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NTP Working Group                                              D. Franke
Internet-Draft                                                    Akamai
Intended status: Standards Track                               D. Sibold
Expires: September 19, 2020                                   K. Teichel
                                                                     PTB
                                                             M. Dansarie

                                                             R. Sundblad
                                                                  Netnod
                                                          March 18, 2020


          Network Time Security for the Network Time Protocol
                  draft-ietf-ntp-using-nts-for-ntp-24

Abstract

   This memo specifies Network Time Security (NTS), a mechanism for
   using Transport Layer Security (TLS) and Authenticated Encryption
   with Associated Data (AEAD) to provide cryptographic security for the
   client-server mode of the Network Time Protocol (NTP).

   NTS is structured as a suite of two loosely coupled sub-protocols.
   The first (NTS-KE) handles initial authentication and key
   establishment over TLS.  The second handles encryption and
   authentication during NTP time synchronization via extension fields
   in the NTP packets, and holds all required state only on the client
   via opaque cookies.

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 https://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on September 19, 2020.






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Copyright Notice

   Copyright (c) 2020 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
   (https://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  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Objectives  . . . . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Protocol Overview . . . . . . . . . . . . . . . . . . . .   5
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   7
   3.  TLS profile for Network Time Security . . . . . . . . . . . .   7
   4.  The NTS Key Establishment Protocol  . . . . . . . . . . . . .   8
     4.1.  NTS-KE Record Types . . . . . . . . . . . . . . . . . . .  10
       4.1.1.  End of Message  . . . . . . . . . . . . . . . . . . .  11
       4.1.2.  NTS Next Protocol Negotiation . . . . . . . . . . . .  11
       4.1.3.  Error . . . . . . . . . . . . . . . . . . . . . . . .  11
       4.1.4.  Warning . . . . . . . . . . . . . . . . . . . . . . .  12
       4.1.5.  AEAD Algorithm Negotiation  . . . . . . . . . . . . .  12
       4.1.6.  New Cookie for NTPv4  . . . . . . . . . . . . . . . .  13
       4.1.7.  NTPv4 Server Negotiation  . . . . . . . . . . . . . .  13
       4.1.8.  NTPv4 Port Negotiation  . . . . . . . . . . . . . . .  14
     4.2.  Key Extraction (generally)  . . . . . . . . . . . . . . .  14
   5.  NTS Extension Fields for NTPv4  . . . . . . . . . . . . . . .  15
     5.1.  Key Extraction (for NTPv4)  . . . . . . . . . . . . . . .  15
     5.2.  Packet Structure Overview . . . . . . . . . . . . . . . .  15
     5.3.  The Unique Identifier Extension Field . . . . . . . . . .  16
     5.4.  The NTS Cookie Extension Field  . . . . . . . . . . . . .  16
     5.5.  The NTS Cookie Placeholder Extension Field  . . . . . . .  17
     5.6.  The NTS Authenticator and Encrypted Extension Fields
           Extension Field . . . . . . . . . . . . . . . . . . . . .  17
     5.7.  Protocol Details  . . . . . . . . . . . . . . . . . . . .  19
   6.  Suggested Format for NTS Cookies  . . . . . . . . . . . . . .  23
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  25
     7.1.  Service Name and Transport Protocol Port Number Registry   25
     7.2.  TLS Application-Layer Protocol Negotiation (ALPN)
           Protocol IDs Registry . . . . . . . . . . . . . . . . . .  25
     7.3.  TLS Exporter Labels Registry  . . . . . . . . . . . . . .  26



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     7.4.  NTP Kiss-o'-Death Codes Registry  . . . . . . . . . . . .  26
     7.5.  NTP Extension Field Types Registry  . . . . . . . . . . .  26
     7.6.  Network Time Security Key Establishment Record Types
           Registry  . . . . . . . . . . . . . . . . . . . . . . . .  27
     7.7.  Network Time Security Next Protocols Registry . . . . . .  28
     7.8.  Network Time Security Error and Warning Codes Registries   29
   8.  Implementation Status - RFC EDITOR: REMOVE BEFORE PUBLICATION  30
     8.1.  Implementation 1  . . . . . . . . . . . . . . . . . . . .  30
       8.1.1.  Coverage  . . . . . . . . . . . . . . . . . . . . . .  30
       8.1.2.  Licensing . . . . . . . . . . . . . . . . . . . . . .  31
       8.1.3.  Contact Information . . . . . . . . . . . . . . . . .  31
       8.1.4.  Last Update . . . . . . . . . . . . . . . . . . . . .  31
     8.2.  Implementation 2  . . . . . . . . . . . . . . . . . . . .  31
       8.2.1.  Coverage  . . . . . . . . . . . . . . . . . . . . . .  31
       8.2.2.  Licensing . . . . . . . . . . . . . . . . . . . . . .  31
       8.2.3.  Contact Information . . . . . . . . . . . . . . . . .  31
       8.2.4.  Last Update . . . . . . . . . . . . . . . . . . . . .  31
     8.3.  Implementation 3  . . . . . . . . . . . . . . . . . . . .  32
       8.3.1.  Coverage  . . . . . . . . . . . . . . . . . . . . . .  32
       8.3.2.  Licensing . . . . . . . . . . . . . . . . . . . . . .  32
       8.3.3.  Contact Information . . . . . . . . . . . . . . . . .  32
       8.3.4.  Last Update . . . . . . . . . . . . . . . . . . . . .  32
     8.4.  Implementation 4  . . . . . . . . . . . . . . . . . . . .  32
       8.4.1.  Coverage  . . . . . . . . . . . . . . . . . . . . . .  32
       8.4.2.  Licensing . . . . . . . . . . . . . . . . . . . . . .  33
       8.4.3.  Contact Information . . . . . . . . . . . . . . . . .  33
       8.4.4.  Last Update . . . . . . . . . . . . . . . . . . . . .  33
     8.5.  Implementation 5  . . . . . . . . . . . . . . . . . . . .  33
       8.5.1.  Coverage  . . . . . . . . . . . . . . . . . . . . . .  33
       8.5.2.  Licensing . . . . . . . . . . . . . . . . . . . . . .  33
       8.5.3.  Contact Information . . . . . . . . . . . . . . . . .  33
       8.5.4.  Last Update . . . . . . . . . . . . . . . . . . . . .  33
     8.6.  Implementation 6  . . . . . . . . . . . . . . . . . . . .  33
       8.6.1.  Coverage  . . . . . . . . . . . . . . . . . . . . . .  34
       8.6.2.  Licensing . . . . . . . . . . . . . . . . . . . . . .  34
       8.6.3.  Contact Information . . . . . . . . . . . . . . . . .  34
       8.6.4.  Last Update . . . . . . . . . . . . . . . . . . . . .  34
     8.7.  Interoperability  . . . . . . . . . . . . . . . . . . . .  34
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  34
     9.1.  Protected Modes . . . . . . . . . . . . . . . . . . . . .  34
     9.2.  Sensitivity to DDoS Attacks . . . . . . . . . . . . . . .  35
     9.3.  Avoiding DDoS Amplification . . . . . . . . . . . . . . .  35
     9.4.  Initial Verification of Server Certificates . . . . . . .  36
     9.5.  Delay Attacks . . . . . . . . . . . . . . . . . . . . . .  37
     9.6.  NTS Stripping . . . . . . . . . . . . . . . . . . . . . .  37
   10. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  38
     10.1.  Unlinkability  . . . . . . . . . . . . . . . . . . . . .  38
     10.2.  Confidentiality  . . . . . . . . . . . . . . . . . . . .  39



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   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  39
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  39
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  39
     12.2.  Informative References . . . . . . . . . . . . . . . . .  41
   Appendix A.  Terms and Abbreviations  . . . . . . . . . . . . . .  42
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  43

1.  Introduction

   This memo specifies Network Time Security (NTS), a cryptographic
   security mechanism for network time synchronization.  A complete
   specification is provided for application of NTS to the client-server
   mode of the Network Time Protocol (NTP) [RFC5905].

