system clock is
stepped, the key lists for all associations are regenerated.
Autokey Protocols
This section describes the Autokey protocols supporting
cryptographically secure server proventication. There are three
subprotocols, called dances, corresponding to the NTP client/server,
broadcast and symmetric active/passive modes. While Autokey messages are
piggybacked in NTP packets, the NTP protocol assumes clients poll
servers at a relatively low rate, such as once per minute, and where
possible avoids large packets. In particular, it is assumed that a
request sent at one poll opportunity will normally result in a response
before the next poll opportunity.
It is important to observe that, while the Autokey dances are obtaining
and validating cryptographic values, the underlying NTP protocol
continues to operate. Most packets used during the dances contain
signatures, so the values can be believed even before the dance has
concluded. Since signatures are valid once the certificate has been
validated during the initial steps of the dance, by the time the Autokey
values are validated the clock is usually already set. In this way the
sometimes intricate Autokey dance interactions do not delay the
accumulation of time values that will eventually set the clock. Each
autokey dance is designed to be nonintrusive and to require no
additional packets other than for regular NTP operations. Therefore, the
phrase "some time later" in the descriptions applies to the next poll
opportunity.
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. Some requests and most responses are protected by
timestamped signatures. The signature covers the data, timestamp and
filestamp, where applicable. The timestamp is set to the default (zero)
when the sender is not proventicated; otherwise, it is set to the NTP
seconds when the signature was generated. The following rules are
designed to detect invalid header or data fields and to deflect clogging
attacks. Each extension field is validated in the following order and
discarded if:
1. The request or response code is invalid or the data field has
incorrect length.
2. The signature field is either missing or has incorrect length.
3. The public key is missing or has incorrect length.
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4. In the case of the agreement algorithm, the agreement parameterss are
missing or have incorrect lengths.
5. The signature timestamp is earlier than the last received timestamp
of the same type or the two timestamps are equal and the proventic bit
is set..
6. Where applicable, the filestamp is earlier than the last received
filiestamp of the same type.
Only if the extension field passes all the above tests is the signature
verified using PKI algorithms. Otherwise and in general, a response is
generated for every request, even if the requestor is not proventicated.
However, some responses may have truncated data or signature fields
under certain conditions. If these fields are present and have correct
length, signatures are present and verifiable.
In the Autokey protocol every transmitted packet is associated with an
autokey previously computed and stored in the key list. When the last
entry in the list is used, a new list is constructed as described above.
This requires knowledge of the cookie value. If for some reason the
cookie value is changed, the remaining entries in the key list are
purged and a new one constructed. However, if an extension field is
present, the current autokey is discarded and the autokey reconstructed
using a cookie value of zero.
A timestamp-based agreement protocol is used to manage the distribution
of the certificate, agreement parameters and leapseconds table. The
association ID request and response messages include the certificate,
agreement and leapseconds bits from the system status word. one or more
of these bits are set when the associated data are present, either
loaded from local files or retrieved from another server at some earlier
time. If any of these bits are set in the association ID response to a
client in client/server mode or a peer in symmetric mode, the data are
requested from the server or peer and, once obtained, the bits are
reset. However, the response data are stored only if more recent than
the data already stored.
In the descriptions below, it is assumed that the client and server have
loaded their own private key and public key, as well as certificate,
agreement parameters and leapseconds table, where available. Public keys
for other servers, as well as the agreement parameters and leapseconds
table, can be loaded from local files or retrieved from any server.
Further information on generating and managing these files is in
Appendix B.
Preliminaries
The first thing the server needs to do is obtain the system status word,
which reveals which cryptographic values the server is prepared to
offer, and then the public key and certificate. These steps are
independent of which mode the server is operating in - client/server,
broadcast or symmetric modes.
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The following pseudo-code describes the client state machine operations.
Note that the packet can one request and one or more responses. The
machine requires the association ID, public key and optional
certificate, in that order. While not further specified in this
memorandum, an optional parameter request message can be used to
negotiate algorithm identifiers, parameters and alternate identification
values. Note that the association ID response message also contains the
system status word, which contains the certificate bit.
if (response_pending)
send_response;
if (!parameters)
request_parameters;
if (!association_ID)
request_association_ID;
else if (!public_key)
request_public_key;
else if (certificate_bit)
request_certificate;
The following diagram shows the preliminary protocol dance. In this and
following diagrams the NTP packet type is shown above the arrow and the
extension field(s) message type shown below. Note that in the
client/server mode the server responds immediately to the request, but
in the symmetric modes the response may be delayed for a period up to
the current poll interval. The following cryptographic values are
instantiated by the dance:
public key server public key
host name server host name
CA name certificate authority host name (optional)
filestamp generation time of public key file
secure bit set when the public key is stored and validated
server client
| |
| NTP client |
1 |<-----------------| mobilize client association; generate key list
| assoc ID req | with default cookie; send status word
| |
| NTP server |
2 |----------------->| store status word
| assoc ID rsp |
| |
| NTP client |
3 |<-----------------| request public key and host name
| key/name req |
| |
| NTP server |
4 |----------------->| store public key, host name, filestamp and
| key/name rsp | timestamp
| ... |
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| |
| NTP client |
5 |<-----------------| request certificate
| certif req |
| |
| NTP server |
6 |----------------->| store certificate; verify credentials; set
| certif rsp | secure bit
| ... |
The dance begins when the client (or symmetric-active peer) on the right
mobilizes an association, generates a key list using the default cookie
and sends an association ID request message (1) to the server (or
symmetric-passive peer) on the left. The server responds with an
association ID response message (2) including the server association ID
and status word. To protect against a clogging attack, the transmit
timestamp in the NTP header in the request must be identical to the
originate timestamp in the response. The client retransmits request (1)
at every poll opportunity until receiving a valid response (2) or
association timeout.
