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Versions: 00 01 02 03 04 05 06 07 08 RFC 5906

Network Working Group                                   B. Haberman, Ed.
Internet-Draft                                                   JHU/APL
Obsoletes: RFC 1305                                             D. Mills
(if approved)                                                U. Delaware
Intended status: Informational                        September 25, 2007
Expires: March 28, 2008


         Network Time Protocol Version 4 Autokey Specification
                       draft-ietf-ntp-autokey-00

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

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   This Internet-Draft will expire on March 28, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This memo describes the Autokey security model for authenticating
   servers to clients using the Network Time Protocol (NTP) and public
   key cryptography.  Its design is based on the premise that IPSEC
   schemes cannot be adopted intact, since that would preclude stateless
   servers and severely compromise timekeeping accuracy.  In addition,
   PKI schemes presume authenticated time values are always available to



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   enforce certificate lifetimes; however, cryptographically verified
   timestamps require interaction between the timekeeping and
   authentication functions.

   This memo includes the Autokey requirements analysis, design
   principles and protocol specification.  A detailed description of the
   protocol states, events and transition functions is included.  A
   prototype of the Autokey design based on this report has been
   implemented, tested and documented in the NTP Version 4 (NTPv4)
   software distribution for Unix, Windows and VMS at
   http://www.ntp.org.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Requirements Notation  . . . . . . . . . . . . . . . . . .  4
   2.  NTP Security Model . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Approach . . . . . . . . . . . . . . . . . . . . . . . . . . .  8
   4.  Autokey Cryptography . . . . . . . . . . . . . . . . . . . . .  9
   5.  Secure Groups  . . . . . . . . . . . . . . . . . . . . . . . . 12
   6.  Identity Schemes . . . . . . . . . . . . . . . . . . . . . . . 18
   7.  Autokey Operations . . . . . . . . . . . . . . . . . . . . . . 20
   8.  Public Key Signatures and Timestamps . . . . . . . . . . . . . 22
   9.  Autokey Protocol Overview  . . . . . . . . . . . . . . . . . . 24
   10. Autokey State Machine  . . . . . . . . . . . . . . . . . . . . 26
     10.1. Status Word  . . . . . . . . . . . . . . . . . . . . . . . 26
     10.2. Host State Variables . . . . . . . . . . . . . . . . . . . 28
     10.3. Client State Variables (all modes) . . . . . . . . . . . . 30
     10.4. Server State Variables (broadcast and symmetric modes) . . 31
     10.5. Autokey Protocol Messages  . . . . . . . . . . . . . . . . 31
       10.5.1.  No-Operation  . . . . . . . . . . . . . . . . . . . . 33
       10.5.2.  Association Message (ASSOC) . . . . . . . . . . . . . 33
       10.5.3.  Certificate Message (CERT)  . . . . . . . . . . . . . 34
       10.5.4.  Cookie Message (COOKIE) . . . . . . . . . . . . . . . 34
       10.5.5.  Autokey Message (AUTO)  . . . . . . . . . . . . . . . 34
       10.5.6.  Leapseconds Table Message (LEAP)  . . . . . . . . . . 35
       10.5.7.  Sign Message (SIGN) . . . . . . . . . . . . . . . . . 35
       10.5.8.  Identity Messages (IFF, GQ, MV) . . . . . . . . . . . 35
     10.6. Protocol State Transitions . . . . . . . . . . . . . . . . 35
       10.6.1.  Server Dance  . . . . . . . . . . . . . . . . . . . . 36
       10.6.2.  Broadcast Dance . . . . . . . . . . . . . . . . . . . 37
       10.6.3.  Symmetric Dance . . . . . . . . . . . . . . . . . . . 38
   11. Error Recovery . . . . . . . . . . . . . . . . . . . . . . . . 40
   12. Security Considerations  . . . . . . . . . . . . . . . . . . . 42
     12.1. Protocol Vulnerability . . . . . . . . . . . . . . . . . . 42
     12.2. Clogging Vulnerability . . . . . . . . . . . . . . . . . . 44
   13. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 45



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   14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 45
   15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 46
     15.1. Normative References . . . . . . . . . . . . . . . . . . . 46
     15.2. Informative References . . . . . . . . . . . . . . . . . . 46
   Appendix A.  Cryptographic Key and Certificate Management  . . . . 47
   Appendix B.  Autokey Error Checking  . . . . . . . . . . . . . . . 48
     B.1.  Packet Processing Rules  . . . . . . . . . . . . . . . . . 49
     B.2.  Timestamps, Filestamps and Partial Ordering  . . . . . . . 51
   Appendix C.  Certificates  . . . . . . . . . . . . . . . . . . . . 52
   Appendix D.  Identity Schemes  . . . . . . . . . . . . . . . . . . 53
     D.1.  Private Certificate (PC) Scheme  . . . . . . . . . . . . . 53
     D.2.  Trusted Certificate (TC) Scheme  . . . . . . . . . . . . . 54
     D.3.  Schnorr (IFF) Scheme . . . . . . . . . . . . . . . . . . . 56
     D.4.  Guillard-Quisquater (GQ) . . . . . . . . . . . . . . . . . 58
     D.5.  Mu-Varadharajan (MV) Identity Scheme . . . . . . . . . . . 60
     D.6.  Interoperability Issues  . . . . . . . . . . . . . . . . . 64
   Appendix E.  ASN.1 Encoding Rules  . . . . . . . . . . . . . . . . 66
     E.1.  COOKIE request, IFF response, GQ response, MV response . . 66
     E.2.  CERT response, SIGN request and response . . . . . . . . . 67
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 68
   Intellectual Property and Copyright Statements . . . . . . . . . . 70






























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1.  Introduction

   A distributed network service requires reliable, ubiquitous and
   survivable provisions to prevent accidental or malicious attacks on
   the servers and clients in the network or the values they exchange.
   Reliability requires that clients can determine that received packets
   are authentic; that is, were actually sent by the intended server and
   not manufactured or modified by an intruder.  Ubiquity requires that
   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.

   This memo describes a cryptographically sound and efficient
   methodology for use in the Network Time Protocol (NTP) [1].  The
   various key agreement schemes [2][3][4] proposed require per-
   association state variables, which contradicts the principles of the
   remote procedure call (RPC) paradigm in which servers keep no state
   for a possibly large client population.  An evaluation of the PKI
   model and algorithms as implemented in the OpenSSL library leads to
   the conclusion that any scheme requiring every NTP packet to carry a
   PKI digital signature would result in unacceptably poor timekeeping
   performance.  The Autokey protocol is based on a combination of PKI
   and a pseudo-random sequence generated by repeated hashes of a
   cryptographic value involving both public and private components.
   This scheme has been implemented, tested and widely deployed in the
   Internet of today.  A detailed description of the security model,
   design principles and implementation experience is presented in this
   report.

1.1.  Requirements Notation

   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 [5].


2.  NTP Security Model

   NTP security requirements are even more stringent than most other
   distributed services.  First, the operation of the authentication
   mechanism and the time synchronization mechanism are inextricably
   intertwined.  Reliable time synchronization requires cryptographic
   media which are valid only over designated time intervals; but, time



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   intervals can be enforced only when participating servers and clients
   are reliably synchronized to UTC.  In addition, the NTP subnet is
   hierarchical by nature, so time and trust flow from the primary
   servers at the root through secondary servers to the clients at the
   leaves.

   A client can claim authentic to dependent applications only if all
   servers on the path to the primary servers are bone-fide authentic.
   In order to emphasize this requirement, in this report 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 (authenticates by induction) the lowest
   stratum (primary) servers.  Serious computer linguists would
   correctly interpret the proventic relation as the transitive closure
   of the authentic relation.

   It is important to note that the notion of proventic does not
   necessarily imply the time is correct.  A NTP client mobilizes a
   number of concurrent associations with different servers and uses
   crafted mitigation algorithm to pluck truechimers from the population
   possibly including falsetickers.  A particular association is
   proventic if the server certificate and identity have been verified
   by the means described in this report.  However, the statement "the
   client is synchronized to proventic sources" means that the system
   clock has been set using the time values of one or more proventic
   associations and according to the NTP mitigation algorithms.  While a
   certificate authority (CA) must satisfy this requirement when signing
   a certificate request, the certificate itself can be stored in public
   directories and retrieved over unsecured network paths.

   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) [6] and Authentication Header (AH) [7] 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 recipient, remanufactured in
   various ways and replayed in whole or part.  These packets can cause
   the client to believe or produce incorrect information, cause
   protocol operations to fail, interrupt network service or consume
   precious network and processor resources.




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   In the case of NTP, the assumed goal of the intruder is to inject
   false time values, disrupt the protocol or clog the network, servers
   or clients with spurious packets that exhaust resources and deny
   service to legitimate applications.  The mission of the algorithms
   and protocols described in this report is to detect and discard
   spurious packets sent by other than the intended sender or sent by
   the intended sender, but modified or replayed by an intruder.  The
   cryptographic means of the reference implementation are based on the
   OpenSSL cryptographic software library available at www.openssl.org,
   but other libraries with equivalent functionality could be used as
   well.  It is important for distribution and export purposes that the
   way in which these algorithms are used precludes encryption of any
   data other than incidental to the construction of digital signatures.

   There are a number of defense mechanisms already built in the NTP
   architecture, protocol and algorithms.  The fundamental timestamp
   exchange scheme is inherently resistant to spoofing and replay
   attacks.  The engineered clock filter, selection and clustering
   algorithms are designed to defend against evil cliques of Byzantine
   traitors.  While not necessarily designed to defeat determined
   intruders, these algorithms and accompanying sanity checks have
   functioned well over the years to deflect improperly operating but
   presumably friendly scenarios.  However, these mechanisms do not
   securely identify and authenticate servers to clients.  Without
   specific further protection, an intruder can inject any or all of the
   following attacks.

   1.  An intruder can intercept and archive packets forever, as well as
       all the public values ever generated and transmitted over the
       net.

   2.  An intruder can generate packets faster than the server, network
       or client can process them, especially if they require expensive
       cryptographic computations.

   3.  In a wiretap attack the intruder can intercept, modify and replay
       a packet.  However, it cannot permanently prevent onward
       transmission of the original packet; that is, it cannot break the
       wire, only tell lies and congest it.  Except in unlikely cases
       considered in Section 12, the modified packet cannot arrive at
       the victim before the original packet, nor does it have the
       server private keys or identity parameters.

   4.  In a middleman or masquerade attack the intruder is positioned
       between the server and client, so it can intercept, modify and
       replay a packet and prevent onward transmission of the original
       packet.  Except in unlikely cases considered in Section 12, the
       middleman does not have the server private keys or identity



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       parameters.

   The NTP security model assumes the following possible limitations.

   1.  The running times for public key algorithms are relatively long
       and highly variable.  In general, the performance of the time
       synchronization function is badly degraded if these algorithms
       must be used for every NTP packet.

   2.  In some modes of operation it is not feasible for a server to
       retain state variables for every client.  It is however feasible
       to efficiently 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 with the single exception of encrypted
       signatures and cookies intended only to authenticate the source.
       Private values must never be disclosed beyond the machine on
       which they were created except in the case of a special trusted
       agent (TA) assigned for this purpose.

   5.  Public encryption keys and certificates must be retrievable
       directly from servers without requiring secured channels;
       however, the fundamental security of identification credentials
       and public values bound to those credentials must be a function
       of certificate authorities and/or webs of trust.

   6.  Error checking must be at the enhanced paranoid level, as network
       terrorists may be able to craft errored packets that consume
       excessive cycles with needless result.  While this report
       includes an informal vulnerability analysis and error protection
       paradigm, a formal model based on communicating finite-state
       machine analysis remains to be developed.

   Unlike the Secure Shell security model, where the client must be
   securely authenticated to the server, in NTP the server must be
   securely authenticated to the client.  In ssh each different
   interface address can be bound to a different name, as returned by a
   reverse-DNS query.  In this design separate public/private key pairs
   may be required for each interface address with a distinct name.  A
   perceived advantage of this design is that the security compartment
   can be different for each interface.  This allows a firewall, for



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   instance, to require some interfaces to authenticate the client and
   others not.

   However, the NTP security model specifically assumes that access
   control is performed by means external to the protocol and that all
   time values and cryptographic values are public, so there is no need
   to associate each interface with different cryptographic values.  To
   do so would create the possibility of a two-faced clock, which is
   ordinarily considered a Byzantine hazard.  In other words, there is
   one set of private secrets for the host, not one for each interface.
   In the NTP design the host name, by default the string returned by
   the Unix 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.


3.  Approach

   The Autokey protocol described in this report is designed to meet the
   following objectives.

   1.  It must interoperate with the existing NTP architecture model and
       protocol design.  In particular, it must support the symmetric
       key scheme described in [8].

   2.  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.

   3.  It must be resistant to cryptographic attacks, specifically those
       identified in the security model above.  In particular, it must
       be tolerant of operational or implementation variances, such as
       packet loss or misorder, or suboptimal configurations.

   4.  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 cookie encryption operations.

   5.  It must function in all the modes supported by NTP, including
       server, symmetric and broadcast modes.

   6.  It must not require intricate per-client or per-server
       configuration other than the availability of the required
       cryptographic keys and certificates.



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4.  Autokey Cryptography

   Autokey public key cryptography is based on the PKI algorithms
   commonly used in the Secure Shell and Secure Sockets Layer
   applications.  As in these applications Autokey uses keyed message
   digests to detect packet modification, digital signatures to verify
   the source and public key algorithms to encrypt cookies.  What makes
   Autokey cryptography unique is the way in which these algorithms are
   used to deflect intruder attacks while maintaining the integrity and
   accuracy of the time synchronization function.

   NTPv3 and NTPv4 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 with NTPv3, the NTPv4 key ID space is
   partitioned in two subspaces at a pivot point of 65536.  Symmetric
   key IDs have values less than the pivot and indefinite lifetime.
   Autokey key IDs have pseudo-random values equal to or greater than
   the pivot and are expunged immediately after use.  Both symmetric key
   and public key cryptography authenticate as shown in Figure 1.  The
   server looks up the key associated with the key ID and calculates the
   message digest from the NTP header and extension fields together with
   the key value.  The key ID and digest form the message authentication
   code (MAC) included with the message.  The client does the same
   computation using its local copy of the key and compares the result
   with the digest in the MAC.  If the values agree, the message is
   assumed authentic.

                +------------------+
                | NTP Header and   |
                | Extension Fields |
                +------------------+   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                      |       |        |    Message Authenticator Code |
                     \|/     \|/       +              (MAC)            +
                ********************   | +-------------------------+   |
                *   Compute Hash   *<----| Key ID | Message Digest |   +
                ********************   | +-------------------------+   |
                          |            +-+-+-+-+-+-+-|-+-+-+-+-+-+-+-+-+
                         \|/                        \|/
                +------------------+       +-------------+
                |  Message Digest  |------>|   Compare   |
                +------------------+       +-------------+

                     Figure 1: Message Authentication

   There are three Autokey protocol variants corresponding to each of
   the three NTP modes: server, symmetric and broadcast.  All three
   variants make use of specially contrived session keys, called
   autokeys, and a precomputed pseudo-random sequence of autokeys which



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   are saved along with the key IDs in a key list.  As in the original
   NTPv3 authentication scheme, the Autokey protocol operates separately
   for each association, so there may be several autokey sequences
   operating independently at the same time.

                   +-------------+-------------+--------+--------+
                   | Src Address | Dst Address | Key ID | Cookie |
                   +-------------+-------------+--------+--------+

                          Figure 2: NTPv4 Autokey

   An autokey is computed from four fields in network byte order as
   shown in Figure 2.  The four values are hashed by the MD5 message
   digest algorithm to produce the 128-bit autokey value, which in the
   reference implementation is stored along with the key ID in a cache
   used for symmetric keys as well as autokeys.  Keys are retrieved from
   the cache by key ID using hash tables and a fast lookup algorithm.

