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NTP Working Group                                              D. Sibold
Internet-Draft                                                       PTB
Intended status: Standards Track                             S. Roettger
Expires: September 22, 2016                                  Google Inc.
                                                              K. Teichel
                                                                     PTB
                                                          March 21, 2016


                         Network Time Security
                draft-ietf-ntp-network-time-security-14

Abstract

   This document describes Network Time Security (NTS), a collection of
   measures that enable secure time synchronization with time servers
   using protocols like the Network Time Protocol (NTP) or the 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 Networks [RFC7384].

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 September 22, 2016.








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Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   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.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Terms and Abbreviations . . . . . . . . . . . . . . . . .   4
     2.2.  Common Terminology for PTP and NTP  . . . . . . . . . . .   4
   3.  Security Threats  . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Objectives  . . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  NTS Overview  . . . . . . . . . . . . . . . . . . . . . . . .   6
   6.  Protocol Messages . . . . . . . . . . . . . . . . . . . . . .   7
     6.1.  Unicast Time Synchronisation Messages . . . . . . . . . .   7
       6.1.1.  Preconditions for the Unicast Time Synchronization
               Exchange  . . . . . . . . . . . . . . . . . . . . . .   7
       6.1.2.  Goals of the Unicast Time Synchronization Exchange  .   8
       6.1.3.  Message Type: "time_request"  . . . . . . . . . . . .   8
       6.1.4.  Message Type: "time_response" . . . . . . . . . . . .   8
       6.1.5.  Procedure Overview of the Unicast Time
               Synchronization Exchange  . . . . . . . . . . . . . .   9
     6.2.  Broadcast Time Synchronization Exchange . . . . . . . . .  10
       6.2.1.  Preconditions for the Broadcast Time Synchronization
               Exchange  . . . . . . . . . . . . . . . . . . . . . .  10
       6.2.2.  Goals of the Broadcast Time Synchronization Exchange   11
       6.2.3.  Message Type: "server_broad"  . . . . . . . . . . . .  11
       6.2.4.  Procedure Overview of Broadcast Time Synchronization
               Exchange  . . . . . . . . . . . . . . . . . . . . . .  12
     6.3.  Broadcast Keycheck  . . . . . . . . . . . . . . . . . . .  13
       6.3.1.  Preconditions for the Broadcast Keycheck Exchange . .  13
       6.3.2.  Goals of the Broadcast Keycheck Exchange  . . . . . .  14
       6.3.3.  Message Type: "client_keycheck" . . . . . . . . . . .  14
       6.3.4.  Message Type: "server_keycheck" . . . . . . . . . . .  14
       6.3.5.  Procedure Overview of the Broadcast Keycheck Exchange  15
   7.  Server Seed, MAC Algorithms and Generating MACs . . . . . . .  16
     7.1.  Server Seed . . . . . . . . . . . . . . . . . . . . . . .  16



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     7.2.  MAC Algorithms  . . . . . . . . . . . . . . . . . . . . .  16
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
     9.1.  Privacy . . . . . . . . . . . . . . . . . . . . . . . . .  17
     9.2.  Initial Verification of the Server Certificates . . . . .  18
     9.3.  Revocation of Server Certificates . . . . . . . . . . . .  18
     9.4.  Mitigating Denial-of-Service for broadcast packets  . . .  18
     9.5.  Delay Attack  . . . . . . . . . . . . . . . . . . . . . .  18
     9.6.  Random Number Generation  . . . . . . . . . . . . . . . .  20
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  20
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  20
     11.2.  Informative References . . . . . . . . . . . . . . . . .  21
   Appendix A.  (informative) TICTOC Security Requirements . . . . .  22
   Appendix B.  (normative) Inherent Association Protocol Messages .  23
     B.1.  Overview of NTS with Inherent Association Protocol  . . .  23
     B.2.  Access Message Exchange . . . . . . . . . . . . . . . . .  24
       B.2.1.  Goals of the Access Message Exchange  . . . . . . . .  24
       B.2.2.  Message Type: "client_access" . . . . . . . . . . . .  24
       B.2.3.  Message Type: "server_access" . . . . . . . . . . . .  24
       B.2.4.  Procedure Overview of the Access Exchange . . . . . .  24
     B.3.  Association Message Exchange  . . . . . . . . . . . . . .  25
       B.3.1.  Goals of the Association Exchange . . . . . . . . . .  25
       B.3.2.  Message Type: "client_assoc"  . . . . . . . . . . . .  25
       B.3.3.  Message Type: "server_assoc"  . . . . . . . . . . . .  26
       B.3.4.  Procedure Overview of the Association Exchange  . . .  26
     B.4.  Cookie Message Exchange . . . . . . . . . . . . . . . . .  27
       B.4.1.  Goals of the Cookie Exchange  . . . . . . . . . . . .  28
       B.4.2.  Message Type: "client_cook" . . . . . . . . . . . . .  28
       B.4.3.  Message Type: "server_cook" . . . . . . . . . . . . .  28
       B.4.4.  Procedure Overview of the Cookie Exchange . . . . . .  29
       B.4.5.  Broadcast Parameter Messages  . . . . . . . . . . . .  30
   Appendix C.  (normative) Using TESLA for Broadcast-Type Messages   32
     C.1.  Server Preparation  . . . . . . . . . . . . . . . . . . .  32
     C.2.  Client Preparation  . . . . . . . . . . . . . . . . . . .  34
     C.3.  Sending Authenticated Broadcast Packets . . . . . . . . .  35
     C.4.  Authentication of Received Packets  . . . . . . . . . . .  35
   Appendix D.  (informative) Dependencies . . . . . . . . . . . . .  37
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  39

1.  Introduction

   Time synchronization protocols are increasingly utilized to
   synchronize clocks in networked infrastructures.  Successful attacks
   against the time synchronization protocol can seriously degrade the
   reliable performance of such infrastructures.  Therefore, time
   synchronization protocols have to be secured if they are applied in
   environments that are prone to malicious attacks.  This can be



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   accomplished either by utilization of external security protocols,
   like IPsec or TLS, or by intrinsic security measures of the time
   synchronization protocol.

   The two most popular time synchronization protocols, the Network Time
   Protocol (NTP) [RFC5905] and the Precision Time Protocol (PTP)
   [IEEE1588], currently do not provide adequate intrinsic security
   precautions.  This document specifies generic security measures which
   enable these and possibly other protocols to verify the authenticity
   of the time server/master and the integrity of the time
   synchronization protocol packets.  The utilization of these measures
   for a given specific time synchronization protocol has to be
   described in a separate document.

   [RFC7384] specifies that a security mechanism for timekeeping must be
   designed in such a way that it does not degrade the quality of the
   time transfer.  This implies that for time keeping the increase in
   bandwidth and message latency caused by the security measures should
   be small.  Also, NTP as well as PTP work via UDP and connections are
   stateless on the server/master side.  Therefore, all security
   measures in this document are designed in such a way that they add
   little demand for bandwidth, that the necessary calculations can be
   executed in a fast manner, and that the measures do not require a
   server/master to keep state of a connection.

2.  Terminology

2.1.  Terms and Abbreviations

   MITM   Man In The Middle

   NTS    Network Time Security

   TESLA  Timed Efficient Stream Loss-tolerant Authentication

   MAC  Message Authentication Code

2.2.  Common Terminology for PTP and NTP

   This document refers to different time synchronization protocols, in
   particular to both the PTP and the NTP.  Throughout the document the
   term "server" applies to both a PTP master and an NTP server.
   Accordingly, the term "client" applies to both a PTP slave and an NTP
   client.