Some time later the client sends a public key/host name request (3) to
the server. The server responds with the requested data and associated
timestamp and filestamp (4). The client checks the timestamp and
filestamp, verifies the signature and initializes the public key and
host name. If the certificate bit in the status word is zero, indicating
the server is not prepared to send one, and if the client concurs, the
secure bit is set at this time and the certificate exchange is bypassed.
The client retransmits request (3) at every poll opportunity until
receiving a valid response (4) or association timeout.
The public key/host name message can be interpreted as a poor-man's
certificate, since it is signed and timestamped. However, strong
security requires a CA sign the host name and public key values and
establish a period of validity for the signature. As an optional
feature, the client sends a certificate request (5) to the server. The
server responds with the requested data and assciated timestamp and
filestamp (6). The response is signed by the CA's public key, so a
further step may be necessary to obtain the CA's certificate, which
contains its public key. The details for these additional steps are for
further study.
Since (4) is the first signed message received, the timestamp and
filestamp have only marginal utility, but do serve to avoid messages
from unsynchronized servers and deflect replays. The interesting
question is whether to provide automatic update when the server makes a
new key generation, since the new generation would have a later
filestamp and instantly deprecate all cryptographic values with earlier
timestamps. This brings up the question of a distributed greeting
protocol, which may be a topic for future study. Meanwhile, the
reference implementation accepts only the first message received and
discards all others.
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When the secure bit is set, data in packets with signatures are valid
and the NTP protocol continues in parallel with the Autokey protocol.
Client/Server Modes (3/4)
In client/server modes the server keeps no state variables specific to
each of possibly very many clients and mobilizes no associations. The
server regenerates a cookie for each packet received from the client.
For this purpose, the server hashes the cookie from the IP addresses and
private value with the key ID field set to zero, as described
previously, then provides it to the client. Both the client and server
use the cookie to generate the autokey which validates each packet
received. To further strengthen the validation process, the client
selects a new key ID for every packet and verifies that it matches the
key ID in the server response to that packet.
Before proceeding to the full protocol description, it should be noted
that in the case of lightweight SNTP protocol associations, it is not
necessary to proceed beyond the preliminary protocol defined above. Most
if not all SNTP implementations send only a single client-mode packet
and expect only a single NTP server-mode packet in return. Since the
Autokey protocol is piggybacked in the NTP packet, the clock can be set
and the server authenticated with a single packet exchange if a
certificate is not required and in two exchanges if it is. Details of
this simplified protocol remain to be determined.
The following pseudo-code describes the client state machine operations.
The machine requires the association ID, public key, optional
certificate, cookie, autokey values and leapseconds table in that order,
but the autokey values are required only if broadcast client mode.
if (response_pending)
send_response;
if (!cookie)
request_cookie;
else if (!autokey_values && broadcast_client))
request_autokey_values;
else if (!leapseconds_table)
request_leapseconds_table;
The following diagram shows the protocol dance in client/server mode.
The following cryptographic values are instantiated by the dance:
public key server public key
host name server host name
filestamp generation time of public key file
timestamp signature time of public key/host name values
cookie cookie determined by the server for this client
timestamp signature time of cookie
proventic bit set when client clock is synchronized to source
server client
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| |
| NTP client |
7 |<-----------------| request cookie
| cookie req |
| |
| NTP server |
8 |----------------->| store cookie and timestamp; set proventic bit;
| cookie rsp |
| ... |
| |
| NTP client |
9 |<-----------------| regenerate key list with server cookie
| |
| NTP server |
10 |----------------->|
| |
| continue |
= client/server =
The dance begins when the client on the right mobilizes an association
and validates the public key as in the preliminary dance above. Some
time later the client sends a cookie request (7). The server immediately
responds with the cookie and timestamp (8). The client checks the
timestamp, verifies the signature and initializes the cookie and cookie
timestamp, then sets the proventic bit. Since the cookie has changed,
the client regenerates the key list with this cookie when the next
packet is sent. The client retransmits request (7) at every poll
opportunity until receiving a valid response (8) or association timeout.
After successful verification, there is no further need for extension
fields, unless an error occurs or the server generates a new private
value. When this happens, the server fails to authenticate packet (9)
and, following the original NTP protocol, responds with a NAK packet
(10), which the client ignores. Eventually, an association timeout and
general reset occurs and the dance restarts from the beginning. Of
course, the NAK client could interpret the NAK message to restart the
protocol immediately and avoid the timeout. However, this invites the
opportunity for an intruder to destabilize the state machine with
spurious NAK messages.
Broadcast Mode (5)
In broadcast mode, packets are always sent with an extension field.
Since the autokey values for these packets use a well known default
cookie (zero), they can in principle be remanufactured with a new MAC
acceptable to the receiver; however, the key list provides the
authentication function as described earlier. The broadcast server keeps
no state variables specific to each of possibly very many clients and
mobilizes no associations for them.