1.1.  Objectives

   The objectives of NTS are as follows:

   o  Identity: Through the use of a X.509 public key infrastructure,
      implementations may cryptographically establish the identity of
      the parties they are communicating with.

   o  Authentication: Implementations may cryptographically verify that
      any time synchronization packets are authentic, i.e., that they
      were produced by an identified party and have not been modified in
      transit.

   o  Confidentiality: Although basic time synchronization data is
      considered non-confidential and sent in the clear, NTS includes
      support for encrypting NTP extension fields.

   o  Replay prevention: Client implementations may detect when a
      received time synchronization packet is a replay of a previous
      packet.

   o  Request-response consistency: Client implementations may verify
      that a time synchronization packet received from a server was sent
      in response to a particular request from the client.

   o  Unlinkability: For mobile clients, NTS will not leak any
      information additional to NTP which would permit a passive
      adversary to determine that two packets sent over different
      networks came from the same client.

   o  Non-amplification: Implementations (especially server
      implementations) may avoid acting as distributed denial-of-service
      (DDoS) amplifiers by never responding to a request with a packet
      larger than the request packet.



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   o  Scalability: Server implementations may serve large numbers of
      clients without having to retain any client-specific state.

   o  Performance: NTS must not significantly degrade the quality of the
      time transfer.  The encryption and authentication used when
      actually transferring time should be lightweight (see RFC 7384,
      Section 5.7 [RFC7384]).

1.2.  Protocol Overview

   The Network Time Protocol includes many different operating modes to
   support various network topologies (see RFC 5905, Section 3
   [RFC5905]).  In addition to its best-known and most-widely-used
   client-server mode, it also includes modes for synchronization
   between symmetric peers, a control mode for server monitoring and
   administration, and a broadcast mode.  These various modes have
   differing and partly contradictory requirements for security and
   performance.  Symmetric and control modes demand mutual
   authentication and mutual replay protection.  Additionally, for
   certain message types control mode may require confidentiality as
   well as authentication.  Client-server mode places more stringent
   requirements on resource utilization than other modes, because
   servers may have vast number of clients and be unable to afford to
   maintain per-client state.  However, client-server mode also has more
   relaxed security needs, because only the client requires replay
   protection: it is harmless for stateless servers to process replayed
   packets.  The security demands of symmetric and control modes, on the
   other hand, are in conflict with the resource-utilization demands of
   client-server mode: any scheme which provides replay protection
   inherently involves maintaining some state to keep track of what
   messages have already been seen.

   This memo specifies NTS exclusively for the client-server mode of
   NTP.  To this end, NTS is structured as a suite of two protocols:

      The "NTS Extensions for NTPv4" define a collection of NTP
      extension fields for cryptographically securing NTPv4 using
      previously-established key material.  They are suitable for
      securing client-server mode because the server can implement them
      without retaining per-client state.  All state is kept by the
      client and provided to the server in the form of an encrypted
      cookie supplied with each request.  On the other hand, the NTS
      Extension Fields are suitable *only* for client-server mode
      because only the client, and not the server, is protected from
      replay.

      The "NTS Key Establishment" protocol (NTS-KE) is a mechanism for
      establishing key material for use with the NTS Extension Fields



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      for NTPv4.  It uses TLS to exchange keys, provide the client with
      an initial supply of cookies, and negotiate some additional
      protocol options.  After this exchange, the TLS channel is closed
      with no per-client state remaining on the server side.

   The typical protocol flow is as follows: The client connects to an
   NTS-KE server on the NTS TCP port and the two parties perform a TLS
   handshake.  Via the TLS channel, the parties negotiate some
   additional protocol parameters and the server sends the client a
   supply of cookies along with an address of an NTP server for which
   the cookies are valid.  The parties use TLS key export [RFC5705] to
   extract key material which will be used in the next phase of the
   protocol.  This negotiation takes only a single round trip, after
   which the server closes the connection and discards all associated
   state.  At this point the NTS-KE phase of the protocol is complete.
   Ideally, the client never needs to connect to the NTS-KE server
   again.

   Time synchronization proceeds with the indicated NTP server over the
   NTP UDP port.  The client sends the server an NTP client packet which
   includes several extension fields.  Included among these fields are a
   cookie (previously provided by the key exchange server) and an
   authentication tag, computed using key material extracted from the
   NTS-KE handshake.  The NTP server uses the cookie to recover this key
   material and send back an authenticated response.  The response
   includes a fresh, encrypted cookie which the client then sends back
   in the clear in a subsequent request.  (This constant refreshing of
   cookies is necessary in order to achieve NTS's unlinkability goal.)

   Figure 1 provides an overview of the high-level interaction between
   the client, the NTS-KE server, and the NTP server.  Note that the
   cookies' data format and the exchange of secrets between NTS-KE and
   NTP servers are not part of this specification and are implementation
   dependent.  However, a suggested format for NTS cookies is provided
   in Section 6.
















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                                                        +--------------+
                                                        |              |
                                                    +-> | NTP Server 1 |
                                                    |   |              |
                              Shared cookie         |   +--------------+
   +---------------+      encryption parameters     |   +--------------+
   |               |    (Implementation dependent)  |   |              |
   | NTS-KE Server | <------------------------------+-> | NTP Server 2 |
   |               |                                |   |              |
   +---------------+                                |   +--------------+
          ^                                         |          .
          |                                         |          .
          | 1. Negotiate parameters,                |          .
          |    receive initial cookie               |   +--------------+
          |    supply, generate AEAD keys,          |   |              |
          |    and receive NTP server IP            +-> | NTP Server N |
          |    addresses using "NTS Key                 |              |
          |    Establishment" protocol.                 +--------------+
          |                                                    ^
          |                                                    |
          |             +----------+                           |
          |             |          |                           |
          +-----------> |  Client  | <-------------------------+
                        |          |  2. Perform authenticated
                        +----------+     time synchronization
                                         and generate new
                                         cookies using "NTS
                                         Extension Fields for
                                         NTPv4".

           Figure 1: Overview of High-Level Interactions in NTS

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  TLS profile for Network Time Security

   Network Time Security makes use of TLS for NTS key establishment.

   Since the NTS protocol is new as of this publication, no backward-
   compatibility concerns exist to justify using obsolete, insecure, or
   otherwise broken TLS features or versions.  Implementations MUST
   conform with [RFC7525] or with a later revision of BCP 195.  In



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   particular, failure to use cipher suites that provide forward secrecy
   will make all negotiated NTS keys recoverable by anyone that gains
   access to the NTS-KE server's private key.  Furthermore:

   Implementations MUST NOT negotiate TLS versions earlier than 1.2,
   SHOULD negotiate TLS 1.3 [RFC8446] or later when possible, and MAY
   refuse to negotiate any TLS version which has been superseded by a
   later supported version.

