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


                         Network Time Security
              draft-ietf-ntp-network-time-security-06.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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

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








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Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Security Threats  . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Objectives  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Terms and Abbreviations . . . . . . . . . . . . . . . . . . .   4
   5.  NTS Overview  . . . . . . . . . . . . . . . . . . . . . . . .   4
   6.  Protocol Messages . . . . . . . . . . . . . . . . . . . . . .   5
     6.1.  Association Messages  . . . . . . . . . . . . . . . . . .   6
       6.1.1.  Message Type: "client_assoc"  . . . . . . . . . . . .   6
       6.1.2.  Message Type: "server_assoc"  . . . . . . . . . . . .   6
     6.2.  Cookie Messages . . . . . . . . . . . . . . . . . . . . .   7
       6.2.1.  Message Type: "client_cook" . . . . . . . . . . . . .   7
       6.2.2.  Message Type: "server_cook" . . . . . . . . . . . . .   7
     6.3.  Unicast Time Synchronisation Messages . . . . . . . . . .   8
       6.3.1.  Message Type: "time_request"  . . . . . . . . . . . .   8
       6.3.2.  Message Type: "time_response" . . . . . . . . . . . .   8
     6.4.  Broadcast Parameter Messages  . . . . . . . . . . . . . .   9
       6.4.1.  Message Type: "client_bpar" . . . . . . . . . . . . .   9
       6.4.2.  Message Type: "server_bpar" . . . . . . . . . . . . .   9
     6.5.  Broadcast Messages  . . . . . . . . . . . . . . . . . . .  10
       6.5.1.  Message Type: "server_broad"  . . . . . . . . . . . .  10
     6.6.  Broadcast Key Check . . . . . . . . . . . . . . . . . . .  10
       6.6.1.  Message Type: "client_keycheck" . . . . . . . . . . .  10
       6.6.2.  Message Type: "server_keycheck" . . . . . . . . . . .  11
   7.  Message Dependencies  . . . . . . . . . . . . . . . . . . . .  11
   8.  Server Seed Considerations  . . . . . . . . . . . . . . . . .  12
   9.  Hash Algorithms and MAC Generation  . . . . . . . . . . . . .  13
     9.1.  Hash Algorithms . . . . . . . . . . . . . . . . . . . . .  13
     9.2.  MAC Calculation . . . . . . . . . . . . . . . . . . . . .  13
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  13
     11.1.  Privacy  . . . . . . . . . . . . . . . . . . . . . . . .  13



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     11.2.  Initial Verification of the Server Certificates  . . . .  14
     11.3.  Revocation of Server Certificates  . . . . . . . . . . .  14
     11.4.  Mitigating Denial-of-Service for broadcast packets . . .  14
     11.5.  Delay Attack . . . . . . . . . . . . . . . . . . . . . .  15
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  16
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  16
     13.2.  Informative References . . . . . . . . . . . . . . . . .  17
   Appendix A.  TICTOC Security Requirements . . . . . . . . . . . .  17
   Appendix B.  Using TESLA for Broadcast-Type Messages  . . . . . .  19
     B.1.  Server Preparation  . . . . . . . . . . . . . . . . . . .  19
     B.2.  Client Preparation  . . . . . . . . . . . . . . . . . . .  20
     B.3.  Sending Authenticated Broadcast Packets . . . . . . . . .  21
     B.4.  Authentication of Received Packets  . . . . . . . . . . .  21
   Appendix C.  Random Number Generation . . . . . . . . . . . . . .  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

1.  Introduction

   Time synchronization protocols are increasingly utilized to
   synchronize clocks in networked infrastructures.  The reliable
   performance of such infrastructures can be degraded seriously by
   successful attacks against the time synchronization protocol.
   Therefore, time synchronization protocols have to be secured if they
   are applied in environments that are prone to malicious attacks.
   This can be accomplished either by utilization of external security
   protocols, 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 protocols to verify the authenticity of the time server 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, for time
   synchronization messages.  This prerequisite corresponds with the
   requirement that a security mechanism for timekeeping must be
   designed in such a way that it does not degrade the quality of the
   time transfer [RFC7384].

   Note:





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

2.  Security Threats

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

3.  Objectives

   The objectives of the NTS specification are as follows:

   o  Authenticity: NTS enables a client/slave to authenticate its time
      server(s)/master(s).

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

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

   o  Integration with protocols: NTS can be used to secure different
      time synchronization protocols, specifically at least NTP and PTP.
      An 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 and to exchange a symmetric key, the so-called cookie.
   This cookie is then used to protect the authenticity and the
   integrity of subsequent unicast-type time synchronization packets.
   This is done by means of a Message Authentication Code (MAC), which
   is attached to each time synchronization packet.  The calculation of
   the MAC includes the whole time synchronization packet and the cookie
   which is shared between client and server.  The cookie is calculated
   according to:



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      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 server seed is a random value
   of bit length b that the server possesses, which has to be kept
   secret.  The cookie never changes as long as the server seed stays
   the same, but the server seed has to be refreshed periodically in
   order to provide key freshness as required in [RFC7384].  See
   Section 8 for details on 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 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 of the key, the client has
   to be loosely time synchronized with the server.  This has to be
   accomplished during the initial client server exchange between the
   broadcast client and the server.  In addition, NTS uses another, more
   rigorous check than what is 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



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   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

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

6.1.1.  Message Type: "client_assoc"

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

   o  the NTS message ID "client_assoc",

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

   o  the hostname of the client,

   o  a selection of accepted hash algorithms, and

   o  a selection of accepted encryption algorithms.