The following pseudo-code describes the broadcast server state machine
operations. Each broadcast packet includes one response message
containing either the signed autokey values, if the first autokey on the
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key list, or the association ID and status word otherwise. Note however,
when a broadcast client first comes up, the state machine also responds
to client requests as in client/server mode without affecting the
broadcast packets. Note that the association ID request and response
messages also contain the system status word.
if (new_list)
send_autokey_values;
else
send_association_ID;
The server on the left in the diagram below sends packets that are
received by each of a possibly large number of clients, one of which is
shown on the right. Ordinarily, clients do not send packets to the
server, except to calibrate the propagation delay and to obtain
cryptographic values such as the cookie and autokey values. The
following diagram shows the protocol dance in broadcast mode. The
following cryptographic values are instantiated by the dance:
public key server public key
host name server host name
filestamp generation time of public key file
timestamp signature time of public key/host name values
cookie cookie determined by the server for this client
timestamp signature time of cookie
autokey values initial key ID, initial autokey
timestamp signature time of autokey values
proventic bit set when client clock is synchronized to source
server client
| |
| NTP broadcast |
1 |----------------->| mobilize broadcast client association; set
| assoc ID rsp | initially to operate in client/server mode
| |
| ... | continue as in preliminary protocol above
| |
| NTP client |
7 |<-----------------| request cookie
| cookie req |
| |
| NTP server |
8 |----------------->| store cookie and timestamp
| cookie rsp |
| ... |
| |
| NTP client |
9 |<-----------------| regenerate key list with server cookie
| autokey req |
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| |
| NTP server |
10 |----------------->| store autokey values and timestamp; set
| autokey rsp | proventic bit
| ... |
| |
| NTP client |
|<-----------------| continue to accumulate time values
| |
| NTP server |
|----------------->|
| |
| continue |
= volley =
| |
| NTP client |
|<-----------------|
| |
| NTP server |
|----------------->| set clock and propagation estimate; discard
| | remaining keys; switch to broadcast client mode
| continue |
= broadcast =
| |
| NTP broadcast |
|----------------->| server rolls new key list; client refreshes
| autokey rsp | autokey values
| |
= =
The server sends broadcast packets (1) continuously at intervals of
about one minute using the key list and regenerating the list as
required. The first packet sent after regenerating the list includes the
autokey values and signature; other packets include only the association
ID and status word.
The dance begins when the client on the right receives a broadcast
message (1). It mobilizes a broadcast client association set initially
to operate in client/server mode. It then continues to operate as in the
prelimiary protocol to obtain and validate the public key and host name
values. However, the client does not initiate the dance until some time
later (to avoid implosion at the server). However, in addition to the
status word, the association ID response includes the association ID of
the server, so the correct association, if more than one, can be
identified.
Some time later the client sends a cookie request (7). The server
immediately responds with the requested value (8). The client checks the
timestamp, verifies the signature and initializes the cookie and cookie
timestamp. Since the cookie has changed, the client regenerates the key
list with this cookie when the next packet is sent. The client
retransmits request (7) at every poll opportunity until receiving a
valid response (8) or association timeout.
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If an autokey response happens to be in one of the server packets (1),
the client has stored the autokey values and autokey timestamp, so can
switch immediately to broadcast client mode and send no further packets.
Otherwise, some time later the client sends an autokey request (9). The
server immediately responds with the values (10). The client checks the
timestamp, verifies the signature and initializes the autokey values and
autokey timestamp and sets the proventic bit. The client retransmits
packet (9) until receiving a valid response (10) or association timeout.
After successful verification, there is no further need for extension
fields and the client can switch to broadcast client mode and send no
additional packets. However, it is the usual practice to send additional
client/server packets in order for the client mitigation algorithms to
refine the clock offset/delay estimates. When a sufficient number of
estimates are available, the client discards the cookie and remaining
keys on the key list, switches to broadcast client mode, calculates the
propagation delay and sets the clock.
When the server regenerates the key list, it sends an autokey response
in the first packet, which allows the clients to validate it and reset
the autokey values. Unless this packet happens to be lost, the clients
can continue with no further interaction with the server. Otherwise, the
client fails to authenticate the packets (1). Eventually, an association
timeout and general reset occurs and the dance restarts from the
beginning.
Symmetric Active/Passive Mode (1/2)
In symmetric modes there is no explicit client/server relationship,
since each peer in the relationship can operate as a server with the
other operating as a client. Which peer acts as the server depends on
which peer has the smallest root synchronization distance to its
ultimate reference source, and the choice may change from time to time.
This requirement results in a quite complex interaction between the
peers, especially when considering the many possibilities of failure and
recovery.
There are two protocol scenarios involving symmetric modes. The simplest
scenario is where both peers have configured associations that operate
continuously in symmetric active mode and cryptographic values such as
the public key/host name, certificate, agreement parameters and public
value can be configured in advance. A more interesting scenario is when
a symmetric active peer with a configured association begins operation
with a symmetric-passive peer initially without such an association.
The following pseudo-code describes the symmetric state machine
operations. Note that the packet can contain one request and one or two
responses. The machine requires the association ID, public key,
certificate, agreement parameters, agreement public value, autokey
values and leapseconds table in that order. There is a provision to send
the current autokey values when the peer has not requested them. This
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happens when a peer first proventicates and recomputes the key list
using the agreed cookie.
