[Docs] [txt|pdf|xml|html] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15

NTP Working Group                                              D. Sibold
Internet-Draft                                                       PTB
Intended status: Standards Track                             S. Roettger
Expires: April 21, 2014                                            TU-BS
                                                              K. Teichel
                                                                     PTB
                                                        October 18, 2013


                         Network Time Security
              draft-ietf-ntp-network-time-security-01.txt

Abstract

   This document describes the Network Time Security (NTS) protocol that
   enables secure authentication of time servers using Network Time
   Protocol (NTP) or Precision Time Protocol (PTP).  Its design
   considers the special requirements of precise timekeeping, which are
   described in Security Requirements of Time Protocols in Packet
   Switched Network [I-D.ietf-tictoc-security-requirements].

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 April 21, 2014.

Copyright Notice

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



Sibold, et al.           Expires April 21, 2014                 [Page 1]


Internet-Draft                    NTS                       October 2013


   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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Security Threats  . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Objectives  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Terms and Abbreviations . . . . . . . . . . . . . . . . . . .   4
   5.  NTS Overview  . . . . . . . . . . . . . . . . . . . . . . . .   4
     5.1.  Symmetric and Client/Server Mode  . . . . . . . . . . . .   4
     5.2.  Broadcast Mode  . . . . . . . . . . . . . . . . . . . . .   5
   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  . . . . . . . . . . . . . . . . . .   7
     6.6.  Broadcast Message . . . . . . . . . . . . . . . . . . . .   7
     6.7.  Server Seed Refresh . . . . . . . . . . . . . . . . . . .   7
   7.  Hash Algorithms and MAC Generation  . . . . . . . . . . . . .   8
     7.1.  Hash Algorithms . . . . . . . . . . . . . . . . . . . . .   8
     7.2.  MAC Calculation . . . . . . . . . . . . . . . . . . . . .   8
   8.  Server Seed Considerations  . . . . . . . . . . . . . . . . .   8
     8.1.  Server Seed Algorithm . . . . . . . . . . . . . . . . . .   9
     8.2.  Server Seed Live Time . . . . . . . . . . . . . . . . . .   9
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   10. Security Considerations . . . . . . . . . . . . . . . . . . .   9
     10.1.  Initial Verification of the Server Certificates  . . . .   9
     10.2.  Revocation of Server Certificates  . . . . . . . . . . .   9
     10.3.  Usage of NTP Pools . . . . . . . . . . . . . . . . . . .  10
     10.4.  Denial-of-Service in Broadcast Mode  . . . . . . . . . .  10
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     12.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Appendix A.  TICTOC Security Requirements . . . . . . . . . . . .  11
   Appendix B.  Broadcast Mode . . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12





Sibold, et al.           Expires April 21, 2014                 [Page 2]


Internet-Draft                    NTS                       October 2013


1.  Introduction

   Time synchronization protocols are more and more utilized to
   synchronize clocks in networked infrastructures.  The reliable
   performance of such infrastructures can be degraded seriously by
   successful attacks against the time synchronization protocol.
   Therefore, time synchronization protocols applied in critical
   infrastructures have to provide security measures to defeat possible
   adversaries.  Consequently, the widespread Network Time Protocol
   (NTP) [RFC5905] was supplemented by the autokey protocol [RFC5906]
   which shall ensure authenticity of the NTP server and integrity of
   the protocol packets.  Unfortunately,the autokey protocol exhibits
   various severe security vulnerabilities as revealed in a thorough
   analysis of the protocol [Roettger].  For the Precision Time Protocol
   (PTP) Annex K of the standard document IEEE 1588 [IEEE1588] defines
   an informative security protocol that is still in experimental state.

   Because of autokey's security vulnerabilities and the absence of a
   standardized security protocol for PTP these protocols cannot be
   applied in environments in which compliance requirements demand
   authenticity and integrity protection.  This document specifies a
   security protocol which ensures authenticity of the time server via a
   Public Key Infrastructure and integrity of the time synchronization
   protocol packets and which therefore enables the usage of NTP and PTP
   in such environments.

   The protocol is specified with the prerequisite in mind 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].

   Note:

      It is intended to formulate the protocol to be applicable to NTP
      as well as PTP.  In the current state the draft focuses on the
      application to NTP.

2.  Security Threats

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

3.  Objectives

   The objectives of the NTS specifications are as follows:



Sibold, et al.           Expires April 21, 2014                 [Page 3]


Internet-Draft                    NTS                       October 2013


   o  Authenticity: NTS enables the client to authenticate its time
      server.