   For use with IPv4, the Source Address and Dest Address fields contain
   32 bits; for use with IPv6, these fields contain 128 bits.  In either
   case the Key ID and Cookie fields contain 32 bits.  Thus, an IPv4
   autokey has four 32-bit words, while an IPv6 autokey has ten 32-bit
   words.  The source and destination addresses and key ID are public
   values visible in the packet, while the cookie can be a public value
   or private value, depending on the mode.

   The NTP packet format has been augmented to include one or more
   extension fields piggybacked between the original NTP header and the
   MAC at the end of the packet.  For packets without extension fields,
   the cookie is a shared private value conveyed in encrypted form.  For
   packets with extension fields, the cookie has a default public value
   of zero, since these packets can be validated independently using
   digital signatures.

   There are some scenarios where the use of endpoint IP addresses may
   be difficult or impossible.  These include configurations where
   network address translation (NAT) devices are in use or when
   addresses are changed during an association lifetime due to mobility
   constraints.  For Autokey, the only restriction is that the address
   fields visible in the transmitted packet must be the same as those
   used to construct the autokey sequence and key list and that these
   fields be the same as those visible in the received packet.

   For scenarios where the endpoint IP addresses are not available, an
   optional public identification value could be used instead of the
   addresses.  Examples include the Interplanetary Internet, where
   bundles are identified by name rather than address.  Specific
   provisions are for further study.



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+-----------+-----------+------+------+   +---------+  +-----+------+
|Src Address|Dst Address|Key ID|Cookie|-->|         |  |Final|Final |
+-----------+-----------+------+------+   | Session |  |Index|Key ID|
     |           |         |        |     | Key ID  |  +-----+------+
    \|/         \|/       \|/      \|/    |  List   |     |       |
   *************************************  +---------+    \|/     \|/
   *          COMPUTE HASH             *             *******************
   *************************************             *COMPUTE SIGNATURE*
     |                    Index n                    *******************
    \|/                                                       |
   +--------+                                                 |
   |  Next  |                                                \|/
   | Key ID |                                           +-----------+
   +--------+                                           | Signature |
   Index n+1                                            +-----------+

                    Figure 3: Constructing the Key List

   Figure 3 shows how the autokey list and autokey values are computed.
   The key list consists of a sequence of key IDs starting with a random
   32-bit nonce (autokey seed) equal to or greater than the pivot as the
   first key ID.  The first autokey is computed as above using the given
   cookie and the first 32 bits of the result in network byte order
   become the next key ID.  Operations continue to generate the entire
   list.  It may happen that a newly generated key ID is less than the
   pivot or collides with another one already generated (birthday
   event).  When this happens, which occurs only rarely, the key list is
   terminated at that point.  The lifetime of each key is set to expire
   one poll interval after its scheduled use.  In the reference
   implementation the list is terminated when the maximum key lifetime
   is about one hour, so for poll intervals above one hour a new key
   list containing only a single entry is regenerated for every poll.

                   +------------------+
                   |  NTP Header and  |
                   | Extension Fields |
                   +------------------+
                        |       |
                       \|/     \|/                     +---------+
                     ****************    +--------+    | Session |
                     * COMPUTE HASH *<---| Key ID |<---| Key ID  |
                     ****************    +--------+    |  List   |
                             |                |        +---------+
                            \|/              \|/
                   +----------------------------------+
                   | Message Authenticator Code (MAC) |
                   +----------------------------------+




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                      Figure 4: Transmitting Messages

   The index of the last key ID in the list is saved along with the next
   key ID for that entry, collectively called the autokey values.  The
   autokey values are then signed.  The list is used in reverse order as
   shown in Figure 4, so that the first autokey used is the last one
   generated.  The Autokey protocol includes a message to retrieve the
   autokey values and signature, so that subsequent packets can be
   validated using one or more hashes that eventually match the last key
   ID (valid) or exceed the index (invalid).  This is called the autokey
   test in the following and is done for every packet, including those
   with and without extension fields.  In the reference implementation
   the most recent key ID received is saved for comparison with the
   first 32 bits in network byte order of the next following key value.
   This minimizes the number of hash operations in case a single packet
   is lost.


5.  Secure Groups

   A digital signature scheme provides an authentic certificate trail,
   but does not provide protection against masquerade, unless the server
   identity is verified by other means.  The PKI security model assumes
   each host is able to verify the certificate trail to a trusted
   certificate authority (CA), where each ascendant host must prove
   identity to the immediately descendant host by independent means,
   such as a credit card number or PIN.  While Autokey supports this
   model by default, in a hierarchical ad-hoc network, especially with
   server discovery schemes like NTP Manycast, proving identity at each
   rest stop on the trail must be an intrinsic capability of Autokey
   itself.

   In the NTP security model every member of a closed group, such as
   might be operated by a timestamping service, be in possession of a
   secret group key.  This could take the form of a private certificate
   or one or another identification schemes described in the literature
   and Appendix D.  Certificate trails and identification schemes are at
   the heart of the NTP security model preventing masquerade and
   middleman attacks.  The Autokey protocol operates to hike the trails
   and then run the identity schemes.

   A NTP secure group consists of a number of hosts dynamically
   assembled as a forest with roots the trusted hosts at the lowest
   stratum of the group.  The trusted hosts do not have to be, but often
   are, primary (stratum 1) servers.  A trusted authority (TA), usually
   one of the trusted hosts, generates private and public identity media
   and deploys selected values to the group members.  The identity
   values are of two kinds, encrypted private keys used by servers with



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   dependent clients and nonencrypted public parameters used by all
   hosts, including those that are not group members, but depend on
   group members for authentication purposes.Figure 5

                     +-------------+ +-------------+ +-------------+
                     |   Alice     | |   Brenda    | |   Denise    |
                     |             | |             | |             |
                     | +-+-+-+-+   | | +-+-+-+-+   | | +-+-+-+-+   |
   Certificate       | | Alice |   | | | Brenda|   | | | Denise|   |
   +-+-+-+-+-+       | +-+-+-+-+   | | +-+-+-+-+   | | +-+-+-+-+   |
   | Subject |       | | Alice*| 1 | | | Alice | 4 | | | Carol | 4 |
   +-+-+-+-+-+       | +-+-+-+-+   | | +-+-+-+-+   | | +-+-+-+-+   |
   | Issuer  | S     |             | |             | |             |
   +-+-+-+-+-+       | +=======+   | | +-+-+-+-+   | | +-+-+-+-+   |
                     | ||Alice|| 3 | | | Alice |   | | | Carol |   |
    Group Key        | +=======+   | | +-+-+-+-+   | | +-+-+-+-+   |
   +=========+       +-------------+ | | Alice*| 2 | | | Carol*| 2 |
   || Group || S     |     Carol   | | +-+-+-+-+   | | +-+-+-+-+   |
   +=========+       |             | |             | |             |
                     | +-+-+-+-+   | | +-+-+-+-+   | | +-+-+-+-+   |
    S = step         | | Carol |   | | | Brenda|   | | | Denise|   |
    * = trusted      | +-+-+-+-+   | | +-+-+-+-+   | | +-+-+-+-+   |
                     | | Carol*| 1 | | | Brenda| 1 | | | Denise| 1 |
                     | +-+-+-+-+   | | +-+-+-+-+   | | +-+-+-+-+   |
                     |             | |             | |             |
                     | +=======+   | | +=======+   | | +=======+   |
                     | ||Alice|| 3 | | ||Alice|| 3 | | ||Alice|| 3 |
                     | +=======+   | | +=======+   | | +=======+   |
                     +-------------+ +-------------+ +-------------+
                        Stratum 1                Stratum 2

                     +---------------------------------------------+
                     |                  Eileen                     |
                     |                                             |
                     |           +-+-+-+-+   +-+-+-+-+             |
                     |           | Eileen|   | Eileen|             |
                     |           +-+-+-+-+   +-+-+-+-+             |
                     |           | Brenda|   | Carol | 4           |
                     |           +-+-+-+-+   +-+-+-+-+             |
                     |                                             |
                     |           +-+-+-+-+   +-+-+-+-+             |
                     |           | Alice |   | Carol |             |
                     |           +-+-+-+-+   +-+-+-+-+             |
                     |           | Alice*|   | Carol*| 2           |
                     |           +-+-+-+-+   +-+-+-+-+             |
                     |                                             |
                     |           +-+-+-+-+   +-+-+-+-+             |
                     |           | Brenda|   | Denise|             |



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                     |           +-+-+-+-+   +-+-+-+-+             |
                     |           | Alice |   | Carol | 2           |
                     |           +-+-+-+-+   +-+-+-+-+             |
                     |                                             |
                     |                 +-+-+-+-+                   |
                     |                 | Eileen|                   |
                     |                 +-+-+-+-+                   |
                     |                 | Eileen| 1                 |
                     |                 +-+-+-+-+                   |
                     |                                             |
                     |                 +=======+                   |
                     |                 ||Alice|| 3                 |
                     |                 +=======+                   |
                     +---------------------------------------------+
                                       Stratum 3

                        Figure 5: NTP Secure Groups

   The steps in hiking the certificate trails and verifying identity are
   as follows.  Note the step number in the description matches the step
   number in the figure.

   1.  At startup each host loads its self-signed certificate from a
       local file.  By convention the lowest stratum certificates are
       marked trusted in a X.509 extension field.  As Alice and Carol
       have trusted certificates, they need do nothing further to
       validate the time.  It could be that the trusted hosts depend on
       servers in other groups; this scenario is discussed later.

   2.  The girls begin the Autokey protocol which establishes the server
       name, signature scheme, certificate and identity scheme for each
       group host.  They continue to load certificates recursively until
       a self-signed trusted certificate is found.  Brenda and Denise
       immediately find self-signed trusted certificates for Alice and
       Carol, respectively, but Eileen will loop because neither Brenda
       nor Denise have their own certificates signed by either Alice or
       Carol.

   3.  Brenda and Denise continue with one of the identity schemes
       described below to verify each has the group keys previously
       deployed by Alice.  If this succeeds, each continues in step 4.

   4.  Brenda and Denise present their certificates to Alice and Carol
       for signature.  If this succeeds, either or both Brenda and
       Denise can now provide these signed certificates to Eileen, which
       may be looping in step 2.  When Eileen receives them, she can now
       follow the trail via either Brenda or Denise to the trusted
       certificates for Alice and Carol.  Once this is done, Eileen can



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       complete the protocol just as Brenda and Denise.

   The NTP security model is based on multiple, hierarchical,
   overlapping security compartments or groups.  The example above
   illustrates how groups can be used to construct closed compartments,
   depending on how the identity credentials are deployed.  The rules
   can be summarized:

   1.  Every host holds the public parameters generated by a TA.  Those
       hosts with clients hold in addition the secret group key.

   2.  A host is trusted if it operates at the lowest stratum in the
       group and has a trusted, self-signed certificate.

   3.  A host uses the identity scheme to prove to another host it has
       the same group key, even as in some (zero knowledge) schemes
       neither knows the exact key value.

   4.  A client verifies group membership if the server can prove it has
       the same key as the client and has an unbroken certificate trail
       to a trusted host.

   For various reasons it may be convenient for the trusted hosts to
   hold parameters for one or more groups operating at a lower stratum.
   For example, Figure 6 shows three secure groups Alice, Helen and
   Carol arranged in a hierarchy.  Hosts A, B, C and D belong to the
   Alice group, hosts R and S to the Helen group and hosts X, Y and Z to
   the Carol group.  While not strictly necessary, hosts A, B and R are
   stratum 1 and presumed trusted; the TA generating the group keys and
   parameters could be one of them or another not shown.





















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                         *****     *****     @@@@@
           Stratum 1     * A *     * B *     @ R @
                         *****     *****     @@@@@
                             \     /         /
                              \   /         /
                              *****     @@@@@                *********
                   2          * C *     @ S @                * Alice *
                              *****     @@@@@                *********
                              /   \     /
                             /     \   /                     @@@@@@@@@
                         *****     #####                     @ Helen @
                   3     * D *     # X #                     @@@@@@@@@
                         *****     #####
                                   /   \                     #########
                                  /     \                    # Carol #
                              #####     #####                #########
                   4          # Y #     # Z #
                              #####     #####

                 Figure 6: Hierarchical Overlapping Groups

   The intent of the design is to provide security separation, so that
   servers cannot masquerade as TAs and clients cannot masquerade as
   servers.  Assume for example that Alice and Helen belong to national
   standards laboratories and their group keys are used to confirm
   identity between members of each group.  Carol is a prominent
   corporation receiving standards products via broadcast satellite and
   requiring cryptographic authentication.

   Perhaps under contract, host X belonging to the Carol group has
   rented group parameters for both Alice and Helen and has group keys
   for Carol.  The Autokey protocol operates for each group separately
   while preserving security separation.  Host X can prove identity in
   Carol to clients Y and Z, but cannot prove to anybody that it belongs
   to either Alice or Helen.

   In some scenarios it would be desirable that servers cannot
   masquerade as the TA and clients cannot masquerade as servers.  This
   can be assured using the MV identity scheme described later.  It
   allows the same broadcast transmission to be authenticated by more
   than one key, one used internally by the laboratories (Alice and/or
   Helen) and the other handed out to clients like Carol.  In the MV
   scheme these keys can be separately activated upon subscription and
   deactivated if the subscriber fails to pay the bill.  In the MV
   scheme a key can be deactivated without requiring the other keys to
   be recomputed.

   Figure 7 shows operational details where more than one group is



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   involved, in this case Carol and Alice.  As in the previous example,
   Brenda has configured Alice as her time source and Denise has
   configured Carol as her time source.  Alice and Brenda have group
   keys and parameters for the Alice group; Carol and Denise have group
   keys and parameters for the Carol group.  Eileen has parameters for
   both groups.  The protocol operates as previously described to verify
   Alice to Brenda and Carol to Denise.