if (response_pending)
send_response;
if (!agreement_parameters)
request_agreement_parameters;
else if (!agreement)
send_agreement;
else if (!autokey_values)
request_autokey_values;
else if (!new_list)
send_autokey_values;
else if (!leapseconds_table)
request_leapseconds_table;
The following diagrams show the protocol dance in symmetric
active/passive mode. The dance in symmetric active/active mode is much
simpler and similar to two independent client/server modes, one for each
direction, but with the cookie requests replaced by an agreement
algorithm. Note that in the following the NTP client header is replaced
by the NTP symmetric active header and the NTP server header is replaced
by the NTP symmetric passive header. The following cryptographic values
are instantiated by each peer in the dance:
public key server public key
host name server host name
filestamp generation time of public key file
timestamp signature time of public key/host name values
cookie cookie determined by the agreement algorithm
timestamp signature time of cookie
autokey values initial key ID, initial autokey
timestamp signature time of autokey values
proventic bit set when client clock is synchronized to source
passive active
| |
| NTP active |
1 |<-----------------| mobilize symmetric active association; generate
| assocID req | key list with default cookie; send status word
| |
| ... | continue as in preliminary protocol above
| |
| NTP passive |
2 |----------------->| store status word
| assoc ID rsp |
| |
| NTP active |
1 |<-----------------| generate key list with default cookie; request
| key/name req | passive key/name
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| ... |
| |
| NTP passive |
2 |----------------->| verify passive credentials
| key/name rsp |
| key/name req |
| ... |
| |
| NTP active |
3 |<-----------------| send active key/name; request agreement
| key/name rsp | parameters
| param req |
| ... |
| |
| NTP passive |
4 |----------------->| store agreement parameters; and timestamp; set
| param rsp | proventic bit
| agree rsp |
| ... |
| |
| NTP active |
3 |<-----------------| send active key/name; request agreement
| key/name rsp | parameters
| param req |
| ... |
| |
| NTP passive |
4 |----------------->| store autokey values and timestamp; set
| key/name req | proventic bit
| autokey rsp |
| ... |
| |
| NTP active |
5 |<-----------------| continue to accumulate time values
| key/name rsp |
| |
= continue =
| |
| NTP passive |
6 |----------------->| set clock
| key/name req |
| |
| continue below |
= =
The dance begins when the active peer on the right generates a key list
with default cookie and timestamp and sends a public key/host name
request to the passive peer on the left (1). The passive peer checks its
access control list and (optionally) queries the DNS using the server IP
address to obtain related cryptographic values. If successful, the peer
mobilizes an association in symmetric passive mode, but takes no further
action until the next poll interval, as required by the NTP protocol.
From this point the passive peer responds to requests, but otherwise
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ignores all time values until the active peer has set its clock and can
provide valid timestamps.
Some time later the passive peer generates a key list with default
cookie and timestamp and sends its public key/host name values along
with a request for the public key/host name values of the active peer
(2). Subsequently, the active peer sends these values, but they are
ignored since the timestamps are invalid. Meanwhile, the active peer
checks the timestamp, verifies the signature and initializes the public
key/host name values, filestamp and timestamp. The active peer
retransmits request (1) at every poll opportunity until receiving a
valid response (2) or until association timeout.
Some time later the active peer sends the requested public key/host name
values along with an autokey request (3). The passive peer retransmits
request (2) at every poll opportunity until receiving a valid timestamp
and verified signature or until association timeout. Since the cookies
for each peer already have a common value and the active peer is
unsynchronized, it is pointless to run the agreement algorithm.
Some time later the passive peer sends the requested autokey values (4).
The active peer checks the timestamp, verifies the signature and
initializes the autokey values and timestamp and sets the proventic bit.
At this point the active peer has authenticated the passive peer, but
may not have accumulated sufficient time values to set the clock and
provide valid timestamps. Operation continues in rounds where the
passive peer requests the public key/host name values and the active
peer returns them, but the passive peer ignores them. Eventually, the
active peer accumulates sufficient time values to set the clock. While
the cookie has not changed, the timestamp has, so the key list is
regenerated with the default key (strictly speaking, only the signature
needs to be recomputed). The active peer is now proventicated, but the
passive peer has not yet authenticated the active peer.
Some understanding of the tricky actions to follow can be gained from
the observation that, up until this point every message received by the
active peer had a signed response field, so that the cookie value is the
default. However, at this point the active peer has all the
cryptographic means at hand and does not need to request anything
further from the passive peer. Thus, the passive peer sends nothing but
requests and these are not signed or timestamped. Since the cryptograhic
security relies entirely on the autokey test, it is important that both
peers generate key lists with the same cookie.
The steps now taken are shown below with the active peer on the left and
the passive peer on the right.
active passive
| |
| NTP active |
1 |----------------->| validate active peer, compute agreed key,
| key/name rsp | regenerate key list with peer key
| public req |
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Internet Draft Public-Key Cryptography for the NTP April, 2001
| |
| NTP passive |
2 |<-----------------| active computes agreed key, regenerates key
| public rsp | list with agreed key
| autokey req |
| ... |
| |
| NTP active |
3 |----------------->| set authentic
| autokey rsp |
| autokey req |
| ... |
| |
| NTP passive |
4 |<-----------------|
| autokey rsp |
| ... |
| |
| NTP active |
5 |----------------->| regular operation (no extension fields)
| ... |
| |
| NTP passive |
6 |<-----------------|
| |
| continue |
= active/passive =
The agreement parameters must have been previously obtained by at least
one of the peers, either directly from a file or indirectly from another
server running the Autokey protocol. A peer needing the parameters sends
an agreement parameters request to the other peer and that peer responds
with the requested data. This exchange, along with the leapseconds table
exchange, is similar to the public key/host name exchange, but not shown
here.
Once the proventic bit is set, the next message sent by the active peer
contains the public key/host name requested by the passive peer, but now
with valid timestamp, plus a public value request containing the active
peer public value (1). The passive peer checks the public key/host name
filestamp and timestamp, verifies the signature and initializes the
values. Optionally, it checks its access control list and queries the
DNS using the server IP address to obtain related cryptographic values.
Conceivably, the active peer could be found bogus at this time; what to
do in this case is for further study.