   Use of the Application-Layer Protocol Negotiation Extension [RFC7301]
   is integral to NTS and support for it is REQUIRED for
   interoperability.

4.  The NTS Key Establishment Protocol

   The NTS key establishment protocol is conducted via TCP port
   [[TBD1]].  The two endpoints carry out a TLS handshake in conformance
   with Section 3, with the client offering (via an ALPN [RFC7301]
   extension), and the server accepting, an application-layer protocol
   of "ntske/1".  Immediately following a successful handshake, the
   client SHALL send a single request as Application Data encapsulated
   in the TLS-protected channel.  Then, the server SHALL send a single
   response followed by a TLS "Close notify" alert and then discard the
   channel state.

   The client's request and the server's response each SHALL consist of
   a sequence of records formatted according to Figure 2.  The request
   and a non-error response each SHALL include exactly one NTS Next
   Protocol Negotiation record.  The sequence SHALL be terminated by a
   "End of Message" record.  The requirement that all NTS-KE messages be
   terminated by an End of Message record makes them self-delimiting.

   Clients and servers MAY enforce length limits on requests and
   responses, however, servers MUST accept requests of at least 1024
   octets and clients SHOULD accept responses of at least 65536 octets.
















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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |C|         Record Type         |          Body Length          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                                                               .
   .                           Record Body                         .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 2: NTS-KE Record Format

   The fields of an NTS-KE record are defined as follows:

      C (Critical Bit): Determines the disposition of unrecognized
      Record Types.  Implementations which receive a record with an
      unrecognized Record Type MUST ignore the record if the Critical
      Bit is 0 and MUST treat it as an error if the Critical Bit is 1
      (see Section 4.1.3).

      Record Type Number: A 15-bit integer in network byte order.  The
      semantics of record types 0-7 are specified in this memo.
      Additional type numbers SHALL be tracked through the IANA Network
      Time Security Key Establishment Record Types registry.

      Body Length: The length of the Record Body field, in octets, as a
      16-bit integer in network byte order.  Record bodies MAY have any
      representable length and need not be aligned to a word boundary.

      Record Body: The syntax and semantics of this field SHALL be
      determined by the Record Type.

   For clarity regarding bit-endianness: the Critical Bit is the most-
   significant bit of the first octet.  In the C programming language,
   given a network buffer `unsigned char b[]` containing an NTS-KE
   record, the critical bit is `b[0] >> 7` while the record type is
   `((b[0] & 0x7f) << 8) + b[1]`.

   Note that, although the Type-Length-Body format of an NTS-KE record
   is similar to that of an NTP extension field, the semantics of the
   length field differ.  While the length subfield of an NTP extension
   field gives the length of the entire extension field including the
   type and length subfields, the length field of an NTS-KE record gives
   just the length of the body.





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   Figure 3 provides a schematic overview of the key exchange.  It
   displays the protocol steps to be performed by the NTS client and
   server and record types to be exchanged.

                   +---------------------------------------+
                   | - Verify client request message.      |
                   | - Extract TLS key material.           |
                   | - Generate KE response message.       |
                   |   - Include Record Types:             |
                   |       o NTS Next Protocol Negotiation |
                   |       o AEAD Algorithm Negotiation    |
                   |       o NTP Server Negotiation        |
                   |       o New Cookie for NTPv4          |
                   |       o <New Cookie for NTPv4>        |
                   |       o End of Message                |
                   +-----------------+---------------------+
                                     |
                                     |
   Server -----------+---------------+-----+----------------------->
                     ^                      \
                    /                        \
                   /    TLS application       \
                  /     data                   \
                 /                              \
                /                                V
   Client -----+---------------------------------+----------------->
               |                                 |
               |                                 |
               |                                 |
   +-----------+----------------------+   +------+-----------------+
   |- Generate KE request message.    |   |- Verify server response|
   | - Include Record Types:          |   |  message.              |
   |  o NTS Next Protocol Negotiation |   |- Extract cookie(s).    |
   |  o AEAD Algorithm Negotiation    |   |                        |
   |  o <NTP Server Negotiation>      |   |                        |
   |  o End of Message                |   |                        |
   +----------------------------------+   +------------------------+

                    Figure 3: NTS Key Exchange Messages

4.1.  NTS-KE Record Types

   The following NTS-KE Record Types are defined:








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4.1.1.  End of Message

   The End of Message record has a Record Type number of 0 and a zero-
   length body.  It MUST occur exactly once as the final record of every
   NTS-KE request and response.  The Critical Bit MUST be set.

4.1.2.  NTS Next Protocol Negotiation

   The NTS Next Protocol Negotiation record has a Record Type number of
   1.  It MUST occur exactly once in every NTS-KE request and response.
   Its body consists of a sequence of 16-bit unsigned integers in
   network byte order.  Each integer represents a Protocol ID from the
   IANA Network Time Security Next Protocols registry.  The Critical Bit
   MUST be set.

   The Protocol IDs listed in the client's NTS Next Protocol Negotiation
   record denote those protocols which the client wishes to speak using
   the key material established through this NTS-KE session.  Protocol
   IDs listed in the NTS-KE server's response MUST comprise a subset of
   those listed in the request and denote those protocols which the NTP
   server is willing and able to speak using the key material
   established through this NTS-KE session.  The client MAY proceed with
   one or more of them.  The request MUST list at least one protocol,
   but the response MAY be empty.

4.1.3.  Error

   The Error record has a Record Type number of 2.  Its body is exactly
   two octets long, consisting of an unsigned 16-bit integer in network
   byte order, denoting an error code.  The Critical Bit MUST be set.

   Clients MUST NOT include Error records in their request.  If clients
   receive a server response which includes an Error record, they MUST
   discard any key material negotiated during the initial TLS exchange
   and MUST NOT proceed to the Next Protocol.

   The following error codes are defined:

      Error code 0 means "Unrecognized Critical Record".  The server
      MUST respond with this error code if the request included a record
      which the server did not understand and which had its Critical Bit
      set.  The client SHOULD NOT retry its request without
      modification.

      Error code 1 means "Bad Request".  The server MUST respond with
      this error if the request is not complete and syntactically well-
      formed, or, upon the expiration of an implementation-defined
      timeout, it has not yet received such a request.



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      Error code 2 means "Internal Server Error".  The server MUST
      respond with this error if it is unable to respond properly due to
      an internal condition.

4.1.4.  Warning

   The Warning record has a Record Type number of 3.  Its body is
   exactly two octets long, consisting of an unsigned 16-bit integer in
   network byte order, denoting a warning code.  The Critical Bit MUST
   be set.

   Clients MUST NOT include Warning records in their request.  If
   clients receive a server response which includes a Warning record,
   they MAY discard any negotiated key material and abort without
   proceeding to the Next Protocol.  Unrecognized warning codes MUST be
   treated as errors.

   This memo defines no warning codes.

4.1.5.  AEAD Algorithm Negotiation

   The AEAD Algorithm Negotiation record has a Record Type number of 4.
   Its body consists of a sequence of unsigned 16-bit integers in
   network byte order, denoting Numeric Identifiers from the IANA AEAD
   Algorithms registry [IANA-AEAD].  The Critical Bit MAY be set.

   If the NTS Next Protocol Negotiation record offers Protocol ID 0 (for
   NTPv4), then this record MUST be included exactly once.  Other
   protocols MAY require it as well.

