Network Working Group David L. Mills
Internet Draft University of Delaware
Document: < draft-ietf-stime-ntpauth-00.txt > June 2000
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.
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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 [12].
1. Abstract
This memorandum describes a scheme for authenticating servers to
clients for 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 timestamps are always available using authenticated means;
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 the memorandum contains a description of the
security model, approach rationale, protocol design and vulnerability
analysis. It obsoletes a previous report [10] primarily in the
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schemes for distributing public keys and related values. A detailed
description of the protocol states, events and transition functions
is included. Detailed packet formats and field descriptions are given
in the appendix. A prototype of the Autokey design based on this
document and improvements described in this memorandum has been
implemented, tested and documented in the NTP Version 4 software
distribution for Unix, Windows and VMS.
While not strictly a security function, the Autokey scheme 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.
2. Introduction
A reliable distributed network service requires provisions to prevent
accidental or malicious attacks on the servers and clients in the
network. 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. This is especially important in
such ubiquitous network services as directory services, cryptographic
key management and time synchronization.
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 [1, 4, 11] 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
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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 [10]. It is based on a combination of PKI and a pseudo-random
sequence generated by repeated hashes of a cryptographic value
involving both public and private components. This scheme has been
tested and evaluated in a local environment and 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.
3. Security Model
Over the last several years the IETF has defined and evolved the
IPSEC infrastructure for the protection of privacy and authentication
of sources in the Internet, The infrastructure includes the
Encapsulating Security Payload (ESP) [3] and Authentication Header
(AH) [2] 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 and
receiver 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
purposes that the way in which these algorithms are used precludes
encryption of any data other than incidental to the construction of
digital signatures.
There are a number of defense mechanisms already built in the NTP
architecture, protocol and algorithms. The fundamental timestamp-
exchange scheme is inherently resistant to replay attacks. The
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engineered clock filter, intersection and clustering algorithms are
designed to fend off Byzantine sources and cliques. While not
necessarily designed to defeat determined intruders, these algorithms
and accompanying eleven 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
the servers themselves. Without specific further protection, an
intruder can do any or all of the following mischiefs. Further
discussion on the assumed intruder model is given in [8], 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 if they require expensive PKI operations.
3. An intruder can intercept, modify and replay a packet. However, it
cannot permanently prevent 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 authenticated.
This circular interdependence of the timekeeping and authentication
functions requires special handling.
4. All authentication 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 webs of trust.
4. 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.
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 synchronized to an authentic
source 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.
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, multicast/manycast 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 webs of trust.
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4.1 Autokey Authentication Scheme
The 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
MD5 message digests to detect packet modification, timestamped RSA
digital signatures to verify the source, and Diffie-Hellman key
agreements to construct a private key from public values. However,
there is no reason why alternative signature schemes and agreement
algorithms could be supported. What makes the Autokey scheme unique
is the way in which these algorithms are used to deflect intruder
attacks while maintaining the integrity and accuracy of the time
synchronization function.
The NTP Version 3 symmetric-key cryptography uses keyed-MD5 message
digests with a 128-bit private key and 32-bit key ID. In order to
retain backward compatibility, the key ID space is partitioned in two
subspaces at a pivot point of 65536. Symmetric key IDs have values
less than 65536 and indefinite lifetime. Autokey keys 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, multicast/manycast 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 scheme operates separately for each association,
so there may be several key lists operating independently at the same
time and with associated values and signatures.
An autokey consists of four 32-bit fields in the network order shown
below:
+-----------+-----------+-----------+-----------+
| Source IP | Dest IP | key ID | cookie |
+-----------+-----------+-----------+-----------+
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.
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 is a public value (0), since these packets can be
validated independently using signed data in an extension field. The
four values are hashed by the message digest algorithm to produce the
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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 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. When this
happens, which can 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). In the
reference implementation the most recent key ID received is saved for
comparison with the first 32 bits of the next following key value.
This minimizes the number of hash operations in case a packet is
lost.
