NTP Working Group                                              D. Sibold
Internet-Draft                                                       PTB
Intended status: Standards Track                             S. Roettger
Expires: July 20, September 4, 2015                                   Google Inc.
                                                              K. Teichel
                                                                     PTB
                                                        January 16,
                                                           March 3, 2015

                         Network Time Security
              draft-ietf-ntp-network-time-security-06.txt
              draft-ietf-ntp-network-time-security-07.txt

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
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   This Internet-Draft will expire on July 20, September 4, 2015.

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Security Threats  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Objectives
     2.1.  Terms and Abbreviations . . . . . . . . . . . . . . . . .   4
     2.2.  Common Terminology for PTP and NTP  . . . . . . . . . . .   4
   4.  Terms and Abbreviations
   3.  Security Threats  . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Objectives  . . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  NTS Overview  . . . . . . . . . . . . . . . . . . . . . . . .   4   5
   6.  Protocol Messages . . . . . . . . . . . . . . . . . . . . . .   5   6
     6.1.  Association Messages Message Exchange  . . . . . . . . . . . . . .   7
       6.1.1.  Goals of the Association Exchange . . . .   6
       6.1.1. . . . . . .   7
       6.1.2.  Message Type: "client_assoc"  . . . . . . . . . . . .   6
       6.1.2.   7
       6.1.3.  Message Type: "server_assoc"  . . . . . . . . . . . .   6   7
       6.1.4.  Procedure Overview of the Association Exchange  . . .   8
     6.2.  Cookie Messages . . . . . . . . . . . . . . . . . . . . .   7   9
       6.2.1.  Goals of the Cookie Exchange  . . . . . . . . . . . .   9
       6.2.2.  Message Type: "client_cook" . . . . . . . . . . . . .   7
       6.2.2.  10
       6.2.3.  Message Type: "server_cook" . . . . . . . . . . . . .   7  10
       6.2.4.  Procedure Overview of the Cookie Exchange . . . . . .  11
     6.3.  Unicast Time Synchronisation Messages . . . . . . . . . .   8  12
       6.3.1.  Goals of the Unicast Time Synchronization Exchange  .  12
       6.3.2.  Message Type: "time_request"  . . . . . . . . . . . .   8
       6.3.2.  12
       6.3.3.  Message Type: "time_response" . . . . . . . . . . . .   8  13
       6.3.4.  Procedure Overview of the Unicast Time
               Synchronization Exchange  . . . . . . . . . . . . . .  13
     6.4.  Broadcast Parameter Messages  . . . . . . . . . . . . . .   9  14
       6.4.1.  Goals of the Broadcast Parameter Exchange . . . . . .  15
       6.4.2.  Message Type: "client_bpar" . . . . . . . . . . . . .   9
       6.4.2.  15
       6.4.3.  Message Type: "server_bpar" . . . . . . . . . . . . .   9
     6.5.  15
       6.4.4.  Procedure Overview of the Broadcast Messages Parameter
               Exchange  . . . . . . . . . . . . . . . . . . .  10
       6.5.1.  Message Type: "server_broad" . . .  16
     6.5.  Broadcast Time Synchronization Exchange . . . . . . . . .  10
     6.6.  17
       6.5.1.  Goals of the Broadcast Key Check Time Synchronization Exchange   17
       6.5.2.  Message Type: "server_broad"  . . . . . . . . . . . .  17
       6.5.3.  Procedure Overview of Broadcast Time Synchronization
               Exchange  . . . . . . .  10
       6.6.1.  Message Type: "client_keycheck" . . . . . . . . . . .  10
       6.6.2.  Message Type: "server_keycheck" . . . .  18
     6.6.  Broadcast Keycheck  . . . . . . .  11
   7.  Message Dependencies . . . . . . . . . . . .  19
       6.6.1.  Goals of the Broadcast Keycheck Exchange  . . . . . .  19
       6.6.2.  Message Type: "client_keycheck" . .  11
   8.  Server Seed Considerations . . . . . . . . .  20
       6.6.3.  Message Type: "server_keycheck" . . . . . . . .  12
   9.  Hash . . .  20
       6.6.4.  Procedure Overview of the Broadcast Keycheck Exchange  20
   7.  Server Seed Considerations  . . . . . . . . . . . . . . . . .  21
   8.  Hash Algorithms and MAC Generation  . . . . . . . . . . . . .  13
     9.1.  22
     8.1.  Hash Algorithms . . . . . . . . . . . . . . . . . . . . .  13
     9.2.  22
     8.2.  MAC Calculation . . . . . . . . . . . . . . . . . . . . .  13
   10.  22
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   11.  22
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  13
     11.1.  22
     10.1.  Privacy  . . . . . . . . . . . . . . . . . . . . . . . .  13
     11.2.  22
     10.2.  Initial Verification of the Server Certificates  . . . .  14
     11.3.  23
     10.3.  Revocation of Server Certificates  . . . . . . . . . . .  14
     11.4.  23
     10.4.  Mitigating Denial-of-Service for broadcast packets . . .  14
     11.5.  23
     10.5.  Delay Attack . . . . . . . . . . . . . . . . . . . . . .  15
   12.  24
     10.6.  Random Number Generation . . . . . . . . . . . . . . . .  25
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  16
   13.  25
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     13.1.  25
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  16
     13.2.  25
     12.2.  Informative References . . . . . . . . . . . . . . . . .  17  26
   Appendix A.  (informative) TICTOC Security Requirements . . . . . . . . . . . .  17  27
   Appendix B.  (normative) Using TESLA for Broadcast-Type Messages  . . . . . .  19   28
     B.1.  Server Preparation  . . . . . . . . . . . . . . . . . . .  19  28
     B.2.  Client Preparation  . . . . . . . . . . . . . . . . . . .  20  30
     B.3.  Sending Authenticated Broadcast Packets . . . . . . . . .  21  31
     B.4.  Authentication of Received Packets  . . . . . . . . . . .  21  31
   Appendix C.  Random Number Generation .  (informative) Dependencies . . . . . . . . . . . . .  23  32
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23  35