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3.  Security Threats

   The document "Security Requirements of Time Protocols in Packet
   Switched Networks" [RFC7384] contains a profound analysis of security
   threats and requirements for time synchronization protocols.

4.  Objectives

   The objectives of the NTS specification are as follows:

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

   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  Authorization: NTS enables the client to verify its time server's
      authorization.  NTS optionally enables the server to verify the
      client's authorization as well.

   o  Request-Response-Consistency: NTS enables a client to match an
      incoming response to a request it has sent.  NTS also enables the
      client to deduce from the response whether its request to the
      server has arrived without alteration.

   o  Applicability to Protocols: NTS can be used to secure different
      time synchronization protocols, specifically at least NTP and PTP.

   o  Integration with Protocols: A client or server running an NTS-
      secured version of a time protocol does not negatively affect
      other participants who are running unsecured versions of that
      protocol.

   o  Server-Side Statelessness: All security measures of NTS work
      without creating the necessity for a server to keep state of a
      connection.

   o  Prevention of Amplification Attacks: All communication introduced
      by NTS offers protection against abuse for amplification denial-
      of-service attacks.








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5.  NTS Overview

   NTS initially verifies the authenticity of the time server and
   exchanges a symmetric key, the so-called cookie, as well as a key
   input value (KIV).  The KIV can be opaque for the client.  After the
   cookie and the KIV are exchanged, the client then uses them to
   protect the authenticity and the integrity of subsequent unicast-type
   time synchronization packets.  In order to do this, a Message
   Authentication Code (MAC) is attached to each time synchronization
   packet.  The calculation of the MAC includes the whole time
   synchronization packet and the cookie which is shared between client
   and server.

   The cookie is calculated according to:

      cookie = MSB_<b> (MAC(server seed, KIV)),

   with the server seed as the key, where KIV is the client's key input
   value, and where the application of the function MSB_<b> returns only
   the b most significant bits.  The server seed is a random value of
   bit length b that the server possesses, which has to remain secret.
   The cookie deterministically depends on KIV 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
   [RFC7384].  See Section 7 for details on seed refreshing.

   Since the server does not keep a state of the client, it has to
   recalculate the cookie each time it receives a unicast time
   synchronization request from the client.  To this end, the client has
   to attach its KIV to each request (see Section 6.1).

   Note:  The communication of the KIV and the cookie can be performed
      between client and server directly, or via a third party key
      distribution entity.

   For broadcast-type messages, authenticity and integrity of the time
   synchronization packets are also ensured by a MAC, which is attached
   to the time synchronization packet by the sender.  Verification of
   the broadcast-type packets' authenticity is based on the TESLA
   protocol, in particular on its "not re-using keys" scheme, see
   Section 3.7.2 of [RFC4082].  TESLA uses a one-way chain of keys,
   where each key is the output of a one-way function applied to the
   previous key in the chain.  The server securely shares the last
   element of the chain with all clients.  The server splits time into
   intervals of uniform duration and assigns each key to an interval in
   reverse order.  At each time interval, the server sends a broadcast
   packet appended by a MAC, calculated using the corresponding key, and
   the key of the previous disclosure interval.  The client verifies the



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   MAC by buffering the packet until disclosure of the key in its
   associated disclosure interval occurs.  In order to be able to verify
   the timeliness of the packets, the client has to be loosely time
   synchronized with the server.  This has to be accomplished before
   broadcast associations can be used.  For checking timeliness of
   packets, NTS uses another, more rigorous check in addition to just
   the clock lookup used in the TESLA protocol.  For a more detailed
   description of how NTS employs and customizes TESLA, see Appendix C.

6.  Protocol Messages

   This section describes the types of messages needed for secure time
   synchronization with NTS.

   For some guidance on how these message types can be realized in
   practice, and integrated into the communication flow of existing time
   synchronization protocols, see [I-D.ietf-ntp-cms-for-nts-message], a
   companion document for NTS.  Said document describes ASN.1 encodings
   for those message parts that have to be added to a time
   synchronization protocol for security reasons.

6.1.  Unicast Time Synchronisation Messages

   In this message exchange, the usual time synchronization process is
   executed, with the addition of integrity protection for all messages
   that the server sends.  This message exchange can be repeatedly
   performed as often as the client desires and as long as the integrity
   of the server's time responses is verified successfully.

6.1.1.  Preconditions for the Unicast Time Synchronization Exchange

   Before this message exchange is available, there are some
   requirements that the client and server need to meet:

   o  They MUST negotiate the algorithm for the MAC used in the time
      synchronization messages.  Authenticity and integrity of the
      communication MUST be ensured.

   o  The client MUST know a key input value KIV.  Authenticity and
      integrity of the communication MUST be ensured.

   o  Client and server MUST exchange the cookie (which depends on the
      KIV as described in section Section 5).  Authenticity,
      confidentiality and integrity of the communication MUST be
      ensured.

   One way of realizing these requirements is to use the Association and
   Cookie Message Exchanges described in Appendix B.



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6.1.2.  Goals of the Unicast Time Synchronization Exchange

   The unicast time synchronization exchange:

   o  exchanges time synchronization data as specified by the
      appropriate time synchronization protocol,

   o  guarantees authenticity and integrity of the request to the
      server,

   o  guarantees authenticity and integrity of the response to the
      client,

   o  guarantees request-response-consistency to the client.

6.1.3.  Message Type: "time_request"

   This message is sent by the client when it requests a time exchange.
   It contains

   o  the NTS message ID "time_request",

   o  the negotiated version number,

   o  a nonce,

   o  the negotiated MAC algorithm,

   o  the client's key input value (for which the client knows the
      associated cookie),

   o  optional: a MAC (generated with the cookie as key) for
      verification of all of the above data.

6.1.4.  Message Type: "time_response"

   This message is sent by the server after it has received a
   time_request message.  Prior to this the server MUST recalculate the
   client's cookie by using the received key input value and the
   transmitted MAC algorithm.  The message contains

   o  the NTS message ID "time_response",

   o  the version number as transmitted in time_request,

   o  the server's time synchronization response data,

   o  the nonce transmitted in time_request,



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   o  a MAC (generated with the cookie as key) for verification of all
      of the above data.

6.1.5.  Procedure Overview of the Unicast Time Synchronization Exchange

   For a unicast time synchronization exchange, the following steps are
   performed:

   1.  The client sends a time_request message to the server.  The
       client MUST save the included nonce and the transmit_timestamp
       (from the time synchronization data) as a correlated pair for
       later verification steps.  Optionally, the client protects the
       request message with an appended MAC.

   2.  Upon receipt of a time_request message, the server performs the
       following steps:

       *  It re-calculates the cookie.

       *  If the request message contains a MAC the server re-calculates
          the MAC and compares this value with the MAC in the received
          data.

          +  If the re-calculated MAC does not match the MAC in the
             received data the server MUST stop the processing of the
             request.

          +  If the re-calculated MAC matches the MAC in the received
             data the server continues to process the request.

       *  The server computes the necessary time synchronization data
          and constructs a time_response message as given in
          Section 6.1.4.

