NTP Working Group                                              D. Sibold
Internet-Draft                                                       PTB
Intended status: Standards Track                             S. Roettger
Expires: January 5, April 26, 2015                                       Google Inc
                                                              K. Teichel
                                                                     PTB
                                                           July 04,
                                                        October 23, 2014

                         Network Time Security
              draft-ietf-ntp-network-time-security-04.txt
              draft-ietf-ntp-network-time-security-05.txt

Abstract

   This document describes the Network Time Security (NTS) protocol that
   enables secure authentication of time synchronization with time servers using Network
   Time Protocol (NTP) or Precision Time Protocol (PTP).  Its design
   considers the special requirements of precise timekeeping, which are
   described in Security Requirements of Time Protocols in Packet
   Switched Networks [I-D.ietf-tictoc-security-requirements]. [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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   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 5, April 26, 2015.

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
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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
   5.  NTS Overview  . . . . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Symmetric and Client/Server Mode  . . . . . . . . . . . .   5
     5.2.  Broadcast Mode  . . . . . . . . . . . . . . . . . . . . .   6   5
   6.  Protocol Messages . . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Association Messages  . . . . . . . . . . . . . . . . . .   6
       6.1.1.  Message Type: "client_assoc"  . . . . . . . . . . . .   7
       6.1.2.  Message Type: "server_assoc"  . . . . . . . . . . . .   7
     6.2.  Certificate  Cookie Messages . . . . . . . . . . . . . . . . . .   7 . . .   8
       6.2.1.  Message Type: "client_cert" "client_cook" . . . . . . . . . . . . .   8
       6.2.2.  Message Type: "server_cert" "server_cook" . . . . . . . . . . . . .   8
     6.3.  Cookie  Unicast Time Synchronisation Messages . . . . . . . . . . . . . . . . . . . . .   9
       6.3.1.  Message Type: "client_cook" . "time_request"  . . . . . . . . . . . .   9
       6.3.2.  Message Type: "server_cook" . "time_response" . . . . . . . . . . . .   9
     6.4.  Unicast Time Synchronisation  Broadcast Parameter Messages  . . . . . . . . . . . . . .  10
       6.4.1.  Message Type: "time_request" "client_bpar" . . . . . . . . . . . . .  10
       6.4.2.  Message Type: "time_response" "server_bpar" . . . . . . . . . . . . .  10
     6.5.  Broadcast Parameter Messages  . . . . . . . . . . . . . . . . . . .  11
       6.5.1.  Message Type: "client_bpar" . "server_broad"  . . . . . . . . . . . .  11
       6.5.2.  Message Type: "server_bpar" . . .
     6.6.  Broadcast Key Check . . . . . . . . . .  11
     6.6.  Broadcast Message . . . . . . . . .  11
       6.6.1.  Message Type: "client_keycheck" . . . . . . . . . . .  12
       6.6.1.  11
       6.6.2.  Message Type: "server_broad"  . "server_keycheck" . . . . . . . . . . .  12
   7.  Protocol Sequence . . . . . . . . . . . . . . . . . . . . . .  13  12
     7.1.  The Client  . . . . . . . . . . . . . . . . . . . . . . .  13  12
       7.1.1.  The Client in Unicast Mode  . . . . . . . . . . . . .  13  12
       7.1.2.  The Client in Broadcast Mode  . . . . . . . . . . . .  15  14
     7.2.  The Server  . . . . . . . . . . . . . . . . . . . . . . .  16
       7.2.1.  The Server in Unicast Mode  . . . . . . . . . . . . .  16
       7.2.2.  The Server in Broadcast Mode  . . . . . . . . . . . .  17  16
   8.  Server Seed Considerations  . . . . . . . . . . . . . . . . .  17
     8.1.  Server Seed Refresh . . . . . . . . . . . . . . . . . . .  17
     8.2.  Server Seed Algorithm . . . . . . . . . . . . . . . . . .  17
     8.3.  Server Seed Lifetime  . . . . . . . . . . . . . . . . . .  17
   9.  Hash Algorithms and MAC Generation  . . . . . . . . . . . . .  17
     9.1.  Hash Algorithms . . . . . . . . . . . . . . . . . . . . .  17
     9.2.  MAC Calculation . . . . . . . . . . . . . . . . . . . . .  18
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  18
     11.1.  Initial Verification of the Server Certificates  . . . .  18
     11.2.  Revocation of Server Certificates  . . . . . . . . . . .  19  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  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  20  21
     13.2.  Informative References . . . . . . . . . . . . . . . . .  21
   Appendix A.  Flow Diagrams of Client Behaviour  . . . . . . . . .  22
   Appendix B.  Extension Fields . . . . . . . . . . . . . . . . . .  25
   Appendix C.  TICTOC Security Requirements . . . . . . . . . . . .  25  24
   Appendix D. C.  Broadcast Mode . . . . . . . . . . . . . . . . . . .  26
     D.1.  25
     C.1.  Server Preparations . . . . . . . . . . . . . . . . . . .  27
     D.2.  25
     C.2.  Client Preparation  . . . . . . . . . . . . . . . . . . .  28
     D.3.  27
     C.3.  Sending Authenticated Broadcast Packets . . . . . . . . .  29
     D.4.  27
     C.4.  Authentication of Received Packets  . . . . . . . . . . .  29  28
   Appendix E. D.  Random Number Generation . . . . . . . . . . . . . .  30  29
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  30  29