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  Successful attacks
   against the time synchronization protocol. 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
   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 security measures which enable
   these and possibly other protocols to verify the authenticity of the
   time server server/master and the integrity of the time synchronization
   protocol packets.  The 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, like IPsec or TLS, these measures for a given
   specific time
   synchronization messages.  This prerequisite corresponds with the
   requirement synchronisation 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 [RFC7384].

   Note:

      It is recommended transfer.  This implies that details on how to apply NTS to specific for time synchronization protocols be formulated keeping the increase in separate
      documents, with one separate document for each protocol.

2.  Security Threats

   A profound analysis of
   bandwidth and message latency caused by the security threats measures should
   be small.  Also, NTP as well as PTP work via UDP and requirements 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 time
   synchronization protocols bandwidth, that the necessary calculations can be found
   executed in the 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

   HMAC  Keyed-Hash 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.

3.  Security Threats

   The document "Security Requirements of Time Protocols in Packet
   Switched Networks" [RFC7384].

3. [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 a client/slave client to authenticate its time
      server(s)/master(s).
      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 optionally enables the server to verify the
      client's authorization.

   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  Integration with protocols: NTS can be used to secure different
      time synchronization protocols, specifically at least NTP and PTP.
      An
      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.

4.  Terms and Abbreviations

   MITM   Man In The Middle

   NTS    Network Time Security

   TESLA  Timed Efficient Stream Loss-tolerant Authentication

5.  NTS Overview

   NTS applies X.509 certificates to verify the authenticity of the time
   server/master
   server and to exchange a symmetric key, the so-called cookie.
   This cookie is  It
   then used uses the cookie to protect the authenticity and the integrity of
   subsequent unicast-type time synchronization packets.
   This is done by means of  In order to do
   this, a Message Authentication Code (MAC), which (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> (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_<b> cuts off the b most significant bits of
   the result of the HMAC function.  The client's certificate contains
   the client's public key and enables the server to identify the
   client, if client authorization is desired.  The server seed is a
   random value of bit length b that the server possesses, which has to be kept
   remain 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 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 the hash value of its certificate to each request (see
   Section 6.3).