   3.  The client awaits a reply in the form of a time_response message.
       Upon receipt, it checks:

       *  that the transmitted version number matches the one negotiated
          previously,

       *  that the transmitted nonce belongs to a previous time_request
          message,

       *  that the transmit_timestamp in that time_request message
          matches the corresponding time stamp from the synchronization
          data received in the time_response, and





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       *  that the appended MAC verifies the received synchronization
          data, version number and nonce.

       If at least one of the first three checks fails (i.e.  if the
       version number does not match, if the client has never used the
       nonce transmitted in the time_response message, or if it has used
       the nonce with initial time synchronization data different from
       that in the response), then the client MUST ignore this
       time_response message.  If the MAC is invalid, the client MUST do
       one of the following: abort the run or send another cookie
       request (because the cookie might have changed due to a server
       seed refresh).  If both checks are successful, the client SHOULD
       continue time synchronization.

            +-----------------------+
            | o Re-generate cookie  |
            | o Assemble response   |
            | o Generate MAC        |
            +-----------+-----------+
                        |
                      <-+->

    Server ----------------------------------------------->
                    /|     \
           time_    /       \  time_
           request /         \ response
                  /          \|
    Client ----------------------------------------------->

           <------ Unicast time ------>  <- Client-side ->
                  synchronization            validity
                     exchange                checks

   Procedure for unicast time synchronization exchange.

6.2.  Broadcast Time Synchronization Exchange

6.2.1.  Preconditions for the Broadcast Time Synchronization Exchange

   Before this message exchange is available, there are some
   requirements that the client and server need to meet:

   o  The client MUST receive all the information necessary to process
      broadcast time synchronization messages from the server.  This
      includes

      *  the one-way functions used for building the key chain,




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      *  the last key of the key chain,

      *  time interval duration,

      *  the disclosure delay (number of intervals between use and
         disclosure of a key),

      *  the time at which the next time interval will start, and

      *  the next interval's associated index.

   o  The communication of the data listed above MUST guarantee
      authenticity of the server, as well as integrity and freshness of
      the broadcast parameters to the client.

6.2.2.  Goals of the Broadcast Time Synchronization Exchange

   The broadcast time synchronization exchange:

   o  transmits (broadcast) time synchronization data from the server to
      the client as specified by the appropriate time synchronization
      protocol,

   o  guarantees to the client that the received synchronization data
      has arrived in a timely manner as required by the TESLA protocol
      and is trustworthy enough to be stored for later checks,

   o  additionally guarantees authenticity of a certain broadcast
      synchronization message in the client's storage.

6.2.3.  Message Type: "server_broad"

   This message is sent by the server over the course of its broadcast
   schedule.  It is part of any broadcast association.  It contains

   o  the NTS message ID "server_broad",

   o  the version number that the server is working under,

   o  time broadcast data,

   o  the index that belongs to the current interval (and therefore
      identifies the current, yet undisclosed, key),

   o  the disclosed key of the previous disclosure interval (current
      time interval minus disclosure delay),





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   o  a MAC, calculated with the key for the current time interval,
      verifying

      *  the message ID,

      *  the version number, and

      *  the time data.

6.2.4.  Procedure Overview of Broadcast Time Synchronization Exchange

   A broadcast time synchronization message exchange consists of the
   following steps:

   1.  The server follows the TESLA protocol by regularly sending
       server_broad messages as described in Section 6.2.3, adhering to
       its own disclosure schedule.

   2.  The client awaits time synchronization data in the form of a
       server_broadcast message.  Upon receipt, it performs the
       following checks:

       *  Proof that the MAC is based on a key that is not yet disclosed
          (packet timeliness).  This is achieved via a combination of
          checks.  First, the disclosure schedule is used, which
          requires loose time synchronization.  If this is successful,
          the client obtains a stronger guarantee via a key check
          exchange (see below).  If its timeliness is verified, the
          packet will be buffered for later authentication.  Otherwise,
          the client MUST discard it.  Note that the time information
          included in the packet will not be used for synchronization
          until its authenticity could also be verified.

       *  The client checks that it does not already know the disclosed
          key.  Otherwise, the client SHOULD discard the packet to avoid
          a buffer overrun.  If this check is successful, the client
          ensures 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 shown.  If it is falsified, the
          client MUST discard the packet.

       *  If the disclosed key is legitimate, then the client verifies
          the authenticity of any packet that it has received during the
          corresponding time interval.  If authenticity of a packet is
          verified, then it is released from the buffer and its time
          information can be utilized.  If the verification fails, then
          authenticity is not given.  In this case, the client MUST
          request authentic time from the server by means other than



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          broadcast messages.  Also, the client MUST re-initialize the
          broadcast sequence with a "client_bpar" message if the one-way
          key chain expires, which it can check via the disclosure
          schedule.

       See RFC 4082[RFC4082] for a detailed description of the packet
       verification process.

    Server ---------------------------------->
            \
             \  server_
              \ broad
              \|
    Client ---------------------------------->

            < Broadcast >  <- Client-side  ->
              time sync.      validity and
               exchange        timeliness
                                 checks

   Procedure for broadcast time synchronization exchange.

6.3.  Broadcast Keycheck

   This message exchange is performed for an additional check of packet
   timeliness in the course of the TESLA scheme, see Appendix C.

6.3.1.  Preconditions for the Broadcast Keycheck Exchange

   Before this message exchange is available, there are some
   requirements that the client and server need to meet:

   o  They MUST negotiate the algorithm for the MAC used in the time
      synchronization messages.  Authenticity and integrity of the
      communication MUST be ensured.

   o  The client MUST know a key input value KIV.  Authenticity and
      integrity of the communication MUST be ensured.

   o  Client and server MUST exchange the cookie (which depends on the
      KIV as described in section Section 5).  Authenticity,
      confidentiality and integrity of the communication MUST be
      ensured.

   These requirements conform to those for the unicast time
   synchronization exchange.  Accordingly, they too can be realized via
   the Association and Cookie Message Exchanges described in Appendix B
   (Appendix B).



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6.3.2.  Goals of the Broadcast Keycheck Exchange

   The keycheck exchange:

   o  guarantees to the client that the key belonging to the respective
      TESLA interval communicated in the exchange had not been disclosed
      before the client_keycheck message was sent.

   o  guarantees to the client the timeliness of any broadcast packet
      secured with this key if it arrived before client_keycheck was
      sent.

6.3.3.  Message Type: "client_keycheck"

   A message of this type is sent by the client in order to initiate an
   additional check of packet timeliness for the TESLA scheme.  It
   contains

   o  the NTS message ID "client_keycheck",

   o  the NTS version number negotiated during association,

   o  a nonce,

   o  an interval number from the TESLA disclosure schedule,

   o  the MAC algorithm negotiated during association,

   o  the client's key input value KIV, and

   o  optional: a MAC (generated with the cookie as key) for
      verification of all of the above data.

6.3.4.  Message Type: "server_keycheck"

   A message of this type is sent by the server upon receipt of a
   client_keycheck message during the broadcast loop of the server.
   Prior to this, the server MUST recalculate the client's cookie by
   using the received key input value and the transmitted MAC algorithm.
   It contains

   o  the NTS message ID "server_keycheck"

   o  the version number as transmitted in "client_keycheck,

   o  the nonce transmitted in the client_keycheck message,





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   o  the interval number transmitted in the client_keycheck message,
      and

   o  a MAC (generated with the cookie as key) for verification of all
      of the above data.

6.3.5.  Procedure Overview of the Broadcast Keycheck Exchange

   A broadcast keycheck message exchange consists of the following
   steps:

   1.  The client sends a client_keycheck message.  It MUST memorize the
       nonce and the time interval number that it sends as a correlated
       pair.