The passive peer next checks the public value request timestamp,
verifies the signature and runs the agreement algorithm to construct the
shared cookie. Since the cookie has changed, the peer regenerates the
key list with this cookie when the next packet is sent.
Some time later the passive peer sends a public value response including
its own public value together with an autokey request (2). The active
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Internet Draft Public-Key Cryptography for the NTP April, 2001
peer checks the timestamp, verifies the signature and runs the agreement
algorithm to construct the shared cookie. Since the cookie has changed,
the peer regenerates the key list with this cookie when the next packet
is sent. The active peer retransmits the public value request (only) (1)
at every poll opportunity until receiving a valid response (2) or
association timeout.
Some time later the active peer sends its autokey values as requested
together with an autokey request (3). The passive peer checks the
timestamp, verifies the signature, initializes the autokey values and
sets its proventic bit. The passive peer retransmits request (2) at
every poll opportunity until receiving a valid response (3) or
association timeout.
Some time later the passive peer sends its autokey values as requested
(4). The active peer checks the timestamp, verifies the signature, and
initializes the autokey values (the proventic bit is already set). The
active retransmits the autokey request (only) (3) until receiving a
valid response (4) or association timeout.
At this point both peers have completed the Autokey dance and each is
authenticated to the other. However, note that the NTP rules require a
peer operating at a lower stratum disregards time values from a hither
stratum peer; so, while the peers continue to exchange time values, the
values will not be used unless the passive server for some reason loses
its synchronization source.
After successful authentication, there is no further need for extension
fields, unless an error occurs or one of the peers generates new public
values. The protocol requires that, if a peer receives a public value
resulting in a different cookie, it must send its own public value.
Since the autokey values are included in an extension field when a new
key list is generated, there is ordinarily no need to request these
values, unless one or the other peer restarts the protocol or the packet
containing the autokey values is lost. Eventually, an association
timeout and general reset occurs and the dance restarts from the
beginning.
Security Analysis
This section discusses the most obvious security vulnerabilities in the
various modes and phases of operation. Throughout the discussion the
cryptographic algorithms themselves are assumed secure; that is, a
successful brute force attack on the algorithms or public/private keys
or agreement values is unlikely. However, vulnerabilities remain in the
way the actual cryptographic data, including the cookie and autokey
values, are computed and used.
While the protocol has not been subjected to a formal analysis, a few
preliminary observations are warranted. 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 will start from the beginning. A general reset is performed on
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all associations when the clock is first set and when it is stepped
after that. This purges all cryptographic values and time values
dependent on unproventicated sources.
The first exchange in all protocol modes involves an association ID
request and response cycle. Bits in the server status word indicate
whether the server has the agreement paramters and/or leapseconds table.
The association ID messages are not protected by a signature, so
presumably an intruder can manufacture fake bits causing a client
livelock or deadlock condition. To protect against this vulnerability,
the transmit timestamp of the request is matched against the originate
timestamp of the response. The response is accepted only if the two
values match. An intruder is unlikely to predict the transmit timestamp,
which in this case is an effective nonce.
Once the clock is set, and except for the special cases summarized
below, no old or duplicate values will be accepted in any state and an
intruder cannot induce a clogging attack, since the MAC, autokey and
timestamp tests will discard packets before a clogging vulnerability is
exposed. While significant vulnerabilities exist during the initial
protocol states while the necessary values are being obtained, the most
an intruder can do is prevent the protocol dance from completing. If it
does complete, it must complete correctly.
The cryptographic values are always obtained in the same order and in
the same order as the dependency relationships between them. No
cryptographic variables or time variables are instantiated unless the
server is proventic and proventicated. The public key and host name must
be obtained first and no other messages are accepted until they have
been obtained. The cookie must be obtained before the autokey values
that depend on them, etc. Finally, in symmetric modes, both peers obtain
cryptographic values in the same order, so deadlock cannot occur.
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.
There are three tiers of defense against hostile intruder interference.
The first is the message authentication code (MAC) based on a keyed
message digest or autokey generated as the hash of the IP address
fields, key ID field and a special cookie, which can be public or the
result of an agreement algorithm. If the message digest computed by the
client does not match the value in the MAC, either the autokey used a
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Internet Draft Public-Key Cryptography for the NTP April, 2001
different cookie than the server or the packet was modified by an
intruder. Packets that fail this test are discarded without further
processing; in particular, without spending processor cycles on
expensive public-key algorithms.
The second tier of defense involves the key list, which is generated as
a repeated hash of autokeys and used in the reverse order. While any
receiver can authenticate a message by hashing to match a previous key
ID, as a practical matter an intruder cannot predict the next key ID and
thus cannot spoof a packet acceptable to the client. In addition,
tedious hashing operations provoked by replays of old packets are
suppressed because of the basic NTP protocol design. Finally, spurious
public-key computations provoked by replays of old packets with
extension fields are suppressed because of the signature timestamp
check.
The third tier of defense is represented by the Autokey protocol and
extension fields with timestamped signatures. The signatures are used to
reliably bind the autokey values to the private key of a trusted server.
Once these values are instantiated, the key list authenticates each
packet relative to its predecessors and by induction to the instantiated
autokey values.
In addition to the three-tier defense strategy, all packets are
protected by the NTP sanity checks. Since all packets carry time values,
replays of old or bogus packets can be deflected once the client has
synchronized to proventic sources. However, the NTP sanity checks are
only effective once the packet has passed all cryptographic tests. This
is why the signature timestamp is necessary to avoid expensive
calculations that might be provoked by replays. Since the signature and
verify operations have a high manufacturing cost, in all except
client/server modes the protocol design protects against a clogging
attack by signing cryptographic values only when they are created or
changed and not on request.