   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 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 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
   disclosure of the key in its associated disclosure interval occurs.
   In order to be able to verify the validity timeliness of the key, packets, the
   client has to be loosely time synchronized with the server.  This has
   to be accomplished during the initial client server exchange between the before broadcast client and the server.  In addition, associations can be used.  For
   checking timeliness of packets, NTS uses another, more rigorous check than what is
   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 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, 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 as well as CMS
   (Cryptographic Message Syntax, see [RFC5652]) conventions that can be
   used to get the cryptographic aspects right.

6.1.  Association Messages Message Exchange

   In this message exchange, the participants negotiate the hash and
   encryption algorithms that are used throughout the protocol are negotiated. protocol.  In addition ,
   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  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 of the negotiation result to the client.

6.1.2.  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  a nonce,

   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.

6.1.3.  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 hash 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 hostname of the server,

   o  the server's choice of algorithm for encryption and for
      cryptographic hashing, 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.

6.1.4.  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 constructs and
       sends a reply in the form of a server_assoc message as described
       in Section 6.1.3.  Upon unsuccessful negotiation for version
       number or algorithms the server_assoc message MUST contain an
       error code.

   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 also verifies that the server has chosen the encryption and
          hash algorithms from its proposal sent in the client_assoc
          message.

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

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

            +------------------------+
            | 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.

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  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 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.

6.2.2.  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  a nonce,

   o  the negotiated hash algorithm H,

   o  the client's certificate.

6.2.2.

6.2.3.  Message Type: "server_cook"

   This message is sent by the 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.  This message contains

   o  the NTS message ID "server_cook"

   o  the version number as transmitted in client_cook,

   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.

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

6.2.4.  Procedure Overview of the
   server's time responses is verified successfully.

6.3.1.  Message Type: "time_request"

   This message is sent by Cookie Exchange

   For a cookie exchange, the following steps are performed:

   1.  The client when it requests sends a time exchange.
   It contains

   o  the NTS client_cook message ID "time_request",

   o to the negotiated version number,

   o  a nonce,

   o server.  The client
       MUST save the negotiated hash algorithm H,

   o included nonce until the hash reply has been processed.

   2.  Upon receipt of the client's certificate under H.

6.3.2.  Message Type: "time_response"

   This message is sent by a client_cook message, the server after checks whether
       it has received a
   time_request message.  Prior to this supports the server MUST recalculate given cryptographic algorithms.  It then
       calculates the
   client's cookie by using according to the hash of formula given in
       Section 5.  The server MAY use the client's certificate and to check
       that the
   transmitted hash algorithm.  The message contains

   o client is authorized to use the NTS message ID "time_response",

   o  the version number as transmitted in time_request,

   o  the server's secure time
       synchronization response data,
   o  the nonce transmitted in time_request,

   o service.  With this, it MUST construct a MAC (generated with the cookie as key) for verification of all
      of the above data.

6.4.  Broadcast Parameter Messages

   In this
       server_cook message exchange, the client receives the necessary
   information to execute the TESLA protocol as described in a secured broadcast
   association. Section 6.2.3.

   3.  The client can only initiate a secure broadcast
   association after awaits a successful unicast run.

   See Appendix B for more details on TESLA.

6.4.1.  Message Type: "client_bpar"

   This message is sent by the client reply in order to establish the form of a secured
   time broadcast association with server_cook message;
       upon receipt it executes the server. following actions:

       *  It contains

   o  the NTS message ID "client_bpar",

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

   o matches the client's hostname, and

   o one
          negotiated beforehand.