In client/server mode 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, where 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 addresses reversed. 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 multicast/manycast 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
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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 multicast
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.
The server authentic bit is set only when the cookie or autokey
values, depending on mode, and signature are both valid. If the bit
is set, the client sends valid timestamps in signed responses. If the
bit is not set, the data and signature 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 authentic bit is set, the NTP time values are processed, so
that eventually the client will synchronize to an authentic source.
In client/server and symmetric modes, packets are normally sent
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 data and time values
for all associations are cleared and the synchronization and
authentication procedures start over from scratch. This insures that
old cryptographic and synchronization values never propagate beyond a
clock reset.
4.2 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.
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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 the source is
authentic 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:
1. The public agreement 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. For
convenience, the public key/host name, agreement parameters and leap
table signatures are recomputed at the same time as the public value
signature and carries 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 request
message. On request, the cookie, signature and timestamp are returned
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 three signature types is
saved for comparison. Once a signature with valid timestamp has been
received, packets carrying extension fields with invalid timestamps
or older valid timestamps of the same type are discarded before any
values are used or signatures 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.
While files carrying cryptographic data not specifically signed, the
file names have timestamp extensions which reliably determine the
time of generation. As the data are forwarded from machine to
machine, the filestamps are preserved. This 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.
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In order to further reduce the window of opportunity, even for a
fanatical intruder, additional causality constraints can be checked.
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 multicast 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.
5. Timestamps for each cryptographic data type must be later than the
filestamps for that type.
In the above constraints, note the public key timestamp and signature
timestamps have a granularity of one second, so that a timestamp
difference of zero seconds is ambiguous. Furthermore, timestamps 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 operates with polling intervals much longer
than one second, so that successive timestamps for the same data type
can never by ambiguous.
4.3 Filestamps
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. For these
reasons, the name of every cryptographic value file includes a
filestamp consisting of the decimal NTP seconds at the time of
generation.
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 parameter file and constructs the
public and private values to be used later in the agreement protocol.
Also optionally, it reads the leap table file. When loading these
files it checks the filestamps for consistency and validity. When the
client is eventually synchronized to an authentic source, it checks
the filestamps for validity relative to the system clock time. If the
filestamps are later than the clock time, something is seriously
wrong and either the time has synchronized in error or the data files
are defective.
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When a client mobilizes an association, it retrieves the server host
name and public key from the server using the Autokey protocol.
Optionally, it retrieves the agreement parameters and leap table, if
required, and constructs the public and private values. Optionally,
and before going further, it verifies the server credentials using
certificate authorities and a web of trust. As above, when the client
is eventually synchronized to an authentic source, it again checks
the filestamps for validity.
4.4 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 key or agreement parameter files, corrupted
data, protocol state mismatches and packet loss or misorder, not to
mention hostile intrusion. There are two mechanisms which maintain
the liveness state of the protocol, the reachability register defined
in RFC-1305 and the watchdog timer, which is new in NTP Version 4.
The reachability register is an 8-bit register that shifts left with
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. Thus, the pattern of ones and zeros in this register
reveals the reachability status of the server for the last eight poll
intervals.
With respect to the issues at hand, if this register is nonzero, 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 server 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 cryptographically
authenticated packet, 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.
The general approach to Autokey error recovery is to retry the
request message at intervals of about one minute until the watchdog
timer expires and then restart the protocol from the beginning. At
each state in the protocol the client expects a particular variable
to be received from the server. A NTP packet including the
appropriate request is sent at every poll interval until the variable
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is received or a general reset occurs. While this behavior might be
considered rather conservative, the advantage is that old
cryptographic values can never persist from one mobilization to the
next.
There are a number of situations where some action 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 is generated when the next NTP message is sent,
assuming there is one. Following is a list of these situations.
1. When a client switches from client/server mode to multicast client
mode. There is no further need for the key list, since the client
will not transmit again.
2. When the poll interval is changed in an association. In this case
the calculated expiration times for the keys become invalid.