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 critical
   infrastructures have environments that are prone to provide malicious attacks.
   This can be accomplished by utilization of external security
   protocols like IPsec or by intrinsic security measures to defeat possible
   adversaries.  Consequently, of the time
   synchronization protocol.

   The two most popular time synchronization protocols, the widespread Network Time
   Protocol (NTP) [RFC5905] was supplemented by the autokey protocol [RFC5906]
   which shall ensure authenticity of the NTP server and integrity of
   the protocol packets.  Unfortunately, the autokey protocol exhibits
   various severe security vulnerabilities as revealed in a thorough
   analysis of the protocol [Roettger].  For the Precision Time Protocol
   (PTP), Annex K of the standard document IEEE 1588 [IEEE1588] defines
   an informative security protocol that is still in experimental state.

   Because of autokey's security vulnerabilities and the absence of a
   standardized (PTP)
   [IEEE1588], currently do not provide adequate intrinsic security protocol for PTP, these protocols cannot be
   applied in environments in which compliance requirements demand
   authenticity and integrity protection.
   precautions.  This document specifies a security protocol measures for NTP and
   PTP which ensures enable these protocols to verify authenticity of the time
   server via a
   Public Key Infrastructure (PKI) and integrity of the time synchronization protocol packets and which therefore enables the
   usage of NTP and PTP in such environments. packets.

   The protocol is 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 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 [I-D.ietf-tictoc-security-requirements]. [RFC7384].

   Note:

      The intent is to formulate the protocol to be applicable to NTP
      and also PTP.  In the current state the draft specification focuses on
      the application to NTP.

2.  Security Threats

   A profound analysis of security threats and requirements for NTP and
   PTP can be found in the "Security Requirements of Time Protocols in
   Packet Switched Networks" [I-D.ietf-tictoc-security-requirements]. [RFC7384].

3.  Objectives

   The objectives of the NTS specification are as follows:

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

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

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

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

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

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

   o  Compatibility:

      *  Unsecured NTP associations shall not be affected.

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

4.  Terms and Abbreviations

   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 Authentication

5.  NTS Overview

5.1.  Symmetric and Client/Server Mode

   NTS applies X.509 certificates to verify the authenticity of the time
   server and to exchange a symmetric key, the so-called cookie.  This
   cookie is then used to protect authenticity and integrity of the
   subsequent time synchronization packets 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.  It  The cookie is calculated according to:

      cookie = MSB_128 (HMAC(server seed, H(certificate of client))),

   with the server seed as key, where H is a hash function, and where
   the function MSB_128 cuts off the 128 most significant bits of the
   result of the HMAC function.  The server seed is a 128 bit random
   value of the server, 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 [I-D.ietf-tictoc-security-requirements]. [RFC7384].  See Section 8 for details on the
   seed refresh and Section 7.1.1 for the client's reaction to it.

   The server does not keep a state of the client.  Therefore it has to
   recalculate the cookie each time it receives a request from the
   client.  To this end, the client has to attach the hash value of its
   certificate to each request (see Section 6.4). 6.3).

5.2.  Broadcast Mode

   Just as in the case of the client server mode and symmetric mode,
   authenticity and integrity of the NTP packets are ensured by a MAC,
   which is attached to the NTP packet by the sender.  The verification  Verification of
   the packets' authenticity is based on the TESLA protocol, in
   particular on its "Not Re-using Keys" "not re-using keys" scheme, see section 3.7.2 of

   [RFC4082].  TESLA is based on uses a one-way chain of keys, where each key is the
   output of a one-way function applied on 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
   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.  In
   order to be able to verify the validity of the key, the client has to
   be loosely time synchronized to the server.  This has to be
   accomplished during the initial client server exchange between
   broadcast client and server.  In addition, NTS uses another, more
   rigorous check to what is used in the TESLA protocol.  For a more
   detailed description of the
   TESLA protocol how NTS employs and customizes TESLA, see
   Appendix D. C.