   When included in a request, this record denotes which AEAD algorithms
   the client is willing to use to secure the Next Protocol, in
   decreasing preference order.  When included in a response, this
   record denotes which algorithm the server chooses to use.  It is
   empty if the server supports none of the algorithms offered.  In
   requests, the list MUST include at least one algorithm.  In
   responses, it MUST include at most one.  Honoring the client's
   preference order is OPTIONAL: servers may select among any of the
   client's offered choices, even if they are able to support some other
   algorithm which the client prefers more.

   Server implementations of NTS extension fields for NTPv4 (Section 5)
   MUST support AEAD_AES_SIV_CMAC_256 [RFC5297] (Numeric Identifier 15).
   That is, if the client includes AEAD_AES_SIV_CMAC_256 in its AEAD
   Algorithm Negotiation record and the server accepts Protocol ID 0
   (NTPv4) in its NTS Next Protocol Negotiation record, then the
   server's AEAD Algorithm Negotiation record MUST NOT be empty.




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4.1.6.  New Cookie for NTPv4

   The New Cookie for NTPv4 record has a Record Type number of 5.  The
   contents of its body SHALL be implementation-defined and clients MUST
   NOT attempt to interpret them.  See Section 6 for a suggested
   construction.

   Clients MUST NOT send records of this type.  Servers MUST send at
   least one record of this type, and SHOULD send eight of them, if the
   Next Protocol Negotiation response record contains Protocol ID 0
   (NTPv4) and the AEAD Algorithm Negotiation response record is not
   empty.  The Critical Bit SHOULD NOT be set.

4.1.7.  NTPv4 Server Negotiation

   The NTPv4 Server Negotiation record has a Record Type number of 6.
   Its body consists of an ASCII-encoded [RFC0020] string.  The contents
   of the string SHALL be either an IPv4 address, an IPv6 address, or a
   fully qualified domain name (FQDN).  IPv4 addresses MUST be in dotted
   decimal notation.  IPv6 addresses MUST conform to the "Text
   Representation of Addresses" as specified in RFC 4291 [RFC4291] and
   MUST NOT include zone identifiers [RFC6874].  If a label contains at
   least one non-ASCII character, it is an internationalized domain name
   and an A-LABEL MUST be used as defined in section 2.3.2.1 of RFC 5890
   [RFC5890].  If the record contains a domain name, the recipient MUST
   treat it as a FQDN, e.g. by making sure it ends with a dot.

   When NTPv4 is negotiated as a Next Protocol and this record is sent
   by the server, the body specifies the hostname or IP address of the
   NTPv4 server with which the client should associate and which will
   accept the supplied cookies.  If no record of this type is sent, the
   client SHALL interpret this as a directive to associate with an NTPv4
   server at the same IP address as the NTS-KE server.  Servers MUST NOT
   send more than one record of this type.

   When this record is sent by the client, it indicates that the client
   wishes to associate with the specified NTP server.  The NTS-KE server
   MAY incorporate this request when deciding what NTPv4 Server
   Negotiation records to respond with, but honoring the client's
   preference is OPTIONAL.  The client MUST NOT send more than one
   record of this type.

   If the client has sent a record of this type, the NTS-KE server
   SHOULD reply with the same record if it is valid and the server is
   able to supply cookies for it.  If the client has not sent any record
   of this type, the NTS-KE server SHOULD respond with either an NTP
   server address in the same family as the NTS-KE session or a FQDN




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   that can be resolved to an address in that family, if such
   alternatives are available.

   Servers MAY set the Critical Bit on records of this type; clients
   SHOULD NOT.

4.1.8.  NTPv4 Port Negotiation

   The NTPv4 Port Negotiation record has a Record Type number of 7.  Its
   body consists of a 16-bit unsigned integer in network byte order,
   denoting a UDP port number.

   When NTPv4 is negotiated as a Next Protocol and this record is sent
   by the server, the body specifies the port number of the NTPv4 server
   with which the client should associate and which will accept the
   supplied cookies.  If no record of this type is sent, the client
   SHALL assume a default of 123 (the registered port number for NTP).

   When this record is sent by the client in conjunction with a NTPv4
   Server Negotiation record, it indicates that the client wishes to
   associate with the NTP server at the specified port.  The NTS-KE
   server MAY incorporate this request when deciding what NTPv4 Server
   Negotiation and NTPv4 Port Negotiation records to respond with, but
   honoring the client's preference is OPTIONAL.

   Servers MAY set the Critical Bit on records of this type; clients
   SHOULD NOT.

4.2.  Key Extraction (generally)

   Following a successful run of the NTS-KE protocol, key material SHALL
   be extracted using the TLS pseudorandom function (PRF) [RFC5705] for
   TLS version 1.2, or the HMAC-based Extract-and-Expand Key Derivation
   Function (HKDF) [RFC5869] in accordance with RFC 8446, Section 7.5
   [RFC8446] for TLS version 1.3.  Inputs to the exporter function are
   to be constructed in a manner specific to the negotiated Next
   Protocol.  However, all protocols which utilize NTS-KE MUST conform
   to the following two rules:

      The disambiguating label string MUST be "EXPORTER-network-time-
      security/1".

      The per-association context value MUST be provided and MUST begin
      with the two-octet Protocol ID which was negotiated as a Next
      Protocol.






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5.  NTS Extension Fields for NTPv4

5.1.  Key Extraction (for NTPv4)

   Following a successful run of the NTS-KE protocol wherein Protocol ID
   0 (NTPv4) is selected as a Next Protocol, two AEAD keys SHALL be
   extracted: a client-to-server (C2S) key and a server-to-client (S2C)
   key.  These keys SHALL be computed with the PRF defined in RFC 5705
   [RFC5705] for TLS version 1.2, or the HKDF defined in RFC 8446,
   Section 7.5 [RFC8446] for TLS version 1.3, using the following
   inputs.

      The disambiguating label string (for PRF) or label (for HKDF)
      SHALL be "EXPORTER-network-time-security/1".

      The context value SHALL consist of the following five octets:

         The first two octets SHALL be zero (the Protocol ID for NTPv4).

         The next two octets SHALL be the Numeric Identifier of the
         negotiated AEAD Algorithm in network byte order.

         The final octet SHALL be 0x00 for the C2S key and 0x01 for the
         S2C key.

   Implementations wishing to derive additional keys for private or
   experimental use MUST NOT do so by extending the above-specified
   syntax for per-association context values.  Instead, they SHOULD use
   their own disambiguating label string.  Note that RFC 5705 [RFC5705]
   provides that disambiguating label strings beginning with
   "EXPERIMENTAL" MAY be used without IANA registration.

5.2.  Packet Structure Overview

   In general, an NTS-protected NTPv4 packet consists of:

      The usual 48-octet NTP header which is authenticated but not
      encrypted.

      Some extension fields which are authenticated but not encrypted.

      An extension field which contains AEAD output (i.e., an
      authentication tag and possible ciphertext).  The corresponding
      plaintext, if non-empty, consists of some extension fields which
      benefit from both encryption and authentication.






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      Possibly, some additional extension fields which are neither
      encrypted nor authenticated.  In general, these are discarded by
      the receiver.

   Always included among the authenticated or authenticated-and-
   encrypted extension fields are a cookie extension field and a unique
   identifier extension field, as described in Section 5.7.  The purpose
   of the cookie extension field is to enable the server to offload
   storage of session state onto the client.  The purpose of the unique
   identifier extension field is to protect the client from replay
   attacks.