                     +-------------+ +-------------+ +-------------+
                     |   Alice     | |   Brenda    | |   Denise    |
                     |             | |             | |             |
                     | +-+-+-+-+   | | +-+-+-+-+   | | +-+-+-+-+   |
   Certificate       | | Alice |   | | | Brenda|   | | | Denise|   |
   +-+-+-+-+-+       | +-+-+-+-+   | | +-+-+-+-+   | | +-+-+-+-+   |
   | Subject |       | | Alice*| 1 | | | Alice | 4 | | | Carol | 4 |
   +-+-+-+-+-+       | +-+-+-+-+   | | +-+-+-+-+   | | +-+-+-+-+   |
   | Issuer  | S     |             | |             | |             |
   +-+-+-+-+-+       | +=======+   | | +-+-+-+-+   | | +-+-+-+-+   |
                     | ||Alice|| 3 | | | Alice |   | | | Carol |   |
    Group Key        | +=======+   | | +-+-+-+-+   | | +-+-+-+-+   |
   +=========+       +-------------+ | | Alice*| 2 | | | Carol*| 2 |
   || Group || S     |     Carol   | | +-+-+-+-+   | | +-+-+-+-+   |
   +=========+       |             | |             | |             |
                     | +-+-+-+-+   | | +-+-+-+-+   | | +-+-+-+-+   |
    S = step         | | Carol |   | | | Brenda|   | | | Denise|   |
    * = trusted      | +-+-+-+-+   | | +-+-+-+-+   | | +-+-+-+-+   |
                     | | Carol*| 1 | | | Brenda| 1 | | | Denise| 1 |
                     | +-+-+-+-+   | | +-+-+-+-+   | | +-+-+-+-+   |
                     |             | |             | |             |
                     | +=======+   | | +=======+   | | +=======+   |
                     | ||Carol|| 3 | | ||Alice|| 3 | | ||Carol|| 3 |
                     | +=======+   | | +=======+   | | +=======+   |
                     +-------------+ +-------------+ +-------------+
                        Stratum 1                Stratum 2

                     +---------------------------------------------+
                     |                  Eileen                     |
                     |                                             |
                     |           +-+-+-+-+   +-+-+-+-+             |
                     |           | Eileen|   | Eileen|             |
                     |           +-+-+-+-+   +-+-+-+-+             |
                     |           | Brenda|   | Carol | 4           |
                     |           +-+-+-+-+   +-+-+-+-+             |
                     |                                             |
                     |           +-+-+-+-+   +-+-+-+-+             |
                     |           | Alice |   | Carol |             |
                     |           +-+-+-+-+   +-+-+-+-+             |
                     |           | Alice*|   | Carol*| 2           |



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                     |           +-+-+-+-+   +-+-+-+-+             |
                     |                                             |
                     |           +-+-+-+-+   +-+-+-+-+             |
                     |           | Brenda|   | Denise|             |
                     |           +-+-+-+-+   +-+-+-+-+             |
                     |           | Alice |   | Carol | 2           |
                     |           +-+-+-+-+   +-+-+-+-+             |
                     |                                             |
                     |                 +-+-+-+-+                   |
                     |                 | Eileen|                   |
                     |                 +-+-+-+-+                   |
                     |                 | Eileen| 1                 |
                     |                 +-+-+-+-+                   |
                     |                                             |
                     |          +=======+    +=======+             |
                     |          ||Alice||    ||Carol|| 3           |
                     |          +=======+    +=======+             |
                     +---------------------------------------------+
                                       Stratum 3

                   Figure 7: Multiple Overlapping Groups

   The interesting case is Eileen, who may verify identity either via
   Brenda or Denise or both.  To do that she uses the public pameters of
   either Alice and Carol or both.  But, Eileen doesn't know which of
   the two parameters to use until hiking the certificate trail to find
   the trusted certificate of either Alice or Carol and then loading the
   associated parameters.  This scenario can of course become even more
   complex as the number of servers and depth of the tree increase.  The
   bottom line is that every host must have the parameters for all the
   lowest-stratum trusted hosts it is ever likely to find.


6.  Identity Schemes

   While the identity scheme described in [9] is based on a ubiquitous
   Diffie-Hellman infrastructure, it is expensive to generate and use
   when compared to others described in Appendix D.  There are five
   schemes now implemented in the NTPv4 reference implementation to
   prove identity: (1) private certificate (PC), (2) trusted certificate
   (TC), (3) a modified Schnorr algorithm (IFF aka Identify Friendly or
   Foe), (4) a modified Guillou-Quisquater algorithm (GQ), and (5) a
   modified Mu-Varadharajan algorithm (MV).  Following is a summary
   description of each; details are given in Appendix D.

   The PC scheme involves a private certificate as group key.  A
   certificate is designated private by a X509 Version 3 extension field
   when generated by utility routines in the NTP software distribution.



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   The certificate is distributed to all other group members by secure
   means and is never revealed outside the group.  A client is marked
   trusted when the first signature is verified.  In effect, the private
   certificate is used as a symmetric key.  This scheme is
   cryptographically strong as long as the private certificate is
   protected; however, it can be very awkward to refresh the keys or
   certificate, since new values must be securely distributed to a
   possibly large population and activated simultaneously.  It is
   included here primarily as a yardstick and protocol exercise.

   All other schemes involve a conventional certificate trail as
   described in RFC 4210 [10], where each certificate is signed by an
   issuer one step closer to the trusted hosts, which have self-signed
   trusted certificates.  A certificate is designated trusted by a X509
   Version 3 extension field when generated by utility routines in the
   NTP software distribution.  A host obtains the certificates of all
   other hosts along the trail ending at a trusted host, then requests
   the immediately ascendant host to sign its own certificate.
   Subsequently, these certificates are provided to descendent hosts by
   the Autokey protocol.  In this scheme keys, parameters and
   certificates can be refreshed at any time, but a masquerade
   vulnerability remains unless a request to sign a client certificate
   is validated by some means such as reverse-DNS.  If no specific
   identity scheme is specified, this is the default TC identity scheme.

   The three remaining schemes IFF, GQ and MV involve a
   cryptographically strong challenge-response exchange where an
   intruder cannot learn the group key, even after repeated observations
   of multiple exchanges.  In addition, the IFF and MV schemes are
   properly described as zero-knowledge proofs, because the client can
   verify the server has the group key without the client knowing its
   value.

   These schemes start when the client sends a nonce to the server,
   which then rolls its own nonce, performs a mathematical operation and
   sends the results along with a message digest to the client.  The
   client performs another mathematical operation and verifies the
   results match the message digest.  The IFF scheme is used when the
   certificate is generated by a third party, such as a commercial
   service and in general has the same refreshment and distribution
   problems as PC.  However, this scheme has the advantage that the
   group key is not known to the clients.

   On the other hand, when certificates are generated by routines in the
   NTPv4 distribution, the GQ scheme may be a better choice.  In this
   scheme the server further obscures the secret group key using a
   public/private key pair which can be refreshed at any time.  The
   public member is conveyed in the certificate by a X509 Version 3



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   extension field which changes for each regeneration of key pair and
   certificate.

   The MV scheme is perhaps the most interesting and flexible of the
   three challenge/response schemes.  It can be used when a small number
   of servers provide synchronization to a large client population where
   there might be considerable risk of compromise between and among the
   servers and clients.  The TA generates a deliciously intricate
   cryptosystem involving public and private encryption keys, together
   with a number of activation keys and associated private client
   decryption keys.  The activation keys are used by the TA to activate
   and revoke individual client decryption keys without changing the
   decryption keys themselves.

   The TA provides the server with a private encryption key and public
   decryption key.  The server adjusts the keys by a nonce for each
   plaintext encryption, so they appear different on each use.  The
   encrypted ciphertext and adjusted public decryption key are provided
   in the client message.  The client computes the decryption key from
   its private decryption key and the public decryption key in the
   message.


7.  Autokey Operations

   The Autokey protocol has three variations or dances corresponding to
   the NTP server, symmetric and broadcast modes.  The server dance was
   suggested by Steve Kent over lunch some time ago, but considerably
   modified since that meal.  The server keeps no state for each client,
   but uses a fast algorithm and a 32-bit random private value (server
   seed) to regenerate the cookie upon arrival of a client packet.  The
   cookie is calculated as the first 32 bits of the autokey computed
   from the client and server addresses, a key ID of zero and the server
   seed as cookie.  The cookie is used for the actual autokey
   calculation by both the client and server and is thus specific to
   each client separately.

   In previous Autokey versions the cookie was transmitted in clear on
   the assumption it was not useful to a wiretapper other than to launch
   an ineffective replay attack.  However, a middleman could intercept
   the cookie and manufacture bogus messages acceptable to the client.
   In order to reduce the risk of such an attack, the Autokey Version 2
   server encrypts the cookie using a public key supplied by the client.
   While requiring additional processor resources for the encryption,
   this makes it effectively impossible to spoof a cookie or masquerade
   as the server.

   In the server dance the client uses the cookie and each key ID on the



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   key list in turn to retrieve the autokey and generate the MAC in the
   NTP packet.  The server uses the same values to generate the message
   digest and verifies it matches the MAC in the packet.  It then
   generates the MAC for the response using the same values, but with
   the client and server addresses exchanged.  The client generates the
   message digest and verifies it matches the MAC in the packet.  In
   order to deflect old replays, the client verifies the key ID matches
   the last one sent.  In this mode the sequential structure of the key
   list is not exploited, but doing it this way simplifies and
   regularizes the implementation while making it nearly impossible for
   an intruder to guess the next key ID.

   In the broadcast dance clients normally do not send packets to the
   server, except when first starting up to verify credentials and
   calibrate the propagation delay.  At the same time the client runs
   the broadcast dance to obtain the autokey values.  The dance requires
   the association ID of the particular server association, since there
   can be more than one operating in the same server.  For this purpose,
   the server packet includes the association ID in every response
   message sent and, when sending the first packet after generating a
   new key list, it sends the autokey values as well.  After obtaining
   and verifying the autokey values, the client verifies further server
   packets using the autokey sequence.

   The symmetric dance is similar to the server dance and keeps only a
   small amount of state between the arrival of a packet and departure
   of the reply.  The key list for each direction is generated
   separately by each peer and used independently, but each is generated
   with the same cookie.  The cookie is conveyed in a way similar to the
   server dance, except that the cookie is a random value.  There exists
   a possible race condition where each peer sends a cookie request
   message before receiving the cookie response from the other peer.  In
   this case, each peer winds up with two values, one it generated and
   one the other peer generated.  The ambiguity is resolved simply by
   computing the working cookie as the EXOR of the two values.

   Autokey choreography includes one or more exchanges, each with a
   specific purpose, that must be completed in order.  The client
   obtains the server host name, digest/signature scheme and identity
   scheme in the parameter exchange.  It recursively obtains and
   verifies certificates on the trail leading to a trusted certificate
   in the certificate exchange and verifies the server identity in the
   identity exchange.  In the values exchange the client obtains the
   cookie and autokey values, depending on the particular dance.
   Finally, the client presents its self-signed certificate to the
   server for signature in the sign exchange.

   Once the certificates and identity have been validated, subsequent



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   packets are validated by digital signatures and autokey sequences.
   These packets are presumed to contain valid time values; however,
   unless the system clock has already been set by some other proventic
   means, it is not known whether these values actually represent a
   truechime or falsetick source.  As the protocol evolves, the NTP
   associations continue to accumulate time values until a majority
   clique is available in a population of at least three servers.  At
   this point the NTP mitigation algorithms cull the falsetickers and
   cluster outlyers from the population and the survivors are allowed to
   discipline the system clock.

   The time values for truechimer sources form a proventic partial
   ordering relative to the applicable signature timestamps.  This
   raises the interesting issue of how to mitigate between the
   timestamps of different associations.  It might happen, for instance,
   that the timestamp of some Autokey message is ahead of the system
   clock by some presumably small amount.  For this reason, timestamp
   comparisons between different associations and between associations
   and the system clock are avoided, except in the NTP intersection and
   clustering algorithms and when determining whether a certificate has
   expired.

   Once the Autokey values have been instantiated, the dances are
   normally dormant.  In all except the broadcast dance, packets are
   normally sent without extension fields, unless the packet is the
   first one sent after generating a new key list or unless the client
   has requested the cookie or autokey values.  If for some reason the
   client clock is stepped, rather than slewed, all cryptographic and
   time values for all associations are purged and the dances in all
   associations restarted from scratch.  This insures that stale values
   never propagate beyond a clock step.  At intervals of about one day
   the reference implementation purges all associations, refreshes all
   signatures, garbage collects expired certificates and refreshes the
   server seed.


8.  Public Key Signatures and Timestamps

   While public key signatures provide strong protection against
   misrepresentation of source, computing them is expensive.  This
   invites the opportunity for an intruder to clog the client or server
   by replaying old messages or to originate bogus messages.  A client
   receiving such messages might be forced to verify what turns out to
   be an invalid signature and consume significant processor resources.

   In order to foil such attacks, every signed extension field carries a
   timestamp in the form of the NTP seconds at the signature epoch.  The
   signature spans the entire extension field including the timestamp.



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   If the Autokey protocol has verified a proventic source and the NTP
   algorithms have validated the time values, the system clock can be
   synchronized and signatures will then carry a nonzero (valid)
   timestamp.  Otherwise the system clock is unsynchronized and
   signatures carry a zero (invalid) timestamp.  The protocol detects
   and discards replayed extension fields with old or duplicate
   timestamps, as well as fabricated extension fields with bogus
   timestamps, before any values are used or signatures verified.

   There are three signature types currently defined:

   1.  Cookie signature/timestamp.  Each association has a cookie for
       use when generating a key list.  The cookie value is determined
       along with the cookie signature and timestamp upon arrival of a
       cookie request message.  The values are returned in a a cookie
       response message.

   2.  Autokey signature/timestamp.  Each association has a key list for
       generating the autokey sequence.  The autokey values are
       determined along with the autokey signature and timestamp when a
       new key list is generated, which occurs about once per hour in
       the reference implementation.  The values are returned in a
       autokey response message.

   3.  Public values signature/timestamp.  All public key and
       certificate values are signed at the time of generation, which
       occurs when the system clock is first synchronized to a proventic
       source, when the values have changed and about once per day after
       that, even if these values have not changed.  During protocol
       operations, each of these values and associated signatures and
       timestamps are returned in the associated request or response
       message.  While there are in fact several public value
       signatures, depending on the number of entries on the certificate
       list, the values are all signed at the same time, so there is
       only one public value timestamp.

   The most recent timestamp received of each type is saved for
   comparison.  Once a valid signature with valid timestamp has been
   received, messages with invalid timestamps or earlier valid
   timestamps of the same type are discarded before the signature is
   verified.  For signed messages this deflects replays that otherwise
   might consume significant processor resources.  For other messages
   the Autokey protocol deflects message modification or replay by a
   wiretapper, but not necessarily by a middleman.  In addition, the NTP
   protocol itself is inherently resistant to replays and consumes only
   minimal processor resources.

   All cryptographic values used by the protocol are time sensitive and



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   are regularly refreshed.  In particular, files containing
   cryptographic basis values used by signature and encryption
   algorithms are regenerated from time to time.  It is the intent that
   file regenerations occur without specific advance warning and without
   requiring prior distribution of the file contents.  While
   cryptographic data files are not specifically signed, every file is
   associated with a filestamp in the form of the NTP seconds at the
   creation epoch.  It is not the intent in this report to specify file
   formats or names or encoding rules; however, whatever conventions are
   used must support a NTP filestamp in one form or another.  Additional
   details specific to the reference implementation are in Appendix A.

   Filestamps and timestamps can be compared in any combination and use
   the same conventions.  It is necessary to compare them from time to
   time to determine which are earlier or later.  Since these quantities
   have a granularity only to the second, such comparisons are ambiguous
   if the values are in the same second.  Thus, the ambiguity must be
   resolved for each comparison operation as described in Appendix B.

   It is important that filestamps be proventic data; thus, they cannot
   be produced unless the producer has been synchronized to a proventic
   source.  As such, the filestamps throughout the NTP subnet represent
   a partial ordering of all creation epochs and serve as means to
   expunge old data and insure new data are consistent.  As the data are
   forwarded from server to client, the filestamps are preserved,
   including those for certificate files.  Packets with older filestamps
   are discarded before spending cycles to verify the signature.


9.  Autokey Protocol Overview

   This section presents an overview of the three dances: server,
   broadcast and symmetric.  Each dance is designed to be nonintrusive
   and to require no additional packets other than for regular NTP
   operations.  The NTP and Autokey protocols operate independently and
   simultaneously and use the same packets.  When the preliminary dance
   exchanges are complete, subsequent packets are validated by the
   autokey sequence and thus considered proventic as well.  Autokey
   assumes hosts poll servers at a relatively low rate, such as once per
   minute or slower.  In particular, it is assumed that a request sent
   at one poll opportunity will normally result in a response before the
   next poll opportunity; however the protocol is robust against a
   missed or duplicate response.

   The 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 or
   without data.  To avoid deadlocks, any number of responses can be



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   included in a packet, but only one request.  A response is generated
   for every request, even if the requestor is not synchronized to a
   proventic source, but contain meaningful data only if the responder
   is synchronized to a proventic source.  Some requests and most
   responses carry timestamped signatures.  The signature covers the
   entire extension field, including the timestamp and filestamp, where
   applicable.  Only if the packet passes all extension field tests are
   cycles spent to verify the signature.

