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Versions: 00 02

NTP Working Group                                              D. Sibold
Internet-Draft                                                       PTB
Intended status: Standards Track                             S. Roettger
Expires: August 25, 2013                                           TU-BS
                                                       February 23, 2013

         Network Time Protocol: autokey Version 2 Specification
                        draft-sibold-autokey-02

Abstract

   This document describes a security protocol that enables
   authenticated time synchronization using Network Time Protocol (NTP).
   Autokey Version 2 obsoletes NTP autokey protocol RFC 5906 [RFC5906]
   which suffers from various security vulnerabilities.  Its design
   considers the special requirements that are related to the task of
   precise timekeeping.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 25, 2013.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.








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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (http://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
     1.1.  Differences from the original autokey  . . . . . . . . . .  3
   2.  Security Threats . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Objectives . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   4.  Terms and abbreviations  . . . . . . . . . . . . . . . . . . .  4
   5.  Autokey Overview . . . . . . . . . . . . . . . . . . . . . . .  4
     5.1.  Symmetric and Client/Server Mode . . . . . . . . . . . . .  4
     5.2.  Broadcast Mode . . . . . . . . . . . . . . . . . . . . . .  4
   6.  Protocol Sequence  . . . . . . . . . . . . . . . . . . . . . .  5
     6.1.  Association Message  . . . . . . . . . . . . . . . . . . .  5
     6.2.  Certificate Message  . . . . . . . . . . . . . . . . . . .  5
     6.3.  Cookie Message . . . . . . . . . . . . . . . . . . . . . .  6
     6.4.  Broadcast Parameter Message  . . . . . . . . . . . . . . .  6
     6.5.  Time Request Message . . . . . . . . . . . . . . . . . . .  6
     6.6.  Broadcast Message  . . . . . . . . . . . . . . . . . . . .  6
   7.  Hash algorithms and MAC generation . . . . . . . . . . . . . .  7
     7.1.  Hash algorithms  . . . . . . . . . . . . . . . . . . . . .  7
     7.2.  MAC Calculation  . . . . . . . . . . . . . . . . . . . . .  7
   8.  Server Seed Considerations . . . . . . . . . . . . . . . . . .  8
     8.1.  Server Seed algorithm  . . . . . . . . . . . . . . . . . .  8
     8.2.  Server Seed Live Time  . . . . . . . . . . . . . . . . . .  8
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   10. Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  8
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     12.1.  Normative References  . . . . . . . . . . . . . . . . . .  8
     12.2.  Informative References  . . . . . . . . . . . . . . . . .  9
   Appendix A. TICTOC Security Requirements . . . . . . . . . . . . .  9
   Appendix B. Broadcast Mode . . . . . . . . . . . . . . . . . . . . 10
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10

1.  Introduction

   In NTP [RFC5905] the autokey protocol [RFC5906] was introduced to
   provide authenticity to NTP servers and to ensure integrity of time
   synchronization.  It is designed to meet the specific communication
   requirements of precise timekeeping and therefore does not compromise
   timekeeping precision.





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   This document focuses on a new definition of the autokey protocol for
   NTP, autokey version 2. The necessity to renew the autokey
   specification arises from various severe security vulnerabilities
   that have been found in a thorough analysis of the protocol
   [Roettger].  The new specification is based on the same assumptions
   as the original autokey specification.  In particular, the
   prerequisite is that precise timekeeping can only be accomplished
   with stateless time synchronization communication, which excludes
   standard security protocols like IPSec or TLS. This prerequisite
   corresponds with the requirement that a security mechanism for
   timekeeping must be designed in such a way that it does not degrade
   the quality of the time transfer [I-D.ietf-tictoc-security-
   requirements].

1.1.  Differences from the original autokey

   Autokey version 2 is a major redraft of the original autokey
   specification.  It is intended to mitigate security vulnerabilities
   of the original specification and it is based on the suggestions in
   the analysis of Roettger [Roettger].  The major changes are:

   o  The bit length of server seed and cookie has been increased.

   o  The IP addresses of the synchronization partners in the
      calculation of the cookie have been replaced by the hash value of
      the client's public key.

   o  The identity schemes for the verification of the NTP server
      authenticity have been replaced by a hierarchical public key
      infrastructure (PKI) based on X.509 certificates.