Specific Attacks
While the above arguments suggest that the vulnerability of the Autokey
protocols to cryptanalysis is suitably hard, the same cannot be said
about the vulnerability to a replay or clogging attack, especially when
a client is first mobilized and has not yet proventicated. In the
following discussion a clogging attack is considered a replay attack at
high speed which can clog the network and deny service to other network
users or clog the processor and deny service to other users on the same
machine. While a clogging attack can be concentrated on any function or
algorithm of the Autokey protocol, the must vulnerable target is the
public key routines to sign and verify public values. It is vital to
shield these routines from a clogging attack.
In all modes the cryptographic seed data used to generate cookies and
autokey values are changed from time to time. Thus, a determined
intruder could save old request and response packets containing these
values and replay them before or after the seed data have changed. Once
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the client has proventicated, the client will detect replays due to the
old timestamp and discard the data. This is why the timestamp test is
done first and before the signature is computed. However, before this
happens, the client is vulnerable to replays whether or not they result
in clogging.
There are two vulnerabilities exposed in the protocol design: a sign
attack where the intruder hopes to clog the victim with needless
signature computations, and a verify attack where the intruder attempts
to clog the victim with needless verification computations. The
reference implementation uses the RSA public key algorithms for both
sign and verify functions 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
agreement values are regenerated, which happens about once per day.
However, a server is vulnerable to a sign attack where the intruder can
clog the server with cookie-request messages. The protocol design
precludes server state variables stored on behalf of any client, so the
signature must be recomputed for every cookie request. Ordinarily,
cookie requests are seldom used, except when the private values are
regenerated. However, a determined intruder could replay intercepted
cookie requests at high rate, which may very well clog the server. There
appears no easy countermeasure for this particular attack.
The intruder might be more successful with a verify attack. Once the
client has proventicated, replays are detected and discarded before the
signature is verified. However, if the cookie is known or compromised,
the intruder can replace the timestamp in an old message with one in the
future and construct a packet with a MAC acceptable to a client, even if
it has bogus signature and incorrect autokey sequence. The packet passes
the MAC test, but then tricks the client to verify the signature, which
of course fails. What makes this kind of attack more serious is the fact
that the cookie used when extension fields are present is well known
(zero). Since all broadcast packets have an extension field, all the
intruder has to do is clog the clients with responses including
timestamps in the future. Assuming the intruder has joined the NTP
broadcast group, the attack could clog all other members of the group.
This attack can be deflected by the autokey test, which in the reference
implementation is after extension field processing, but this requires
very intricate protocol engineering and is left for a future refinement.
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 the special NAK packet. A naive client might
conclude the server had refreshed its private value and so attempt to
refresh the server cookie using a cookie-request message. This results
in the server and client burning spurious machine cycles and invites a
clogging attack. This is why the reference implementation simply
discards all protocol and procedure errors and waits for timeout in
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order to refresh the values. However, a more clever client may notice
that the NTP originate timestamp does not match the most recent client
packet sent, so can discard the bogus NAK immediately.
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 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 earlier in this memorandum, 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.
Present Status and Unifinished Business
The Autokey protocol described in this memorandu has been implemented in
the public software distribution for NTP Version 4 and has been tested
in machines of either endian persuasion and both 32- and 64-bit
architectures and kernels. Testing the implementation has been
complicated by the many combinations of modes and failure/recovery
mechanisms, including daemon restarts, key expiration, communication
failures and various management mistakes. The experience points up the
fact that many little gotchas that are survivable in ordinary protocol
designs become showstoppers when strong cryptographic assurance is
required.
The analysis, design and implementation of the Autokey protocol is
basically mature; however, There are several remaining implementation
issues. One has to do with cryptographic parameter negotiation, as in
IPSEC protocols such as Photuris. As with Photuris, there may be a need
to offer and agree to one of possibly several hashing algorithms,
signature algorithms and agreement algorithms. A message type has been
defined for this purpose, but its syntax and semantics remain to be
provoked.
Another issue is support for certificates and certificate authorities,
in particular Secure DNS services. In the reference implementation a
complicating factor is the existing banal state of the configuration and
resolver code. Over the years this code has sprouted to a fractal-like
state where possibly the only correct repair is a complete rewrite.
Appendix A. Packet Formats
The NTP Version 4 packet consists of a number of fields made up of 32-
bit (4 octet) words. The packet consists of three components, the
header, one or more optional extension fields and an optional message
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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 bits.
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 (64) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Originate Timestamp (64) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Receive Timestamp (64) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Transmit Timestamp (64) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
= Extension Field(s) =
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
| Message Digest (128) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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
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
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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 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. While this algorithm and mode of operation is supported in
NTP Version 4, the DES algorithm has been removed from the standard
software distribution and must be obtained via other sources. The
preferred replacement for NTP Version 4 is the Message Digest 5 (MD5)
algorithm, which is included in the distribution. The Message Digest
field is 64 bits for DES-CBC and 128 bits for MD5, while the Key ID
field is 32 bits for either algorithm.
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. Each extension field contains a request or response
message, which consists of a 16-bit length field, an 8-bit control
field, an 8-bit flags field and a variable length data field, all in
network byte order:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
= Data =
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 Version field identifies the version number of
the extension field protocol; this memorandum specifies version 1. The
Code field specifies the operation in request and response messages. The
length includes all octets in the extension field, including the length
field itself. Each extension field is rounded up to the next multiple of
4 octets and the last field rounded up to the next multiple of 8 octets.
The extension fields can occur in any order; however, in some cases
there is a preferred order which improves the protocol efficiency.