       *  It verifies the signature algorithm negotiated during unicast.

6.4.2.  Message Type: "server_bpar"

   This message is sent by using the server upon receipt of a client_bpar
   message during server's public key.  The
          signature has to authenticate the broadcast loop of encrypted data.

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

       *  It contains

   o checks that the NTS decrypted message ID "server_bpar",

   o  the version number as transmitted in the client_bpar message,

   o is of the one-way functions used for building expected
          format: the key chain, concatenation of a 128 bit nonce and

   o a 128 bit
          cookie.

       *  It verifies that the disclosure schedule of received nonce matches the keys.  This contains:

      * nonce sent in
          the last key client_cook message.

       If one of those checks fails, the key chain,

      *  time interval duration,

      * client MUST abort the disclosure delay (number of intervals between use run.

        +----------------------------+
        | 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
         disclosure of a key),

      * cookie exchange.

6.3.  Unicast Time Synchronisation Messages

   In this message exchange, the usual time at which synchronization process is
   executed, with the next time interval will start, and

      * addition of integrity protection for all messages
   that the next interval's associated index.

   o  The server sends.  This message also contains a signature signed by can be repeatedly exchanged as
   often as the server with
      its private key, verifying all client desires and as long as the integrity of the
   server's time responses is verified successfully.

6.3.1.  Goals of the Unicast Time Synchronization Exchange

   The unicast time synchronization exchange:

   o  exchanges (unicast) time synchronization data listed above.

6.5.  Broadcast Messages

   Via this message, as specified by the server keeps sending broadcast
      appropriate time synchronization messages protocol,

   o  guarantees to all participating clients.

6.5.1. the client that the response originates from the
      server and is based on the client's original, unaltered request.

6.3.2.  Message Type: "server_broad" "time_request"

   This message is sent by the server over the course of its broadcast
   schedule.  It is part of any broadcast association. client when it requests a time exchange.
   It contains

   o  the NTS message ID "server_broad", "time_request",

   o  the negotiated version number that the server is working under, number,
   o  time broadcast data,  a nonce,

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

   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 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 negotiated during association in unicast
      mode,

   o  a nonce,

   o  an interval number from the TESLA disclosure schedule,

   o  the hash algorithm H negotiated in unicast mode, and

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

6.6.2.

6.3.3.  Message Type: "server_keycheck"

   A "time_response"

   This message of this type is sent by the server upon receipt of after it has received a
   client_keycheck message during the broadcast loop of the server.
   time_request message.  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  The message contains

   o  the NTS message ID "server_keycheck" "time_response",

   o  the version number as transmitted in "client_keycheck, time_request,

   o  the nonce transmitted in the client_keycheck message, server's time synchronization response data,

   o  the interval number nonce transmitted in the client_keycheck message,
      and time_request,

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

7.  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
                       |
                       v
   +----------------------------------------+
   |Broadcast

6.3.4.  Procedure Overview of the Unicast Time Synchronization Reception|
   +----------------------------------------+
                       |
           Whenever deemed necessary
                       |
                       v
              +-----------------+
              |Keycheck Exchange|
              +-----------------+

8.  Server Seed Considerations Exchange

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

   1.  The server has to calculate client sends a random seed which has time_request message to be kept
   secret. the server.  The server
       client MUST generate save the included nonce and the transmit_timestamp
       (from the time synchronization data) as a seed correlated pair for each supported hash
   algorithm, see Section 9.1.

   According to the requirements in [RFC7384],
       later verification steps.

   2.  Upon receipt of a time_request message, the server MUST refresh
   each server seed periodically.  Consequently, re-calculates
       the cookie memorized by cookie, then computes the client becomes obsolete.  In this case, necessary time synchronization data
       and constructs a time_response message as given in Section 6.3.3.

   3.  It awaits a reply in the client cannot verify form of a time_response message.  Upon
       receipt, it checks:

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

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

       *  that the transmit_timestamp in that time_request message
          matches the corresponding time response messages and has to
   respond accordingly by re-initiating stamp from the protocol with a cookie
   request (Section 6.2).