6.  Protocol Messages

   Note that this

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

   For some guidance on how these message types can be realized in
   practice, for use with existing time synchronization protocols, see
   [I-D.ietf-ntp-cms-for-nts-messages], a companion document for NTS.
   Said document describes ASN.1 encodings for those message format 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 its utilization for 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 would will have to be developed
   accordingly.

   The steps described in Section 6.1 - Section 6.4 6.3 belong to the
   unicast mode, while Section 6.5 6.4 and Section 6.6 6.5 explain the steps
   involved in the broadcast mode of NTS.

6.1.  Association Messages

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

   o  a selection of accepted encryption algorithms, and

   o  a selection of accepted algorithms for the signatures.

   For NTP, this message is realized as a packet with an extension field
   of type "association", which contains all this data. 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, for the
      signatures encryption and for
      cryptographic hashing , hashing, all of which MUST be chosen from the
      client's proposals.

   In the case of NTP,

   o  a signature, calculated over the data listed above, with the
      server's private key and according to the signature algorithm
      which is enclosed in a packet's extension
   field, also of type "association".

6.2.  Certificate Messages

   In this message exchange, the client receives used for 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,
   authenticity of future NTS messages from the server is ensured.

   Discussion:

      Note that in this step 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 [I-D.ietf-tictoc-security-requirements] depends on the chosen
      trust anchor.

6.2.1.  Message Type: "client_cert"

   This message is sent by the client, after it successfully verified
   the content of the received server_assoc message (see Section 7.1.1).
   It contains

   o  the NTS message ID "client_cert",

   o  the negotiated version number,

   o  the client's hostname, and

   o  the signature algorithm negotiated during the association
      messages.

   In the case of NTP, the data is enclosed in a packet's extension
   field of type "certificate request".

6.2.2.  Message Type: "server_cert"

   This message is sent by the server, upon receipt of a client_cert
   message, if the version number and choice of methods communicated in
   that message are actually supported by the server.  It contains

   o  the NTS message ID "server_cert",

   o  the version number as transmitted in client_cert,

   o  a signature, calculated over the data listed above, with the
      server's private key and according to the signature algorithm
      transmitted in server_cert,

   o  all the information necessary to authenticate the server to the
      client.  This is a certificates which are included (see
      below),

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

   This message is realized for NTP as a packet with extension field of
   type "certificate" which holds all of the data listed above.

6.3.

6.2.  Cookie Messages

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

6.3.1.

6.2.1.  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  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.

   For NTP, an extension field of type "cookie request" holds the listed
   data.

6.3.2.

6.2.2.  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 5.1.  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

   o  the NTS message ID "server_cook"

   o  the version number as transmitted in client_cook,

   o

      *  the nonce transmitted in client_cook,

   o  the cookie, and

      *  the cookie.

   o  a signature, created with the server's private key, calculated
      over

      * all of the data listed above, and also

      *  the hash of the client's certificate. above.  This signature MUST be
      calculated according to the transmitted signature algorithm from
      the client_cook message.

   In the case of NTP, this is a packet with an extension field of type
   "cookie transmit".

6.4.

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.

6.4.1.

6.3.1.  Message Type: "time_request"

   This message is sent by the client when it requests 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.

   In the case of NTP the data is enclosed in the packet's extension
   field of type "time request".

6.4.2.

6.3.2.  Message Type: "time_response"

   This message is sent by the server, after it 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.

   In the case of NTP, this is a packet with the necessary time
   synchronization data and a new extension field of type "time
   response".  This packet has an appended MAC that is generated over
   the time synchronization data and the extension field, with the
   cookie as the key.

6.5. 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.

   See Appendix D C for more details on TESLA.

6.5.1.

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 version number negotiated during association in unicast mode,

   o  the client's hostname, and

   o  the signature algorithm negotiated during unicast.

   For NTP, this message is realized as a packet with an extension field
   of type "broadcast request".

6.5.2.

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

   o  the last key of the one-way 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.

   It is realized for NTP as a packet with an extension field of type
   "broadcast parameters", which contains all the given data.

6.6.

6.5.  Broadcast Message 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 is working
      under,

   o  time broadcast data,

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

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

   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.