   2.  Upon receipt of a client_keycheck message the server performs as
       follows: If the client_keycheck message contains a MAC the server
       re-calculates the MAC and compares this value with the MAC in the
       received data.

       *  If the re-calculated MAC does not match the MAC in the
          received data the server MUST stop the processing of the
          request.

       *  If the re-calculated MAC matches the MAC in the received data
          the server continues to process the request: It looks up
          whether it has already disclosed the key associated with the
          interval number transmitted in that message.  If it has not
          disclosed it, it constructs and sends the appropriate
          server_keycheck message as described in Section 6.3.4.  For
          more details, see also Appendix C.

   3.  The client awaits a reply in the form of a server_keycheck
       message.  On receipt, it performs the following checks:

       *  that the transmitted version number matches the one negotiated
          previously,

       *  that the transmitted nonce belongs to a previous
          client_keycheck message,

       *  that the TESLA interval number in that client_keycheck message
          matches the corresponding interval number from the
          server_keycheck, and

       *  that the appended MAC verifies the received data.





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                             +----------------------+
                             | o Assemble response  |
                             | o Re-generate cookie |
                             | o Generate MAC       |
                             +-----------+----------+
                                         |
                                       <-+->
    Server --------------------------------------------->
            \                        /|     \
             \  server_    client_   /       \  server_
              \ broad      keycheck /         \ keycheck
              \|                   /          \|
    Client --------------------------------------------->
             <-------- Extended broadcast time  ------->
                      synchronization exchange

                            <---- Keycheck exchange --->

   Procedure for extended broadcast time synchronization exchange.

7.  Server Seed, MAC Algorithms and Generating MACs

7.1.  Server Seed

   The server has to calculate a random seed which has to be kept
   secret.  The server MUST generate a seed for each supported MAC
   algorithm, see Section 7.2.

   According to the requirements in [RFC7384], the server MUST refresh
   each server seed periodically.  Consequently, the cookie memorized by
   the client becomes obsolete.  In this case, the client cannot verify
   the MAC attached to subsequent time response messages and has to
   respond accordingly by re-initiating the protocol with a cookie
   request (Appendix B.4).

7.2.  MAC Algorithms

   MAC algorithms are used for calculation of the cookie and the actual
   MAC.  The client and the server negotiate a MAC algorithm during the
   association phase at the beginning.  The selected algorithm MUST be
   used for all cookie and MAC creation processes in that run.

   Note:  Any MAC algorithm is prone to be compromised in the future.  A
      successful attack on a MAC algorithm would enable any NTS client
      to derive the server seed from its own cookie.  Therefore, the
      server MUST have separate seed values for its different supported
      MAC algorithms.  This way, knowledge gained from an attack on a




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      MAC algorithm can at least only be used to compromise such clients
      who use this algorithm as well.

8.  IANA Considerations

   As mentioned, this document generically specifies security measures
   whose utilization for any given specific time synchronization
   protocol requires a separate document.  Consequently, this document
   itself does not have any IANA actions (TO BE REVIEWED).

9.  Security Considerations

   Aspects of security for time synchronization protocols are treated
   throughout this document.  For a comprehensive discussion of security
   requirements in time synchronization contexts, refer to [RFC7384].
   See Appendix A for a tabular overview of how NTS deals with those
   requirements.

   Additional NTS specific discussion of security issues can be found in
   the following subsections.

   Note:  Any separate document describing the utilization of NTS to a
      specific time synchronization protocol may additionally introduce
      discussion of its own specific security considerations.

9.1.  Privacy

   The payload of time synchronization protocol packets of two-way time
   transfer approaches like NTP and PTP consists basically of time
   stamps, which are not considered secret [RFC7384].  Therefore,
   encryption of the time synchronization protocol packet's payload is
   not considered in this document.  However, an attacker can exploit
   the exchange of time synchronization protocol packets for topology
   detection and inference attacks as described in [RFC7624].  To make
   such attacks more difficult, that draft recommends the encryption of
   the packet payload.  Yet, in the case of time synchronization
   protocols the confidentiality protection of time synchronization
   packet's payload is of secondary importance since the packet's meta
   data (IP addresses, port numbers, possibly packet size and regular
   sending intervals) carry more information than the payload.  To
   enhance the privacy of the time synchronization partners, the usage
   of tunnel protocols such as IPsec and MACsec, where applicable, is
   therefore more suited than confidentiality protection of the payload.








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9.2.  Initial Verification of the Server Certificates

   The client may wish to verify the validity of certificates during the
   initial association phase.  Since it generally has no reliable time
   during this initial communication phase, it is impossible to verify
   the period of validity of the certificates.  To solve this chicken-
   and-egg problem, the client has to rely on external means.

9.3.  Revocation of Server Certificates

   According to Section 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 (Appendix B.3.3).
   Furthermore, certificates may also be revoked prior to the normal
   expiration date.  To increase security the client MAY periodically
   verify the state of the server's certificate via Online Certificate
   Status Protocol (OCSP) Online Certificate Status Protocol (OCSP)
   [RFC6960].

9.4.  Mitigating Denial-of-Service for broadcast packets

   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, the client and the server 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 already knows, thus
   preventing memory flooding attacks.

   Discussion:  Note that 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.

9.5.  Delay Attack

   In a packet delay attack, an adversary with the ability to act as a
   MITM delays time synchronization packets between client and server
   asymmetrically [RFC7384].  This prevents the client from accurately
   measuring the network delay, and hence its time offset to the server
   [Mizrahi].  The delay attack does not modify the content of the
   exchanged synchronization packets.  Therefore, cryptographic means do
   not provide a feasible way to mitigate this attack.  However, several



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   non-cryptographic precautions can be taken in order to detect this
   attack.

   1.  Usage of multiple time servers: this enables the client to detect
       the attack, provided that the adversary is unable to delay the
       synchronization packets between the majority of servers.  This
       approach is commonly used in NTP to exclude incorrect time
       servers [RFC5905].

   2.  Multiple communication paths: The client and server utilize
       different paths for packet exchange as described in the I-D
       [I-D.ietf-tictoc-multi-path-synchronization].  The client can
       detect the attack, provided that the adversary is unable to
       manipulate the majority of the available paths [Shpiner].  Note
       that this approach is not yet available, neither for NTP nor for
       PTP.

   3.  Usage of an encrypted connection: the client exchanges all
       packets with the time server over an encrypted connection (e.g.
       IPsec).  This measure does not mitigate the delay attack, but it
       makes it more difficult for the adversary to identify the time
       synchronization packets.

   4.  For unicast-type messages: Introduction of a threshold value for
       the delay time of the synchronization packets.  The client can
       discard a time server if the packet delay time of this time
       server is larger than the threshold value.

   Additional provision against delay attacks has to be taken for
   broadcast-type messages.  This mode relies on the TESLA scheme which
   is based on the requirement that a client and the broadcast server
   are loosely time synchronized.  Therefore, a broadcast client has to
   establish time synchronization with its broadcast server before it
   starts utilizing broadcast messages for time synchronization.

   One possible way to achieve this initial synchronization is to
   establish a unicast association with its broadcast server until time
   synchronization and calibration of the packet delay time is achieved.
   After that, the client can establish a broadcast association with the
   broadcast server and utilizes TESLA to verify integrity and
   authenticity of any received broadcast packets.