2.  Security Threats

   A profound analysis of security threats and requirements for NTP and
   Precision Time Protocol (PTP) can be found in the I-D [I-D.ietf-
   tictoc-security-requirements].

3.  Objectives

   The objectives of the autokey specifications are as follows:

   o  Authenticity: Autokey enables the client to authenticate its NTP
      server or peer.

   o  Integrity: Autokey protects the integrity of time synchronization
      packets via a message authentication code (MAC).

   o  Confidentiality: Autokey does not provide confidentiality






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      protection of the NTP packets.

   o  Modes of operation: All operational modes of NTP are supported
      (client server, symmetric, broadcast).

   o  Hybrid mode: Both secure and insecure communication modes are
      possible for NTP servers and clients, respectively.

   o  Compatibility:

      *  Interoperation with autokey version 1 is not given.

      *  NTP associations without authentication shall not be affected.

      *  An NTP server that does not support autokey version 2 shall not
         be affected by autokey version 2 authentication requests.

4.  Terms and abbreviations

   o  Throughout this document the term "autokey" refers to autokey
      version 2.

   o  TESLA: Time efficient stream loss-tolerant authentication

5.  Autokey Overview

5.1.  Symmetric and Client/Server Mode

   Authenticity and integrity of the NTP packets are ensured by a
   Message Authentication Code (MAC), which is attached to the NTP
   packet.  The calculation of the MAC includes the whole NTP packet and
   the cookie which is shared between client and server.  It is
   calculated according to:

      cookie = MSB_128 (H(server seed || H(public key of client))),

   where || indicates concatenation and in which H is a hash algorithm.
   The function MSB_128 cuts off the 128 most significant bits of the
   result of the hash function.  The server seed is a 128 bit random
   value of the server, which has to be kept secret.  The cookie thus
   never changes.  The server seed has to be refreshed periodically.
   The server does not keep a state of the client.  Therefore it has to
   recalculate the cookie each time it receives a request from the
   client.  To this end, the client has to attach the hash value of its
   public key to each request (see Section 6.5).

5.2.  Broadcast Mode








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   Just as in the case of the client server mode and symmetric mode,
   authenticity and integrity of the NTP packets are ensured by a MAC,
   which is attached to the NTP packet by the sender.  The verification
   of the authenticity is based on the TESLA protocol [RFC4082].  TESLA
   is based on a one-way chain of keys, where each key is the output of
   a one-way function applied on the previous key in the chain.  The
   last element of the chain is shared securely with all clients.  The
   server splits time into intervals of uniform duration and assigns
   each key to an interval in reverse order, starting with the
   penultimate.  At each time interval, the server sends an NTP
   broadcast packet appended by a MAC, calculated using the
   corresponding key, and the key of the previous interval.  The client
   verifies the MAC by buffering the packet until the disclosure of the
   key in the next interval.  In order to be able to verify the validity
   of the key, the client has to be loosely time synchronized to the
   server.  This has to be accomplished during the initial client server
   exchange between broadcast client and server.

6.  Protocol Sequence

6.1.  Association Message

   The protocol sequence starts with the association message, in which
   the client sends an NTP packet with an extension field of type
   association.  It contains the hostname of the client and a status
   word which contains the algorithms used for the signatures and the
   status of the connection.  The response contains the hostname of the
   server and the algorithms for the signatures.  The server notifies
   the cryptographic hash algorithms which it supports.

6.2.  Certificate Message

   In this step, the client receives the certification chain up to the
   trusted authority (TA). To this end, the client requests the
   certificate for the subject name (hostname) of the NTP server.  The
   response contains the certificate with the issuer name.  If the
   issuer name is different from the subject name, the client requests
   the certificate for the issuer.  This continues until it receives a
   certificate which is issued by a TA. The client recognizes the TA
   because it has a list of certificates which are accepted as TAs.  The
   client has to check that each issuer is authorized to issue new
   certificates.  To this end, the certificates have to include the
   X.509v3 extension field "CA:TRUE".  With the established
   certification chain the client is able to verify the server
   signatures and, hence, the authenticity of the server messages with
   extension fields is ensured.

   Discussion:






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      Note that in this step the client validate the authenticity of its
      NTP-server only.  It does not recursively validate the
      authenticity of each NTP server on the time synchronization chain.
      But each NTP server on the time synchronization chain validates
      the NTP server to which it is synchronized.  This conforms to the
      recursive authentication requirement in the TICTOC security
      requirements [I-D.ietf-tictoc-security-requirements].