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 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 12 (3 words), a MAC (DES-CBC) is present, but no extension
field; if 20 (5 words), a MAC (MD5) is present, but no extension field;
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If the length is 8 (2 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 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 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. However, it is not
a protocol error to send an unsolicited response with no matching
request.
There are currently five request and six response messages. All request
messages except the Association ID request message have 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 | Code | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Association ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Association ID field is used to match a client request to a
particular server association. By convention, servers set the
association ID in the response and clients include the same value in
requests. Also by convention, until a client has received a response
from a server, the client sets the Association ID field to 0. 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.
The following request and response messages have been defined.
Parameter Negotiation (1)
This extension field is reserved for future use as an algorithm and
algorithm parameter offer/select exchange, as well as to provide the
optional identification value to use in lieu of endpoint IP addresses
when calculating the autokey. The format, encoding and use of these data
remain to be specified. The command code is reserved.
Association ID (2)
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A client sends the request to obtain the association ID and status
flags. A broadcast server sends an unsolicited response for all except
the first autokey sent from the key list. The request and response have
the following format (except for the response bit):
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|E| 1 | 2 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Association ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Association ID field contains the association ID of the server. The
status flags currently defined are
Bit Function
================
31 autokey is enabled
30 public and private keys have been loaded
29 agreement parameters have been loaded
28 leapseconds table has been loaded
Additional bits may be defined in future, so for now bits 0-27 should be
set to zero. There is no timestamp or signature associated with this
message.
Autokey (3)
A broadcast server or symmetric peer sends the request to obtain the
autokey values. The response 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 | 4 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Association ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initial Sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initial Key ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Signature Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
= Signature =
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Internet Draft Public-Key Cryptography for the NTP April, 2001
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The response is also sent unsolicited when the server or peer generates
a new key list. The Initial Sequence field contains the first key number
in the current key list and the Initial Key ID field contains the next
key ID associated with that number. If the server is not synchronized to
a proventicated source, the Timestamp field contains 0; otherwise, it
contains the NTP seconds when the key list was generated and signed. The
signature covers all fields from the Timestamp field through the Initial
Key ID field. If for some reason these values are unavailable or the
signing operation fails, the Initial Sequence and Initial Key ID fields
contain 0 and the extension field is truncated following the Initial Key
ID field.
Cookie (4)
A client sends the request to obtain the server cookie. The response 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 | 3 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Association ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cookie |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Signature Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
= Signature =
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Since there is no server association matching the client, the
association ID field for the request and response is 0. The Cookie field
contains the cookie used in client/server modes. If the server is not
synchronized to a proventicated source, the Timestamp field contains 0;
otherwise, it contains the NTP seconds when the cookie was computed and
signed. The signature covers the Timestamp and Cookie fields. If for
some reason the cookie value is unavailable or the signing operation
fails, the Cookie field contains 0 and the extension field is truncated
following this field.
Diffie-Hellman Parameters (5)
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A symmetric peer uses the request and response to send the public value
and signature to its peer. The response 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 | 5 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Association ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Parameters Filestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Parameters Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
= Parameters =
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Signature Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
= Signature =
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Parameters field contains the Diffie-Hellman parameters used to
compute the public and private values. The Parameters Filestamp field
contains the NTP seconds when the Diffie-Hellman parameter file was
generated. If the server is not synchronized to a proventicated source,
the Timestamp field contains 0; otherwise, it contains the NTP seconds
when the public value was generated and signed. The signature covers the
Timestamp, Parameters Length and Parameters fields. If for some reason
these values are unavailable or the signing operation fails, the
Parameters Length field contains 0 and the extension field is truncated
following this field.
Public Value (6)
A symmetric peer uses the request and response to send the public value
and signature to its peer. The response 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 | 5 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Association ID |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Filestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Public Value Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
= Public Value =
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Signature Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
= Signature =
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Public Value field contains the Diffie-Hellman public value used to
compute the agreed key.
The Filestamp field contains the NTP seconds when the Diffie-Hellman
parameter file was generated. If the server is not synchronized to a
proventicated source, the Timestamp field contains 0; otherwise, it
contains the NTP seconds when the public value was generated and signed.
The signature covers all fields from the Timestamp field through the
Public Value field. If for some reason these values are unavailable or
the signing operation fails, the Public Value Length field contains 0
and the extension field is truncated following this field.
Public Key/Host Name (7)
A client uses the request to retrieve the public key, host name and
signature. The response 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 | 7 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Public Key ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Association ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Filestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Public Key Length |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
= Public Key =
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Host Name Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
= Host Name =
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Signature Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
= Signature =
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Since the public key and host name are a property of the server and not
any particular association, the association ID field for the request and
response is 0. The Public Key field contains the RSA public key in
rsaref2.0 format; that is, the modulus length (in bits) as the first
word followed by the modulus bits. Note that in some architectures the
rsaref2.0 modulus word may be something other than 32 bits. The Host
Name field contains the host name string returned by the Unix
gethostname() library function.
The Filestamp field contains the NTP seconds when the public/private key
files were generated. If the server is not synchronized to a
proventicated source, the Timestamp field contains 0; otherwise, it
contains the NTP seconds when the public value was generated and signed.
The signature covers all fields from the Timestamp field through the
Host Name field. If for some reason these values are unavailable or the
signing operation fails, the Host Name Length field contains 0 and the
extension field is truncated following this field.