   An adversary who is able to delay broadcast packets can cause a time
   adjustment at the receiving broadcast clients.  If the adversary
   delays broadcast packets continuously, then the time adjustment will
   accumulate until the loose time synchronization requirement is
   violated, which breaks the TESLA scheme.  To mitigate this
   vulnerability the security condition in TESLA has to be supplemented



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   by an additional check in which the client, upon receipt of a
   broadcast message, verifies the status of the corresponding key via a
   unicast message exchange with the broadcast server (see Appendix C.4
   for a detailed description of this check).  Note that a broadcast
   client should also apply the above-mentioned precautions as far as
   possible.

9.6.  Random Number Generation

   At various points of the protocol, the generation of random numbers
   is required.  The employed methods of generation need to be
   cryptographically secure.  See [RFC4086] for guidelines concerning
   this topic.

10.  Acknowledgements

   The authors would like to thank Tal Mizrahi, Russ Housley, Steven
   Bellovin, David Mills, Kurt Roeckx, Rainer Bermbach, Martin Langer
   and Florian Weimer for discussions and comments on the design of NTS.
   Also, thanks go to Harlan Stenn and Richard Welty for their technical
   review and specific text contributions to this document.

11.  References

11.1.  Normative References

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997,
              <http://www.rfc-editor.org/info/rfc2104>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC4082]  Perrig, A., Song, D., Canetti, R., Tygar, J., and B.
              Briscoe, "Timed Efficient Stream Loss-Tolerant
              Authentication (TESLA): Multicast Source Authentication
              Transform Introduction", RFC 4082, DOI 10.17487/RFC4082,
              June 2005, <http://www.rfc-editor.org/info/rfc4082>.

   [RFC7384]  Mizrahi, T., "Security Requirements of Time Protocols in
              Packet Switched Networks", RFC 7384, DOI 10.17487/RFC7384,
              October 2014, <http://www.rfc-editor.org/info/rfc7384>.






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11.2.  Informative References

   [I-D.ietf-ntp-cms-for-nts-message]
              Sibold, D., Teichel, K., Roettger, S., and R. Housley,
              "Protecting Network Time Security Messages with the
              Cryptographic Message Syntax (CMS)", draft-ietf-ntp-cms-
              for-nts-message-04 (work in progress), July 2015.

   [I-D.ietf-tictoc-multi-path-synchronization]
              Shpiner, A., Tse, R., Schelp, C., and T. Mizrahi, "Multi-
              Path Time Synchronization", draft-ietf-tictoc-multi-path-
              synchronization-02 (work in progress), April 2015.

   [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.

   [Mizrahi]  Mizrahi, T., "A game theoretic analysis of delay attacks
              against time synchronization protocols", in Proceedings
              of Precision Clock Synchronization for Measurement Control
              and Communication, ISPCS 2012, pp. 1-6, September 2012.

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,
              <http://www.rfc-editor.org/info/rfc4086>.

   [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
              "Network Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
              <http://www.rfc-editor.org/info/rfc5905>.

   [RFC6960]  Santesson, S., Myers, M., Ankney, R., Malpani, A.,
              Galperin, S., and C. Adams, "X.509 Internet Public Key
              Infrastructure Online Certificate Status Protocol - OCSP",
              RFC 6960, DOI 10.17487/RFC6960, June 2013,
              <http://www.rfc-editor.org/info/rfc6960>.

   [RFC7624]  Barnes, R., Schneier, B., Jennings, C., Hardie, T.,
              Trammell, B., Huitema, C., and D. Borkmann,
              "Confidentiality in the Face of Pervasive Surveillance: A
              Threat Model and Problem Statement", RFC 7624,
              DOI 10.17487/RFC7624, August 2015,
              <http://www.rfc-editor.org/info/rfc7624>.





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   [Shpiner]  Shpiner, A., Revah, Y., and T. Mizrahi, "Multi-path Time
              Protocols", in Proceedings of Precision Clock
              Synchronization for Measurement Control and Communication,
              ISPCS 2013, pp. 1-6, September 2013.

Appendix A.  (informative) TICTOC Security Requirements

   The following table compares the NTS specifications against the
   TICTOC security requirements [RFC7384].

   +---------+------------------------------+-------------+------------+
   | Section | Requirement from RFC 7384    | Requirement | NTS        |
   |         |                              | level       |            |
   +---------+------------------------------+-------------+------------+
   | 5.1.1   | Authentication of Servers    | MUST        | OK         |
   +---------+------------------------------+-------------+------------+
   | 5.1.1   | Authorization of Servers     | MUST        | OK         |
   +---------+------------------------------+-------------+------------+
   | 5.1.2   | Recursive Authentication of  | MUST        | OK         |
   |         | Servers (Stratum 1)          |             |            |
   +---------+------------------------------+-------------+------------+
   | 5.1.2   | Recursive Authorization of   | MUST        | OK         |
   |         | Servers (Stratum 1)          |             |            |
   +---------+------------------------------+-------------+------------+
   | 5.1.3   | Authentication and           | MAY         | Optional,  |
   |         | Authorization of Clients     |             | Limited    |
   +---------+------------------------------+-------------+------------+
   | 5.2     | Integrity protection         | MUST        | OK         |
   +---------+------------------------------+-------------+------------+
   | 5.3     | Spoofing Prevention          | MUST        | OK         |
   +---------+------------------------------+-------------+------------+
   | 5.4     | Protection from DoS attacks  | SHOULD      | OK         |
   |         | against the time protocol    |             |            |
   +---------+------------------------------+-------------+------------+
   | 5.5     | Replay protection            | MUST        | OK         |
   +---------+------------------------------+-------------+------------+
   | 5.6     | Key freshness                | MUST        | OK         |
   +---------+------------------------------+-------------+------------+
   |         | Security association         | SHOULD      | OK         |
   +---------+------------------------------+-------------+------------+
   |         | Unicast and multicast        | SHOULD      | OK         |
   |         | associations                 |             |            |
   +---------+------------------------------+-------------+------------+
   | 5.7     | Performance: no degradation  | MUST        | OK         |
   |         | in quality of time transfer  |             |            |
   +---------+------------------------------+-------------+------------+
   |         | Performance: lightweight     | SHOULD      | OK         |
   |         | computation                  |             |            |



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   +---------+------------------------------+-------------+------------+
   |         | Performance: storage         | SHOULD      | OK         |
   +---------+------------------------------+-------------+------------+
   |         | Performance: bandwidth       | SHOULD      | OK         |
   +---------+------------------------------+-------------+------------+
   | 5.8     | Confidentiality protection   | MAY         | NO         |
   +---------+------------------------------+-------------+------------+
   | 5.9     | Protection against Packet    | MUST        | Limited*)  |
   |         | Delay and Interception       |             |            |
   |         | Attacks                      |             |            |
   +---------+------------------------------+-------------+------------+
   | 5.10    | Secure mode                  | MUST        | OK         |
   +---------+------------------------------+-------------+------------+
   |         | Hybrid mode                  | SHOULD      | -          |
   +---------+------------------------------+-------------+------------+

   *) See discussion in Section 9.5.

   Comparison of NTS specification against Security Requirements of Time
             Protocols in Packet Switched Networks (RFC 7384)

Appendix B.  (normative) Inherent Association Protocol Messages

   This appendix presents a procedure that performs the association, the
   cookie, and also the broadcast parameter message exchanges between a
   client and a server.  This procedure is one possible way to achieve
   the preconditions listed in Sections Section 6.1.1, Section 6.2.1,
   and Section 6.3.1 while taking into account the objectives given in
   Section Section 4.