9.  Hash Algorithms synchronization
          data received in the time_response, and

       *  that the appended MAC Generation

9.1.  Hash Algorithms

   Hash algorithms are used verifies the received synchronization
          data, version number and nonce.

       If at different points: calculation least one of the
   cookie and first three checks fails (i.e.  if the MAC, and hashing of
       version number does not match, if the client's certificate.  The client and the server negotiate a hash algorithm H during has never used the
   association message exchange (Section 6.1) at
       nonce transmitted in the beginning of a
   unicast run.  The selected algorithm H is time_response message, or if it has used for all hashing
   processes in
       the nonce with initial time synchronization data different from
       that run.

   In in the TESLA scheme, hash algorithms are used as pseudo-random
   functions to construct response), then the one-way key chain.  Here, client MUST ignore this
       time_response message.  If the utilized
   hash algorithm MAC is communicated by invalid, the server and is non-negotiable.

   Note:

      Any hash algorithm is prone to be compromised in client MUST do
       one of the future.  A
      successful attack on a hash algorithm would enable any NTS client following: abort the run or go back to derive step 5 (because
       the cookie might have changed due to a server seed from its own cookie.  Therefore, refresh).  If
       both checks are successful, 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 client SHOULD continue time
       synchronization by going back to compromise such
      clients who use hash algorithm H as well.

9.2. step 7.

            +-----------------------+
            | o Re-generate cookie  |
            | o Assemble response   |
            | o Generate MAC Calculation

   For the calculation of        |
            +-----------+-----------+
                        |
                      <-+->

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

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

   Procedure for unicast time synchronization exchange.

6.4.  Broadcast Parameter Messages

   In this message exchange, the MAC, client and server use receives the necessary
   information to execute the TESLA protocol in a Keyed-Hash
   Message Authentication Code (HMAC) approach [RFC2104]. secured broadcast
   association.  The HMAC 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
   generated with the hash algorithm specified by roughly synchronized to
   the client (see
   Section 9.1).

10.  IANA Considerations

11.  Security Considerations

11.1.  Privacy

   tbd

11.2.  Initial Verification server's.

   See Appendix B for more details on TESLA.

6.4.1.  Goals of the Server Certificates Broadcast Parameter Exchange

   The broadcast parameter exchange

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

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

6.4.2.  Message Type: "client_bpar"

   This 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, sent by the client MUST use one of in order to establish a secured
   time broadcast association with the following approaches: server.  It contains

   o  The validity of the certificates is preconditioned.  Usually this
      will be  the case in corporate networks. NTS message ID "client_bpar",

   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]. NTS version number negotiated during association,

   o  The client requests  a different service to get an initial time
      stamp in order to be able to verify nonce,

   o  the certificates' periods of
      validity.  To this end, it can, e.g., use a secure shell
      connection to a reliable host.  Another alternative client's hostname, and

   o  the signature algorithm negotiated during association.

6.4.3.  Message Type: "server_bpar"

   This message is to request
      a time stamp from a Time Stamping Authority (TSA) sent by means of the
      Time-Stamp Protocol (TSP) defined in [RFC3161].

11.3.  Revocation server upon receipt of Server Certificates

   According to Section 8, 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 client_bpar
   message during the expiration date broadcast loop of the
   certificate during server.  It contains

   o  the certificate NTS message exchange (Section 6.1.2).
   Furthermore, certificates may also be revoked prior to ID "server_bpar",

   o  the normal
   expiration date.  To increase security version number as transmitted in the client MAY periodically
   verify client_bpar message,

   o  the state of nonce transmitted in client_bpar,

   o  the server's certificate via OCSP.

