NTP Working Group                                              D. Sibold
Internet-Draft                                                       PTB
Intended status: Standards Track                             S. Roettger
Expires: April 26, July 20, 2015                                       Google Inc Inc.
                                                              K. Teichel
                                                                     PTB
                                                        October 23, 2014
                                                        January 16, 2015

                         Network Time Security
              draft-ietf-ntp-network-time-security-05.txt
              draft-ietf-ntp-network-time-security-06.txt

Abstract

   This document describes the Network Time Security (NTS) protocol (NTS), a collection of
   measures that
   enables 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, 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
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   This Internet-Draft will expire on April 26, July 20, 2015.

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   Copyright (c) 2014 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Security Threats  . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Objectives  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Terms and Abbreviations . . . . . . . . . . . . . . . . . . .   5   4
   5.  NTS Overview  . . . . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Symmetric and Client/Server Mode  . . . . . . . . . . . .   5
     5.2.  Broadcast Mode  . . . . . . . . . . . . . . . . . . . . .   5   4
   6.  Protocol Messages . . . . . . . . . . . . . . . . . . . . . .   6   5
     6.1.  Association Messages  . . . . . . . . . . . . . . . . . .   6
       6.1.1.  Message Type: "client_assoc"  . . . . . . . . . . . .   7   6
       6.1.2.  Message Type: "server_assoc"  . . . . . . . . . . . .   7   6
     6.2.  Cookie Messages . . . . . . . . . . . . . . . . . . . . .   8   7
       6.2.1.  Message Type: "client_cook" . . . . . . . . . . . . .   8   7
       6.2.2.  Message Type: "server_cook" . . . . . . . . . . . . .   8   7
     6.3.  Unicast Time Synchronisation Messages . . . . . . . . . .   9   8
       6.3.1.  Message Type: "time_request"  . . . . . . . . . . . .   9   8
       6.3.2.  Message Type: "time_response" . . . . . . . . . . . .   9   8
     6.4.  Broadcast Parameter Messages  . . . . . . . . . . . . . .  10   9
       6.4.1.  Message Type: "client_bpar" . . . . . . . . . . . . .  10   9
       6.4.2.  Message Type: "server_bpar" . . . . . . . . . . . . .  10   9
     6.5.  Broadcast Messages  . . . . . . . . . . . . . . . . . . .  11  10
       6.5.1.  Message Type: "server_broad"  . . . . . . . . . . . .  11  10
     6.6.  Broadcast Key Check . . . . . . . . . . . . . . . . . . .  11  10
       6.6.1.  Message Type: "client_keycheck" . . . . . . . . . . .  11  10
       6.6.2.  Message Type: "server_keycheck" . . . . . . . . . . .  12  11
   7.  Protocol Sequence  Message Dependencies  . . . . . . . . . . . . . . . . . . . .  11
   8.  Server Seed Considerations  . . . .  12
     7.1.  The Client . . . . . . . . . . . . .  12
   9.  Hash Algorithms and MAC Generation  . . . . . . . . . .  12
       7.1.1.  The Client in Unicast Mode . . .  13
     9.1.  Hash Algorithms . . . . . . . . . .  12
       7.1.2.  The Client in Broadcast Mode . . . . . . . . . . .  13
     9.2.  MAC Calculation .  14
     7.2.  The Server . . . . . . . . . . . . . . . . . . . .  13
   10. IANA Considerations . . .  16
       7.2.1.  The Server in Unicast Mode . . . . . . . . . . . . .  16
       7.2.2.  The Server in Broadcast Mode . . . . .  13
   11. Security Considerations . . . . . . .  16
   8.  Server Seed Considerations . . . . . . . . . . . .  13
     11.1.  Privacy  . . . . .  17
     8.1.  Server Seed Refresh . . . . . . . . . . . . . . . . . . .  17
     8.2.  13
     11.2.  Initial Verification of the Server Seed Algorithm Certificates  . . . .  14
     11.3.  Revocation of Server Certificates  . . . . . . . . . . .  14
     11.4.  Mitigating Denial-of-Service for broadcast packets . . .  17
     8.3.  Server Seed Lifetime  14
     11.5.  Delay Attack . . . . . . . . . . . . . . . . . .  17
   9.  Hash Algorithms and MAC Generation . . . .  15
   12. Acknowledgements  . . . . . . . . .  17
     9.1.  Hash Algorithms . . . . . . . . . . . . .  16
   13. References  . . . . . . . .  17
     9.2.  MAC Calculation . . . . . . . . . . . . . . . . .  16
     13.1.  Normative References . . . .  18
   10. IANA Considerations . . . . . . . . . . . . . .  16
     13.2.  Informative References . . . . . . .  18
   11. Security Considerations . . . . . . . . . .  17
   Appendix A.  TICTOC Security Requirements . . . . . . . . .  18
     11.1.  Initial Verification of the Server Certificates . . .  17
   Appendix B.  Using TESLA for Broadcast-Type Messages  .  18
     11.2.  Revocation of Server Certificates . . . . .  19
     B.1.  Server Preparation  . . . . . .  18
     11.3.  Usage of NTP Pools . . . . . . . . . . . . . . . . . . .  19
     11.4.  Denial-of-Service in Broadcast Mode  . . . . . . . . . .  19
     11.5.  Delay Attack . . . . . . . . . . . . . . . . . . . . . .  19
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  20
   13. References  . . . . . .  19
     B.2.  Client Preparation  . . . . . . . . . . . . . . . . . . .  20
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  21
     13.2.  Informative References . . . . . . . . . . . . . . . . .  21
   Appendix A.  Flow Diagrams of Client Behaviour  . . . . . . . . .  22
   Appendix B.  TICTOC Security Requirements . . . . . . . . . . . .  24
   Appendix C.  Broadcast Mode . . . . . . . . . . . . . . . . . . .  25
     C.1.  Server Preparations . . . . . . . . . . . . . . . . . . .  25
     C.2.  Client Preparation  . . . . . . . . . . . . . . . . . . .  27
     C.3.
     B.3.  Sending Authenticated Broadcast Packets . . . . . . . . .  27
     C.4.  21
     B.4.  Authentication of Received Packets  . . . . . . . . . . .  28  21
   Appendix D. C.  Random Number Generation . . . . . . . . . . . . . .  29  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  29  23