   All dances begin with the parameter exchange where the host obtains
   the server host name and status word specifying the digest/signature
   scheme it will use and the identity schemes it supports.  The dance
   continues with the certificate exchange where the host obtains and
   verifies the certificates along the trail to a trusted, self-signed
   certificate usually, but not necessarily, provided by a primary
   (stratum 1) server.  Primary servers are by design proventic with
   trusted, self-signed certificates.

   However, the certificate trail is not sufficient protection against
   middleman attacks unless an identity scheme such as described in
   Appendix D or proof-of-possession scheme in [9] is available.  While
   the protocol for a generic challenge/response scheme is defined in
   this report, the choice of one or another required or optional
   identification schemes is yet to be determined.  If all certificate
   signatures along the trail are verified and the server identity is
   confirmed, the host continues with the cookie and autokey exchanges
   as necessary to complete the protocol.  Upon completion the host
   verifies packets using digital signatures and/or the autokey
   sequence.

   Once synchronized to a proventic source, the host continues with the
   sign exchange where the server acting as CA signs the host
   certificate.  The CA interprets the certificate as a X.509v3
   certificate request, but verifies the signature if it is self-signed.
   The CA extracts the subject, issuer, extension fields and public key,
   then builds a new certificate with these data along with its own
   serial number and begin and end times, then signs it using its own
   public key.  The host uses the signed certificate in its own role as
   CA for dependent clients.

   A final exchange occurs when the server has the NIST leapseconds
   table, as indicated in the host status word.  If so, the host
   requests the table and compares the filestamp with its own
   leapseconds table filestamp, if available.  If the server table is
   newer than the host table, the host replaces its table with the
   server table.  The host, acting as server, can now provide the most
   recent table to its dependent clients.  In symmetric mode, this
   results in both peers having the newest table.



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10.  Autokey State Machine

   This section describes the formal model of the Autokey state machine,
   its state variables and the state transition functions.

10.1.  Status Word

   Each server and client operating also as a server implements a host
   status word, while each client implements an association status word
   for each server.  Both words have the format and content shown in
   Figure 8.  The low order 16 bits of the status word define the state
   of the Autokey protocol, while the high order 16 bits specify the
   message digest/signature encryption scheme. as encoded in the OpenSSL
   library.  Bits 24-31 of the status word are reserved for server use,
   while bits 16-23 are reserved for client association use.  In the
   host portion bits 24-27 specify the available identity schemes, while
   bits 28-31 specify the server capabilities.  There are four
   additional bits implemented separately.

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Digest / Signature NID     |    Client     | Ident |  Host |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                           Figure 8: Status Word

   The host status word is included in the ASSOC request and response
   messages.  The client copies this word to the association status word
   and then lights additional status bits as the dance proceeds.  Once
   enabled, these bits never come dim unless a general reset occurs and
   the protocol is restarted from the beginning.  The status bits are
   defined as follows:

   o  ENB (31) - Lit if the server implements the Autokey protocol

   o  LPF (30) - Lit if the server has loaded a valid leapseconds file

   o  IDN (24-27) - These four bits select which identity scheme is in
      use.  While specific coding for various schemes is yet to be
      determined, the schemes available in the reference implementation
      and described in Appendix D include the following:

      *  0x0 Trusted Certificate (TC) Scheme (default)

      *  0x1 Private Certificate (PC) Scheme





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      *  0x2 Schnorr aka Identify-Friendly-or-Foe (IFF) Scheme

      *  0x4 Guillard-Quisquater (GC) Scheme

      *  0x8 Mu-Varadharajan (MV) Scheme

   The PC scheme is exclusive of any other scheme.  Otherwise, the IFF,
   GQ and MV bits can be lit in any combination.

   The association status bits are defined as follows:

   o  VAL (0x0100) - Lit when the server certificate and public key are
      validated.

   o  IFF (0x0200) - Lit when the server identity credentials are
      confirmed.

   o  PRV (0x0400) - Lit when the server signature is verified using the
      public key and identity credentials.  Also called the proventic
      bit elsewhere in this report.  When enabled, signed values in
      subsequent messages are presumed proventic.

   o  CKY (0x0800) - Lit when the cookie is received and validated.
      When enabled, key lists can be generated.

   o  AUT (0x1000) - Lit when the autokey values are received and
      validated.  When enabled, clients can validate packets without
      extension fields according to the autokey sequence.

   o  SGN (0x2000) - Lit when the host certificate is signed by the
      server.

   o  LPT (0x4000) - Lit when the leapseconds table is received and
      validated.

   There are four additional status bits LST, LBK, DUP, and SYN not
   included in the status word.  All except SYN are association
   properties, while SYN is a host property.  These bits may be enabled
   (set to 1) or disabled (set to 0) as the protocol proceeds; all
   except LST are active whether or not the protocol is running.  LST is
   enabled when the key list is regenerated and signed and comes dim
   after the autokey values have been transmitted.  This is necessary to
   avoid livelock under some conditions.  SYN is enabled when the client
   has synchronized to a proventic source and never dim after that.
   There are two error bits maintained by the NTP on-wire protocol:

   o  LBK - indicates the received packet does not match the last one
      sent



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   o  DUP - indicates a duplicate packet

   These bits, which are described in Appendix B, are lit if the
   corresponding error has occurred for the current packet and dim
   otherwise.

10.2.  Host State Variables

   Following is a list of state variables used by the server protocol.

   o  Host Name - The name of the server; by default, the string
      returned by the Unix gethostname() library function.  The name
      must agree with the subject name in the server certificate

   o  Host Status Word - This word is initialized when the host first
      starts up.  The format is described above

   o  Host Key - The RSA public/private key pair used to encrypt/decrypt
      cookies.  This is also the default sign key

   o  Sign Key - The RSA or DSA public/private key pair used to encrypt/
      decrypt signatures when the host key is not used for this purpose

   o  Sign Digest - The message digest algorithm used to compute the
      signature before encryption

   o  IFF identity - The keys and parameters used in the optional IFF
      identity scheme described in Appendix D

   o  GQ identity - The keys and parameters used in the optional GQ
      identity scheme described in Appendix D

   o  MV identity - The keys and parameters used in the optional MV
      identity scheme described in Appendix D

   o  Server Seed - The private value hashed with the IP addresses to
      construct the cookie

   o  Certificate Information Structure (CIS) - Certificates are used to
      construct certificate information structures (CIS) which are
      stored on the certificate list.  The structure includes certain
      information fields from an X.509v3 certificate, together with the
      certificate itself encoded in ASN.1 syntax.  Each structure
      carries the public value timestamp and the filestamp of the
      certificate file when it was generated.  Elsewhere in this report
      the CIS will not be distinguished from the certificate unless
      noted otherwise.  A flags field in the CIS determines the status
      of the certificate.  The field is encoded as follows:



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      *  TRST (0x01) - The certificate has been signed by a trusted
         issuer.  If the certificate is self-signed and contains
         "trustRoot" in the Extended Key Usage field, this bit will be
         lit when the CIS is constructed

      *  SIGN (0x02) - The certificate signature has been verified.  If
         the certificate is self-signed and verified using the contained
         public key, this bit will lit when the CIS is constructed

      *  VALD (0x04) - The certificate is valid and can be used to
         verify signatures.  This bit is lit when a trusted certificate
         has been found on a valid certificate trail

      *  PRIV (0x08) - The certificate is private and not to be
         revealed.  If the certificate is self-signed and contains
         "Private" in the Extended Key Usage field, this bit will be lit
         when the CIS is constructed

      *  ERRR (0x80) - The certificate is defective and not to be used
         in any way

   o  Certificate List - CIS structures are stored on the certificate
      list in order of arrival, with the most recently received CIS
      placed first on the list.  The list is initialized with the CIS
      for the host certificate, which is read from the certificate file.
      Additional CIS entries are pushed on the list as certificates are
      obtained from the servers during the certificate exchange.  CIS
      entries are discarded if overtaken by newer ones or expire due to
      old age

   o  Host Certificate - The self-signed X.509v3 certificate for the
      host.  The subject and issuer fields consist of the host name,
      while the message digest/signature encryption scheme consists of
      the sign key and message digest defined above.  Optional
      information used in the identity schemes is carried in X.509v3
      extension fields compatible with [11]

   o  Public Key Values - The public encryption key for the COOKIE
      request, which consists of the public value of the host key.  It
      carries the public values timestamp and the filestamp of the host
      key file

   o  Leapseconds Table Values.  The NIST leapseconds table from the
      NIST leapseconds file.  It carries the public values timestamp and
      the filestamp of the leapseconds file






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10.3.  Client State Variables (all modes)

   Following is a list of state variables used by the client association
   protocol in all modes.

   o  Association ID - The association ID used in responses.  It is
      assigned when the association is mobilized

   o  Server Association ID - The server association ID used in
      requests.  It is copied from the first nonzero association ID
      field in a response

   o  Server Host Name - The server host name determined in the
      parameter exchange

   o  Server Issuer Name - The host name signing the certificate.  It is
      extracted from the current server certificate upon arrival and
      used to request the next item on the certificate trail

   o  Association Status Word - The host status word of the server
      determined in the parameter exchange

   o  Server Public Key - The public key used to decrypt signatures.  It
      is extracted from the first certificate received, which by design
      is the server host certificate

   o  Server Message Digest - The digest/signature scheme determined in
      the parameter exchange

   o  Identification Challenge - A 512-bit nonce used in the
      identification exchange

   o  Group Key - A set of values used by the identification exchange.
      It identifies the cryptographic compartment shared by the server
      and client

   o  Receive Cookie Values - The cookie returned in a COOKIE response,
      together with its timestamp and filestamp

   o  Receive Autokey Values - The autokey values returned in an AUTO
      response, together with its timestamp and filestamp

   o  Receive Leapsecond Values - The leapsecond table returned by a
      LEAP response, together with its timestamp and filestamp







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10.4.  Server State Variables (broadcast and symmetric modes)

   Following is a list of server state variables used in broadcast and
   symmetric modes.

   o  Send Cookie Values - The cookie encryption values, signature and
      timestamps

   o  Send Autokey Values - The autokey values, signature and timestamps

   o  Key List - A sequence of key IDs starting with the autokey seed
      and each pointing to the next.  It is computed, timestamped and
      signed at the next poll opportunity when the key list becomes
      empty

   o  Current Key Number - The index of the entry on the Key List to be
      used at the next poll opportunity

10.5.  Autokey Protocol Messages

   There are currently eight Autokey requests and eight corresponding
   responses.  The NTPv4 packet format is described in [1] and the
   extension field format used for these messages is illustrated in
   Figure 9.



























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    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Field Type           |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Association ID                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Timestamp                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Filestamp                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Value Length                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               /
   /                             Value                             \
   \                                                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Signature Length                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               /
   /                           Signature                           \
   \                                                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               /
   /                      Padding (if needed)                      \
   \                                                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 9: NTPv4 Extension Field Format

   Each extension field is zero-padded to a 4-octet boundary.  The
   Length field covers the entire extension field, including the Length
   and Padding fields.  While the minimum field length is 8 octets, a
   maximum field length remains to be established.  The reference
   implementation discards any packet with a total field length more
   than 1024 octets.

   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 MUST be discarded; otherwise, the
   parser increments the pointer by the length value.  The parser now
   uses the same rules as above to determine whether a MAC is present
   and/or another extension field.

   The 8-bit Code field specifies the request or response operation,
   while the 4-bit Version Number (VN) field is 2 for the current
   protocol version.  There are four flag bits: bit 0 is the Response
   Flag (R) and bit 1 is the Error Flag (E); the other two bits are
   presently unused and should be set to 0.  The remaining fields will



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   be described later.

   In the most common protocol operations, a client sends a request to a
   server with an operation code specified in the Code field and both
   the R bit and E bit dim.  The Association ID field is set to the
   value previously received from the server or 0 otherwise.  The server
   returns a response with the same operation code in the Code field and
   lights the R bit.  The server can also light the E bit in case of
   error.  The Association ID field is set to the association ID of the
   server as a handle for subsequent exchanges.  If for some reason the
   association ID value in a request does not match the association ID
   of any mobilized association, the server returns the request with
   both the R and E bits lit.  Note that it is not necessarily a
   protocol error to send an unsolicited response with no matching
   request.

   In some cases not all fields may be present.  For requests, until a
   client has synchronized to a proventic source, signatures are not
   valid.  In such cases the Timestamp and Signature Length fields are 0
   and the Signature field is empty.  Responses are generated only when
   the responder has synchronized to a proventic source; otherwise, an
   empty response message is sent.  Some request and error response
   messages carry no value or signature fields, so in these messages
   only the first two words are present.

   The Timestamp and Filestamp words carry the seconds field of an NTP
   timestamp.  The Timestamp field establishes the signature epoch of
   the data field in the message, while the filestamp establishes the
   generation epoch of the file that ultimately produced the data that
   is signed.  A signature and timestamp are valid only when the signing
   host is synchronized to a proventic source; otherwise, the timestamp
   is zero.  A cryptographic data file can only be generated if a
   signature is possible; otherwise, the filestamp is zero, except in
   the ASSOC response message, where it contains the server status word.

10.5.1.  No-Operation

   A No-operation request (Field Type = 0) does nothing except return an
   empty reply which can be used as a crypto-ping.

10.5.2.  Association Message (ASSOC)

   An Association Message (Field Type = 1) is used in the parameter
   exchange to obtain the host name and related values.  The request
   contains the host status word in the filestamp field.  The response
   contains the server status word in the filestamp field and in
   addition the host name, which by default is the string returned by
   the Unix gethostname() library function.  While minimum and maximum



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   host name lengths remain to be established, the reference
   implementation uses the values 4 and 256, respectively.  The
   remaining fields are defined previously in this report.

   If the server response is acceptable and both server and client share
   the same identity scheme, ENB is lit.  When the PC identity scheme is
   in use, the ASSOC response lights VAL, IFF, and SIG, since the PC
   exchange is complete at this point.

10.5.3.  Certificate Message (CERT)

   A Certificate Message (Field Type = 2) is used in the certificate
   exchange to obtain a certificates and related values by subject name.
   The request contains the subject name.  For the purposes of
   interoperability with older Autokey versions, if only the first two
   words are sent, the request is for the server host certificate.  The
   response contains the certificate encoded in X.509 format with ASN.1
   syntax as described in Appendix E.

   If the subject name in the response does not match the issuer name,
   the exchange continues with the issuer name replacing the subject
   name in the request.  The exchange continues until either the subject
   name matches the issuer name, indicating a self-signed certificate,
   or the trst bit is set in the CIS, indicating a trusted certificate.
   If a trusted certificate is found, the client stops the exchange and
   lights VAL.  If a self-signed certificate is found but not trusted,
   the protocol loops at polite intervals until one is found or timeout.

10.5.4.  Cookie Message (COOKIE)

   The Cookie Message (Field Type = 3) is used in server and symmetric
   modes to obtain the server cookie.  The request contains the host
   public key encoded with ASN.1 syntax as described in Appendix E.  The
   response contains the cookie encrypted by the public key in the
   request.  The signature and timestamps are determined when the cookie
   is encrypted.  If the response is valid, the client lights CKY.

10.5.5.  Autokey Message (AUTO)

   The Autokey Message (Field Type = 4) is used to obtain the autokey
   values.  The request contains no value.  The response contains two
   32-bit words in network order.  The first word is the final key ID,
   while the second is the index of the final key ID.  The signature and
   timestamps are determined when the key list is generated.  If the
   response is valid, the client lights AUT.






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10.5.6.  Leapseconds Table Message (LEAP)

   The Leapseconds Table Message (Field Type = 5) is used to exchange
   leapseconds tables.  The request and response messages have the same
   format, except that the R bit is dim in the request and lit in the
   response.  Both the request and response contains the leapseconds
   table as parsed from the NIST leapseconds file.  If the client
   already has a copy of the leapseconds data, it uses the one with the
   latest filestamp and discards the other.  If the response is valid,
   the client lights LPT.