B.1.  Overview of NTS with Inherent Association Protocol

   This inherent association protocol applies X.509 certificates to
   verify the authenticity of the time server and to exchange the
   cookie.  This is done in two separate message exchanges, described
   below.  An additional required exchange in advance serves to limit
   the amplification potential of the association message exchange.

   A client needs a public/private key pair for encryption, with the
   public key enclosed in a certificate.  A server needs a public/
   private key pair for signing, with the public key enclosed in a
   certificate.  If a participant intends to act as both a client and a
   server, it MUST have two different key pairs for these purposes.

   If this protocol is employed, the hash value of the client's
   certificate is used as the client's key input value, i.e. the cookie
   is calculated according to:




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      cookie = MSB_<b> (MAC(server seed, H(certificate of client))),

   Where the hash function H is the one used in the MAC algorithm.  The
   client's certificate contains the client's public key and enables the
   server to identify the client, if client authorization is desired.

B.2.  Access Message Exchange

   This message exchange serves only to prevent the next (association)
   exchange from being abusable for amplification denial-of-service
   attacks.

B.2.1.  Goals of the Access Message Exchange

   The access message exchange:

   o  transfers a secret value from the server to the client
      (initiator),

   o  the secret value permits the client to initiate an association
      message exchange.

B.2.2.  Message Type: "client_access"

   This message is sent by a client who intends to perform an
   association exchange with the server in the future.  It contains:

   o  the NTS message ID "client_access".

B.2.3.  Message Type: "server_access"

   This message is sent by the server on receipt of a client_access
   message.  It contains:

   o  the NTS message ID "server_access",

   o  an access key.

B.2.4.  Procedure Overview of the Access Exchange

   For an access exchange, the following steps are performed:

   1.  The client sends a client_access message to the server.

   2.  Upon receipt of a client_access, the server calculates the access
       key.  It then sends a reply in the form of a server_access
       message.  The server must either memorize the access key or
       alternatively apply a means by which it can reconstruct the



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       access key.  Note that in both cases the access key must be
       correlated with the address of the requester.  Note also that if
       the server memorizes the access key for a requester, it has to
       keep state for a certain amount of time.

   3.  The client waits for a response in the form of a server_access
       message.  Upon receipt of one, it MUST memorize the included
       access key.

B.3.  Association Message Exchange

   In this message exchange, the participants negotiate the MAC and
   encryption algorithms that are used throughout the protocol.  In
   addition, the client receives the certification chain up to a trusted
   anchor.  With the established certification chain the client is able
   to verify the server's signatures and, hence, the authenticity of
   future NTS messages from the server is ensured.

B.3.1.  Goals of the Association Exchange

   The association exchange:

   o  enables the client to verify any communication with the server as
      authentic,

   o  lets the participants negotiate NTS version and algorithms,

   o  guarantees authenticity and integrity of the negotiation result to
      the client,

   o  guarantees to the client that the negotiation result is based on
      the client's original, unaltered request.

B.3.2.  Message Type: "client_assoc"

   This message is sent by the client if it wants to perform association
   with a server.  It contains

   o  the NTS message ID "client_assoc",

   o  a nonce,

   o  the access key obtained earlier via an access message exchange,

   o  the version number of NTS that the client wants to use (this
      SHOULD be the highest version number that it supports),

   o  a selection of accepted MAC algorithms, and



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   o  a selection of accepted encryption algorithms.

B.3.3.  Message Type: "server_assoc"

   This message is sent by the server upon receipt of client_assoc.  It
   contains

   o  the NTS message ID "server_assoc",

   o  the nonce transmitted in client_assoc,

   o  the client's proposal for the version number, selection of
      accepted MAC algorithms and selection of accepted encryption
      algorithms, as transmitted in client_assoc,

   o  the version number used for the rest of the protocol (which SHOULD
      be determined as the minimum over the client's suggestion in the
      client_assoc message and the highest supported by the server),

   o  the server's choice of algorithm for encryption and for MAC
      creation, all of which MUST be chosen from the client's proposals,

   o  a signature, calculated over the data listed above, with the
      server's private key and according to the signature algorithm
      which is also used for the certificates that are included (see
      below), and

   o  a chain of certificates, which starts at the server and goes up to
      a trusted authority; each certificate MUST be certified by the one
      directly following it.

B.3.4.  Procedure Overview of the Association Exchange

   For an association exchange, the following steps are performed:

   1.  The client sends a client_assoc message to the server.  It MUST
       keep the transmitted values for the version number and algorithms
       available for later checks.

   2.  Upon receipt of a client_assoc message, the server checks the
       validity of the included access key.  If it is not valid, the
       server MUST abort communication.  If it is valid, the server
       constructs and sends a reply in the form of a server_assoc
       message as described in Appendix B.3.3.  Upon unsuccessful
       negotiation for version number or algorithms the server_assoc
       message MUST contain an error code.





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   3.  The client waits for a reply in the form of a server_assoc
       message.  After receipt of the message it performs the following
       checks:

       *  The client checks that the message contains a conforming
          version number.

       *  It checks that the nonce sent back by the server matches the
          one transmitted in client_assoc,

       *  It also verifies that the server has chosen the encryption and
          MAC algorithms from its proposal sent in the client_assoc
          message and that this proposal was not altered.

       *  Furthermore, it performs authenticity checks on the
          certificate chain and the signature.

       If one of the checks fails, the client MUST abort the run.

            +------------------------+
            | o Check access key     |
            +------------------------+
            | o Choose version       |
            | o Choose algorithms    |
            | o Acquire certificates |
            | o Assemble response    |
            | o Create signature     |
            +-----------+------------+
                        |
                      <-+->

    Server --------------------------->
                    /|     \
           client_  /       \ server_
           assoc   /         \ assoc
                  /          \|
    Client --------------------------->

           <------ Association ----->
                    exchange

   Procedure for association and cookie exchange.

B.4.  Cookie Message Exchange

   During this message exchange, the server transmits a secret cookie to
   the client securely.  The cookie will later be used for integrity
   protection during unicast time synchronization.



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B.4.1.  Goals of the Cookie Exchange

   The cookie exchange:

   o  enables the server to check the client's authorization via its
      certificate (optional),

   o  supplies the client with the correct cookie and corresponding KIV
      for its association to the server,

   o  guarantees to the client that the cookie originates from the
      server and that it is based on the client's original, unaltered
      request.

   o  guarantees that the received cookie is unknown to anyone but the
      server and the client.

B.4.2.  Message Type: "client_cook"

   This message is sent by the client upon successful authentication of
   the server.  In this message, the client requests a cookie from the
   server.  The message contains

   o  the NTS message ID "client_cook",

   o  a nonce,

   o  the negotiated version number,

   o  the negotiated signature algorithm,

   o  the negotiated encryption algorithm,

   o  the negotiated MAC algorithm,

   o  the client's certificate.

B.4.3.  Message Type: "server_cook"

   This message is sent by the server upon receipt of a client_cook
   message.  The server generates the hash (the used hash function is
   the one used for the MAC algorithm) of the client's certificate, as
   conveyed during client_cook, in order to calculate the cookie
   according to Section 5.  This message contains

   o  the NTS message ID "server_cook"

   o  the version number as transmitted in client_cook,



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   o  a concatenated datum which is encrypted with the client's public
      key, according to the encryption algorithm transmitted in the
      client_cook message.  The concatenated datum contains

      *  the nonce transmitted in client_cook, and

      *  the cookie.

   o  a signature, created with the server's private key, calculated
      over all of the data listed above.  This signature MUST be
      calculated according to the transmitted signature algorithm from
      the client_cook message.