3. When a general reset is performed in an association.
4. If a problem is detected when an entry is fetched from the key
list for an association. This could happen if the key was marked non-
trusted or timed out, either of which implies a software bug.
5. When the cryptographic values are refreshed, the key lists for all
associations are regenerated.
6. When the client is first synchronized to an authentic source or
the system clock is stepped, the key lists for all associations are
regenerated.
5 Autokey Protocols
This section describes the Autokey protocols supporting
cryptographically secure server and peer authentication. There are
three protocols corresponding to the NTP client/server,
multicast/manycast and symmetric active/passive modes.
In the descriptions below, it is assumed that the client has the
public key and agreement parameters, where required, for the server.
These data can be loaded from local files or automatically retrieved
from the server as described later in this memorandum. Further
information on generating and managing these files is in Appendix B.
The Autokey protocol data unit is the extension field, which 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,
which is set valid (nonzero) only if the sender is synchronized to an
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authentic source. An extension fields are discarded before the
signature is verified if a signature timestamp is the same as or
earlier than the last received timestamp of the same type.
An extension field is also discarded if a protocol or procedure error
occurs or the required cryptographic data are incomplete or
unavailable or the field values are inconsistent with these data.
Otherwise and in general, a response is generated for every request,
even if the requestor is not synchronized to an authentic source.
However, some responses may have truncated data fields under certain
conditions, although the signatures are always present and
verifiable.
5.1 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 cookie is hashed from the IP addresses
and private value with the key ID field set to zero, as described
previously. 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.
The following diagram shows the protocol dance in client/server mode.
In this and following diagrams the NTP packet type is shown above the
arrow and the extension field(s) message type shown below. There are
three cryptographic values instantiated by the dance: the cookie,
signature timestamp and authentic bit.
server client
| |
| NTP client |
1 |<-----------------| mobilize association; generate key list;
| cookie req | DNS lookup for canonical name, certificate
| | and public key
| NTP server |
2 |----------------->| store cookie; verify server credentials
| cookie rsp |
| ... |
| |
| NTP client |
3 |<-----------------|
| |
| NTP server |
4 |----------------->|
| |
| continue |
= client/server =
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The dance begins when the client on the right sends a packet (1)
including a cookie request to the server on the left. The server
immediately responds with the cookie, signature and timestamp. Upon
arrival of this packet (2), the client checks the timestamp, verifies
the signature and, if successful, initializes the cookie and
signature timestamp and sets the authentic bit. The client will
retransmit packet (1) until receiving a valid timestamp and verified
signature (2) or until 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 the packet
(3) and, following the original NTP protocol, responds with a NAK
packet (4), which the client ignores. Eventually, a general reset
occurs and the dance restarts from the beginning.
5.2 Multicast/Manycast Mode (5)
In multicast mode, packets are always sent with an extension field.
Since the autokey values for these packets use a well known 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 multicast server
keeps no state variables specific to each of possibly very many
clients and mobilizes no associations for them. 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 multicast mode.
There are four cryptographic values instantiated by the dance: the
signature timestamp, cookie, autokey values and authentic bit.
server client
| |
| NTP multicast |
1 |----------------->| mobilize association; generate key list;
| assoc ID rsp | reverse DNS lookup for canonical name,
| ... | certificate and public key
| |
| NTP client |
2 |<-----------------|
| cookie req |
| |
| NTP server |
3 |----------------->| store cookie; verify server credentials
| cookie rsp |
| ... |
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| |
| NTP client |
4 |<-----------------|
| autokey req |
| |
| NTP server |
5 |----------------->| store autokey
| autokey rsp |
| ... |
| |
| NTP client |
|<-----------------|
| |
| NTP server |
|----------------->|
| |
| continue |
= volley =
| |
| NTP client |
|<-----------------|
| |
| NTP server |
|----------------->| initialize delay estimate; discard cookie
| | and remaining keys; switch to multicast
| | client mode
| continue |
= multicast =
| |
| NTP multicast |
|----------------->| server rolls new key list; client refreshes
| autokey rsp | autokey
| signature |
= =
The server sends multicast packets continuously at intervals of about
one minute (1) using the key list and regenerating the list as
required. The first packet sent after regenerating the list includes
an extension field containing the autokey values and signature; other
packets include an extension field containing only the association
ID.