5.3.  The Unique Identifier Extension Field

   The Unique Identifier extension field provides the client with a
   cryptographically strong means of detecting replayed packets.  It has
   a Field Type of [[TBD2]].  When the extension field is included in a
   client packet (mode 3), its body SHALL consist of a string of octets
   generated uniformly at random.  The string MUST be at least 32 octets
   long.  When the extension field is included in a server packet (mode
   4), its body SHALL contain the same octet string as was provided in
   the client packet to which the server is responding.  All server
   packets generated by NTS-implementing servers in response to client
   packets containing this extension field MUST also contain this field
   with the same content as in the client's request.  The field's use in
   modes other than client-server is not defined.

   This extension field MAY also be used standalone, without NTS, in
   which case it provides the client with a means of detecting spoofed
   packets from off-path attackers.  Historically, NTP's origin
   timestamp field has played both these roles, but for cryptographic
   purposes this is suboptimal because it is only 64 bits long and,
   depending on implementation details, most of those bits may be
   predictable.  In contrast, the Unique Identifier extension field
   enables a degree of unpredictability and collision resistance more
   consistent with cryptographic best practice.

5.4.  The NTS Cookie Extension Field

   The NTS Cookie extension field has a Field Type of [[TBD3]].  Its
   purpose is to carry information which enables the server to recompute
   keys and other session state without having to store any per-client
   state.  The contents of its body SHALL be implementation-defined and
   clients MUST NOT attempt to interpret them.  See Section 6 for a
   suggested construction.  The NTS Cookie extension field MUST NOT be
   included in NTP packets whose mode is other than 3 (client) or 4
   (server).




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5.5.  The NTS Cookie Placeholder Extension Field

   The NTS Cookie Placeholder extension field has a Field Type of
   [[TBD4]].  When this extension field is included in a client packet
   (mode 3), it communicates to the server that the client wishes it to
   send additional cookies in its response.  This extension field MUST
   NOT be included in NTP packets whose mode is other than 3.

   Whenever an NTS Cookie Placeholder extension field is present, it
   MUST be accompanied by an NTS Cookie extension field.  The body
   length of the NTS Cookie Placeholder extension field MUST be the same
   as the body length of the NTS Cookie extension field.  This length
   requirement serves to ensure that the response will not be larger
   than the request, in order to improve timekeeping precision and
   prevent DDoS amplification.  The contents of the NTS Cookie
   Placeholder extension field's body SHOULD be all zeros and, aside
   from checking its length, MUST be ignored by the server.

5.6.  The NTS Authenticator and Encrypted Extension Fields Extension
      Field

   The NTS Authenticator and Encrypted Extension Fields extension field
   is the central cryptographic element of an NTS-protected NTP packet.
   Its Field Type is [[TBD5]].  It SHALL be formatted according to
   Figure 4 and include the following fields:

      Nonce Length: Two octets in network byte order, giving the length
      of the Nonce field, excluding any padding, interpreted as an
      unsigned integer.

      Ciphertext Length: Two octets in network byte order, giving the
      length of the Ciphertext field, excluding any padding, interpreted
      as an unsigned integer.

      Nonce: A nonce as required by the negotiated AEAD Algorithm.  The
      end of the field is zero-padded to a word (four octets) boundary.

      Ciphertext: The output of the negotiated AEAD Algorithm.  The
      structure of this field is determined by the negotiated algorithm,
      but it typically contains an authentication tag in addition to the
      actual ciphertext.  The end of the field is zero-padded to a word
      (four octets) boundary.

      Additional Padding: Clients which use a nonce length shorter than
      the maximum allowed by the negotiated AEAD algorithm may be
      required to include additional zero-padding.  The necessary length
      of this field is specified below.




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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Nonce Length         |      Ciphertext Length        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                                                               .
   .          Nonce, including up to 3 octets padding              .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                                                               .
   .        Ciphertext, including up to 3 octets padding           .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                                                               .
   .                      Additional Padding                       .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 4: NTS Authenticator and Encrypted Extension Fields Extension
                               Field Format

   The Ciphertext field SHALL be formed by providing the following
   inputs to the negotiated AEAD Algorithm:

      K: For packets sent from the client to the server, the C2S key
      SHALL be used.  For packets sent from the server to the client,
      the S2C key SHALL be used.

      A: The associated data SHALL consist of the portion of the NTP
      packet beginning from the start of the NTP header and ending at
      the end of the last extension field which precedes the NTS
      Authenticator and Encrypted Extension Fields extension field.

      P: The plaintext SHALL consist of all (if any) NTP extension
      fields to be encrypted; if multiple extension fields are present
      they SHALL be joined by concatenation.  Each such field SHALL be
      formatted in accordance with RFC 7822 [RFC7822], except that,
      contrary to the RFC 7822 requirement that fields have a minimum
      length of 16 or 28 octets, encrypted extension fields MAY be
      arbitrarily short (but still MUST be a multiple of 4 octets in
      length).




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      N: The nonce SHALL be formed however required by the negotiated
      AEAD algorithm.

   The purpose of the Additional Padding field is to ensure that servers
   can always choose a nonce whose length is adequate to ensure its
   uniqueness, even if the client chooses a shorter one, and still
   ensure that the overall length of the server's response packet does
   not exceed the length of the request.  For mode 4 (server) packets,
   no Additional Padding field is ever required.  For mode 3 (client)
   packets, the length of the Additional Padding field SHALL be computed
   as follows.  Let `N_LEN` be the padded length of the Nonce field.
   Let `N_MAX` be, as specified by RFC 5116 [RFC5116], the maximum
   permitted nonce length for the negotiated AEAD algorithm.  Let
   `N_REQ` be the lesser of 16 and N_MAX, rounded up to the nearest
   multiple of 4.  If N_LEN is greater than or equal to N_REQ, then no
   Additional Padding field is required.  Otherwise, the Additional
   Padding field SHALL be at least N_REQ - N_LEN octets in length.
   Servers MUST enforce this requirement by discarding any packet which
   does not conform to it.

   Senders are always free to include more Additional Padding than
   mandated by the above paragraph.  Theoretically, it could be
   necessary to do so in order to bring the extension field to the
   minimum length required by [RFC7822].  This should never happen in
   practice because any reasonable AEAD algorithm will have a nonce and
   an authenticator long enough to bring the extension field to its
   required length already.  Nonetheless, implementers are advised to
   explicitly handle this case and ensure that the extension field they
   emit is of legal length.

   The NTS Authenticator and Encrypted Extension Fields extension field
   MUST NOT be included in NTP packets whose mode is other than 3
   (client) or 4 (server).

5.7.  Protocol Details

   A client sending an NTS-protected request SHALL include the following
   extension fields as displayed in Figure 5:

      Exactly one Unique Identifier extension field which MUST be
      authenticated, MUST NOT be encrypted, and whose contents MUST be
      the output of a cryptographically secure random number generator.
      [RFC4086]

      Exactly one NTS Cookie extension field which MUST be authenticated
      and MUST NOT be encrypted.  The cookie MUST be one which has been
      previously provided to the client, either from the key exchange




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      server during the NTS-KE handshake or from the NTP server in
      response to a previous NTS-protected NTP request.