1.  Introduction

   Time synchronization protocols are increasingly 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 have to be secured if they
   are applied in environments that are prone to malicious attacks.
   This can be accomplished either by utilization of external security
   protocols
   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 security measures for NTP and
   PTP which enable
   these protocols to verify the authenticity of the time server and the
   integrity of the time synchronization protocol packets.

   The protocol is given measures are specified with the prerequisite in mind that
   precise timekeeping can only be accomplished with stateless time
   synchronization communication, which excludes the utilization of
   standard security protocols protocols, like IPsec or TLS TLS, for time
   synchronization messages.  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 [RFC7384].

   Note:

      The intent

      It is recommended that details on how to formulate the protocol apply NTS to specific
      time synchronization protocols be applicable to NTP
      and also PTP.  In the current state the specification focuses on
      the application to NTP. formulated in separate
      documents, with one separate document for each protocol.

2.  Security Threats

   A profound analysis of security threats and requirements for NTP and
   PTP time
   synchronization protocols can be found in the "Security Requirements
   of Time Protocols in Packet Switched Networks" [RFC7384].

3.  Objectives

   The objectives of the NTS specification are as follows:

   o  Authenticity: NTS enables the client a client/slave to authenticate its time
      servers.
      server(s)/master(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  Modes of operation: All operational modes of NTP are supported.

   o  Operational modes of PTP should  Integration with protocols: NTS can be supported as far as possible.

   o  Hybrid mode: Both used to secure and insecure communication modes are
      possible for different
      time synchronization protocols, specifically at least NTP servers and clients, respectively.

   o  Compatibility:

      *  Unsecured NTP associations shall not be affected.