Upon arrival of the first packet (1), the multicast client mobilizes
an association and loads the canonical name and public key as
described above. Alternately, it queries the DNS and loads the
canonical name, certificate and server public key.
Some time later the client generates a key list and sends a packet
(2) requesting the cookie as in client/server mode. The server
immediately responds (3) with the cookie, signature and timestamp.
The client checks the timestamp, verifies the signature and, if
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successful, initializes the cookie and signature timestamp. The
client retransmits packet (2) until receiving a valid timestamp and
verified signature (3) or until a general reset occurs.
If an autokey response happens to be in one of the server packets (1,
3), the client can switch to multicast client mode and send no
further packets. Otherwise, some time later the client sends a packet
(4) requesting the autokey values. The server immediately responds
(5) with the values. The client checks the timestamp, verifies the
signature and, if successful, initializes the autokey values and
signature timestamp and sets the authentic bit. The client
retransmits packet (4) until receiving a valid timestamp and verified
signature (5) or until a general reset occurs.
After successful verification, there is no further need for extension
fields, unless the server regenerates the cookie or the server
regenerates the key list and the Autokey response message happens to
be lost. When this happens, the server fails to authenticate the
packets (1). Eventually, a general reset occurs and the dance
restarts from the beginning. 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 have been accumulated, the
client discards the cookie and remaining keys on the key list,
switches to multicast client mode and sets the clock.
5.3 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. The particular choice of 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.
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 host name and public key can be configured in advance.
The other scenario is when one peer operates with a configured
association and begins operation with another peer without a
configured association and begins operation in symmetric-passive
mode.
The following diagram shows the protocol dance in symmetric-
active/passive mode. The exchange is similar in the symmetric-
active/active mode, although the order can change depending on which
peer starts the dance. There are four cryptographic values
instantiated by the dance: the signature timestamp, cookie, autokey
values and authentic bit.
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active passive
| |
| NTP active |
1 |----------------->| mobilize association; query DNS for
| public rsp | canonical name, certificate and public key;
| | verify public signature, compute and
| public req | initialize agreed key
| ... |
| |
| NTP passive |
2 |<-----------------|
| public rsp |
| autokey req |
| ... |
| |
| NTP active |
3 |----------------->| verify autokey signature, initialize
| autokey rsp | autokey key; sign public values
| autokey req |
| ... |
| |
| NTP passive |
4 |<-----------------|
| autokey rsp |
| ... |
| |
| NTP active |
|----------------->| regular operation
| ... |
| |
| NTP passive |
|<-----------------|
| |
| continue |
= active/passive =
The dance begins when the active peer on the left in the diagram
sends a packet (1) to the passive peer on the right. Before sending
the first packet, the active peer generates a key list using the
default key (zero) and initializes the autokey values and signature
along with the public agreement value and signature.
The first packet from the active peer includes its public value and
signature along with a request for the public value of the passive
peer. Upon arrival of this packet, the passive peer mobilizes an
association and loads the canonical name and public key as described
above. Alternately, it queries the DNS and loads the canonical name,
certificate and public key of the active peer. The passive peer
checks the timestamp, verifies the signature and, if successful,
executes the agreements algorithm and initializes the cookie and
signature timestamp. As the cookie affects the autokey values, the
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key list is regenerated with the cookie. The active peer retransmits
packet (1) until receiving a valid public value (2) or until a
general reset occurs.
Some time later the passive peer sends a packet (2) to the active
peer including its public value and signature along with a request
for the autokey values of the active peer. Upon arrival of this
packet, the active peer checks the timestamp, verifies the signature
and, if successful, executes the agreements algorithm and initializes
the cookie and signature timestamp. As the cookie affects the autokey
values, the key list is regenerated with the cookie. The passive peer
retransmits packet (2) until receiving a valid autokey values (3) or
until a general reset occurs.