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

   o  Confidentiality: NTS does not provide confidentiality protection
      of the time synchronization packets.

   o  Modes of operation: All operational modes of NTP are supported.

   o  Operational modes of PTP should be supported as far as possible.

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

   o  Compatibility:

      *  Unsecured NTP associations shall not be affected.

      *  An NTP server that does not support NTS shall not be affected
         by NTS authentication requests.

4.  Terms and Abbreviations

   o  TESLA: Time efficient stream loss-tolerant authentication

5.  NTS Overview

5.1.  Symmetric and Client/Server Mode

   Authenticity of the time server is verified once by a Public Key
   Infrastructure.  Authenticity and integrity of the NTP packets are
   then 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 as long as the server seed stays the same.  The server
   seed has to be refreshed periodically in order to provide key
   freshness as required in [I-D.ietf-tictoc-security-requirements].
   The server does not keep a state of the client.  Therefore it has to



Sibold, et al.           Expires April 21, 2014                 [Page 4]


Internet-Draft                    NTS                       October 2013


   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

   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.  For a more detailed
   description of the TESLA protocol see Appendix B.

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 a
   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 in which the subject name and the issuer name are
   identical, which indicates that it is issued by a TA.  The client
   then checks that the issuer is indeed on its list of issuers which



Sibold, et al.           Expires April 21, 2014                 [Page 5]


Internet-Draft                    NTS                       October 2013


   are accepted as TAs.  The client has to check that each issuer in the
   certificate chain 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:

      Note that in this step the client validates 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
   and a signature of the cookie signed with the server's private key,
   both encrypted with the client's 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.



Sibold, et al.           Expires April 21, 2014                 [Page 6]


Internet-Draft                    NTS                       October 2013


6.5.  Time Request Message

   The client request includes a new extension field "time request"
   which contains the hash of its public key, a 128-bit nonce, and the
   chosen hash algorithm . The server needs the hash of the public key
   and the notified hash algorithm to recalculate the cookie for the
   client.  The response is a NTP packet with a new extension field
   "time response" which contains the nonce and a MAC generated over the
   time synchronization data, the cookie and the nonce.

6.6.  Broadcast Message

   In broadcast mode the NTP 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.
      This is achieved via a disclosure schedule, so this is where loose
      time synchronization is required.  If verified the packet will be
      buffered for later authentication otherwise it has to be
      discarded.  Note that the time information included in the packet
      will not be used for synchronization until their authenticity
      could be verified.

   o  The client checks whether it already knows the disclosed key.  If
      so, the packet is discarded to avoid a buffer overrun.  If not,
      the client verifies that the disclosed key belongs to the one-way
      key chain by applying the one-way function until equality with a
      previous disclosed key is verified.  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 and the packet's time
      information can be utilized.  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 RFC 4082[RFC4082] for a detailed description of the packet
   verification process.

6.7.  Server Seed Refresh




Sibold, et al.           Expires April 21, 2014                 [Page 7]


Internet-Draft                    NTS                       October 2013


   According to the requirements in
   [I-D.ietf-tictoc-security-requirements] the server has to refresh its
   server seed periodically.  As a consequence the cookie used in the
   time request messages becomes invalid.  In this case the client
   cannot verify the attached MAC and has to respond accordingly by re-
   initiating the protocol with a cookie request (Section 6.3).  This is
   true for the unicast and broadcast mode, respectively.

   Additionally, in broadcast mode the client has to restart the
   broadcast sequence with a time request message if the one-way key
   chain expires.

   During certificate message exchange the client reads the expiration
   date of the period of validity of the server certificate.  The client
   MAY restart the protocol sequence with the association message before
   the server certificate expires.

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
   functions to construct the one-way key chain.

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

   o  it MUST NOT include 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).

8.  Server Seed Considerations





Sibold, et al.           Expires April 21, 2014                 [Page 8]


Internet-Draft                    NTS                       October 2013


   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

10.1.  Initial Verification of the Server Certificates

   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 corporate 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].

10.2.  Revocation of Server Certificates

   According to Section Section 6.7 it is the client's responsibility to
   initiate a new association with the server after the server's
   certificate expires.  To this end the client reads the expiration
   date of the certificate during the certificate message exchange
   (Section 6.2).  Besides, certificates may also be revoked prior to
   the normal expiration date.  To increase security the client MAY
   verify the state of the server's certificate via OCSP periodically.



Sibold, et al.           Expires April 21, 2014                 [Page 9]


Internet-Draft                    NTS                       October 2013


10.3.  Usage of NTP Pools

   The certification based authentication scheme described in Section 6
   is not applicable to the concept of NTP pools.  Therefore, NTS is not
   able to provide secure usage of NTP pools.