11.4.  Mitigating Denial-of-Service for broadcast packets

   TESLA authentication buffers packets one-way functions used for delayed authentication.
   This makes building the protocol vulnerable to flooding attacks, causing key chain, and

   o  the
   client to buffer excessive numbers disclosure schedule of packets.  To add stronger DoS
   protection to the protocol, keys.  This contains:

      *  the client and last key of the server use key chain,

      *  time interval duration,

      *  the "not
   re-using keys" scheme disclosure delay (number of TESLA as pointed out in Section 3.7.2 intervals between use and
         disclosure of RFC
   4082 [RFC4082].  In this scheme the server never uses a key for key),

      *  the
   MAC generation more than once.  Therefore, time at which the client can discard any
   packet that next time interval will start, and

      *  the next interval's associated index.

   o  The message also contains a disclosed key it already knows, thus
   preventing memory flooding attacks.

   Note that an alternative approach to enhance TESLA's resistance
   against DoS attacks involves signature signed by the addition server with
      its private key, verifying all the data listed above.

6.4.4.  Procedure Overview of a group MAC to each
   packet.  This requires the Broadcast Parameter Exchange

   A broadcast parameter exchange consists of an additional shared key
   common to the whole group.  This adds additional complexity following steps:

   1.  The client sends a client_bpar message to the
   protocol and hence is currently not considered in this document.

11.5.  Delay Attack

   In a packet delay attack, an adversary with server.  It MUST
       remember the ability to act as a
   MITM delays time synchronization packets between client and server
   asymmetrically [RFC7384].  This prevents transmitted values for the client from accurately
   measuring nonce, the network delay, version number
       and hence its time offset to the signature algorithm.

   2.  Upon receipt of a client_bpar message, the server
   [Mizrahi]. constructs and
       sends a server_bpar message as described in Section 6.4.3.

   3.  The delay attack does not modify client waits for a reply in the content form of the
   exchanged synchronization packets.  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, provided that the adversary is unable to delay the
       synchronization packets between server_bpar
       message, on which it performs the majority of servers.  This
       approach is commonly used in NTP to exclude incorrect time
       servers [RFC5905].

   2.  Multiple communication paths: following checks:

       *  The client and server utilize
       different paths message must contain all the necessary information for packet exchange the
          TESLA protocol, as described listed in the I-D
       [I-D.shpiner-multi-path-synchronization]. Section 6.4.3.

       *  The client can detect
       the attack, provided that the adversary is unable message must contain a nonce belonging to manipulate a client_bpar
          message that the majority client has previously sent.

       *  Verification of the available paths [Shpiner].  Note that this
       approach message's signature.

       If any information is not yet available, neither for NTP nor for PTP.

   3.  Usage of an encrypted connection: missing or if the server's signature cannot
       be verified, the client exchanges MUST abort the broadcast run.  If all
       packets with
       checks are successful, the time server over an encrypted connection (e.g.
       IPsec).  This measure does not mitigate client MUST remember all the delay attack, but it
       makes it more difficult broadcast
       parameters received for the adversary to identify the 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 packets.

   4.  For unicast-type messages: Introduction of exchange.

6.5.  Broadcast Time Synchronization Exchange

   Via a threshold value for stream of messages of the delay following message type, the server
   keeps sending broadcast time synchronization messages to all
   participating clients.

6.5.1.  Goals of the synchronization packets. Broadcast Time Synchronization Exchange

   The client can
       discard a time server if the packet delay broadcast time of this synchronization exchange:

   o  transmits (broadcast) time
       server is larger than synchronization data from the threshold value.

   Additional provision against delay attacks has server to be taken for
   broadcast-type messages.  This mode relies on the TESLA scheme which
   is based on
      the requirement that a client and as specified by the broadcast server
   are loosely appropriate time synchronized.  Therefore, a broadcast 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
   establish time synchronization with its broadcast server before it
   starts utilizing broadcast messages be stored for time synchronization.  To
   this end, it initially establishes later checks,

   o  additionally guarantees authenticity of a unicast association with its certain broadcast server until time
      synchronization and calibration of message in the
   packet delay time client's storage.