Some time later the active peer sends a packet (3) to the passive
peer including its autokey values and signature along with a request
for the autokey values of the passive peer. Upon arrival of this
packet, the passive peer checks the timestamp, verifies the signature
and, if successful, initializes the autokey values and sets its
authentic bit. The active peer retransmits packet (3) until receiving
a valid autokey values (4) or until a general reset occurs.
Some time later the passive peer sends a packet (4) to the active
peer including its autokey values and signature. Upon arrival of this
packet, the active peer checks the timestamp, verifies the signature
and, if successful, initializes the autokey values and sets the
authentic bit.
After successful verification, 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. In any
case, the request will be retransmitted at intervals until a general
reset occurs.
6. Additional Protocols
While not mentioned in the above protocol descriptions, there are
provisions to negotiate the algorithms and algorithm parameters,
retrieve the public key and host name, and retrieve the agreement
parameters and ancillary data using the defined requests summarized
in Appendix A. Ordinarily, a client or peer requests the public key
and host name in the first message from an association to a server or
peer. The response includes the filestamp and is signed by the server
using its private key. The signature and filestamp is useful to
confirm the correct key generation and to verify correct procedure.
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Each association requiring public key authentication cannot proceed
until the response has been received.
The NIST provides a table showing the epoch for all occasions of leap
second insertions since 1972. The table is maintained in a file
called pub/leap-seconds and available for anonymous FTP download.
While not strictly a security function, the table can be retrieved
from an NTP server using the Autokey protocol if the feature is
enabled. If enabled and the leap table is not available, a request is
included in the next Autokey message. The response includes the
original filestamp generated by the NIST and is signed and
timestamped. Note that the table will be requested by all
associations, either configured or not; but, none of the associations
can proceed until one of them has received the response. After this,
the table can be provided on request to other clients and servers.
If any associations are operating in symmetric modes, the agreement
parameters are required to complete the protocol. If the parameters
are needed and not currently available, they are requested in the
next message. The response includes the original filestamp and is
signed as before. Note that the parameters will be requested by all
associations needing them, either configured or not; but, none of the
associations can proceed until one of them has received the response.
After this, the parameters can be provided on request to other
clients and servers.
7 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 parameters is unlikely. However, vulnerabilities
remain in the way the actual cryptographic data, including the cookie
and autokey values, are computed and used.
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 successfulcryptanalysis of these values compromises only a
particular client/server association and does not represent a danger
to the general population.
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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 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 authentic 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.
7.1 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
synchronized to an authentic source. 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
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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 the client has synchronized to an authentic source, 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 synchronized to an authentic source, 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 multicast 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
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multicast 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 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 multicast 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.
8 Present Status
The Autokey scheme has been implemented in the public software
distribution for NTP Version 4 and has been tested in all machines of
either endian persuasion and both 32- and 64-bit architectures.
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.
9 Future Plans
The analysis, design and implementation of the Autokey scheme is
basically mature; however, There are two remaining implementation
issues. One has to do with the Unix sockets semantics used for
multicast. The problem is how to set the source address when more
than one interface is present. Since the Autokey scheme hashes the IP
addresses, as well as the NTP header, it is necessary that the
correct address be known before the hash can be computed. In the
present implementation the address is not known until the first
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packet arrives, which considerably complicates the protocol. Probably
nothing short of a complete rewrite of the I/O code will fix this.
The other issue is support for Secure DNS services, especially the
retrieval of public certificates. A complicating factor is the
existing banal state of the configuration and resolver code in the
NTP daemon. 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
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) =
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| 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 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
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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; 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 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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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.
Public Key (1)
This extension field is reserved for future use as an algorithm and
algorithm parameter offer/select exchange. The command code is
reserved.
Association ID (2)
This message is sent by a multicast server as an unsolicited response
only; there is no corresponding request message of this type. 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 | 2 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Association ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Association ID field contains the association ID of the server.