10.4.  Denial-of-Service in Broadcast Mode

   TESLA authentication buffers packets for delayed authentication.
   This makes the protocol vulnerable to flooding attacks, causing the
   client to buffer excessive numbers of packets.  To add stronger DoS
   protection to the protocol client and server SHALL use the "Not Re-
   using Keys" scheme of TESLA as pointed out in section 3.7.2 of RFC
   4082 [RFC4082].  In this scheme the server never uses a key for the
   MAC generation more than once.  Therefore the client can discard any
   packet that contains a disclosed key it knows already, thus
   preventing memory flooding attacks.

   Note, an alternative approach to enhance TESLA's resistance against
   DoS attacks involves the addition of a group MAC to each packet.
   This requires the exchange of an additional shared key common to the
   whole group.  This adds additional complexity to the protocol and
   hence is currently not considered in this document.

11.  Acknowledgements

12.  References

12.1.  Normative References

   [IEEE1588]
              IEEE Instrumentation and Measurement Society. TC-9 Sensor
              Technology, "IEEE standard for a precision clock
              synchronization protocol for networked measurement and
              control systems", 2008.

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

   [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.





Sibold, et al.           Expires April 21, 2014                [Page 10]


Internet-Draft                    NTS                       October 2013


   [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.

   [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 Protocols in
              Packet Switched Networks", draft-ietf-tictoc-security-
              requirements-05 (work in progress), April 2013.

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

Appendix A.  TICTOC Security Requirements

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

   +---------+--------------------------------+---------------+--------+
   | Section | Requirement from I-D tictoc    | Requirement   | NTS    |
   |         | security-requirements-05       | level         |        |
   +---------+--------------------------------+---------------+--------+
   | 5.1.1   | Authentication of Servers      | MUST          | OK     |
   | 5.1.1   | Authorization of Servers       | MUST          | -      |
   | 5.1.2   | Recursive Authentication of    | MUST          | NO     |
   |         | Servers (Stratum 1)            |               |        |
   | 5.1.2   | Recursive Authorization of     | MUST          | -      |
   |         | Servers (Stratum 1)            |               |        |
   | 5.1.3   | Authentication and             | MAY           | -      |
   |         | Authorization of Slaves        |               |        |
   | 5.2     | Integrity protection.          | MUST          | OK     |
   | 5.3     | Protection against DoS attacks | SHOULD        | OK     |
   | 5.4     | Replay protection              | MUST          | OK     |
   | 5.5.1   | Key freshness.                 | MUST          | OK     |
   | 5.5.2   | Security association.          | SHOULD        | OK     |



Sibold, et al.           Expires April 21, 2014                [Page 11]


Internet-Draft                    NTS                       October 2013


   | 5.5.3   | Unicast and multicast          | SHOULD        | OK     |
   |         | associations.                  |               |        |
   | 5.6     | Performance: no degradation in | MUST          | OK     |
   |         | quality of time transfer.      |               |        |
   |         | Performance: lightweight       | SHOULD        | OK     |
   |         | computation                    |               |        |
   |         | Performance: storage,          | SHOULD        | OK     |
   |         | bandwidth                      |               |        |
   | 5.7     | Confidentiality protection     | MAY           | NO     |
   | 5.8     | Protection against Packet      | SHOULD        | NA*)   |
   |         | Delay and Interception Attacks |               |        |
   | 5.9.1   | Secure mode                    | MUST          | -      |
   | 5.9.2   | Hybrid mode                    | MAY           | -      |
   +---------+--------------------------------+---------------+--------+

   *) Ensured by NTP via multi-source configuration.

         Comparsion of NTS sepecification against TICTOC security
                               requirements.

Appendix B.  Broadcast Mode

Authors' Addresses

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

   Phone: +49-(0)531-592-8420
   Fax:   +49-531-592-698420
   Email: dieter.sibold@ptb.de


   Stephen Roettger
   Technische Universitaet Braunschweig

   Email: stephen.roettger@googlemail.com


   Kristof Teichel
   Physikalisch-Technische Bundesanstalt
   Bundesallee 100
   Braunschweig  D-38116
   Germany

   Email: kristof.teichel@ptb.de



Sibold, et al.           Expires April 21, 2014                [Page 12]


Internet-Draft                    NTS                       October 2013




















































Sibold, et al.           Expires April 21, 2014                [Page 13]


Html markup produced by rfcmarkup 1.129c, available from https://tools.ietf.org/tools/rfcmarkup/