6.3.  Cookie Message

   The client requests a cookie from the server.  It selects a hash
   algorithm from the list of algorithms supported by the server.  The
   request includes its public key and the selected hash algorithm.  The
   hash of the public key is used by the server to calculate the cookie
   (see Section 5.1). The response of the server contains the cookie
   encrypted with the public key.

6.4.  Broadcast Parameter Message

   In the broadcast mode the client requests the following information
   from the server:

   o  the last key of the one-way key chain,

   o  the disclosure schedule of the following keys.  This contains:

      *  time interval duration, time at which the next time interval
         will start and its associated index,

      *  key disclosure delay (number of time intervals for which a key
         is valid).

   The server will sign all transmitted properties so that the client is
   able to verify their authenticity.  For this packet exchange a new
   extension field "broadcast parameters" is used.  The client
   synchronizes its time with the server in the client server mode and
   saves an upper bound of its time offset with respect to the time of
   the server.  See Appendix B for more details.

6.5.  Time Request Message

   The client request includes a new extension field "time request"
   which contains the hash of its public key.  The server needs the hash
   of the public key to recalculate the cookie for the client.  The
   response is a normal NTP packet without extension field.  It contains
   a MAC.

6.6.  Broadcast Message






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   The NTP broadcast packet includes a new extension field "broadcast
   message" which contains the disclosed key of the previous disclosure
   interval (current time interval minus disclosure delay). The NTP
   packet is appended by a MAC, calculated with the key for the current
   time interval.  When a client receives a broadcast message it has to
   perform the following tests:

   o  Proof that the MAC is based on a key that is not yet disclosed.
      If verified the packet will be buffered for later authentication
      otherwise it has to be discarded.

   o  The client checks whether it already knows the disclosed key.  If
      not, the client verifies its legitimacy.  If falsified the packet
      has to be discarded.

   o  If the disclosed key is legitimate the client verifies the
      authenticity of any packet that it received during the
      corresponding time interval.  If authenticity of a packet is
      verified it is released from the buffer.  If the verification
      fails authenticity is no longer given.  In this case the client
      MUST request authentic time from the server by means of a unicast
      time request message.

   See Appendix B or [RFC4082] for a detailed description of the packet
   verification process.

7.  Hash algorithms and MAC generation

7.1.  Hash algorithms

   Hash algorithms are used at different points: calculation of the
   cookie and the MAC, and hashing of the public key.  The client
   selects the hash algorithm from the list of hash algorithms which are
   supported by the server.  This list is notified during the
   association message exchange (Section 6.1). The selected algorithm is
   used for all hashing processes in the protocol.

   In the broadcast mode hash algorithm are used as pseudo random
   function to construct the one-way key chain.

   The list of the server supported hash algorithms has to fulfill
   following requirements:

   o  it MUST NOT contain the MD5 or weaker algorithms,

   o  it MUST include SHA-256 or stronger algorithms.

7.2.  MAC Calculation

   For the calculation of the MAC client and server are using a Keyed-
   Hash Message Authentication Code (HMAC) approach [RFC2104].  The HMAC
   is generated with the hash algorithm specified by the client (see
   Section 7.1).

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8.  Server Seed Considerations

   The server has to calculate a random seed which has to be kept secret
   and which has to be changed periodically.  The server has to generate
   a seed for each supported hash algorithm.

8.1.  Server Seed algorithm

8.2.  Server Seed Live Time

9.  IANA Considerations

   This document makes no request of IANA.

   Note to RFC Editor: this section may be removed on publication as an
   RFC.

10.  Security Considerations

   The client has to verify the validity of the certificates during the
   certification message exchange (Section 6.2). Since it generally has
   no reliable time during this initial communication phase, it is
   impossible to verify the period of validity of the certificates.
   Therefore, the client MUST use one of the following approaches:

   o  The validity of the certificates is preconditioned.  Usually this
      will be the case in corporation networks.

   o  The client ensures that the certificates are not revoked.  To this
      end, the client uses the Online Certificate Status Protocol (OCSP)
      defined in [RFC6277].

   o  The client requests a different service to get an initial time
      stamp in order to be able to verify the certificates' periods of
      validity.  To this end, it can, e.g., use a secure shell
      connection to a reliable host.  Another alternative is to request
      a time stamp from a Time Stamping Authority (TSA) by means of the
      Time-Stamp Protocol (TSP) defined in [RFC3161].