Leapseconds table (8)
The civil timescale (UTC), which is based on Earth rotation, has been
diverging from atomic time (TAI), which is based on an ensemble of
cesium oscillators, at about one second per year. Since 1972 the
International Bureau of Weights and Measures (BIPM) declares on occasion
a leap second to be inserted in the UTC timescale on the last day of
June or December. Sometimes it is necessary to correct UTC as
disseminated by NTP to agree with TAI on the current or some previous
epoch.
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A client uses the request to retrieve the leapseconds table and
signature. The response 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 | 8 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Public Key ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Association ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Filestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Leapseconds table Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
= Leapseconds table =
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Signature Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
= Signature =
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The NTP extension field format consists of a table with one entry in NTP
seconds for each leap second.
Since the leapseconds table is a property of the server and not any
particular association, the association ID field for the request and
response is 0. The Leapseconds table field contains a list of the
historic epoches that leap seconds were inserted in the UTC timescale.
Each list entry is a 32-bit word in NTP seconds, while the table is in
order from the most recent to the oldest insertion. At the first
insertion in January, 1972 UTC was ahead of TAI by 10 s and has
increased by 1 s for each insertion since then. Thus, the table length
in bytes divided by four plus nine is the current offset of UTC relative
to TAI.
The Filestamp field contains the NTP seconds when the leapseconds table
was generated at the original host, in this case one of the public time
servers operated by NIST. If the value of the filestamp is less than the
first entry on the list, the first entry is the epoch of the predicted
next leap insertion. The filestamp must always be greater than the
second entry in the list. If the server is not synchronized to a
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proventicated source, the Timestamp field contains 0; otherwise, it
contains the NTP seconds when the public value was generated and signed.
The signature covers all fields from the Timestamp field through the
Leapseconds table field. If for some reason these values are unavailable
or the signing operation fails, the Host Name Length field contains 0
and the extension field is truncated following this field.
Appendix B. Key Generation and Management
In the reference implementation the lifetimes of various cryptographic
values are carefully managed and frequently refreshed. While permanent
keys have lifetimes that expire only when manually revoked, autokeys
have a lifetime specified at the time of generation. When generating a
key list for an association, the lifetime of each autokey is set to
expire one poll interval later than it is scheduled to be used.
Ordinarily, key lists are regenerated and signed about once per hour and
private cookie values and public agreement values are refreshed and
signed about once per day. The protocol design is specially tailored to
make a smooth transition when these values are refreshed and to avoid
vulnerabilities due to clogging and replay attacks while refreshment is
in progress.
Autokey key management can be handled in much the same way as in the ssh
facility. A set of public and private keys and agreement parameters are
generated by a utility program designed for this purpose. The program
generates four files, one containing random DES/MD5 private keys, which
are not used in the Autokey protocol, a second containing the RSA
private key, a third the RSA public key, and a fourth the Diffie-Hellman
agreement parameters. In addition, the leapseconds table is generated
and stored in public time servers maintained by NIST. The means to do
this are beyond the scope of this memorandum.
All files are based on random strings seeded by the system clock at the
time of generation and are in printable ASCII format with PEM (base-64)
encoding. The name of each file includes an extension consisting of the
NTP seconds at the time of generation. This is interpreted as a key ID
in order to detect incorrect keys and to handle key changeovers in an
orderly way. In the recommended method, all files except the RSA private
key file are installed in a shared directory /usr/local/etc, which is
where the daemon looks for them by default. The private RSA key file is
installed in an unshared directory such as /etc. It is convenient to
install links from the default file names, which do not have filestamp
extensions, to the current files, which do. This way when a new
generation of keys is installed, only the links need to be changed.
When a server or client first initializes, it loads the RSA public and
private key files, which are required for continued operation. It then
attempts to load the agreement parameters file, certificate file and
leapseconds table file. If one or more of these files are present, the
associated bit is set in the system status word. Neither of these files
are necessary at this time, since the data can be retrieved later from
another server. If obtaining these data from another server is
considered a significant vulnerability, the files should be present.
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In the current management model, the keys and parameter files are
generated on each machine separately and the private key obscured. For
the most demanding applications, the public key files for a community of
users can be copied to all of those users, while one of the parameter
files can be selected and copied to all users. However, if security
considerations permit, the public key and parameter values, as well as
the certificate file and leapseconds table file, can be obtained from
other servers during operation. These data completely define the
security community and the servers configured for each client. In
broadcast client and symmetric passive modes the identity of a
particular server may not be known in advance, so the protocol obtains
and verifies the public key and host name directly from the server.
Ultimately, these procedures may be automated using public certificates
retrieved from secure directory services.
Since all files carry a filestamp incorporated in the file name, newer
file generations are detected in the data obtained from the one or more
configured servers. When detected, the newer generations replace the
older ones automatically and the newer ones made available to dependent
clients as required. Since the filestamp signatures are refreshed once
per day, which causes all associations to reset, the newer generations
will eventually overtake all older ones throughout the subnet of servers
and dependent clients.
Where security considerations permit and the public key, certificate and
agreement parameter files can be retrieved directly from the server,
these data can be easily automated. Each server and client runs a shell
script perhaps once per month. The script generates new key and
parameter files, updates the links and then restarts the daemon. The
daemon loads the necessary files and then restarts the protocol with
each of its servers, refreshing public keys and parameter files during
the process. Clients will not be able to authenticate following daemon
restart, but the protocol design is such that they will eventually time
out, restart the protocol and retrieve the latest data.
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.
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Internet Draft Public-Key Cryptography for the NTP April, 2001
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.
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
Mills Expires October, 2001 [page 44]
Internet Draft Public-Key Cryptography for the NTP April, 2001
Edited into Internet-draft form by:
Patrick Cain. Please notify pcain@genuity.com of editorial omissions or
errors.
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