      * PTP.
      An NTP client or server that running an NTS-secured version of a time
      protocol does not support NTS shall not be affected
         by NTS authentication requests. negatively affect other participants who are
      running unsecured versions of that protocol.

4.  Terms and Abbreviations

   MITM   Man In The Middle

   NTP    Network Time Protocol [RFC5905]

   NTS    Network Time Security

   PTP    Precision Time Protocol [IEEE1588]

   TESLA  Timed Efficient Stream Loss-Tolerant Loss-tolerant Authentication

5.  NTS Overview

5.1.  Symmetric and Client/Server Mode

   NTS applies X.509 certificates to verify the authenticity of the time
   server
   server/master and to exchange a symmetric key, the so-called cookie.
   This cookie is then used to protect the authenticity and the
   integrity of the subsequent unicast-type time synchronization packets packets.
   This is done by means of a Message Authentication Code (MAC), which
   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_128 MSB_<b> (HMAC(server seed, H(certificate of client))),

   with the server seed as the key, where H is a hash function, and
   where the function MSB_128 MSB_<b> cuts off the 128 b most significant bits of
   the result of the HMAC function.  The server seed is a 128 bit random value
   of bit length b that the server, server possesses, which has to be kept
   secret.  The cookie never changes as long as the server seed stays
   the same, but the server seed has to be refreshed periodically in
   order to provide key freshness as required in [RFC7384].  See
   Section 8 for details on the seed refresh and Section 7.1.1 for refreshing.

   Since the client's reaction to it.

   The server does not keep a state of the client.  Therefore 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 the hash value of its certificate to each request (see
   Section 6.3).

5.2.  Broadcast Mode

   Just as in the case of the client server mode and symmetric mode,

   For broadcast-type messages, authenticity and integrity of the NTP time
   synchronization packets are also ensured by a MAC, which is attached
   to the NTP 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
   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 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 a broadcast packet appended by a MAC, calculated using
   the corresponding key, and the key of the previous disclosure
   interval.  The client verifies the MAC by buffering the packet until the
   disclosure of the key in its associated disclosure interval. interval occurs.
   In order to be able to verify the validity of the key, the client has
   to be loosely time synchronized to with the server.  This has to be
   accomplished during the initial client server exchange between the
   broadcast client and the server.  In addition, NTS uses another, more
   rigorous check to than what is used in the TESLA protocol.  For a more
   detailed description of how NTS employs and customizes TESLA, see
   Appendix C. B.

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, for use with and integrated into the communication flow of existing time
   synchronization protocols, see
   [I-D.ietf-ntp-cms-for-nts-messages], [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 as well as CMS
   (Cryptographic Message Syntax, see [RFC5652]) conventions that can be
   used to get the cryptographic aspects right.

   Note that currently, the companion document describes realizations of
   NTS messages only for utilization with NTP, in which the NTS specific
   data are enclosed in extension fields on top of NTP packets.  A
   specification of NTS messages for PTP will have to be developed
   accordingly.

   The steps described in Section 6.1 - Section 6.3 belong to the
   unicast mode, while Section 6.4 and Section 6.5 explain the steps
   involved in the broadcast mode of NTS.

6.1.  Association Messages

   In this message exchange, the hash and encryption algorithms that are
   used throughout the protocol are negotiated.  Also,  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.

6.1.1.  Message Type: "client_assoc"

   The protocol sequence starts with the client sending an association
   message, called client_assoc.  This message contains

   o  the NTS message ID "client_assoc",

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

   o  the hostname of the client,

   o  a selection of accepted hash algorithms, and

   o  a selection of accepted encryption algorithms.

6.1.2.  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 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 hostname of the server, and

   o  the server's choice of algorithm for encryption and for
      cryptographic hashing, all of which MUST be chosen from the
      client's proposals. 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 which that are included (see
      below), and

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

6.2.  Cookie Messages

   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.

6.2.1.  Message Type: "client_cook"

   This message is sent by the client, 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  the negotiated version number,

   o  the negotiated signature algorithm,

   o  the negotiated encryption algorithm,

   o  a 128-bit nonce,

   o  the negotiated hash algorithm H,

   o  the client's certificate.