6.1.2.  Message Type: "server_assoc"

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

   o  the NTS message ID "server_assoc",

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

   o  the hostname of the server,

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




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   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.

6.2.  Cookie Messages

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

6.2.1.  Message Type: "client_cook"

   This message is sent by the client 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 nonce,

   o  the negotiated hash algorithm H,

   o  the client's certificate.

6.2.2.  Message Type: "server_cook"

   This message is sent by the server 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,






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

      *  the nonce transmitted in client_cook, and

      *  the cookie.

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

6.3.  Unicast Time Synchronisation Messages

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

6.3.1.  Message Type: "time_request"

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

   o  the NTS message ID "time_request",

   o  the negotiated version number,

   o  a nonce,

   o  the negotiated hash algorithm H,

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

6.3.2.  Message Type: "time_response"

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

   o  the NTS message ID "time_response",

   o  the version number as transmitted in time_request,

   o  the server's time synchronization response data,



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   o  the nonce transmitted in time_request,

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

6.4.  Broadcast Parameter Messages

   In this message exchange, the client receives the necessary
   information to execute the TESLA protocol in a secured broadcast
   association.  The client can only initiate a secure broadcast
   association after a successful unicast run.

   See Appendix B for more details on TESLA.

6.4.1.  Message Type: "client_bpar"

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

   o  the NTS message ID "client_bpar",

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

   o  the client's hostname, and

   o  the signature algorithm negotiated during unicast.

6.4.2.  Message Type: "server_bpar"

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

   o  the NTS message ID "server_bpar",

   o  the version number as transmitted in the client_bpar message,

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

   o  the disclosure schedule of the keys.  This contains:

      *  the last key of the key chain,

      *  time interval duration,

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




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

      *  the next interval's associated index.

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

6.5.  Broadcast Messages

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

6.5.1.  Message Type: "server_broad"

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

   o  the NTS message ID "server_broad",

   o  the version number that the server 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 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



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   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 of the client's certificate under H.

6.6.2.  Message Type: "server_keycheck"

   A message of this type is sent by the server upon receipt of a
   client_keycheck message during the broadcast loop of the server.
   Prior to this, 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.

7.  Message Dependencies

















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             +--------------------+
             |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 Time Synchronization Reception|
   +----------------------------------------+
                       |
           Whenever deemed necessary
                       |
                       v
              +-----------------+
              |Keycheck Exchange|
              +-----------------+

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.

   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



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   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).

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.  The
   client and the 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 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.

9.2.  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 9.1).

10.  IANA Considerations

11.  Security Considerations

11.1.  Privacy

   tbd








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11.2.  Initial Verification of the Server Certificates

   The client has to verify the validity of the certificates during the
   certification message exchange (Section 6.1.2).  Since it generally
   has no reliable time during this initial communication phase, it is
   impossible to verify the period of validity of the certificates.
   Therefore, the client MUST use one of the following approaches:

   o  The validity of the certificates is preconditioned.  Usually this
      will be the case in corporate networks.

   o  The client ensures that the certificates are not revoked.  To this
      end, the client uses the Online Certificate Status Protocol (OCSP)
      defined in [RFC6277].

   o  The client requests a different service to get an initial time
      stamp in order to be able to verify the certificates' periods of
      validity.  To this end, it can, e.g., use a secure shell
      connection to a reliable host.  Another alternative is to request
      a time stamp from a Time Stamping Authority (TSA) by means of the
      Time-Stamp Protocol (TSP) defined in [RFC3161].

11.3.  Revocation 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 the expiration date of the
   certificate during the certificate message exchange (Section 6.1.2).
   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.

11.4.  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




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   common to the whole group.  This adds additional complexity to the
   protocol and hence is currently not considered in this document.

11.5.  Delay Attack

   In a packet delay attack, an adversary with the ability to act as a
   MITM delays time synchronization packets between client and server
   asymmetrically [RFC7384].  This prevents the client 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.  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



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   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.

12.  Acknowledgements

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

13.  References

13.1.  Normative References

   [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.



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

13.2.  Informative References

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

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

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

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

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

   [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.  TICTOC Security Requirements

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

   +---------+------------------------------------+-------------+------+
   | Section | Requirement from I-D tictoc        | Requirement | NTS  |
   |         | security-requirements-05           | level       |      |
   +---------+------------------------------------+-------------+------+



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   | 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 associations | SHOULD      | OK   |
   +---------+------------------------------------+-------------+------+
   | 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 11.5.

   Comparison of NTS specification against TICTOC security requirements.





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Appendix B.  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 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
       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:





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       *  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,




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      *  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 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.

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



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   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 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).



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

Appendix C.  Random Number Generation

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

Authors' Addresses

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

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


   Stephen Roettger
   Google Inc.

   Email: stephen.roettger@googlemail.com


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

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











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