This response is included in every packet sent by a multicast server,
except when a new key list is generated. There is no timestamp or
signature associated with this message.
Cookie (3)
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 |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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 an authenticated 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.
Autokey (4)
A multicast 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 =
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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
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is not synchronized to an authenticated 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.
Diffie-Hellman Parameters (5)
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 an authenticated
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
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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 an
authenticated 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)
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
= 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.
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The Filestamp field contains the NTP seconds when the public/private
key files were generated. If the server is not synchronized to an
authenticated 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.
TAI Leap Second 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.
A client uses the request to retrieve the leap second 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Leap Second Table Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
= Leap Second Table =
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Signature Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
= Signature =
| |
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| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The NTP extension field format consists of a table with one entry in
NTP seconds for each leap second insertion and in the order from the
most recent insertion to the first. Since UTC led TAI by ten seconds
at the first insertion and each insertion since then adds one second,
the current UTC-TAI offset is simply the sum of these values.
Since the leap second table is a property of the server and not any
particular association, the association ID field for the request and
response is 0. The Leap Second 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 leap second
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 an authenticated 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
Leap Second 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.
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In the reference implementation, Autokey key management is 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. From these data the program generates four
files, one containing random DES/MD5 private keys, which are not used
in the Autokey scheme, another containing the RSA private key, a
third the RSA public key, and a fourth the agreement parameters. In
addition, the leap second 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 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 stored in shared directory
/usr/local/etc, which is where the daemon looks for them by default.
The private RSA key file should be stored 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 parameter file and, if enabled by
a configuration file bit, it attempts to load the leap second table
file. 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.
In the current management model, the keys and parameter files are
generated on each machine separately and the private key obscured.
The set of 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 leap second table
can be obtained from other servers during operation. These data
completely define the security community and the servers configured
for each client. In multicast 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 will be automated using
public certificates retrieved from secure directory services.
Where security considerations permit and the public key and parameter
data can be retrieved directly from the server, key and parameter
refreshment can be easily automated. Each server and client runs a
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shell script perhaps once per month to generate new key and parameter
files, update the links and then restarts the daemon. The daemon will
load the necessary files and then restart the protocol with each of
its servers or peers, refreshing public keys and parameter files
during the process.
Clients of the daemon will not be able to authenticate following
daemon restart, of course, but the protocol design is such that they
will eventually time out, restart the protocol and retrieve the
latest data.
The parameters and leap second table files are a special case, since
the expectation is that all servers and clients in the network have
the same versions. Therefore, the scripts should provide for
automatic, secure transfer of these files to all the lowest-stratum
servers in the security compartment.
Unlike ssh, 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 are required for each interface address with
a distinct name. In the NTP design the canonical host name, as
returned by the gethostname() library function, represents all
interface addresses. Since at least in some host configurations the
canonical 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.
10 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. Karn, P., and W. Simpson. Photuris: session-key management
protocol. Request for Comments RFC-2522, Internet Engineering Task
Force, March 1999.
2. Kent, S., R. Atkinson. IP Authentication Header. Request for
Comments RFC-2402, Internet Engineering Task Force, November 1998.
3. Kent, S., and R. Atkinson. IP Encapsulating security payload
(ESP). Request for Comments RFC-2406, Internet Engineering Task
Force, November 1998.
4. 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.
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5. 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.
6. 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.
7. 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.
8. 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.
9. 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.
10. Mills, D.L. Public key cryptography for the Network Time
Protocol. Electrical Engineering Report 00-5-1, University of
Delaware, May 2000. 23 pp.
11. Orman, H. The OAKLEY key determination protocol. Request for
Comments RFC-2412, Internet Engineering Task Force, November 1998.
12. Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997
11. 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
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Full Copyright Statement
"Copyright (C) The Internet Society (date). All Rights Reserved. This
document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implmentation may be prepared, copied, published and
distributed, in whole or in part, without restriction of any kind,
provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
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the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into.
Mills Expire December, 2000 [page 36]