6.2.2.  Message Type: "server_cook"

   This message is sent by the server, server upon receipt of a client_cook
   message.  The server generates the hash of the client's certificate,
   as conveyed during client_cook, in order to calculate the cookie
   according to Section 5.1. 5.  This message contains

   o  the NTS message ID "server_cook"

   o  the version number as transmitted in client_cook,
   o  a concatenated datum, 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.

6.3.  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 can be repeatedly exchanged as
   often as the client desires and as long as the integrity of the
   server's time responses is verified successfully.

6.3.1.  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 128-bit nonce,

   o  the negotiated hash algorithm H,

   o  the hash of the client's certificate under H.

6.3.2.  Message Type: "time_response"

   This message is sent by the server, server after it has received a
   time_request message.  Prior to this the server MUST recalculate the
   client's cookie by using the hash of the client's certificate and the
   transmitted hash 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 128-bit nonce transmitted in time_request,

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

6.4.  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 a successful unicast run, see Section 7.1.2. run.

   See Appendix C B for more details on TESLA.

6.4.1.  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",

   o  the NTS version number negotiated during association in unicast
      mode,

   o  the client's hostname, and

   o  the signature algorithm negotiated during unicast.

6.4.2.  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 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.

6.5.  Broadcast Messages

   Via this message, the server keeps sending broadcast time
   synchronization messages to all participating clients.

6.5.1.  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's broadcast mode server is working under,

   o  time broadcast data,

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

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

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

      *  the message ID,

      *  the version number, and

      *  the time data.

6.6.  Broadcast Key Check

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

6.6.1.  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 chosen for the broadcast, negotiated during association in unicast
      mode,

   o  a 128-bit nonce,

   o  an interval number from the TESLA disclosure schedule,

   o  the hash algorithm H negotiated in unicast mode, and

   o  the hash of the client's certificate under H.

6.6.2.  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 this, the server MUST recalculate the client's cookie by
   using the hash of the client's certificate and the transmitted hash
   algorithm.  It contains

   o  the NTS message ID "server_keycheck"

   o  the version number that the server's broadcast mode is working
      under, as transmitted in "client_keycheck,

   o  the 128-bit nonce transmitted in the client_keycheck message,

   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.

7.  Protocol Sequence

7.1.  The Client

7.1.1.  The Client in Unicast Mode

   For a unicast run, the  Message Dependencies
             +--------------------+
             |Association Exchange|
             +--------------------+
                       |
            At least one successful
                       |
                       v
               +---------------+
               |Cookie Exchange|
               +---------------+
                       |
            At least one successful
                       |
                       v
   +----------------------------------------+
   |Unicast Time Synchronization Exchange(s)|
   +----------------------------------------+
                       |
   Until sufficient accuracy has been reached
                       |
                       v
         +----------------------------+
         |Broadcast Parameter Exchange|
         +----------------------------+
                       |
           One successful per client performs the following steps:

   1.  It sends
                       |
                       v
   +----------------------------------------+
   |Broadcast Time Synchronization Reception|
   +----------------------------------------+
                       |
           Whenever deemed necessary
                       |
                       v
              +-----------------+
              |Keycheck Exchange|
              +-----------------+

8.  Server Seed Considerations

   The server has to calculate a client_assoc message random seed which has to the server.  It be kept
   secret.  The server MUST keep the
       transmitted values for version number and algorithms available
       for later checks.

   2.  It waits for generate a reply seed for each supported hash
   algorithm, see Section 9.1.

   According to the requirements in [RFC7384], the form of a server_assoc message.
       After receipt of server MUST refresh
   each server seed periodically.  Consequently, the message it performs cookie memorized by
   the following checks:

       *  The client checks that becomes obsolete.  In this case, the message contains a conform version
          number.

       *  It also verifies that client cannot verify
   the server MAC attached to subsequent time response messages and has chosen to
   respond accordingly by re-initiating the encryption protocol with a cookie
   request (Section 6.2).

9.  Hash Algorithms and
          hash MAC Generation

9.1.  Hash Algorithms

   Hash algorithms from its proposal sent in the client_assoc
          message.