11.  Acknowledgements

12.  References

12.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3161]  Adams, C., Cain, P., Pinkas, D. and R. Zuccherato,
              "Internet X.509 Public Key Infrastructure Time-Stamp
              Protocol (TSP)", RFC 3161, August 2001.



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   [RFC6277]  Santesson, S. and P. Hallam-Baker, "Online Certificate
              Status Protocol Algorithm Agility", RFC 6277, June 2011.

12.2.  Informative References

   [I-D.ietf-tictoc-security-requirements]
              Mizrahi, T., "Security Requirements of Time
              Synchronization Protocols in Packet Switched Networks",
              Internet-Draft draft-ietf-tictoc-security-requirements-04,
              February 2013.

   [RFC2104]  Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104, February
              1997.

   [RFC4082]  Perrig, A., Song, D., Canetti, R., Tygar, J.D. and B.
              Briscoe, "Timed Efficient Stream Loss-Tolerant
              Authentication (TESLA): Multicast Source Authentication
              Transform Introduction", RFC 4082, June 2005.

   [RFC5905]  Mills, D., Martin, J., Burbank, J. and W. Kasch, "Network
              Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, June 2010.

   [RFC5906]  Haberman, B. and D. Mills, "Network Time Protocol Version
              4: Autokey Specification", RFC 5906, June 2010.

   [Roettger]
              Roettger, S., "Analysis of the NTP Autokey Procedures",
              February 2012.

Appendix A.  TICTOC Security Requirements

   The following table compares the autokey specifications against the
   TICTOC security requirements [I-D.ietf-tictoc-security-requirements].



















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   +----------+------------------------------+---------------+---------+
   | Section  | Requirement from I-D tictoc  | Requirement   | Autokey |
   |          | security-requirements-04     | level         | V2      |
   +----------+------------------------------+---------------+---------+
   | 5.1      | Clock Identity               | MUST          | OK      |
   |          | Authentication and           |               |         |
   |          | Authorization                |               |         |
   | 5.1.1    | Authentication and           | MUST          | OK      |
   |          | Authorization of Masters     |               |         |
   | 5.1.2    | Recursive Authentication and | MUST          | OK      |
   |          | Authorization of Masters     |               |         |
   |          | (Chain of Trust)             |               |         |
   | 5.1.3    | Authentication and           | MAY           | -       |
   |          | Authorization of Slaves      |               |         |
   | 5.2      | Integrity protection.        | MUST          | OK      |
   | 5.3      | Protection against DoS       | SHOULD        | -       |
   |          | attacks                      |               |         |
   | 5.4      | Replay protection            | MUST          | OK      |
   |          |                              |               | (NTP)   |
   | 5.5.1    | Key freshness.               | MUST          | OK      |
   | 5.5.2    | Security association.        | SHOULD        | OK      |
   | 5.5.3    | Unicast and multicast        | SHOULD        | OK      |
   |          | associations.                |               |         |
   | 5.6      | Performance: no degradation  | MUST          | OK      |
   |          | in quality of time transfer. |               |         |
   |          | Performance: lightweight     | SHOULD        | OK      |
   |          | computation                  |               |         |
   |          | Performance: storage,        | SHOULD        | OK      |
   |          | bandwidth                    |               |         |
   | 5.7      | Confidentiality protection   | MAY           | -       |
   | 5.8      | Protection against Packet    | SHOULD        | -       |
   |          | Delay and Interception       |               |         |
   |          | Attacks                      |               |         |
   | 5.9.1    | Secure mode                  | MUST          | OK      |
   |          |                              |               | (NTP)   |
   | 5.9.2    | Hybrid mode                  | MAY           | OK      |
   |          |                              |               | (NTP)   |
   +----------+------------------------------+---------------+---------+

   Comparison between TICTOC security requirements and autokey.

Appendix B.  Broadcast Mode

Authors' Addresses

   Dieter Sibold
   Physikalisch-Technische Bundesanstalt
   Bundesallee 100
   Braunschweig, D-38116
   Germany

   Phone: +49-(0)531-592-8420
   Email: dieter.sibold@ptb.de

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   Stephen Roettger
   Technische Universitaet Braunschweig

   Email: stephen.roettger@googlemail.com
















































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