6.6.1.  Message Type: "server_broad"

   This "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 version number chosen for the broadcast,

   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 over the course upon receipt of its a
   client_keycheck message during the broadcast
   schedule.  It is part loop of any broadcast association. 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_broad", "server_keycheck"

   o  the version number that the server's broadcast mode is working
      under,

   o  time broadcast data,

   o  the index that belongs to  the current interval (and therefore
      identifies 128-bit nonce transmitted in the current, yet undisclosed key), client_keycheck message,

   o  the disclosed key of the previous disclosure interval (current
      time interval minus disclosure delay), number transmitted in the client_keycheck message,
      and

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

      *  the message ID,
      *  the version number, and

      *  the time data.

   For NTP, this message is realized cookie as an NTP broadcast packet with the
   time broadcast data and with an extension field key) for verification of type "broadcast
   message", which contains the rest all
      of the listed above data.  The NTP packet
   is then appended by a MAC verifying its contents.

7.  Protocol Sequence

7.1.  The Client

7.1.1.  The Client in Unicast Mode

   For a unicast run, the client performs the following steps:

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

   2.  It 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 conform version
          number.

       *  It also has to verify verifies that the server has chosen the signature encryption and
          hash algorithms from its proposal sent in the client_assoc
          message.

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

   3.  The client then sends a client_cert message to the server.
       Again,

       *  Furthermore, it MUST remember the transmitted values for version number
       and algorithms for later checks.

   4.  It awaits a reply in the form of a server_cert message and performs authenticity checks on the
          certificate chain and the signature for the version number.

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

          Note that by performing 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 run.

   5. 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.

   6.

   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 message encrypted data with its own private key.

       *  It checks that the decrypted message is of the expected
          format: the concatenation of version number, a 128 bit nonce, nonce and a 128 bit cookie and a signature value.

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

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

       *  It verifies the signature using the server's public key.  The
          signature has to authenticate the version number, the nonce,
          the cookie, and the hash of the client's certificate.

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

   7.

   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.

   8.

   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 6.5.2. 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.
           disclosed (packet timeliness).  This is achieved via a
           combination of checks.  First the disclosure schedule and 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 whether that it does not already knows know the disclosed
           key.
           If so,  Otherwise, the client SHOULD discard the packet to
           avoid a buffer overrun.  If not, verified, the client verifies 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
           verified. shown.  If falsified, the client MUST
           discard the packet.

       3.  If the disclosed key is legitimate 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 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 6.5.1 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_cert message, the server checks whether
      it supports the given signature algorithm.  If so, it constructs
      and sends a server_cert message as described in Section 6.2.2.

   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 6.3.2. 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 6.4.2. 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 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 6.5.2. 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 6.6.1, 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
   [I-D.ietf-tictoc-security-requirements] [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 attachted attached to subsequent time response messages
   and has to respond accordingly by re-initiating the protocol with a
   cookie request (Section 6.3). 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, 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 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

      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 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 a 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.  Initial Verification of the Server Certificates

   The client has to verify the validity of the certificates during the
   certification message exchange (Section 6.2). 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.  Revocation of Server Certificates

   According to Section 8.1, it is the client's responsibility to
   initiate a new association with the server after the server's
   certificate expires.  To this end the client reads the expiration
   date of the certificate during the certificate message exchange
   (Section 6.2). 6.1.2).  Besides, certificates may also be revoked prior to
   the normal expiration date.  To increase security the client MAY
   verify the state of the server's certificate via OCSP periodically.

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.

11.4.  Denial-of-Service in Broadcast Mode

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

   Note,

   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 [I-D.ietf-tictoc-security-requirements]. [RFC7384].  This prevents the client to measure the
   network delay, and hence its time offset to the server, accurately
   [Mizrahi].  The delay attack does not modifiy modify the content of the
   exchanged synchronization packets.  Therefore cryptographic means are do
   not provide a feasible way to mitigate this attack.  However, serveral several
   non-cryptographic precautions can be taken in order to detect this
   attack.

   o

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

   o

   2.  Multiple communication paths: The client and server are utilizing
       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,  Note that this
       approach is not yet available, neither for NTP nor for PTP.

   o  The introduction

   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 the unicast mode: 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 this time
       server is larger than the threshold value.