10.5.7.  Sign Message (SIGN)

   The Sign Message (Field Type = 6) requests the server to sign and
   return the certificate presented in the request.  The request
   contains the client certificate encoded in X.509 format with ASN.1
   syntax as described in Appendix E.  The response contains the client
   certificate signed by the server private key.  If the certificate is
   valid when received by the host, it is linked in the certificate list
   and SGN is lit.

10.5.8.  Identity Messages (IFF, GQ, MV)

   The Identity Messages (Field Type = 7 (IFF), 8 (GQ), or 9 (MV))
   contains the host challenge, usually a 160- or 512-bit nonce.  The
   response contains the result of the mathematical operation defined in
   Appendix D.  The Response is encoded in ASN.1 syntax as described in
   Appendix E.  The response signature and timestamp are determined when
   the response is sent.  If the response is valid, IFF is lit.

10.6.  Protocol State Transitions

   The protocol state machine is very simple but robust.  The state is
   determined by the server status bits defined above.  The state
   transitions of the three dances are shown below.  The capitalized
   truth values represent the server status bits.  All server bits are
   initialized dim and lit upon the arrival of a specific response
   message, as detailed above.

   When the system clock is first set and about once per day after that,
   or when the system clock is stepped, the server seed is refreshed,
   signatures and timestamps updated and the protocol restarted in all
   associations.  When the server seed is refreshed or a new certificate
   or leapseconds table is received, the public values timestamp is
   reset to the current time and all signatures are recomputed.






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10.6.1.  Server Dance

   The server dance begins when the host sends an ASSOC request to the
   server.  It ends when the first signature is verified and PRV is lit.
   Subsequent packets received without extension fields are validated by
   the autokey sequence.  An optional LEAP exchange updates the
   leapseconds table.  Note the order of the identity exchanges and that
   only the first one will be used if multiple schemes are available.
   Note also that the SIGN and LEAP requests are not issued until the
   client has synchronized to a proventic source.

           while (1) {
                   wait_for_next_poll;
                   make_NTP_header;
                   if (response_ready)
                           send_response;
                   if (!ENB)
                           / * parameters exchange */
                           ASSOC_request;
                   else if (!VAL)
                           /* certificate exchange */
                           CERT_request(Host_Name);
                   else if (IDN & GQ && !IFF)
                           /* GQ identity exchange */
                           GQ_challenge;
                   else if (IDN & IFF && !IFF)
                           /* IFF identity exchange */
                           IFF_challenge;
                   else if (!IFF)
                           /* TC identity exchange */
                           CERT_request(Issuer_Name);
                   else if (!CKY)
                           /* cookie exchange */
                           COOKIE_request;
                   else if (SYN && !SIG)
                           /* signe exchange */
                           SIGN_request(Host_Certificate);
                   else if (SYN && LPF & !LPT)
                           /* leapseconds exchange */
                           LEAP_request;

           }

   When the PC identity scheme is in use, the ASSOC response lights VAL,
   IFF, and SIG; the COOKIE response lights CKY and AUT; and the first
   valid signature lights PRV.





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10.6.2.  Broadcast Dance

   The only difference between the broadcast and server dances is the
   inclusion of an autokey values exchange following the cookie
   exchange.  The broadcast dance begins when the host receives the
   first broadcast packet, which includes an ASSOC response with
   association ID.  The host uses the association ID to initiate a
   server dance in order to calibrate the propagation delay.

   The dance ends when the first signature is verified and PRV is lit.
   Subsequent packets received without extension fields are validated by
   the autokey sequence.  An optional LEAP exchange updates the
   leapseconds table.  When the server generates a new key list, the
   server replaces the ASSOC response with an AUTO response in the first
   packet sent.

           while (1) {
                   wait_for_next_poll;
                   make_NTP_header;
                   if (response_ready)
                           send_response;
                   if (!ENB)
                           /* parameters exchange */
                           ASSOC_request;
                   else if (!VAL)
                           /* certificate exchange */
                           CERT_request(Host_Name);
                   else if (IDN & GQ && !IFF)
                           /* GQ identity exchange */
                           GQ_challenge;
                   else if (IDN & IFF && !IFF)
                           /* IFF identity exchange */
                           IFF_challenge;
                   else if (!IFF)
                           /* TC identity exchange */
                           CERT_request(Issuer_Name);
                   else if (!CKY)
                           /* cookie exchange */
                           COOKIE_request;
                   else if (!AUT)
                           /* autokey values exchange */
                           AUTO_request;
                   else if (SYN &&! SIG)
                           /* sign exchange */
                           SIGN_request(Host_Certificate);
                   else if (SYN && LPF & !LPT)
                           /* leapseconds exchange */
                           LEAP_request;



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           }


   When the PC identity scheme is in use, the ASSOC response sets VAL,
   IFF, and SIG to 1; the COOKIE response sets CKY and AUT to 1; and the
   first valid signature set PRV to 1.

10.6.3.  Symmetric Dance

   The symmetric dance is intricately choreographed.  It begins when the
   active peer sends an ASSOC request to the passive peer.  The passive
   peer mobilizes an association and both peers step the same dance from
   the beginning.  Until the active peer is synchronized to a proventic
   source (which could be the passive peer) and can sign messages, the
   passive peer loops waiting for the timestamp in the ASSOC response to
   be lit.  Until then, the active peer dances the server steps, but
   skips the sign, cookie and leapseconds exchanges.


































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           while (1) {
                   wait_for_next_poll;
                   make_NTP_header;
                   if (!ENB)
                           /* parameters exchange */
                           ASSOC_request;
                   else if (!VAL)
                           /* certificate exchange */
                           CERT_request(Host_Name);
                   else if (IDN & GQ && !IFF)
                           /* GQ identity exchange */
                           GQ_challenge;
                   else if (IDN & IFF && !IFF)
                           /* IFF identity exchange */
                           IFF_challenge;
                   else if (!IFF)
                           /* TC identity exchange */
                           CERT_request(Issuer_Name);
                   else if (SYN && !SIG)
                           /* sign exchange */
                           SIGN_request(Host_Certificate);
                   else if (SYN && !CKY)
                           /* cookie exchange */
                           COOKIE_request;
                   else if (!LST)
                           /* autokey values response */
                           AUTO_response;
                   else if (!AUT)
                           /* autokey values exchange */
                           AUTO_request;
                   else if (SYN && LPF & !LPT)
                           /* leapseconds exchange */
                           LEAP_request;
           }


   When the PC identity scheme is in use, the ASSOC response lights VAL,
   IFF, and SIG; the COOKIE response lights CKY and AUT; and the first
   valid signature lights PRV.

   Once the active peer has synchronized to a proventic source, it
   includes timestamped signatures with its messages.  The first thing
   it does after lighting timestamps is dance the sign exchange so that
   the passive peer can survive the default identity exchange, if
   necessary.  This is pretty weird, since the passive peer will find
   the active certificate signed by its own public key.

   The passive peer, which has been stalled waiting for the active



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   timestamps to be lit, now mates the dance.  The initial value of the
   cookie is zero.  If a COOKIE response has not been received by either
   peer, the next message sent is a COOKIE request.  The recipient rolls
   a random cookie, lights CKY and returns the encrypted cookie.  The
   recipient decrypts the cookie and lights CKY.  It is not a protocol
   error if both peers happen to send a COOKIE request at the same time.
   In this case both peers will have two values, one generated by itself
   and the other received from the other peer.  In such cases the
   working cookie is constructed as the EXOR of the two values.

   At the next packet transmission opportunity, either peer generates a
   new key list and lights LST; however, there may already be an AUTO
   request queued for transmission and the rules say no more than one
   request in a packet.  When available, either peer sends an AUTO
   response and dims LST.  The recipient initializes the autokey values
   and lights LST and AUT.  Subsequent packets received without
   extension fields are validated by the autokey sequence.

   The above description assumes the active peer synchronizes to the
   passive peer, which itself is synchronized to some other source, such
   as a radio clock or another NTP server.  In this case, the active
   peer is operating at a stratum level one greater than the passive
   peer and so the passive peer will not synchronize to it unless it
   loses its own sources and the active peer itself has another source.
   Various other intricate scenarios are possible.


11.  Error Recovery

   The Autokey protocol state machine includes provisions for various
   kinds of error conditions that can arise due to missing files,
   corrupted data, protocol violations and packet loss or misorder, not
   to mention hostile intrusion.  This section describes how the
   protocol responds to reachability and timeout events which can occur
   due to such errors.  Appendix B contains an extended discussion on
   error checking and timestamp validation.

   A persistent NTP association is mobilized by an entry in the
   configuration file, while an ephemeral association is mobilized upon
   the arrival of a broadcast, manycast or symmetric active packet with
   no matching association.  If necessary, a general reset reinitializes
   all association variables to the initial state when first mobilized.
   In addition, if the association is ephemeral, the association is
   demobilized and all resources acquired are returned to the system.

   Every NTP association has two variables which maintain the liveness
   state of the protocol, the 8-bit reachability register defined in [8]
   and the watchdog timer, which is new in NTPv4.  At every poll



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   interval the reachability register is shifted left, the low order bit
   is dimmed and the high order bit is lost.  At the same time the
   watchdog counter is incremented by one.  If an arriving packet passes
   all authentication and sanity checks, the rightmost bit of the
   reachability register is lit and the watchdog counter is set to zero.
   If any bit in the reachability register is lit, the server is
   reachable, otherwise it is unreachable.

   When the first poll is sent from an association, the reachability
   register and watchdog counter are zero.  If the watchdog counter
   reaches a preset threshold before the server becomes reachable, a
   general reset occurs.  This resets the protocol and clears any
   acquired resources before trying again.  If the association is
   ephemerable, it is demobilized; otherwise, 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 packet
   sent at each poll interval until a valid response is received or a
   general reset occurs, in which case the protocol restarts from the
   beginning.  A general reset also occurs for an association when an
   unrecoverable protocol error occurs.  A general reset occurs for all
   associations when the system clock is first synchronized or the clock
   is stepped or when the server seed is refreshed.

   There are special cases designed to quickly respond to broken
   associations, such as when a server restarts or refreshes keys.
   Since the client cookie is invalidated, the server rejects the next
   client request and returns a crypto-NAK packet.  A cryoti-NAK packet
   has a runt MAC with a key ID of zero and no message digest.  The
   problem for the host is to determine whether it is legitimate or the
   result of intruder mischief.  This is done using the NTPv4 on-wire
   protocol, which requites the crypto-NAK, as well as all responses, to
   be believed only if the result of a previous packet sent by the host
   and not a replay, as confirmed by the LBK and DUP status bits
   described above.  While this defense can be easily circumvented by a
   middleman, it does deflect other kinds of intruder warfare.

   There are a number of situations where some event happens that causes
   the remaining autokeys on the key list to become invalid.  When one
   of these situations happens, the key list and associated autokeys in
   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.




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   2.  When a client switches from client 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.  If a problem is detected when an entry is fetched from the key
       list.  This could happen if the key was marked non-trusted or
       timed out, either of which implies a software bug.


12.  Security Considerations

   This section discusses the most obvious security vulnerabilities in
   the various Autokey dances.  First, some observations on the
   particular engineering parameters of the Autokey protocol are in
   order.  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
   seed values are regenerated frequently.  However, in most cases even
   a successful cryptanalysis of these values compromises only a
   particular association and does not represent a danger to the general
   population.

   Throughout the following discussion the cryptographic algorithms and
   private values themselves are assumed secure; that is, a brute force
   cryptanalytic attack will not reveal the host private key, sign
   private key, cookie value, identity parameters, server seed or
   autokey seed.  In addition, an intruder will not be able to predict
   random generator values or predict the next autokey.  On the other
   hand, the intruder can remember the totality of all past values for
   all packets ever sent.  Ordinarily, the timestamp/filestamp
   provisions will make such possesions unusable.

12.1.  Protocol Vulnerability

   While the protocol has not been subjected to a formal analysis, a few
   preliminary assertions can be made.  The protocol cannot loop forever
   in any state, since the watchdog counter and general reset insure
   that the association variables will eventually be purged and the
   protocol restarted from the beginning.  However, if something is
   seriously wrong, the timeout/restart cycle could continue



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   indefinitely until whatever is wrong is fixed.  This is not a
   clogging hazard, as the timeout period is very long compared to
   expected network delays.

   The LBK and DUP bits described in the main body and Appendix B are
   effective whether or not cryptographic means are in use.  The DUP bit
   deflects duplicate packets in any mode, while the LBK bit deflects
   bogus packets in all except broadcast mode.  All packets must have
   the correct MAC, as verified with correct key ID and cookie.  In all
   modes the next key ID cannot be predicted by a wiretapper, so are of
   no use for cryptanalysis.

   As long as the client has validated the server certificate trail, a
   wiretapper cannot produce a convincing signature and cannot produce
   cryptographic values acceptable to the client.  As long as the
   identity values are not compromised, a middleman cannot masquerade as
   a legitimate group member and produce convincing certificates or
   signatures.  In server and symmetric modes after the preliminary
   exchanges have concluded, extension fields are no longer used, a
   client validates the packet using the autokey sequence.  A wiretapper
   cannot predict the next Key IDs, so cannot produce a valid MAC.  A
   middleman cannot successfully modify and replay a message, since he
   does not know the cookie and without the cookie cannot produce a
   valid MAC.

   In broadcast mode a wiretapper cannot produce a key list with signed
   autokey values that a client will accept.  The most it can do is
   replay an old packet causing clients to repeat the autokey hash
   operations until exceeding the maximum key number.  However, a
   middleman could intercept an otherwise valid broadcast packet and
   produce a bogus packet with acceptable MAC, since in this case it
   knows the key ID before the clients do.  Of course, the middleman key
   list would eventually be used up and clients would discover the ruse
   when verifying the signature of the autokey values.  There does not
   seem to be a suitable defense against this attack.

   During the exchanges where extension fields are in use, the cookie is
   a public value rather than a shared secret and an intruder can easily
   construct a packet with a valid MAC, but not a valid signature.  In
   the certificate and identity exchanges an intruder can generate fake
   request messages which may evade server detection; however, the LBK
   and DUP bits minimize the client exposure to the resulting rogue
   responses.  A wiretapper might be able to intercept a request,
   manufacture a fake response and loft it swiftly to the client before
   the real server response.  A middleman could do this without even
   being swift.  However, once the identity exchange has completed and
   the PRV bit lit, these attacks are readily deflected.




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   A client instantiates cryptographic variables only if the server is
   synchronized to a proventic source.  A server does not sign values or
   generate cryptographic data files unless synchronized to a proventic
   source.  This raises an interesting issue: how does a client generate
   proventic cryptographic files before it has ever been synchronized to
   a proventic source?  [Who shaves the barber if the barber shaves
   everybody in town who does not shave himself?]  In principle, this
   paradox is resolved by assuming the primary (stratum 1) servers are
   proventicated by external phenomenological means.

   There is a significant vulnerability in the underlying NTP on-wire
   protocol.  Until confirming the first exchange when a host first
   comes up, an intruder can replay an old message, which cause the LBK
   and DUP bits to be reset.  If the intruder does this repeatedly, the
   host may never synchronize.  This vulnerability is the same as with
   TCP.

12.2.  Clogging Vulnerability

   There are two clogging vulnerabilities exposed in the protocol
   design: an encryption attack where the intruder hopes to clog the
   victim server with needless cookie or signature encryptions or
   identity calculations, and a decryption attack where the intruder
   attempts to clog the victim client with needless cookie or
   verification decryptions.  Autokey uses public key cryptography and
   the algorithms that perform these functions consume significant
   processor resources.