      Exactly one NTS Authenticator and Encrypted Extension Fields
      extension field, generated using an AEAD Algorithm and C2S key
      established through NTS-KE.

   To protect the client's privacy, the client SHOULD avoid reusing a
   cookie.  If the client does not have any cookies that it has not
   already sent, it SHOULD initiate a re-run of the NTS-KE protocol.
   The client MAY reuse cookies in order to prioritize resilience over
   unlinkability.  Which of the two that should be prioritized in any
   particular case is dependent on the application and the user's
   preference.  Section 10.1 describes the privacy considerations of
   this in further detail.

   The client MAY include one or more NTS Cookie Placeholder extension
   fields which MUST be authenticated and MAY be encrypted.  The number
   of NTS Cookie Placeholder extension fields that the client includes
   SHOULD be such that if the client includes N placeholders and the
   server sends back N+1 cookies, the number of unused cookies stored by
   the client will come to eight.  The client SHOULD NOT include more
   than seven NTS Cookie Placeholder extension fields in a request.
   When both the client and server adhere to all cookie-management
   guidance provided in this memo, the number of placeholder extension
   fields will equal the number of dropped packets since the last
   successful volley.

   In rare circumstances, it may be necessary to include fewer NTS
   Cookie Placeholder extensions than recommended above in order to
   prevent datagram fragmentation.  When cookies adhere the format
   recommended in Section 6 and the AEAD in use is the mandatory-to-
   implement AEAD_AES_SIV_CMAC_256, senders can include a cookie and
   seven placeholders and still have packet size fall comfortably below
   1280 octets if no non-NTS-related extensions are used; 1280 octets is
   the minimum prescribed MTU for IPv6 and is generally safe for
   avoiding IPv4 fragmentation.  Nonetheless, senders SHOULD include
   fewer cookies and placeholders than otherwise indicated if doing so
   is necessary to prevent fragmentation.












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                   +---------------------------------------+
                   | - Verify time request message         |
                   | - Generate time response message      |
                   |   - Included NTPv4 extension fields   |
                   |      o Unique Identifier EF           |
                   |      o NTS Authentication and         |
                   |        Encrypted Extension Fields EF  |
                   |        - NTS Cookie EF                |
                   |        - <NTS Cookie EF>              |
                   | - Transmit time request packet        |
                   +-----------------+---------------------+
                                     |
                                     |
   Server -----------+---------------+-----+----------------------->
                     ^                      \
                    /                        \
     Time request  /                          \   Time response
     (mode 3)     /                            \  (mode 4)
                 /                              \
                /                                V
   Client -----+---------------------------------+----------------->
               |                                 |
               |                                 |
               |                                 |
   +-----------+----------------------+   +------+-----------------+
   |- Generate time request message   |   |- Verify time response  |
   | - Include NTPv4 Extension fields |   |  message               |
   |    o Unique Identifier EF        |   |- Extract cookie(s)     |
   |    o NTS Cookie EF               |   |- Time synchronization  |
   |    o <NTS Cookie Placeholder EF> |   |  processing            |
   |                                  |   +------------------------+
   |- Generate AEAD tag of NTP message|
   |- Add NTS Authentication and      |
   |  Encrypted Extension Fields EF   |
   |- Transmit time request packet    |
   +----------------------------------+

         Figure 5: NTS-protected NTP Time Synchronization Messages

   The client MAY include additional (non-NTS-related) extension fields
   which MAY appear prior to the NTS Authenticator and Encrypted
   Extension Fields extension fields (therefore authenticated but not
   encrypted), within it (therefore encrypted and authenticated), or
   after it (therefore neither encrypted nor authenticated).  The server
   MUST discard any unauthenticated extension fields.  Future
   specifications of extension fields MAY provide exceptions to this
   rule.




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   Upon receiving an NTS-protected request, the server SHALL (through
   some implementation-defined mechanism) use the cookie to recover the
   AEAD Algorithm, C2S key, and S2C key associated with the request, and
   then use the C2S key to authenticate the packet and decrypt the
   ciphertext.  If the cookie is valid and authentication and decryption
   succeed, the server SHALL include the following extension fields in
   its response:

      Exactly one Unique Identifier extension field which MUST be
      authenticated, MUST NOT be encrypted, and whose contents SHALL
      echo those provided by the client.

      Exactly one NTS Authenticator and Encrypted Extension Fields
      extension field, generated using the AEAD algorithm and S2C key
      recovered from the cookie provided by the client.

      One or more NTS Cookie extension fields which MUST be
      authenticated and encrypted.  The number of NTS Cookie extension
      fields included SHOULD be equal to, and MUST NOT exceed, one plus
      the number of valid NTS Cookie Placeholder extension fields
      included in the request.  The cookies returned in those fields
      MUST be valid for use with the NTP server that sent them.  They
      MAY be valid for other NTP servers as well, but there is no way
      for the server to indicate this.

   We emphasize the contrast that NTS Cookie extension fields MUST NOT
   be encrypted when sent from client to server, but MUST be encrypted
   when sent from server to client.  The former is necessary in order
   for the server to be able to recover the C2S and S2C keys, while the
   latter is necessary to satisfy the unlinkability goals discussed in
   Section 10.1.  We emphasize also that "encrypted" means encapsulated
   within the NTS Authenticator and Encrypted Extensions extension
   field.  While the body of an NTS Cookie extension field will
   generally consist of some sort of AEAD output (regardless of whether
   the recommendations of Section 6 are precisely followed), this is not
   sufficient to make the extension field "encrypted".

   The server MAY include additional (non-NTS-related) extension fields
   which MAY appear prior to the NTS Authenticator and Encrypted
   Extension Fields extension field (therefore authenticated but not
   encrypted), within it (therefore encrypted and authenticated), or
   after it (therefore neither encrypted nor authenticated).  The client
   MUST discard any unauthenticated extension fields.  Future
   specifications of extension fields MAY provide exceptions to this
   rule.

   Upon receiving an NTS-protected response, the client MUST verify that
   the Unique Identifier matches that of an outstanding request, and



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   that the packet is authentic under the S2C key associated with that
   request.  If either of these checks fails, the packet MUST be
   discarded without further processing.

   If the server is unable to validate the cookie or authenticate the
   request, it SHOULD respond with a Kiss-o'-Death (KoD) packet (see RFC
   5905, Section 7.4 [RFC5905]) with kiss code "NTSN", meaning "NTS NAK"
   (NTS negative-acknowledgment).  It MUST NOT include any NTS Cookie or
   NTS Authenticator and Encrypted Extension Fields extension fields.

   If the NTP server has previously responded with authentic NTS-
   protected NTP packets, the client MUST verify that any KoD packets
   received from the server contain the Unique Identifier extension
   field and that the Unique Identifier matches that of an outstanding
   request.  If this check fails, the packet MUST be discarded without
   further processing.  If this check passes, the client MUST comply
   with RFC 5905, Section 7.4 [RFC5905] where required.  A client MAY
   automatically re-run the NTS-KE protocol upon forced disassociation
   from an NTP server.  In that case, it MUST be able to detect and stop
   looping between the NTS-KE and NTP servers by rate limiting the
   retries using e.g. exponential retry intervals.

   Upon reception of the NTS NAK kiss code, the client SHOULD wait until
   the next poll for a valid NTS-protected response and if none is
   received, initiate a fresh NTS-KE handshake to try to renegotiate new
   cookies, AEAD keys, and parameters.  If the NTS-KE handshake
   succeeds, the client MUST discard all old cookies and parameters and
   use the new ones instead.  As long as the NTS-KE handshake has not
   succeeded, the client SHOULD continue polling the NTP server using
   the cookies and parameters it has.