   Additional provision against delay attacks has to be taken in the
   broadcast mode.  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 broadcast mode.  To this
   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 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
      threshold value.

   o  Usage status of an encrypted connection: the client exchanges all packets corresponding key via a
   unicast message exchange with the time broadcast server over an encrypted connection (e.g.  IPsec).
      This measure does not mitigate the delay attack but it makes it
      more difficult (see section
   Appendix C.4 for a detailed description of this check).  Note, that a
   broadcast client should also apply the adversary to identify the time
      synchronization packets. 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 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", RFC 5905, June 2010.

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

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

13.2.  Informative References

   [I-D.ietf-tictoc-security-requirements]
              Mizrahi, T., "Security Requirements of Time Protocols in
              Packet Switched Networks", draft-ietf-tictoc-security-
              requirements-10 (work in progress), July 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.

   [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.

   [Roettger]
              Roettger, S., "Analysis

   [RFC7384]  Mizrahi, T., "Security Requirements of the NTP Autokey Procedures",
              February 2012. 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 |
                        +----------+----------+
                                   |
                                   v
                        +---------------------+
                        |Certificate 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 Time Sync. Message |            |
                            +--------------+--------------+            |
                                           |                           |
   +-------------------------------------->o                           |
   |                                       |                           |
   |                                       v                           |
   |                             +-------------------+                 |
   |                             |Key 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 in NTS broadcast mode.

Appendix B.  Extension Fields

   In Section 6, some new extension fields for NTP packets are
   introduced.  They are listed here again, for reference.

                +------------------------+---------------+
                | name                   | used in       |
                +------------------------+---------------+
                | "association"          | client_assoc  |
                |                        | server_assoc  |
                |                        |               |
                | "certificate request"  | client_cert   |
                |                        |               |
                | "certificate"          | server_cert   |
                |                        |               |
                | "cookie request"       | client_cook   |
                |                        | Previous |                                     | "cookie transmit"
   | server_cook             +-------+-------+                                     |
   |                     |                                             |
   | "time request"            /--------*--------\                                    | time_request
   |           v                   v                                   |
   |      .---------.         .---------.                              |
   | "time response"     ( Verified  )       ( Falsified )                             | time_response
   |      '----+----'         '----+----'                              |
   |           |                   | "broadcast request"                                   | client_bpar
   |           v                   v                                   |
   |    +-------------+   +-----------------+                          |
   | "broadcast parameters"    |Sync. Process|   |Discard Previous | server_bpar                          |
   |    +------+------+   +--------+--------+                          |
   |           | "broadcast message"                   | server_broad                                   |
                +------------------------+---------------+
   +-----------+                   +-----------------------------------+

          Figure 2: The client's behaviour in NTS broadcast mode.

Appendix C. B.  TICTOC Security Requirements

   The following table compares the NTS specifications against the
   TICTOC security requirements [I-D.ietf-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.              | MUST        | OK   |
   +---------+------------------------------------+-------------+------+
   | 5.4     | Protection against DoS attacks     | SHOULD      | OK   |
   +---------+------------------------------------+-------------+------+
   | 5.5     | Replay protection                  | MUST        | OK   |
   +---------+------------------------------------+-------------+------+
   | 5.6     | Key freshness.                     | MUST        | OK   |
   +---------+------------------------------------+-------------+------+
   |         | Security association.              | SHOULD      | OK   |
   +---------+------------------------------------+-------------+------+
   |         | Unicast and multicast              | SHOULD      | OK   |
   |         | associations.                      |             |      |
   +---------+------------------------------------+-------------+------+
   | 5.7     | Performance: no degradation in     | MUST        | OK   |
   |         | quality of time 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 against TICTOC security
                               requirements.

Appendix D. C.  Broadcast Mode

   For the broadcast mode, NTS adopts the TESLA protocol, which is based
   on a one-way key chain. 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 the "not re-using keys" scheme of
   TESLA as described in section 3.7.2. of [RFC4082].

D.1.

C.1.  Server Preparations

   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 smaller 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 still provided for all choices d>0, the
       choice still makes a difference:

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

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

       The server SHOULD calculate d according to

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

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

D.2.

C.2.  Client Preparation

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

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

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

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

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

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

      *  the disclosure delay d.

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

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

   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.

D.3.

C.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.

D.4.

C.4.  Authentication of Received Packets

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

   Next, the client checks 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 by
   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), 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 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 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
   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 verifies identity with some
   earlier disclosed key by recursively applies the one-way
   function F to K_{i-d} until it can verify 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 falsified, the packet MUST be discarded and the client MUST
   reinitialize the broadcast mode with a unicast association (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 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 success, it regards M_{i-d} as authenticated.

Appendix E. D.  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

   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