   In order to reduce exposure to decryption attacks the LBK and DUP
   bits deflect bogus and replayed packets before invoking any
   cryptographic operations.  In order to reduce exposure to encryption
   attacks, 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 one of them changes or the server seed is
   refreshed, which happens about once per day.

   In some Autokey dances the protocol precludes server state variables
   on behalf of an individual client, so a request message must be
   processed and the response message sent without delay.  The identity,
   cookie and sign exchanges are particularly vulnerable to a clogging
   attack, since these exchanges can involve expensive cryptographic
   algorithms as well as digital signatures.  A determined intruder
   could replay identity, cookie or sign requests at high rate, which
   may very well be a useful munition for an encryption attack.
   Ordinarily, these requests are seldom used, except when the protocol
   is restarted or the server seed or public values are refreshed.




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   Once synchronized to a proventic source, a legitimate extension field
   with timestamp the same as or earlier than the most recent received
   of that type is immediately discarded.  This foils a middleman cut-
   and-paste attack using an earlier AUTO response, for example.  A
   legitimate extension field with timestamp in the future is unlikely,
   as that would require predicting the autokey sequence.  In either
   case the extension field is discarded before expensive signature
   computations.  This defense is most useful in symmetric mode, but a
   useful redundancy in other modes.

   The client is vulnerable to a certificate clogging attack until
   declared proventic, after which CERT responses are discarded.  Before
   that, a determined intruder could flood the client with bogus
   certificate responses and force spurious signature verifications,
   which of course would fail.  The intruder could flood the server with
   bogus certificate requests and cause similar mischief.  Once declared
   proventic, further certificate responses are discard, so these
   attacks would fail.  The intruder could flood the server with
   replayed sign requests and cause the server to verify the request and
   sign the response, although the client would drop the response due
   invalid timestamp.

   An interesting adventure is when an intruder replays a recent packet
   with an intentional bit error.  A stateless server will return a
   crypto-NAK message which the client will notice and discard, since
   the LBK bit is lit.  However, a legitimate crypto-NAK is sent if the
   server has just refreshed the server seed.  In this case the LBK bit
   is dim and the client performs a general reset and restarts the
   protocol as expected.  Another adventure is to replay broadcast mode
   packets at high rate.  These will be rejected by the clients by the
   timestamp check and before consuming signature cycles.

   In broadcast and symmetric modes the client must include the
   association ID in the AUTO request.  Since association ID values for
   different invocations of the NTP daemon are randomized over the 16-
   bit space, it is unlikely that a bogus request would match a valid
   association with different IP addresses, for example, and cause
   confusion.


13.  IANA Considerations

   Any IANA registries needed?


14.  Acknowledgements

   ...



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15.  References

15.1.  Normative References

   [1]   Burbank, J., "Network Time Protocol Version 4 Protocol And
         Algorithms Specification", draft-ietf-ntp-ntpv4-proto-07 (work
         in progress), July 2007.

15.2.  Informative References

   [2]   Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
         RFC 4306, December 2005.

   [3]   Orman, H., "The OAKLEY Key Determination Protocol", RFC 2412,
         November 1998.

   [4]   Karn, P. and W. Simpson, "Photuris: Session-Key Management
         Protocol", RFC 2522, March 1999.

   [5]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels", BCP 14, RFC 2119, March 1997.

   [6]   Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303,
         December 2005.

   [7]   Kent, S., "IP Authentication Header", RFC 4302, December 2005.

   [8]   Mills, D., "Network Time Protocol (Version 3) Specification,
         Implementation", RFC 1305, March 1992.

   [9]   Prafullchandra, H. and J. Schaad, "Diffie-Hellman Proof-of-
         Possession Algorithms", RFC 2875, July 2000.

   [10]  Adams, C., Farrell, S., Kause, T., and T. Mononen, "Internet
         X.509 Public Key Infrastructure Certificate Management Protocol
         (CMP)", RFC 4210, September 2005.

   [11]  Housley, R., Polk, W., Ford, W., and D. Solo, "Internet X.509
         Public Key Infrastructure Certificate and Certificate
         Revocation List (CRL) Profile", RFC 3280, April 2002.

   [12]  Schnorr, C., "Efficient signature generation for smart cards",
         1991.

   [13]  Stinson, D., "Cryptography - Theory and Practice", 1995.

   [14]  Guillou, L. and J. Quisquatar, "A "paradoxical" identity-based
         signature scheme resulting from zero-knowledge", 1990.



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   [15]  Mu, Y. and V. Varadharajan, "Robust and secure broadcasting",
         2001.

   [16]  Bassham, L., Polk, W., and R. Housley, "Algorithms and
         Identifiers for the Internet X.509 Public Key Infrastructure
         Certificate and Certificate Revocation List (CRL) Profile",
         RFC 3279, April 2002.


Appendix A.  Cryptographic Key and Certificate Management

   This appendix describes how cryptographic keys and certificates are
   generated and managed in the NTPv4 reference implementation.  These
   means are not intended to become part of any standard that may be
   evolved from this report, but to serve as an example of how these
   functions can be implemented and managed in a typical operational
   environment.

   The ntp-keygen utility program in the NTPv4 software library
   generates public/private key files, certificate files and identity
   key/parameter files.  By default the modulus of all encryption and
   identity keys is 512 bits.  All random cryptographic data are based
   on a pseudo-random number generator seeded in such a way that random
   values are exceedingly unlikely to repeat.  The files are in ASN.1
   format, encrypted is necessary, then PEM encoded in printable ASCII
   format suitable for mailing as MIME objects.

   Every file has a filestamp, which is a string of decimal digits
   representing the NTP seconds when the file was created.  The file
   name is formed from the concatenation of the host name, filestamp and
   constant strings, so files can be copied from one environment to
   another while preserving the original filestamp.  The file header
   includes the file name and date and generation time in printable
   ASCII.  The utility assumes the host is synchronized to a proventic
   source at the time of generation, so that filestamps are proventic
   data.  This raises an interesting circularity issue that will not be
   further explored here.

   The generated files are typically stored in NFS mounted file systems,
   with files containing private keys obscured to all but root.
   Symbolic links are installed from default file names assumed by the
   NTP daemon to the selected files.  Since the files of successive
   generations and different hosts have unique names, there is no
   possibility of name collisions.

   Public/private host and sign keys and certificates must be generated
   by the host to which they belong.  The host key must be RSA, since it
   is used to encrypt the cookie, as well as encrypt signatures by



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   default.  The optional sign key can be RSA or DSA, since it is used
   only to encrypt signatures.  In principle, certificates could be
   generated directly by OpenSSL utility programs, as long as the
   symbolic links are consistent.

   Identity keys and parameters must be generated by the ntp-keygen
   utility, since they have proprietary formats.  Since these are
   private to the group, they are generated by one host acting as a
   trusted authority and then distributed to all other members of the
   group by secure means.

   Certificates are usually, but not necessarily, generated by the host
   to which they belong.  The ntp-keygen utility generates self-signed
   X.509v3 host certificate files with optional extension fields.
   Certificate requests are not used, since the certificate itself is
   used as a request to be signed.  OpenSSL X.509v3 certificates are
   generated as an OpenSSL structure, which is then PEM encoded in ASN.1
   syntax and written to the host certificate file.  The string returned
   by the Unix gethostname() routine is used by default for both the
   subject and issuer fields.  The serial number and begin time fields
   are derived from the filestamp; the end time is one year hence.  The
   host certificate is signed by the sign key or host key if a sign key
   is not present.

   An important design goal is to make cryptographic data refreshment as
   simple and intuitive as possible, so it can be driven by scripts on a
   periodic basis.  When the ntp-keygen utility is run for the first
   time, it creates by default a RSA host key file and RSA-MD5 host
   certificate file and necessary symbolic links.  After that, it
   creates a new certificate file and symbolic link using the existing
   host key.  The program run with given options creates identity key/
   parameter files for one or more IFF, GQ or MV identity schemes.  The
   key files must then be securely copied to all other group members and
   symbolic links installed from the default names to the installed
   files.  In the GQ scheme the next and each subsequent time the ntp-
   keygen utility runs, it automatically creates or updates the private/
   public identity key files and certificate file using the existing
   identity parameters.


Appendix B.  Autokey Error Checking

   Exhaustive examination of possible vulnerabilities at the various
   processing steps of the NTPv3 protocol as specified in [8] have
   resulted in a revised list of packet sanity tests.  There are 12
   tests in the NTPv4 reference implementation, called TEST1 through
   TEST12, which are performed in a specific order designed to gain
   maximum diagnostic information while protecting against an accidental



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   or malicious clogging attacks.  These tests are described in detail
   in the NTP software documentation.  Those relevant to the Autokey
   protocol are described in this appendix.

   The sanity tests are classified in four tiers.  The first tier
   deflects access control and message digest violations.  The second,
   represented by the autokey sequence, deflects spoofed or replayed
   packets.  The third, represented by timestamped digital signatures,
   binds cryptographic values to verifiable credentials.  The fourth
   deflects packets with invalid NTP header fields or out of bounds time
   values.  However, the tests in this last group do not directly affect
   cryptographic protocol vulnerability, so are beyond the scope of
   discussion here.

B.1.  Packet Processing Rules

   Every arriving NTP packet is checked enthusiastically for format,
   content and protocol errors.  Some packet header fields are checked
   by the main NTP code path both before and after the Autokey protocol
   engine cranks.  These include the NTP version number, overall packet
   length and extension field lengths.  The total length of all
   extension fields may be no longer than 1024 octets in the reference
   implementation.  Packets failing any of these checks are discarded
   immediately.  Packets denied by the access control mechanism will be
   discarded later, but processing continues temporarily in order to
   gather further information useful for error recovery and reporting.

   Next, the cookie and session key are determined and the MAC computed
   as described above.  If the MAC fails to match the value included in
   the packet, the action depends on the mode and the type of packet.
   Packets failing the MAC check will be discarded later, but processing
   continues temporarily in order to gather further information useful
   for error recovery and reporting.

   The NTP transmit and receive timestamps are in effect nonces, since
   an intruder cannot effectively guess either value in advance.  To
   minimize the possibility that an intruder can guess the nonces, the
   low order unused bits in all timestamps are obscured with random
   values.  If the transmit timestamp matches the transmit timestamp in
   the last packet received, the packet is a duplicate, so the DUP bit
   is lit.  If the packet mode is not broadcast and the last transmit
   timestamp does not match the originate timestamp in the packet,
   either it was delivered out of order or an intruder has injected a
   rogue packet, so the LBK bit is lit.  Packets with either the DUP or
   LBK bit will be discarded later, but processing continues temporarily
   in order to gather further information useful for error recovery and
   reporting.




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   Further indicators of the server and client state are apparent from
   the transmit and receive timestamps of both the packet and the
   association.  The quite intricate rules take into account these and
   the above error indicators They are designed to discriminate between
   legitimate cases where the server or client are in inconsistent
   states and recoverable, and when an intruder is trying to destabilize
   the protocol or force consumption of needless resources.  The exact
   behavior is beyond the scope of discussion, but is clearly described
   in the source code documentation.

   Next, the Autokey protocol engine is cranked and the dances evolve as
   described above.  Some requests and all responses have value fields
   which carry timestamps and filestamps.  When the server or client is
   synchronized to a proventic source, most requests and responses with
   value fields carry signatures with valid timestamps.  When not
   synchronized to a proventic source, value fields carry an invalid
   (zero) timestamp and the signature field and signature length word
   are omitted.

   The extension field parser checks that the Autokey version number,
   operation code and field length are valid.  If the error bit is lit
   in a request, the request is discarded without response; if an error
   is discovered in processing the request, or if the responder is not
   synchronized to a proventic source, the response contains only the
   first two words of the request with the response and error bits lit.
   For messages with signatures, the parser requires that timestamps and
   filestamps are valid and not a replay, that the signature length
   matches the certificate public key length and only then verifies the
   signature.  Errors are reported via the security logging facility.

   All certificates must have correct ASN.1 encoding, supported digest/
   signature scheme and valid subject, issuer, public key and, for self-
   signed certificates, valid signature.  While the begin and end times
   can be checked relative to the filestamp and each other, whether the
   certificate is valid relative to the actual time cannot be determined
   until the client is synchronized to a proventic source and the
   certificate is signed and verified by the server.

   When the protocol starts the only response accepted is ASSOC with
   valid timestamp, after which the server status word must be nonzero.
   ASSOC responses are discarded if this word is nonzero.  The only
   responses accepted after that and until the PRV bit is lit are CERT,
   IFF and GQ.  Once identity is confirmed and IFF is lit, these
   responses are no longer accepted, but all other responses are
   accepted only if in response to a previously sent request and only in
   the order prescribed in the protocol dances.  Additional checks are
   implemented for each request type and dance step.




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B.2.  Timestamps, Filestamps and Partial Ordering

   When the host starts, it reads the host key and certificate files,
   which are required for continued operation.  It also reads the sign
   key and leapseconds file, when available.  When reading these files
   the host checks the file formats and filestamps for validity; for
   instance, all filestamps must be later than the time the UTC
   timescale was established in 1972 and the certificate filestamp must
   not be earlier than its associated sign key filestamp.  In general,
   at the time the files are read, the host is not synchronized, so it
   cannot determine whether the filestamps are bogus other than these
   simple checks.

   In the following the relation A --> B is Lamport's "happens before"
   relation, which is true if event A happens before event B. When
   timestamps are compared to timestamps, the relation is false if A
   <--> B; that is, false if the events are simultaneous.  For
   timestamps compared to filestamps and filestamps compared to
   filestamps, the relation is true if A <--> B. Note that the current
   time plays no part in these assertions except in (6) below; however,
   the NTP protocol itself insures a correct partial ordering for all
   current time values.

   The following assertions apply to all relevant responses:

   1.  The client saves the most recent timestamp T0 and filestamp F0
       for the respective signature type.  For every received message
       carrying timestamp T1 and filestamp F1, the message is discarded
       unless T0 --> T1 and F0 --> F1.  The requirement that T0 --> T1
       is the primary defense against replays of old messages.

   2.  For timestamp T and filestamp F, F --> T; that is, the filestamp
       must happen before the timestamp.  If not, this could be due to a
       file generation error or a significant error in the system clock
       time.

   3.  For sign key filestamp S, certificate filestamp C, cookie
       timestamp D and autokey timestamp A, S --> C --> D --> A; that
       is, the autokey must be generated after the cookie, the cookie
       after the certificate and the certificate after the sign key.

   4.  For sign key filestamp S and certificate filestamp C specifying
       begin time B and end time E, S --> C--> B --> E; that is, the
       valid period must not be retroactive.

   5.  A certificate for subject S signed by issuer I and with filestamp
       C1 obsoletes, but does not necessarily invalidate, another
       certificate with the same subject and issuer but with filestamp



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       C0, where C0 --> C1.

   6.  A certificate with begin time B and end time E is invalid and can
       not be used to verify signatures if t --> B or E --> t, where t
       is the current proventic time.  Note that the public key
       previously extracted from the certificate continues to be valid
       for an indefinite time.  This raises the interesting possibility
       where a truechimer server with expired certificate or a
       falseticker with valid certificate are not detected until the
       client has synchronized to a clique of proventic truechimers.


Appendix C.  Certificates

   Certificate extension fields are used to convey information used by
   the identity schemes, such as whether the certificate is private,
   trusted or contains a public identity key.  While the semantics of
   these fields generally conforms with conventional usage, there are
   subtle variations.  The fields used by Autokey Version 2 include:

   o  Basic Constraints.  This field defines the basic functions of the
      certificate.  It contains the string "critical,CA:TRUE", which
      means the field must be interpreted and the associated private key
      can be used to sign other certificates.  While included for
      compatibility, Autokey makes no use of this field.

   o  Key Usage.  This field defines the intended use of the public key
      contained in the certificate.  It contains the string
      "digitalSignature,keyCertSign", which means the contained public
      key can be used to verify signatures on data and other
      certificates.  While included for compatibility, Autokey makes no
      use of this field.

   o  Extended Key Usage.  This field further refines the intended use
      of the public key contained in the certificate and is present only
      in self-signed certificates.  It contains the string "Private" if
      the certificate is designated private or the string "trustRoot" if
      it is designated trusted.  A private certificate is always
      trusted.

   o  Subject Key Identifier.  This field contains the public identity
      key used in the GQ identity scheme.  It is present only if the GQ
      scheme is configured.