       *  Furthermore, it performs authenticity checks on are used at different points: calculation of the
          certificate chain
   cookie and the signature for the version number.

       If one MAC, and hashing of the checks fails, the client's certificate.  The
   client MUST abort the run.
       Discussion:

          Note that by performing and the above message exchange and checks,
          the client validates the authenticity of its immediate NTP
          server only.  It does not recursively validate the
          authenticity of each NTP server on the time synchronization
          chain.  Recursive authentication (and authorization) as
          formulated in [RFC7384] depends on the chosen trust anchor.

   3.  Next, it sends a client_cook message to the server.  The client
       MUST save the included nonce until the reply has been processed.

   4.  It 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 before.

       *  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 128 bit nonce and a 128 bit
          cookie.

       *  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.

   5.  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.

   6.  It awaits a reply in the form of a time_response message.  Upon
       receipt, it checks:

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

       *  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

       *  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 go back to step 5 (because
       the cookie might have changed due to a server seed refresh).  If
       both checks are successful, the client SHOULD continue time
       synchronization by going back to step 7.

   The client's behavior in unicast mode is also expressed in Figure 1.

7.1.2.  The Client in Broadcast Mode

   To establish a secure broadcast association with a broadcast server,
   the client MUST initially authenticate the broadcast server and
   securely synchronize its time to it up to an upper bound for its time
   offset in unicast mode.  After that, the client performs the
   following steps:

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

   2.  It waits for a reply in the form of a server_bpar message after
       which it performs the following checks:

       *  The message must contain all the necessary information for the
          TESLA protocol, as listed in Section 6.4.2.

       *  Verification of the message's signature.

       If any information is missing or 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.

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

       1.  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 the loose time synchronization.  If this
           is successful, the client gets a stronger guarantee via a key
           check exchange: it sends a client_keycheck message and waits
           for the appropriate response.  Note that it needs to memorize
           the nonce and the time interval number that it sends as a
           correlated pair.  For more detail on both of the mentioned
           timeliness checks, see Appendix Appendix C.4.  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.

       2.  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 verified, 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 falsified, the client MUST
           discard the packet.

       3.  If the disclosed key is legitimate, then 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, then
           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.

   The client MUST restart the broadcast sequence with a client_bpar
   message Section 6.4.1 if the one-way key chain expires.

   The client's behavior in broadcast mode can also be seen in Figure 2.

7.2.  The Server

7.2.1.  The Server in Unicast Mode

   To support unicast mode, the server MUST be ready to perform the
   following actions:

   o  Upon receipt of a client_assoc message, the server constructs and
      sends a reply in the form of a server_assoc message as described
      in Section 6.1.2.

   o  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 5.1.  With this, it MUST construct a server_cook message
      as described in Section 6.2.2.

   o  Upon receipt of a time_request message, the server re-calculates
      the cookie, then computes the necessary time synchronization data
      and constructs a time_response message as given in Section 6.3.2.

   The server MUST refresh its server seed periodically (see
   Section 8.1).

7.2.2.  The Server in Broadcast Mode

   A broadcast server MUST also support unicast mode, in order to
   provide the initial time synchronization which is a precondition for
   any broadcast association.  To support NTS broadcast, the server MUST
   additionally be ready to perform the following actions:

   o  Upon receipt of a client_bpar message, the server constructs and
      sends a server_bpar message as described in Section 6.4.2.

   o  Upon receipt of a client_keycheck message, the server looks up if
      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 6.6.2.  For more detail, see also Appendix C.

   o  The server follows the TESLA protocol in all other aspects, by
      regularly sending server_broad messages as described in
      Section 6.5.1, adhering to its own disclosure schedule.

   It is also the server's responsibility to watch for the expiration
   date of the one-way key chain and generate a new key chain
   accordingly.

8.  Server Seed Considerations

   The server has to calculate a random seed which has to be kept
   secret.  The server MUST generate a seed for each supported hash
   algorithm, see Section 9.1.