B.4.4.  Procedure Overview of the Cookie Exchange

   For a cookie exchange, the following steps are performed:

   1.  The client sends a client_cook message to the server.  The client
       MUST save the included nonce until the reply has been processed.

   2.  Upon receipt of a client_cook message, the server checks whether
       it supports the given cryptographic algorithms.  It then
       calculates the cookie according to the formula given in
       Section 5.  The server MAY use the client's certificate to check
       that the client is authorized to use the secure time
       synchronization service.  With this, it MUST construct a
       server_cook message as described in Appendix B.4.3.

   3.  The client awaits a reply in the form of a server_cook message;
       upon receipt it executes the following actions:

       *  It verifies that the received version number matches the one
          negotiated beforehand.

       *  It verifies the signature using the server's public key.  The
          signature has to authenticate the encrypted data.

       *  It decrypts the encrypted data with its own private key.

       *  It checks that the decrypted message is of the expected
          format: the concatenation of a nonce and a cookie of the
          expected bit lengths.

       *  It verifies that the received nonce matches the nonce sent in
          the client_cook message.

       If one of those checks fails, the client MUST abort the run.




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        +----------------------------+
        | o OPTIONAL: Check client's |
        |             authorization  |
        | o Generate cookie          |
        | o Encrypt inner message    |
        | o Generate signature       |
        +-------------+--------------+
                      |
                    <-+->

    Server --------------------------->
                  /|     \
          client_ /       \ server_
          cook   /         \ cook
                /          \|
    Client --------------------------->

           <--- Cookie exchange -->

   Procedure for association and cookie exchange.

B.4.5.  Broadcast Parameter Messages

   In this message exchange, the client receives the necessary
   information to execute the TESLA protocol in a secured broadcast
   association.  The client can only initiate a secure broadcast
   association after successful association and cookie exchanges and
   only if it has made sure that its clock is roughly synchronized to
   the server's.

   See Appendix C for more details on TESLA.

B.4.5.1.  Goals of the Broadcast Parameter Exchange

   The broadcast parameter exchange

   o  provides the client with all the information necessary to process
      broadcast time synchronization messages from the server, and

   o  guarantees authenticity, integrity and freshness of the broadcast
      parameters to the client.

B.4.5.2.  Message Type: "client_bpar"

   This message is sent by the client in order to establish a secured
   time broadcast association with the server.  It contains

   o  the NTS message ID "client_bpar",



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   o  the NTS version number negotiated during association,

   o  a nonce, and

   o  the signature algorithm negotiated during association.

B.4.5.3.  Message Type: "server_bpar"

   This message is sent by the server upon receipt of a client_bpar
   message during the broadcast loop of the server.  It contains

   o  the NTS message ID "server_bpar",

   o  the version number as transmitted in the client_bpar message,

   o  the nonce transmitted in client_bpar,

   o  the one-way functions used for building the key chain, and

   o  the disclosure schedule of the keys.  This contains:

      *  the last key of the key chain,

      *  time interval duration,

      *  the disclosure delay (number of intervals between use and
         disclosure of a key),

      *  the time at which the next time interval will start, and

      *  the next interval's associated index.

   o  The message also contains a signature signed by the server with
      its private key, verifying all the data listed above.

B.4.5.4.  Procedure Overview of the Broadcast Parameter Exchange

   A broadcast parameter exchange consists of the following steps:

   1.  The client sends a client_bpar message to the server.  It MUST
       remember the transmitted values for the nonce, the version number
       and the signature algorithm.

   2.  Upon receipt of a client_bpar message, the server constructs and
       sends a server_bpar message as described in Appendix B.4.5.3.

   3.  The client waits for a reply in the form of a server_bpar
       message, on which it performs the following checks:



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       *  The message must contain all the necessary information for the
          TESLA protocol, as listed in Appendix B.4.5.3.

       *  The message must contain a nonce belonging to a client_bpar
          message that the client has previously sent.

       *  Verification of the message's signature.

       If any information is missing or if the server's signature cannot
       be verified, the client MUST abort the broadcast run.  If all
       checks are successful, the client MUST remember all the broadcast
       parameters received for later checks.

            +---------------------+
            | o Assemble response |
            | o Create public-key |
            |   signature         |
            +----------+----------+
                       |
                     <-+->

    Server --------------------------------------------->
                   /|     \
           client_ /       \  server_
           bpar   /         \ bpar
                 /          \|
    Client --------------------------------------------->

           <------- Broadcast ------>  <- Client-side ->
                    parameter              validity
                    exchange                checks

   Procedure for unicast time synchronization exchange.

Appendix C.  (normative) Using TESLA for Broadcast-Type Messages

   For broadcast-type messages, NTS adopts the TESLA protocol with some
   customizations.  This appendix provides details on the generation and
   usage of the one-way key chain collected and assembled from
   [RFC4082].  Note that NTS uses the "not re-using keys" scheme of
   TESLA as described in Section 3.7.2. of [RFC4082].

C.1.  Server Preparation

   Server setup:






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   1.  The server determines a reasonable upper bound B on the network
       delay between itself and an arbitrary client, measured in
       milliseconds.

   2.  It determines the number n+1 of keys in the one-way key chain.
       This yields the number n of keys that are usable to authenticate
       broadcast packets.  This number n is therefore also the number of
       time intervals during which the server can send authenticated
       broadcast messages before it has to calculate a new key chain.

   3.  It divides time into n uniform intervals I_1, I_2, ..., I_n.
       Each of these time intervals has length L, measured in
       milliseconds.  In order to fulfill the requirement 3.7.2. of RFC
       4082, the time interval L has to be shorter than the time
       interval between the broadcast messages.

   4.  The server generates a random key K_n.

   5.  Using a one-way function F, the server generates a one-way chain
       of n+1 keys K_0, K_1, ..., K_{n} according to

          K_i = F(K_{i+1}).

   6.  Using another one-way function F', it generates a sequence of n
       MAC keys K'_0, K'_1, ..., K'_{n-1} according to

          K'_i = F'(K_i).

   7.  Each MAC key K'_i is assigned to the time interval I_i.

   8.  The server determines the key disclosure delay d, which is the
       number of intervals between using a key and disclosing it.  Note
       that although security is provided for all choices d>0, the
       choice still makes a difference:

       *  If d is chosen too short, the client might discard packets
          because it fails to verify that the key used for its MAC has
          not yet been disclosed.

       *  If d is chosen too long, the received packets have to be
          buffered for an unnecessarily long time before they can be
          verified by the client and be subsequently utilized for time
          synchronization.

       It is RECOMMENDED that the server calculate d according to

          d = ceil( 2*B / L) + 1,




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       where ceil yields the smallest integer greater than or equal to
       its argument.

   < - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
                         Generation of Keys

             F              F               F                 F
    K_0  <-------- K_1  <--------  ...  <-------- K_{n-1} <------- K_n
     |              |                              |                |
     |              |                              |                |
     | F'           | F'                           | F'             | F'
     |              |                              |                |
     v              v                              v                v
    K'_0           K'_1            ...           K'_{n-1}         K'_n
             [______________|____       ____|_________________|_______]
                   I_1             ...            I_{n-1}          I_n

                     Course of Time/Usage of Keys
   - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ->

     A schematic explanation of the TESLA protocol's one-way key chain

C.2.  Client Preparation

   A client needs the following information in order to participate in a
   TESLA broadcast:

   o  One key K_i from the one-way key chain, which has to be
      authenticated as belonging to the server.  Typically, this will be
      K_0.

   o  The disclosure schedule of the keys.  This consists of:

      *  the length n of the one-way key chain,

      *  the length L of the time intervals I_1, I_2, ..., I_n,

      *  the starting time T_i of an interval I_i.  Typically this is
         the starting time T_1 of the first interval;

      *  the disclosure delay d.

   o  The one-way function F used to recursively derive the keys in the
      one-way key chain,

   o  The second one-way function F' used to derive the MAC keys K'_0,
      K'_1, ... , K'_n from the keys in the one-way chain.




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   o  An upper bound D_t on how far its own clock is "behind" that of
      the server.

   Note that if D_t is greater than (d - 1) * L, then some authentic
   packets might be discarded.  If D_t is greater than d * L, then all
   authentic packets will be discarded.  In the latter case, the client
   SHOULD NOT participate in the broadcast, since there will be no
   benefit in doing so.

C.3.  Sending Authenticated Broadcast Packets

   During each time interval I_i, the server sends at most one
   authenticated broadcast packet P_i.  Such a packet consists of:

   o  a message M_i,

   o  the index i (in case a packet arrives late),

   o  a MAC authenticating the message M_i, with K'_i used as key,

   o  the key K_{i-d}, which is included for disclosure.

C.4.  Authentication of Received Packets

   When a client receives a packet P_i as described above, it first
   checks that it has not already received a packet with the same
   disclosed key.  This is done to avoid replay/flooding attacks.  A
   packet that fails this test is discarded.

   Next, the client begins to check the packet's timeliness by ensuring
   that according to the disclosure schedule and with respect to the
   upper bound D_t determined above, the server cannot have disclosed
   the key K_i yet.  Specifically, it needs to check that the server's
   clock cannot read a time that is in time interval I_{i+d} or later.
   Since it works under the assumption that the server's clock is not
   more than D_t "ahead" of the client's clock, the client can calculate
   an upper bound t_i for the server's clock at the time when P_i
   arrived.  This upper bound t_i is calculated according to

      t_i = R + D_t,

   where R is the client's clock at the arrival of P_i.  This implies
   that at the time of arrival of P_i, the server could have been in
   interval I_x at most, with

      x = floor((t_i - T_1) / L) + 1,





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   where floor gives the greatest integer less than or equal to its
   argument.  The client now needs to verify that

      x < i+d

   is valid (see also Section 3.5 of [RFC4082]).  If it is falsified, it
   is discarded.

   If the check above is successful, the client performs another more
   rigorous check: it sends a key check request to the server (in the
   form of a client_keycheck message), asking explicitly if K_i has
   already been disclosed.  It remembers the time stamp t_check of the
   sending time of that request as well as the nonce it used correlated
   with the interval number i.  If it receives an answer from the server
   stating that K_i has not yet been disclosed and it is able to verify
   the HMAC on that response, then it deduces that K_i was undisclosed
   at t_check and therefore also at R.  In this case, the client accepts
   P_i as timely.

   Next the client verifies that a newly disclosed key K_{i-d} belongs
   to the one-way key chain.  To this end, it applies the one-way
   function F to K_{i-d} until it can verify the identity with an
   earlier disclosed key (see Clause 3.5 in RFC 4082, item 3).

   Next the client verifies that the transmitted time value s_i belongs
   to the time interval I_i, by checking

      T_i =< s_i, and

      s_i < T_{i+1}.

   If it is falsified, the packet MUST be discarded and the client MUST
   reinitialize its broadcast module by performing time synchronization
   by other means than broadcast messages, and it MUST perform a new
   broadcast parameter exchange (because a falsification of this check
   yields that the packet was not generated according to protocol, which
   suggests an attack).

   If a packet P_i passes all the tests listed above, it is stored for
   later authentication.  Also, if at this time there is a package with
   index i-d already buffered, then the client uses the disclosed key
   K_{i-d} to derive K'_{i-d} and uses that to check the MAC included in
   package P_{i-d}. Upon success, it regards M_{i-d} as authenticated.








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Appendix D.  (informative) Dependencies

   +---------+--------------+--------+-------------------------------+
   | Issuer  |  Type        | Owner  | Description                   |
   +---------+--------------+--------+-------------------------------+
   | Server  | private key  | server | Used for server_assoc,        |
   | PKI     | (signature)  |        | server_cook, server_bpar.     |
   |         +--------------+--------+ The server uses the private   |
   |         | public key   | client | key to sign these messages.   |
   |         | (signature)  |        | The client uses the public    |
   |         +--------------+--------+ key to verify them.           |
   |         | certificate  | server | The certificate is used in    |
   |         |              |        | server_assoc messages, for    |
   |         |              |        | verifying authentication and  |
   |         |              |        | (optionally) authorization.   |
   +---------+--------------+--------+-------------------------------+
   | Client  | private key  | client | The server uses the client's  |
   | PKI     | (encryption) |        | public key to encrypt the     |
   |         +--------------+--------+ content of server_cook        |
   |         | public key   | server | messages. The client uses     |
   |         | (encryption) |        | the private key to decrypt    |
   |         +--------------+--------+ them. The certificate is      |
   |         | certificate  | client | sent in client_cook messages, |
   |         |              |        | where it is used for trans-   |
   |         |              |        | portation of the public key   |
   |         |              |        | as well as (optionally) for   |
   |         |              |        | verification of client        |
   |         |              |        | authorization.                |
   +---------+--------------+--------+-------------------------------+

   This table shows the kind of cryptographic resources that NTS
   participants of server and client role should have ready before NTS
   communication starts.


















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   ++===========================================++
   ||                                           ||
   || Secure Authentication and Cookie Exchange ||
   ||                                           ||
   ++=======_ _=================================++
             |
             |  At least one
             |  successful
             V
   ++=======[ ]=======++
   || Unicast Time    |>-----\   As long as further
   || Synchronization ||      |  synchronization
   || Exchange(s)     |<-----/   is desired
   ++=======_ _=======++
             |
              \                               Other (unspecified)
   Sufficient  \                          /   methods which give
   accuracy     \   either         or    /    sufficient accuracy
                 \----------\ /---------/
                             |
                             |
                             V
                  ++========[ ]=========++
                  || Broadcast          ||
                  || Parameter Exchange ||
                  ++========_ _=========++
                             |
                             |  One successful
                             |  per client
                             V
                   ++=======[ ]=======++
                   || Broadcast Time  |>--------\   As long as further
                   || Synchronization ||         |  synchronization
                   || Reception       |<--------/   is desired
                   ++=======_ _=======++
                             |
                            / \
                  either   /   \       or
               /----------/     \-------------\
              |                               |
              V                               V
   ++========[ ]========++         ++========[ ]========++
   || Keycheck Exchange ||         || Keycheck Exchange ||
   ++===================++         || with TimeSync     ||
                                   ++===================++

   This diagram shows the dependencies between the different message
   exchanges and procedures which NTS offers.



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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
   Google Inc.

   Email: stephen.roettger@googlemail.com


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

   Phone: +49-(0)531-592-8421
   Email: kristof.teichel@ptb.de
























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