   To allow for NTP session restart when the NTS-KE server is
   unavailable and to reduce NTS-KE server load, the client SHOULD keep
   at least one unused but recent cookie, AEAD keys, negotiated AEAD
   algorithm, and other necessary parameters on persistent storage.
   This way, the client is able to resume the NTP session without
   performing renewed NTS-KE negotiation.

6.  Suggested Format for NTS Cookies

   This section is non-normative.  It gives a suggested way for servers
   to construct NTS cookies.  All normative requirements are stated in
   Section 4.1.6 and Section 5.4.

   The role of cookies in NTS is closely analogous to that of session
   cookies in TLS.  Accordingly, the thematic resemblance of this
   section to RFC 5077 [RFC5077] is deliberate and the reader should
   likewise take heed of its security considerations.



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   Servers should select an AEAD algorithm which they will use to
   encrypt and authenticate cookies.  The chosen algorithm should be one
   such as AEAD_AES_SIV_CMAC_256 [RFC5297] which resists accidental
   nonce reuse.  It need not be the same as the one that was negotiated
   with the client.  Servers should randomly generate and store a master
   AEAD key `K`. Servers should additionally choose a non-secret, unique
   value `I` as key-identifier for `K`.

   Servers should periodically (e.g., once daily) generate a new pair
   (I,K) and immediately switch to using these values for all newly-
   generated cookies.  Following each such key rotation, servers should
   securely erase any prevoiusly generated keys that should now be
   expired, e.g.  two or more rotation periods prior.  Servers should
   continue to accept any cookie generated using keys that they have not
   yet erased, even if those keys are no longer current.  Erasing old
   keys provides for forward secrecy, limiting the scope of what old
   information can be stolen if a master key is somehow compromised.
   Holding on to a limited number of old keys allows clients to
   seamlessly transition from one generation to the next without having
   to perform a new NTS-KE handshake.

   The need to keep keys synchronized between NTS-KE and NTP servers as
   well as across load-balanced clusters can make automatic key rotation
   challenging.  However, the task can be accomplished without the need
   for central key-management infrastructure by using a ratchet, i.e.,
   making each new key a deterministic, cryptographically pseudo-random
   function of its predecessor.  A recommended concrete implementation
   of this approach is to use HKDF [RFC5869] to derive new keys, using
   the key's predecessor as Input Keying Material and its key identifier
   as a salt.

   To form a cookie, servers should first form a plaintext `P`
   consisting of the following fields:

      The AEAD algorithm negotiated during NTS-KE.

      The S2C key.

      The C2S key.

   Servers should then generate a nonce `N` uniformly at random, and
   form AEAD output `C` by encrypting `P` under key `K` with nonce `N`
   and no associated data.

   The cookie should consist of the tuple `(I,N,C)`.

   To verify and decrypt a cookie provided by the client, first parse it
   into its components `I`, `N`, and `C`. Use `I` to look up its



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   decryption key `K`. If the key whose identifier is `I` has been
   erased or never existed, decryption fails; reply with an NTS NAK.
   Otherwise, attempt to decrypt and verify ciphertext `C` using key `K`
   and nonce `N` with no associated data.  If decryption or verification
   fails, reply with an NTS NAK.  Otherwise, parse out the contents of
   the resulting plaintext `P` to obtain the negotiated AEAD algorithm,
   S2C key, and C2S key.

7.  IANA Considerations

7.1.  Service Name and Transport Protocol Port Number Registry

   IANA is requested to allocate the following entry in the Service Name
   and Transport Protocol Port Number Registry [RFC6335]:

      Service Name: ntske

      Transport Protocol: tcp

      Assignee: IESG <iesg@ietf.org>

      Contact: IETF Chair <chair@ietf.org>

      Description: Network Time Security Key Exchange

      Reference: [[this memo]]

      Port Number: [[TBD1]], selected by IANA from the User Port range

   [[RFC EDITOR: Replace all instances of [[TBD1]] in this document with
   the IANA port assignment.]]

7.2.  TLS Application-Layer Protocol Negotiation (ALPN) Protocol IDs
      Registry

   IANA is requested to allocate the following entry in the TLS
   Application-Layer Protocol Negotiation (ALPN) Protocol IDs registry
   [RFC7301]:

      Protocol: Network Time Security Key Establishment, version 1

      Identification Sequence:
      0x6E 0x74 0x73 0x6B 0x65 0x2F 0x31 ("ntske/1")

      Reference: [[this memo]], Section 4






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7.3.  TLS Exporter Labels Registry

   IANA is requested to allocate the following entry in the TLS Exporter
   Labels Registry [RFC5705]:

   +--------------------+---------+-------------+---------------+------+
   | Value              | DTLS-OK | Recommended | Reference     | Note |
   +--------------------+---------+-------------+---------------+------+
   | EXPORTER-network-  | Y       | N           | [[this        |      |
   | time-security/1    |         |             | memo]],       |      |
   |                    |         |             | Section 4.2   |      |
   +--------------------+---------+-------------+---------------+------+

7.4.  NTP Kiss-o'-Death Codes Registry

   IANA is requested to allocate the following entry in the registry of
   NTP Kiss-o'-Death Codes [RFC5905]:

   +------+---------------------------------------+--------------------+
   | Code | Meaning                               | Reference          |
   +------+---------------------------------------+--------------------+
   | NTSN | Network Time Security (NTS) negative- | [[this memo]],     |
   |      | acknowledgment (NAK)                  | Section 5.7        |
   +------+---------------------------------------+--------------------+

7.5.  NTP Extension Field Types Registry

   IANA is requested to allocate the following entries in the NTP
   Extension Field Types registry [RFC5905]:

   +----------+-----------------------------+--------------------------+
   | Field    | Meaning                     | Reference                |
   | Type     |                             |                          |
   +----------+-----------------------------+--------------------------+
   | [[TBD2]] | Unique Identifier           | [[this memo]],           |
   |          |                             | Section 5.3              |
   | [[TBD3]] | NTS Cookie                  | [[this memo]],           |
   |          |                             | Section 5.4              |
   | [[TBD4]] | NTS Cookie Placeholder      | [[this memo]],           |
   |          |                             | Section 5.5              |
   | [[TBD5]] | NTS Authenticator and       | [[this memo]],           |
   |          | Encrypted Extension Fields  | Section 5.6              |
   +----------+-----------------------------+--------------------------+

   [[RFC EDITOR: REMOVE BEFORE PUBLICATION - The NTP WG suggests that
   the following values be used:





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   Unique Identifier    0x0104
   NTS Cookie           0x0204
   Cookie Placeholder   0x0304
   NTS Authenticator    0x0404]]

   [[RFC EDITOR: Replace all instances of [[TBD2]], [[TBD3]], [[TBD4]],
   and [[TBD5]] in this document with the respective IANA assignments.]]

7.6.  Network Time Security Key Establishment Record Types Registry

   IANA is requested to create a new registry entitled "Network Time
   Security Key Establishment Record Types".  Entries SHALL have the
   following fields:

      Record Type Number (REQUIRED): An integer in the range 0-32767
      inclusive.

      Description (REQUIRED): A short text description of the purpose of
      the field.