8.1.  Server Seed Refresh

   According to the requirements in [RFC7384] the server MUST refresh
   each server seed periodically.  As a consequence, 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 (Section 6.2).

8.2.  Server Seed Algorithm

8.3.  Server Seed Lifetime

9.  Hash Algorithms and MAC Generation

9.1.  Hash Algorithms

   Hash algorithms are used at different points: calculation of the
   cookie and the MAC, and hashing of the client's certificate.  Client
   and server negotiate a hash algorithm H during the
   association message exchange (Section 6.1) at the beginning of a
   unicast run.  The selected algorithm H is used for all hashing
   processes in that run.

   In broadcast mode, the TESLA scheme, hash algorithms are used as pseudo random pseudo-random
   functions to construct the one-way key chain.  Here, the utilized
   hash algorithm is communicated by the server and is non-negotiable.

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

   o  it MUST NOT include SHA-1 or weaker algorithms,

   o  it MUST include SHA-256 or stronger algorithms.

   Note

   Note:

      Any hash algorithm is prone to be compromised in the future.  A
      successful attack on a hash algorithm would enable any NTS client
      to derive the server seed from their its own cookie.  Therefore, the
      server MUST have separate seed values for its different supported
      hash algorithms.  This way, knowledge gained from an attack on a
      hash algorithm H can at least only be used to compromise such
      clients who use hash algorithm H as well.

9.2.  MAC Calculation

   For the calculation of the MAC, client and server are using use a Keyed-
   Hash Keyed-Hash
   Message Authentication Code (HMAC) approach [RFC2104].  The HMAC is
   generated with the hash algorithm specified by the client (see
   Section 9.1).

10.  IANA Considerations

11.  Security Considerations

11.1.  Privacy

   tbd

11.2.  Initial Verification of the Server Certificates

   The client has to verify the validity of the certificates during the
   certification message exchange (Section 6.1.2).  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].

11.2.

11.3.  Revocation of Server Certificates

   According to Section 8.1, 8, it is the client's responsibility to initiate
   a new association with the server after the server's certificate
   expires.  To this end end, the client reads the expiration date of the
   certificate during the certificate message exchange (Section 6.1.2).  Besides,
   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 OCSP periodically.

11.3.  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. OCSP.

11.4.  Mitigating Denial-of-Service in Broadcast Mode 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 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  Therefore, the client can discard any
   packet that contains a disclosed key it knows already, already knows, thus
   preventing memory flooding attacks.

   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.

11.5.  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 to measure from accurately
   measuring the network delay, and hence its time offset to the server, accurately server
   [Mizrahi].  The delay attack does not modify the content of the
   exchanged synchronization packets.  Therefore  Therefore, cryptographic means do
   not provide a feasible way to mitigate this attack.  However, several
   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 attack, provided that the adversary is unable to delay the
       synchronizations
       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 are utilizing utilize
       different paths for packet exchange as described in the I-D
       [I-D.shpiner-multi-path-synchronization].  The client can detect
       the attack 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 attack, but it
       makes it more difficult for the adversary to identify the time
       synchronization packets.

   4.  For the unicast mode: 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 in the
   broadcast mode. 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 maintains
   time synchronization by utilization of the
   starts utilizing broadcast mode. messages for time synchronization.  To
   this
   end end, it initially establishes a unicast association with its
   broadcast server until time synchronization and calibration of the
   packet delay time is achieved.  After that it establishes a broadcast
   association
   to 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
   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 section Appendix C.4 B.4
   for a detailed description of this check).  Note,  Note that a broadcast
   client should also apply the above mentioned above-mentioned precautions as far as
   possible.

12.  Acknowledgements

   The authors would like to thank Russ Housley, Steven Bellovin, David
   Mills and Kurt Roeckx for discussions and comments on the design of
   NTS.  Also, thanks go to Harlan Stenn for his technical review and
   specific text contributions to this document.

13.  References

13.1.  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.

   [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, June 2005.

   [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, September 2009.

   [RFC5905]  Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network
              Time Protocol Version 4: Protocol and Algorithms
              Specification", Source Authentication
              Transform Introduction", RFC 5905, 4082, June 2010. 2005.