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Appendix D.  Identity Schemes

   The Internet infrastructure model described in [10] is based on
   certificate trails where a subject proves identity to a certificate
   authority (CA) who then signs the subject certificate using the CA
   issuer key.  The CA in turn proves identity to the next CA and
   obtains its own signed certificate.  The trail continues to a CA with
   a self-signed trusted root certificate independently validated by
   other means.  If it is possible to prove identity at each step, each
   certificate along the trail can be considered trusted relative to the
   identity scheme and trusted root certificate.

   The important issue with respect to NTP is the cryptographic strength
   of the identity scheme, since if a middleman could compromise it, the
   trail would have a security breach.  In electric mail and commerce
   the identity scheme can be based on handwritten signatures,
   photographs, fingerprints and other things very hard to counterfeit.
   As applied to NTP subnets and identity proofs, the scheme must allow
   a client to securely verify that a server knows the same secret that
   it does, presuming the secret was previously instantiated by secure
   means, but without revealing the secret to members outside the group.

   While the identity scheme described in RFC-2875 [9] is based on a
   ubiquitous Diffie-Hellman infrastructure, it is expensive to generate
   and use when compared to others described in this appendix.  There
   are five schemes now implemented in the NTPv4 reference
   implementation to prove identity: (1) private certificate (PC), (2)
   trusted certificate (TC), (3) a modified Schnorr algorithm (IFF aka
   Identify Friendly or Foe), (4) a modified Guillou-Quisquater
   algorithm (GQ), and (5) a modified Mu-Varadharajan algorithm (MV).
   The available schemes are selected during the key generation phase,
   with the particular scheme selected during the parameter exchange.

   The IFF, GQ and MV schemes involve a cryptographically strong
   challenge-response exchange where an intruder cannot learn the group
   key, even after repeated observations of multiple exchanges.  These
   schemes begin when the client sends a nonce to the server, which then
   rolls its own nonce, performs a mathematical operation and sends the
   results along with a message digest to the client.  The client
   performs a second mathematical operation to produce a digest that
   must match the one included in the message.  To the extent that a
   server can prove identity to a client without either knowing the
   group key, a scheme is properly described as a zero-knowledge proof.

D.1.  Private Certificate (PC) Scheme

   The PC scheme shown in Figure Figure 13 involves the use of a private
   certificate as group key.  A certificate is designated private by a



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   X509 Version 3 extension field when generated by utility routines in
   the NTP software distribution.  The certificate is distributed to all
   other group members by secure means and is never revealed outside the
   group.  A client is marked trusted in the Parameter Exchange and
   authentic when the first signature is verified.  This scheme is
   cryptographically strong as long as the private certificate is
   protected; however, it can be very awkward to refresh the keys or
   certificate, since new values must be securely distributed to a
   possibly large population and activated simultaneously.


                             Trusted
                            Authority
              Secure     +-------------+    Secure
          +--------------| Certificate |-------------+
          |              +-------------+             |
          |                                          |
         \|/                                        \|/
   +-------------+                            +-------------+
   | Certificate |                            | Certificate |
   +-------------+                            +-------------+
       Server                                     Client


            Figure 13: Private Certificate (PC) Identity Scheme

   The PC scheme uses a private certificate as group key.  A certificate
   is designated private for the purpose of the this scheme if the CIS
   Private bit is lit.  The certificate is distributed to all other
   group members by secret means and never revealed outside the group.
   There is no identity exchange, since the certificate itself is the
   group key.  Therefore, when the parameter exchange completes the VAL,
   IFF and SGN bits are lit in the server status word.  When the
   following cookie exchange is complete, the PRV bit is lit and
   operation continues as described in the main body of this report.

D.2.  Trusted Certificate (TC) Scheme

   All other schemes involve a conventional certificate trail as shown
   in Figure Figure 14.  As described in RFC-4210 [10], each certificate
   is signed by an issuer one step closer to the trusted host, which has
   a self-signed trusted certificate, A certificate is designated
   trusted by a X509 Version 3 extension field when generated by utility
   routines in the NTP software distribution.  A host obtains the
   certificates of all other hosts along the trail leading to a trusted
   host by the Autokey protocol, then requests the immediately ascendant
   host to sign its certificate.  Subsequently, these certificates are
   provided to descendent hosts by the Autokey protocol.  In this scheme



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   keys and certificates can be refreshed at any time, but a masquerade
   vulnerability remains unless a request to sign a client certificate
   is validated by some means such as reverse-DNS.  If no specific
   identity scheme is specified in the Identification Exchange, this is
   the default TC scheme.


                                                           Trusted
                   Host                 Host                 Host
              +-----------+        +-----------+        +-----------+
         +--->|  Subject  |   +--->|  Subject  |   +--->|  Subject  |
         |    +-----------+   |    +-----------+   |    +-----------+
   ...---+    |  Issuer   |---+    |  Issuer   |---+    |  Issuer   |
              +-----------+        +-----------+        +-----------+
              | Signature |        | Signature |        | Signature |
              +-----------+        +-----------+        +-----------+


            Figure 14: Private Certificate (PC) Identity Scheme

   The TC identification exchange follows the parameter exchange in
   which the VAL bit is lit.  It involves a conventional certificate
   trail and a sequence of certificates, each signed by an issuer one
   stratum level lower than the client, and terminating at a trusted
   certificate, as described in [10].  A certificate is designated
   trusted for the purpose of the TC scheme if the CIS Trust bit is lit
   and the certificate is self-signed.  Such would normally be the case
   when the trail ends at a primary (stratum 1) server, but the trail
   can end at a secondary server if the security model permits this.

   When a certificate is obtained from a server, or when a certificate
   is signed by a server, A CIS for the new certificate is pushed on the
   certificate list, but only if the certificate filestamp is greater
   than any with the same subject name and issuer name already on the
   list.  The list is then scanned looking for signature opportunities.
   If a CIS issuer name matches the subject name of another CIS and the
   issuer certificate is verified using the public key of the subject
   certificate, the CIS Sign bit is lit in the issuer CIS.  Furthermore,
   if the Trust bit is lit in the subject CIS, the Trust bit is lit in
   the issuer CIS.

   The client continues to follow the certificate trail to a self-signed
   certificate, lighting the Sign and Trust bits as it proceeds.  If it
   finds a self-signed certificate with Trust bit lit, the client lights
   the IFF and PRV bits and the exchange completes.  It can, however,
   happen that the client finds a self-signed certificate with Trust bit
   dark.  This can happen when a server is just coming up, has
   synchronized to a proventic source, but has not yet completed the



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   Sign exchange.  This is considered a temporary condition, so the
   client simply retries at poll opportunities until the server
   certificate is signed.

D.3.  Schnorr (IFF) Scheme

   The Schnorr (IFF) identity scheme is useful when certificates are
   generated by means other than the NTP software library, such as a
   trusted public authority.  In this case a X.509v3 extension field
   might not be available to convey the identity public key.  The scheme
   involves a set of parameters which persist for the life of the
   scheme.  New generations of these parameters must be securely
   transmitted to all members of the group before use.  The scheme is
   self contained and independent of new generations of host keys, sign
   keys and certificates.

   Certificates can be generated by the OpenSSL library or an external
   public certificate authority, but conveying an arbitrary public value
   in a certificate extension field might not be possible.  The TA
   generates IFF parameters and keys and distributes them by secure
   means to all servers, then removes the group key and redistributes
   these data to dependent clients.  Without the group key a client
   cannot masquerade as a legitimate server.

   The IFF parameters are generated by OpenSSL routines normally used to
   generate DSA keys.  By happy coincidence, the mathematical principles
   on which IFF is based are similar to DSA, but only the moduli p, q
   and generator g are used in identity calculations.  The parameters
   hide in a DSA cuckoo structure and use the same members.  The values
   are used by an identity scheme based on DSA cryptography and
   described in [12] and [13] p. 285.  The p is a 512-bit prime, g a
   generator of the multiplicative group Zp* and q a 160-bit prime that
   divides (p-1) and is a qth root of 1 mod p; that is, g^q mod p.  The
   TA rolls a private random group key b (0 < b < q), then computes
   public client key v = g^(q-b) mod p.  The TA distributes (p, q, g, b)
   to all servers using secure means and (p, q, g, v) to all clients not
   necessarily using secure means.














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                                     Trusted
                                    Authority
                                  +------------+
                                  | Parameters |
                       Secure     +------------+   Insecure
                    +-------------| Group Key  |-----------+
                    |             +------------+           |
                    |             | Client Key |           |
                    |             +------------+           |
                   \|/                                    \|/
              +------------+         Challenge       +------------+
              | Parameters |<------------------------| Parameters |
              +------------+                         +------------+
              |  Group Key |------------------------>| Client Key |
              +------------+         Response        +------------+
                  Server                                 Client


                 Figure 15: Schnorr (IFF) Identity Scheme

   The IFF identity scheme is shown in Figure Figure 15.  The TA
   generates a DSA parameter structure for use as IFF parameters.  The
   IFF parameters are identical to the DSA parameters, so the OpenSSL
   library DSA parameter generation routine can be used directly.  The
   DSA parameter structure shown in Table Figure 16 is written to a file
   as a DSA private key encoded in PEM.  Unused structure members are
   set to one.


              +----------------------------------+-------------+
              |   IFF   |   DSA    |   Item      |   Include   |
              +=========+==========+=============+=============+
              |    p    |    p     | modulus     |    all      |
              +---------+----------+-------------+-------------+
              |    q    |    q     | modulus     |    all      |
              +---------+----------+-------------+-------------+
              |    g    |    g     | generator   |    all      |
              +---------+----------+-------------+-------------+
              |    b    | priv_key | group key   |   server    |
              +---------+----------+-------------+-------------+
              |    v    | pub_key  | client keys |   client    |
              +---------+----------+-------------+-------------+


                 Figure 16: IFF Identity Scheme Parameters

   Alice challenges Bob to confirm identity using the following protocol
   exchange.



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   1.  Alice rolls random r (0 < r < q) and sends to Bob.

   2.  Bob rolls random k (0 < k < q), computes y = k + br mod q and x =
       g^k mod p, then sends (y, hash(x)) to Alice.

   3.  Alice computes z = g^y * v^r mod p and verifies hash(z) =
       hash(x).

   If the hashes match, Alice knows that Bob has the group key b.
   Besides making the response shorter, the hash makes it effectively
   impossible for an intruder to solve for b by observing a number of
   these messages.  The signed response binds this knowledge to Bob's
   private key and the public key previously received in his
   certificate.  On success the IFF and PRV bits are lit in the server
   status word.

D.4.  Guillard-Quisquater (GQ)

   The Guillou-Quisquater (GQ) identity scheme is useful when
   certificates are generated using the NTP software library.  These
   routines convey the GQ public key in a X.509v3 extension field.  The
   scheme involves a set of parameters which persist for the life of the
   scheme and a private/public identity key, which is refreshed each
   time a new certificate is generated.  The utility inserts the client
   key in an X.509 extension field when the certificate is generated.
   The client key is used when computing the response to a challenge.
   The TA generates the GQ parameters and keys and distributes them by
   secure means to all group members.  The scheme is self contained and
   independent of new generations of host keys and sign keys and
   certificates.

   The GQ parameters are generated by OpenSSL routines normally used to
   generate RSA keys.  By happy coincidence, the mathematical principles
   on which GQ is based are similar to RSA, but only the modulus n is
   used in identity calculations.  The parameters hide in a RSA cuckoo
   structure and use the same members.  The values are used in an
   identity scheme based on RSA cryptography and described in [14] and
   [13] p. 300 (with errors).  The 512-bit public modulus n=pq, where p
   and q are secret large primes.  The TA rolls random group key b (0 <
   b < n) and distributes (n, b) to all group members using secure
   means.  The private server key and public client key are constructed
   later.









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                                     Trusted
                                    Authority
                                  +------------+
                                  | Parameters |
                       Secure     +------------+   Secure
                    +-------------| Group Key  |-----------+
                    |             +------------+           |
                   \|/                                    \|/
              +------------+         Challenge       +------------+
              | Parameters |<------------------------| Parameters |
              +------------+                         +------------+
              |  Group Key |                         |  Group Key |
              +------------+         Response        +------------+
              | Server Key |------------------------>| Client Key |
              +------------+                         +------------+
                  Server                                 Client


                 Figure 17: Schnorr (IFF) Identity Scheme

   The GQ identity scheme is shown in Figure Figure 17.  When generating
   new certificates, the server rolls new random private server key u (0
   < u < n) and public client key its inverse obscured by the group key
   v = (u^-1)^b mod n.  These values replace the private and public keys
   normally generated by the RSA scheme.  In addition, the public client
   key is conveyed in a X.509 certificate extension.  The updated GQ
   structure shown in Figure Figure 18 is written as an RSA private key
   encoded in PEM.  Unused structure members are set to one.


              +---------------------------------+-------------+
              |   GQ    |   RSA    |   Item     |   Include   |
              +=========+==========+============+=============+
              |    n    |    n     | modulus    |    all      |
              +---------+----------+------------+-------------+
              |    b    |    e     | group key  |   server    |
              +---------+----------+------------+-------------+
              |    u    |    p     | server key |   server    |
              +---------+----------+------------+-------------+
              |    v    |    q     | client key |   client    |
              +---------+----------+------------+-------------+


                 Figure 18: IFF Identity Scheme Parameters

   Alice challenges Bob to confirm identity using the following
   exchange.




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   1.  Alice rolls random r (0 < r < n) and sends to Bob.

   2.  Bob rolls random k (0 < k < n) and computes y = ku^r mod n and x
       = k^b mod n, then sends (y, hash(x)) to Alice.

   3.  Alice computes z = (v^r)*(y^b) mod n and verifies hash(z) =
       hash(x).

   If the hashes match, Alice knows that Bob has the group key b.
   Besides making the response shorter, the hash makes it effectively
   impossible for an intruder to solve for b by observing a number of
   these messages.  The signed response binds this knowledge to Bob's
   private key and the public key previously received in his
   certificate.  Further evidence is the certificate containing the
   public identity key, since this is also signed with Bob's private
   key.  On success the IFF and PRV bits are lit in the server status
   word.

D.5.  Mu-Varadharajan (MV) Identity Scheme

   The Mu-Varadharajan (MV) scheme was originally intended to encrypt
   broadcast transmissions to receivers which do not transmit.  There is
   one encryption key for the broadcaster and a separate decryption key
   for each receiver.  It operates something like a pay-per-view
   satellite broadcasting system where the session key is encrypted by
   the broadcaster and the decryption keys are held in a tamper proof
   set-top box.  We don't use it this way, but read on.

   The MV scheme is perhaps the most interesting and flexible of the
   three challenge/response schemes.  It can be used when a small number
   of servers provide synchronization to a large client population where
   there might be considerable risk of compromise between and among the
   servers and clients.  The TA generates an intricate cryptosystem
   involving public and private encryption keys, together with a number
   of activation keys and associated private client decryption keys.
   The activation keys are used by the TA to activate and revoke
   individual client decryption keys without changing the decryption
   keys themselves.

   The TA provides the server with a private encryption key and public
   decryption key.  The server adjusts the keys by a nonce for each
   plaintext encryption, so they appear different on each use.  The
   encrypted ciphertext and adjusted public decryption key are provided
   in the client message.  The client computes the decryption key from
   its private decryption key and the public decryption key in the
   message.