6.5.2.  Message Type: "server_broad"

   This message is achieved.  After that it establishes a broadcast
   association with sent by the broadcast server and utilizes TESLA to verify
   integrity and authenticity over the course of any received its broadcast packets.

   An adversary who
   schedule.  It is able to delay broadcast packets can cause a time
   adjustment at the receiving part of any broadcast clients.  If association.  It contains

   o  the adversary
   delays broadcast packets continuously, then NTS message ID "server_broad",
   o  the time adjustment will
   accumulate until version number that the loose time synchronization requirement server is
   violated, which breaks the TESLA scheme.  To mitigate this
   vulnerability working under,

   o  time broadcast data,

   o  the security condition in TESLA has index that belongs to be supplemented
   by an additional check in which the client, upon receipt of a
   broadcast message, verifies current interval (and therefore
      identifies the status of current, yet undisclosed, key),

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

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

      *  the message ID,

      *  the version number, and

      *  the time data.

6.5.3.  Procedure Overview of Broadcast Time Synchronization Exchange

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

   1.  The server (see Appendix B.4
   for a detailed description follows the TESLA protocol by regularly sending
       server_broad messages as described in Section 6.5.2, adhering to
       its own disclosure schedule.

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

       *  Proof that the MAC is based on a broadcast 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 should also apply obtains a stronger guarantee via a key check
          exchange (see below).  If its timeliness is verified, the above-mentioned precautions as far as
   possible.

12.  Acknowledgements

   The authors would like to thank Russ Housley, Steven Bellovin, David
   Mills and Kurt Roeckx
          packet will be buffered for discussions and comments on later authentication.  Otherwise,
          the design of
   NTS.  Also, thanks go to Harlan Stenn client MUST discard it.  Note that the time information
          included in the packet will not be used for his technical review and
   specific text contributions 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 document.

13.  References

13.1.  Normative References

   [RFC2104]  Krawczyk, H., Bellare, M., and 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
          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.

6.6.  Broadcast Keycheck

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

6.6.1.  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.

6.6.2.  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 hash algorithm H negotiated during association, and

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

6.6.3.  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 hash of the client's certificate and the transmitted hash
   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,

   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.

6.6.4.  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 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 6.6.3.  For more
       details, see also Appendix B.

   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.

                             +----------------------+
                             | 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 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 8.1.

   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 (Section 6.2).

8.  Hash Algorithms and MAC Generation

8.1.  Hash Algorithms

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

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

   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 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.

8.2.  MAC Calculation

   For the calculation of the MAC, client and server use a Keyed-Hash
   Message Authentication Code (HMAC) approach [RFC2104].  The HMAC is
   generated with the hash algorithm specified by the client (see
   Section 8.1).

9.  IANA Considerations

10.  Security Considerations

10.1.  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
   [I-D.iab-privsec-confidentiality-threat].  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 role since the packets 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.

10.2.  Initial Verification of the Server Certificates

   The client has to verify the validity of the certificates during the
   certification message exchange (Section 6.1.3).  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 as to rely on external
   means.

10.3.  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 (Section 6.1.3).
   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 OCSP.

10.4.  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.

   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.

10.5.  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
   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.shpiner-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
   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 B.4
   for a detailed description of this check).  Note that a broadcast
   client should also apply the above-mentioned precautions as far as
   possible.

10.6.  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.

11.  Acknowledgements

   The authors would like to thank Tal Mizrahi, 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.

12.  References

12.1.  Normative References

   [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.

   [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.

12.2.  Informative References

   [I-D.iab-privsec-confidentiality-threat]
              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", draft-iab-privsec-
              confidentiality-threat-03 (work in progress), February
              2015.

   [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.