   [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, September 2009.

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

   [RFC7384]  Mizrahi, T., "Security Requirements of Time Protocols in
              Packet Switched Networks", RFC 7384, October 2014.

13.2.  Informative References

   [I-D.ietf-ntp-cms-for-nts-message]
              Sibold, D., Roettger, S., Teichel, K., and R. Housley,
              "Protecting Network Time Security Messages with the
              Cryptographic Message Syntax (CMS)", draft-ietf-ntp-cms-
              for-nts-message-00 (work in progress), October 2014.

   [I-D.shpiner-multi-path-synchronization]
              Shpiner, A., Tse, R., Schelp, C., and T. Mizrahi, "Multi-
              Path Time Synchronization", draft-shpiner-multi-path-
              synchronization-03 (work in progress), February 2014.

   [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, D., Schiller, J., and S. Crocker, "Randomness
              Requirements for Security", BCP 106, RFC 4086, June 2005.

   [RFC7384]  Mizrahi, T., "Security Requirements of Time Protocols in
              Packet Switched Networks", RFC 7384, October 2014.

   [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.  Flow Diagrams of Client Behaviour

                        +---------------------+
                        |Association Messages |
                        +----------+----------+
                                   |
   +------------------------------>o
   |                               |
   |                               v
   |                       +---------------+
   |                       |Cookie Messages|
   |                       +-------+-------+
   |                               |
   |                               o<------------------------------+
   |                               |                               |
   |                               v                               |
   |                     +-------------------+                     |
   |                     |Time Sync. Messages|                     |
   |                     +---------+---------+                     |
   |                               |                               |
   |                               v                               |
   |                            +-----+                            |
   |                            |Check|                            |
   |                            +--+--+                            |
   |                               |                               |
   |            /------------------+------------------\            |
   |           v                   v                   v           |
   |     .-----------.      .-------------.        .-------.       |
   |    ( MAC Failure )    ( Nonce Failure )      ( Success )      |
   |     '-----+-----'      '------+------'        '---+---'       |
   |           |                   |                   |           |
   |           v                   v                   v           |
   |    +-------------+     +-------------+     +--------------+   |
   |    |Discard Data |     |Discard Data |     |Sync. Process |   |
   |    +-------------+     +------+------+     +------+-------+   |
   |           |                   |                   |           |
   |           |                   |                   v           |
   +-----------+                   +------------------>o-----------+

           Figure 1: The client's behavior in NTS unicast mode.

                            +-----------------------------+
                            |Broadcast Parameter Messages |
                            +--------------+--------------+
                                           |
                                           o<--------------------------+
                                           |                           |
                                           v                           |
                            +-----------------------------+            |
                            |Broadcast
              and Communication, ISPCS 2012, pp. 1-6, September 2012.

   [RFC4086]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness
              Requirements for Security", BCP 106, RFC 4086, June 2005.

   [RFC5905]  Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network
              Time Sync. Message |            |
                            +--------------+--------------+            |
                                           |                           |
   +-------------------------------------->o                           |
   |                                       |                           |
   |                                       v                           |
   |                             +-------------------+                 |
   |                             |Key Protocol Version 4: Protocol and Auth. Check|                 |
   |                             +---------+---------+                 |
   |                                       |                           |
   |                      /----------------*----------------\          |
   |                     v                                   v         |
   |                .---------.                         .---------.    |
   |               ( Verified  )                       ( Falsified )   |
   |                '----+----'                         '----+----'    |
   |                     |                                   |         |
   |                     v                                   v         |
   |              +-------------+                        +-------+     |
   |              |Store Message|                        |Discard|     |
   |              +------+------+                        +---+---+     |
   |                     |                                   |         |
   |                     v                                   +---------o
   |             +---------------+                                     |
   |             |Check Previous |                                     |
   |             +-------+-------+                                     |
   |                     |                                             |
   |            /--------*--------\                                    |
   |           v                   v                                   |
   |      .---------.         .---------.                              |
   |     ( Verified  )       ( Falsified )                             |
   |      '----+----'         '----+----'                              |
   |           |                   |                                   |
   |           v                   v                                   |
   |    +-------------+   +-----------------+                          |
   |    |Sync. Process|   |Discard Previous |                          |
   |    +------+------+   +--------+--------+                          |
   |           |                   |                                   |
   +-----------+                   +-----------------------------------+

          Figure 2: The client's behaviour Algorithms
              Specification", RFC 5905, June 2010.