   In the MV scheme the activation keys are known only to the TA.  The



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   TA decides which keys to activate and provides to the servers a
   private encryption key E and public decryption keys g-bar and g-hat
   which depend on the activated keys.  The servers have no additional
   information and, in particular, cannot masquerade as a TA.  In
   addition, the TA provides to each client j individual private
   decryption keys x-bar_j and x-hat_j , which do not need to be changed
   if the TA activates or deactivates this key.  The clients have no
   further information and, in particular, cannot masquerade as a server
   or TA.

   The MV values hide in a DSA cuckoo structure which uses the same
   parameters, but generated in a different way.  The values are used in
   an encryption scheme similar to El Gamal cryptography and a
   polynomial formed from the expansion of product terms (x-x_1)*(x-
   x_2)*(x-x_3)*...*(x-x_n), as described in [15].  The paper has
   significant errors and serious omissions.


                                     Trusted
                                    Authority
                                  +------------+
                                  | Parameters |
                                  +------------+
                                  | Group Key  |
                                  +------------+
                                  | Server Key |
                       Secure     +------------+   Secure
                    +-------------| Client Key |-----------+
                    |             +------------+           |
                   \|/                                    \|/
              +------------+         Challenge       +------------+
              | Parameters |<------------------------| Parameters |
              +------------+                         +------------+
              | Server Key |------------------------>| Client Key |
              +------------+         Response        +------------+
                  Server                                 Client


              Figure 19: Mu-Varadharajan (MV) Identity Scheme

   The MV identity scheme is shown in Figure Figure 19.  The TA writes
   the server parameters, private encryption key and public decryption
   keys for all servers as a DSA private key encoded in PEM, as shown in
   the Figure Figure 20.







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              +---------------------------------+-------------+
              |   MV    |   DSA    |   Item     |   Include   |
              +=========+==========+============+=============+
              |    p    |    p     | modulus    |    all      |
              +---------+----------+------------+-------------+
              |    q    |    q     | modulus    |   server    |
              +---------+----------+------------+-------------+
              |    E    |    g     | private    |   server    |
              |         |          | encrypt    |             |
              +---------+----------+------------+-------------+
              |  g-bar  | priv_key | public     |   server    |
              |         |          | decrypt    |             |
              +---------+----------+------------+-------------+
              |  g-hat  | pub_key  | public     |   server    |
              |         |          | decrypt    |             |
              +---------+----------+------------+-------------+


                  Figure 20: MV Scheme Server Parameters

   The TA writes the client parameters and private decryption keys for
   each client as a DSA private key encoded in PEM.  It is used only by
   the designated recipient(s) who pay a suitably outrageous fee for its
   use.  Unused structure members are set to one, as shown in Table
   Figure 21,


              +---------------------------------+-------------+
              |   MV    |   DSA    |   Item     |   Include   |
              +=========+==========+============+=============+
              |    p    |    p     | modulus    |    all      |
              +---------+----------+------------+-------------+
              | x-bar_j | priv_key | public     |   client    |
              |         |          | decrypt    |             |
              +---------+----------+------------+-------------+
              | x-hat_j | pub_key  | public     |   client    |
              |         |          | decrypt    |             |
              +---------+----------+------------+-------------+


                  Figure 21: MV Scheme Client Parameters

   The devil is in the details.  Let q be the product of n distinct
   primes s'_j (j = 1...n), where each s'_j, also called an activation
   key, has m significant bits.  Let prime p = 2q + 1, so that q and
   each s'_j divide p-1 and p has M=nm+1 significant bits.  Let g be the
   generator of the multiplicative group Zp*; that is, gcd(g, p-1) = 1
   and g^q = 1 mod p.  We do modular arithmetic over Zq and then project



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   into Zp* as powers of g.  Sometimes we have to compute an inverse
   b^-1 of random b in Zq, but for that purpose we require gcd(b, q) =
   1.  We expect M to be in the 500-bit range and n relatively small,
   like 30.  The TA uses a nasty probabilistic algorithm to generate the
   cryptosystem.

   1.  Generate the m-bit primes s'_j (0 < j < n), which may have to be
       replaced later.  As a practical matter, it is tough to find more
       than 30 distinct primes for M=512 or 60 primes for M=1024.  The
       latter can take several hundred iterations and several minutes on
       a Sun Blade 1000.

   2.  Compute modulus q = s'_1 * s'_2 * ... * s'_n, then modulus p = 2q
       + 1.  If p is composite, the TA replaces one of the primes with a
       new distinct prime and tries again.  Note that q will hardly be a
       secret since p is revealed to servers and clients.  However,
       factoring q to find the primes should be adequately hard, as this
       is the same problem considered hard in RSA.  Question: is it as
       hard to find n small prime factors totalling M bits as it is to
       find two large prime factors totalling M bits?  Remember, the bad
       guy doesn't know n.

   3.  Associate with each s'_j an element s_j such that s_j*s'_j = s'_j
       mod q.  One way to find an s_j is the quotient s_j = (q + s'_j) /
       s'_j.

   4.  Compute the generator g of Z_p using a random roll such that
       gcd(g, p-1) = 1 and g^q = 1 mod p.  If not, roll again.

   Once the cryptosystem parameters have been determined, the TA sets up
   a specific instance of the scheme as follows.

   1.  Roll n random roots x_j (0 < x_j < q) for a polynomial of order
       n.  While it may not be strictly necessary, Make sure each root
       has no factors in common with q.

   2.  Expand the n product terms (x-x_0)*(x-x_1)*...*(x-x_n) to form n
       + 1 coefficients a_i mod q (0 <= i < n) in powers of x using a
       fast method contributed by C. Boncelet.

   3.  Generate g_i = g^(a_i) mod p for all i and the generator g.
       Verify the product g_i^(a_i*(x_j)^i) = 1 mod p for all i, j (0 <=
       i < n, 0 <= j < n).  Note the a_i*(x_j)^i exponent is computed
       mod q, but the g_i is computed mod p.  Also note the expression
       given in the paper cited is incorrect.

   4.  Make master encryption key A = product of (g_i)^x_j mod p (0 <= i
       < n, 0 <= j < n).  Keep it around for awhile, since it is



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       expensive to compute.

   5.  Roll private random group key b (0 < b < q), where gcd(b, q) = 1
       to guarantee the inverse exists, then compute b^-1 mod q.  If b
       is changed, all keys must be recomputed.

   6.  Make private client keys x-bar_j = b^-1 * ((x_1)^n + (x_2)^n +
       ... + (x_n)^n) mod q (omitting (x_j)^n from the summation) and
       x-hat_j = s_j * (x_j)^n mod q for all j.  Note that the keys for
       the jth client involve only s_j, but not s'_j or s.  The TA sends
       (p, x-bar_j, x-hat_j) to the jth client(s) using secure means.

   7.  The activation key is initially q by construction.  The TA
       revokes client j by dividing q by s'_j.  The quotient becomes the
       activation key s.  Note we always have to revoke one key;
       otherwise, the plaintext and cryptotext would be identical.  The
       TA computes E = A^s, g-bar = x-bar^s mod p, g-hat = x-hat^sb mod
       p and sends (p, E, g-bar, g-hat to the servers using secure
       means.

   Alice challenges Bob to confirm identity using the following
   exchange.

   1.  Alice rolls random r (0 < r < q) and sends to Bob.

   2.  Bob rolls random k (0 < k < q) and computes the session
       encryption key E' = E^k mod p and public decryption key g-bar' =
       g-bar^k mod p and g-hat' = g-hat^k mod p.  He encrypts x = E' * r
       mod p and sends (hash(x), g-bar', g-hat') to Alice.

   3.  Alice computes the session decryption key E^-1 = (g-bar')^x-hat_j
       * (g-hat')^x-bar_j mod p, recovers the encryption key E' =
       (E^-1)^-1 mod p, encrypts z = E' * r mod p, then verifies that
       hash(z) = hash(x).

D.6.  Interoperability Issues

   A specific combination of authentication scheme (none, symmetric key,
   Autokey), digest/signature scheme and identity scheme (PC, TC, IFF,
   GQ, MV) is called a cryptotype, although not all combinations are
   possible.  There may be management configurations where the servers
   and clients may not all support the same cryptotypes.  A secure NTPv4
   subnet can be configured in several ways while keeping in mind the
   principles explained in this section.  Note however that some
   cryptotype combinations may successfully interoperate with each
   other, but may not represent good security practice.

   The cryptotype of an association is determined at the time of



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   mobilization, either at configuration time or some time later when a
   packet of appropriate cryptotype arrives.  When a client, broadcast
   or symmetric active association is mobilized at configuration time,
   it can be designated non-authentic, authenticated with symmetric key
   or authenticated with some Autokey scheme, and subsequently it will
   send packets with that cryptotype.  When a responding server,
   broadcast client or symmetric passive association is mobilized, it is
   designated with the same cryptotype as the received packet.

   When multiple identity schemes are supported, the parameter exchange
   determines which one is used.  The request message contains bits
   corresponding to the schemes it supports, while the response message
   contains bits corresponding to the schemes it supports.  The client
   matches the server bits with its own and selects a compatible
   identity scheme.  The server is driven entirely by the client
   selection and remains stateless.  When multiple selections are
   possible, the order from most secure to least is GC, IFF, TC.  Note
   that PC does not interoperate with any of the others, since they
   require the host certificate which a PC server will not reveal.

   Following the principle that time is a public value, a server
   responds to any client packet that matches its cryptotype
   capabilities.  Thus, a server receiving a non-authenticated packet
   will respond with a non-authenticated packet, while the same server
   receiving a packet of a cryptotype it supports will respond with
   packets of that cryptotype.  However, new broadcast or manycast
   client associations or symmetric passive associations will not be
   mobilized unless the server supports a cryptotype compatible with the
   first packet received.  By default, non-authenticated associations
   will not be mobilized unless overridden in a decidedly dangerous way.

   Some examples may help to reduce confusion.  Client Alice has no
   specific cryptotype selected.  Server Bob supports both symmetric key
   and Autokey cryptography.  Alice's non-authenticated packets arrive
   at Bob, who replies with non-authenticated packets.  Cathy has a copy
   of Bob's symmetric key file and has selected key ID 4 in packets to
   Bob. If Bob verifies the packet with key ID 4, he sends Cathy a reply
   with that key.  If authentication fails, Bob sends Cathy a thing
   called a crypto-NAK, which tells her something broke.  She can see
   the evidence using the utility programs of the NTP software library.

   Symmetric peers Bob and Denise have rolled their own host keys,
   certificates and identity parameters and lit the host status bits for
   the identity schemes they can support.  Upon completion of the
   parameter exchange, both parties know the digest/signature scheme and
   available identity schemes of the other party.  They do not have to
   use the same schemes, but each party must use the digest/signature
   scheme and one of the identity schemes supported by the other party.



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   It should be clear from the above that Bob can support all the girls
   at the same time, as long as he has compatible authentication and
   identification credentials.  Now, Bob can act just like the girls in
   his own choice of servers; he can run multiple configured
   associations with multiple different servers (or the same server,
   although that might not be useful).  But, wise security policy might
   preclude some cryptotype combinations; for instance, running an
   identity scheme with one server and no authentication with another
   might not be wise.


Appendix E.  ASN.1 Encoding Rules

   Certain value fields in request and response messages contain data
   encoded in ASN.1 distinguished encoding rules (DER).  The BNF grammar
   for each encoding rule is given below along with the OpenSSL routine
   used for the encoding in the reference implementation.  The object
   identifiers for the encryption algorithms and message digest/
   signature encryption schemes are specified in [16].  The particular
   algorithms required for conformance are not specified in this report.

E.1.  COOKIE request, IFF response, GQ response, MV response

   The value field of the COOKIE request message contains a sequence of
   two integers (n, e) encoded by the i2d_RSAPublicKey() routine in the
   OpenSSL distribution.  In the request, n is the RSA modulus in bits
   and e is the public exponent.

   RSAPublicKey ::= SEQUENCE {
           n ::= INTEGER,
           e ::= INTEGER
   }

   The IFF and GQ responses contain a sequence of two integers (r, s)
   encoded by the i2d_DSA_SIG() routine in the OpenSSL distribution.  In
   the responses, r is the challenge response and s is the hash of the
   private value.

   DSAPublicKey ::= SEQUENCE {
           r ::= INTEGER,
           s ::= INTEGER
   }

   The MV response contains a sequence of three integers (p, q, g)
   encoded by the i2d_DSAparams() routine in the OpenSSL library.  In
   the response, p is the hash of the encrypted challenge value and (q,
   g) is the client portion of the decryption key.




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   DSAparameters ::= SEQUENCE {
           p ::= INTEGER,
           q ::= INTEGER,
           g ::= INTEGER
   }

E.2.  CERT response, SIGN request and response

   The value field contains a X509v3 certificate encoded by the
   i2d_X509() routine in the OpenSSL distribution.  The encoding follows
   the rules stated in [11], including the use of X509v3 extension
   fields.

   Certificate ::= SEQUENCE {
           tbsCertificate                  TBSCertificate,
           signatureAlgorithm              AlgorithmIdentifier,
           signatureValue                  BIT STRING
   }

   The signatureAlgorithm is the object identifier of the message
   digest/signature encryption scheme used to sign the certificate.  The
   signatureValue is computed by the certificate issuer using this
   algorithm and the issuer private key.

   TBSCertificate ::= SEQUENCE {
           version                         EXPLICIT v3(2),
           serialNumber                    CertificateSerialNumber,
           signature                       AlgorithmIdentifier,
           issuer                          Name,
           validity                        Validity,
           subject                         Name,
           subjectPublicKeyInfo            SubjectPublicKeyInfo,
           extensions                      EXPLICIT Extensions OPTIONAL
   }

   The serialNumber is an integer guaranteed to be unique for the
   generating host.  The reference implementation uses the NTP seconds
   when the certificate was generated.  The signature is the object
   identifier of the message digest/signature encryption scheme used to
   sign the certificate.  It must be identical to the
   signatureAlgorithm.

   CertificateSerialNumber ::= INTEGER
   Validity ::= SEQUENCE {
           notBefore                       UTCTime,
           notAfter                        UTCTime
   }




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   The notBefore and notAfter define the period of validity as defined
   in Appendix D.

   SubjectPublicKeyInfo ::= SEQUENCE {
           algorithm                       AlgorithmIdentifier,
           subjectPublicKey                BIT STRING
   }

   The AlgorithmIdentifier specifies the encryption algorithm for the
   subject public key.  The subjectPublicKey is the public key of the
   subject.

   Extensions ::= SEQUENCE SIZE (1..MAX) OF Extension
   Extension ::= SEQUENCE {
           extnID                          OBJECT IDENTIFIER,
           critical                        BOOLEAN DEFAULT FALSE,
           extnValue                       OCTET STRING
   }


   Name ::= SEQUENCE {
           OBJECT IDENTIFIER               commonName
           PrintableString                 HostName
   }

   For all certificates, the subject HostName is the unique DNS name of
   the host to which the public key belongs.  The reference
   implementation uses the string returned by the Unix gethostname()
   routine (trailing NUL removed).  For other than self-signed
   certificates, the issuer HostName is the unique DNS name of the host
   signing the certificate.


Authors' Addresses

   Brian Haberman (editor)
   The Johns Hopkins University Applied Physics Laboratory
   11100 Johns Hopkins Road
   Laurel, MD  20723-6099
   US

   Phone: +1 443 778 1319
   Email: brian@innovationslab.net








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   Dr. David L. Mills
   University of Delaware
   Newark, DE  19716
   US

   Phone: +1 302 831 8247
   Email: mills@udel.edu












































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Full Copyright Statement

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