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Versions: 00 01 02 03 04                                                
     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
     Force (IETF), its areas, and its working groups. Note that other groups
     may also distribute working documents as Internet-Drafts.
     
     Internet-Drafts are draft documents valid for a maximum of six months
     and may be updated, replaced, or obsoleted by other documents at any
     time. It is inappropriate to use Internet- Drafts as reference material
     or to cite them other than as "work in progress."
     
     
     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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         |                  |
         |    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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     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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     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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     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     
     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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     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
     
     
     
     
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     Edited into Internet-draft form by:
     
     Patrick Cain. Please notify pcain@genuity.com of editorial omissions or
     errors.
     
     
     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,
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     Mills                   Expires October, 2001               [page 45]