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

Appendix B. A.  TICTOC Security Requirements

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

   +---------+------------------------------------+-------------+------+
   | 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        | OK   |
   +---------+------------------------------------+-------------+------+
   | 5.1.2   | Recursive Authentication of        | MUST        | OK   |
   |         | Servers (Stratum 1)                |             |      |
   +---------+------------------------------------+-------------+------+
   | 5.1.2   | Recursive Authorization of Servers | MUST        | OK   |
   |         | (Stratum 1)                        |             |      |
   +---------+------------------------------------+-------------+------+
   | 5.1.3   | Authentication and Authorization   | MAY         | -    |
   |         | of Slaves                          |             |      |
   +---------+------------------------------------+-------------+------+
   | 5.2     | Integrity protection. protection               | MUST        | OK   |
   +---------+------------------------------------+-------------+------+
   | 5.4     | Protection against DoS attacks     | SHOULD      | OK   |
   +---------+------------------------------------+-------------+------+
   | 5.5     | Replay protection                  | MUST        | OK   |
   +---------+------------------------------------+-------------+------+
   | 5.6     | Key freshness. freshness                      | MUST        | OK   |
   +---------+------------------------------------+-------------+------+
   |         | Security association. association               | SHOULD      | OK   |
   +---------+------------------------------------+-------------+------+
   |         | Unicast and multicast associations | SHOULD      | OK   |
   |         | associations.                      |             |      |
   +---------+------------------------------------+-------------+------+
   | 5.7     | Performance: no degradation in     | MUST        | OK   |
   |         | quality of time transfer. transfer           |             |      |
   +---------+------------------------------------+-------------+------+
   |         | Performance: lightweight           | SHOULD      | OK   |
   |         | computation                        |             |      |
   +---------+------------------------------------+-------------+------+
   |         | Performance: storage, bandwidth    | SHOULD      | OK   |
   +---------+------------------------------------+-------------+------+
   | 5.7     | Confidentiality protection         | MAY         | NO   |
   +---------+------------------------------------+-------------+------+
   | 5.9     | Protection against Packet Delay    | SHOULD      | NA*) |
   |         | and Interception Attacks           |             |      |
   +---------+------------------------------------+-------------+------+
   | 5.10    | Secure mode                        | MUST        | -    |
   +---------+------------------------------------+-------------+------+
   |         | Hybrid mode                        | SHOULD      | -    |
   +---------+------------------------------------+-------------+------+

   *) See discussion in section Section 11.5.

   Comparison of NTS sepecification specification against TICTOC security requirements.

Appendix C.  Broadcast Mode B.  Using TESLA for Broadcast-Type Messages

   For the broadcast mode, 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 is using uses the "not re-using keys" scheme of
   TESLA as described in section Section 3.7.2. of [RFC4082].

C.1.

B.1.  Server Preparations Preparation

   Server setup:

   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
       4082, the time interval L has to be smaller 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+1
       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 their its MAC has
          not been yet been disclosed.

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

       The server SHOULD calculate d according to

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

       where ceil gives 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 schematic explanation on of the TESLA protocol's one-way key chain

C.2.

B.2.  Client Preparation

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

   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.

B.3.  Sending Authenticated Broadcast Packets

   During each time interval I_i, the server sends one authenticated
   broadcast packet P_i.  This 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.

B.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
   before. 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,
   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,

   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 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 timestamp 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 clients 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 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 the its broadcast mode with module by performing a unicast association time
   synchronization as well as 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}. On Upon success, it regards M_{i-d} as authenticated.

Appendix D. C.  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.

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 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