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

   [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 I-D tictoc RFC 7384    | 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         |
   |         | Servers (Stratum 1)          |             |            |
   +---------+------------------------------------+-------------+------+
   +---------+------------------------------+-------------+------------+
   | 5.1.3   | Authentication and Authorization           | MAY         | - Optional,  |
   |         | Authorization of Slaves Clients     |             | Limited    |
   +---------+------------------------------------+-------------+------+
   +---------+------------------------------+-------------+------------+
   | 5.2     | Integrity protection         | MUST        | OK         |
   +---------+------------------------------------+-------------+------+
   +---------+------------------------------+-------------+------------+
   | 5.3     | Spoofing Prevention          | MUST        | OK         |
   +---------+------------------------------+-------------+------------+
   | 5.4     | Protection against 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 associations        | SHOULD      | OK         |
   +---------+------------------------------------+-------------+------+
   |         | associations                 |             |            |
   +---------+------------------------------+-------------+------------+
   | 5.7     | Performance: no degradation in  | MUST        | OK         |
   |         | in quality of time transfer  |             |            |
   +---------+------------------------------------+-------------+------+
   +---------+------------------------------+-------------+------------+
   |         | Performance: lightweight     | SHOULD      | OK         |
   |         | computation                  |             |            |
   +---------+------------------------------------+-------------+------+
   +---------+------------------------------+-------------+------------+
   |         | Performance: storage         | SHOULD      | OK         |
   +---------+------------------------------+-------------+------------+
   |         | Performance: storage, bandwidth       | SHOULD      | OK         |
   +---------+------------------------------------+-------------+------+
   +---------+------------------------------+-------------+------------+
   | 5.7 5.8     | Confidentiality protection   | MAY         | NO         |
   +---------+------------------------------------+-------------+------+
   +---------+------------------------------+-------------+------------+
   | 5.9     | Protection against Packet Delay    | SHOULD MUST        | NA*) Limited*)  |
   |         | Delay and Interception       |             |            |
   |         | Attacks                      |             |            |
   +---------+------------------------------------+-------------+------+
   +---------+------------------------------+-------------+------------+
   | 5.10    | Secure mode                  | MUST        | - OK         |
   +---------+------------------------------------+-------------+------+
   +---------+------------------------------+-------------+------------+
   |         | Hybrid mode                  | SHOULD      | -          |
   +---------+------------------------------------+-------------+------+
   +---------+------------------------------+-------------+------------+

   *) See discussion in Section 11.5. 10.5.

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

Appendix B.  (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].

B.1.  Server Preparation

   Server

   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, 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+1 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.

       The server SHOULD calculate d according to

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

       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

B.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.

   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.

B.3.  Sending Authenticated Broadcast Packets

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

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.  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,

   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 a unicast time synchronization as well as
   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 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.

Appendix C.  (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
   K_{i-d} to derive K'_{i-d} 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 that the client's  |
   | PKI     | (encryption) |        | public key to check encrypt the MAC included     |
   |         +--------------+--------+ 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
   package P_{i-d}. Upon success, client_cook messages, |
   |         |              |        | where it regards M_{i-d} as authenticated.

Appendix C.  Random Number Generation

   At various points is used for trans-   |
   |         |              |        | portation of the protocol, the generation public key   |
   |         |              |        | as well as (optionally) for   |
   |         |              |        | verification of random numbers
   is required.  The employed client        |
   |         |              |        | authorization.                |
   +---------+--------------+--------+-------------------------------+
             +------------<---------------+
             |        At least one        |
             V        successful          |
      ++====[ ]===++               ++=====^=====++
      ||  Cookie  ||               ||Association||
      || Exchange ||               || Exchange  ||
      ++====_ _===++               ++===========++
             |
             |  At least one
             |  successful
             V
   ++=======[ ]=======++
   || Unicast Time    |>-----\   As long as further
   || Synchronization ||      |  synchronization
   || Exchange(s)     |<-----/   is desired
   ++=======_ _=======++
             |
              \                               Other (unspecified)
   Sufficient  \                          /   methods of generation need to be
   cryptographically secure.  See [RFC4086] for guidelines concerning
   this topic. 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     ||
                                   ++===================++

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