      Reference (REQUIRED): A reference to a document specifying the
      semantics of the record.

   The policy for allocation of new entries in this registry SHALL vary
   by the Record Type Number, as follows:

      0-1023: IETF Review

      1024-16383: Specification Required

      16384-32767: Private and Experimental Use

   Applications for new entries SHALL specify the contents of the
   Description and Reference fields as well as which of the above ranges
   the Record Type Number should be allocated from.  Applicants MAY
   request a specific Record Type Number and such requests MAY be
   granted at the registrar's discretion.

   The initial contents of this registry SHALL be as follows:












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   +-------------+-------------------------+---------------------------+
   | Record Type | Description             | Reference                 |
   | Number      |                         |                           |
   +-------------+-------------------------+---------------------------+
   | 0           | End of Message          | [[this memo]],            |
   |             |                         | Section 4.1.1             |
   | 1           | NTS Next Protocol       | [[this memo]],            |
   |             | Negotiation             | Section 4.1.2             |
   | 2           | Error                   | [[this memo]],            |
   |             |                         | Section 4.1.3             |
   | 3           | Warning                 | [[this memo]],            |
   |             |                         | Section 4.1.4             |
   | 4           | AEAD Algorithm          | [[this memo]],            |
   |             | Negotiation             | Section 4.1.5             |
   | 5           | New Cookie for NTPv4    | [[this memo]],            |
   |             |                         | Section 4.1.6             |
   | 6           | NTPv4 Server            | [[this memo]],            |
   |             | Negotiation             | Section 4.1.7             |
   | 7           | NTPv4 Port Negotiation  | [[this memo]],            |
   |             |                         | Section 4.1.8             |
   | 16384-32767 | Reserved for Private &  | [[this memo]]             |
   |             | Experimental Use        |                           |
   +-------------+-------------------------+---------------------------+

7.7.  Network Time Security Next Protocols Registry

   IANA is requested to create a new registry entitled "Network Time
   Security Next Protocols".  Entries SHALL have the following fields:

      Protocol ID (REQUIRED): An integer in the range 0-65535 inclusive,
      functioning as an identifier.

      Protocol Name (REQUIRED): A short text string naming the protocol
      being identified.

      Reference (REQUIRED): A reference to a relevant specification
      document.

   The policy for allocation of new entries in these registries SHALL
   vary by their Protocol ID, as follows:

      0-1023: IETF Review

      1024-32767: Specification Required

      32768-65535: Private and Experimental Use

   The initial contents of this registry SHALL be as follows:



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   +-------------+-------------------------------+---------------------+
   | Protocol ID | Protocol Name                 | Reference           |
   +-------------+-------------------------------+---------------------+
   | 0           | Network Time Protocol version | [[this memo]]       |
   |             | 4 (NTPv4)                     |                     |
   | 32768-65535 | Reserved for Private or       | Reserved by [[this  |
   |             | Experimental Use              | memo]]              |
   +-------------+-------------------------------+---------------------+

7.8.  Network Time Security Error and Warning Codes Registries

   IANA is requested to create two new registries entitled "Network Time
   Security Error Codes" and "Network Time Security Warning Codes".
   Entries in each SHALL have the following fields:

      Number (REQUIRED): An integer in the range 0-65535 inclusive

      Description (REQUIRED): A short text description of the condition.

      Reference (REQUIRED): A reference to a relevant specification
      document.

   The policy for allocation of new entries in these registries SHALL
   vary by their Number, as follows:

      0-1023: IETF Review

      1024-32767: Specification Required

      32768-65535: Private and Experimental Use

   The initial contents of the Network Time Security Error Codes
   Registry SHALL be as follows:

   +-------------+------------------------------+----------------------+
   | Number      | Description                  | Reference            |
   +-------------+------------------------------+----------------------+
   | 0           | Unrecognized Critical        | [[this memo]],       |
   |             | Extension                    | Section 4.1.3        |
   | 1           | Bad Request                  | [[this memo]],       |
   |             |                              | Section 4.1.3        |
   | 2           | Internal Server Error        | [[this memo]],       |
   |             |                              | Section 4.1.3        |
   | 32768-65535 | Reserved for Private or      | Reserved by [[this   |
   |             | Experimental Use             | memo]]               |
   +-------------+------------------------------+----------------------+





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   The Network Time Security Warning Codes Registry SHALL initially be
   empty except for the reserved range, i.e.:

   +-------------+-------------------------------+---------------------+
   | Number      | Description                   | Reference           |
   +-------------+-------------------------------+---------------------+
   | 32768-65535 | Reserved for Private or       | Reserved by [[this  |
   |             | Experimental Use              | memo]]              |
   +-------------+-------------------------------+---------------------+

8.  Implementation Status - RFC EDITOR: REMOVE BEFORE PUBLICATION

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in RFC 7942.
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.  Please note that the listing of any individual implementation
   here does not imply endorsement by the IETF.  Furthermore, no effort
   has been spent to verify the information presented here that was
   supplied by IETF contributors.  This is not intended as, and must not
   be construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may
   exist.

   According to RFC 7942, "this will allow reviewers and working groups
   to assign due consideration to documents that have the benefit of
   running code, which may serve as evidence of valuable experimentation
   and feedback that have made the implemented protocols more mature.
   It is up to the individual working groups to use this information as
   they see fit".

8.1.  Implementation 1

   Organization: Ostfalia University of Applied Science

   Implementor: Martin Langer

   Maturity: Proof-of-Concept Prototype

   This implementation was used to verify consistency and to ensure
   completeness of this specification.

8.1.1.  Coverage

   This implementation covers the complete specification.





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8.1.2.  Licensing

   The code is released under a Apache License 2.0 license.

   The source code is available at: https://gitlab.com/MLanger/nts/

8.1.3.  Contact Information

   Contact Martin Langer: mart.langer@ostfalia.de

8.1.4.  Last Update

   The implementation was updated 25.  February 2019.

8.2.  Implementation 2

   Organization: Netnod

   Implementor: Christer Weinigel

   Maturity: Proof-of-Concept Prototype

   This implementation was used to verify consistency and to ensure
   completeness of this specification.

8.2.1.  Coverage

   This implementation covers the complete specification.

8.2.2.  Licensing

   The source code is available at: https://github.com/Netnod/nts-poc-
   python.

   See LICENSE file for details on licensing (BSD 2).

8.2.3.  Contact Information

   Contact Christer Weinigel: christer@weinigel.se

8.2.4.  Last Update

   The implementation was updated 31.  January 2019.








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8.3.  Implementation 3

   Organization: Red Hat

   Implementor: Miroslav Lichvar

   Maturity: Prototype

   This implementation was used to verify consistency and to ensure
   completeness of this specification.

8.3.1.  Coverage

   This implementation covers the complete specification.

8.3.2.  Licensing

   Licensing is GPLv2.

   The source code is available at: https://github.com/mlichvar/chrony-
   nts

8.3.3.  Contact Information

   Contact Miroslav Lichvar: mlichvar@redhat.com

8.3.4.  Last Update

   The implementation was updated 28.  March 2019.

8.4.  Implementation 4

   Organization: NTPsec

   Implementor: Hal Murray and NTPsec team

   Maturity:Looking for testers.  Servers running at
   ntp1.glypnod.com:123 and ntp2.glypnod.com:123

   This implementation was used to verify consistency and to ensure
   completeness of this specification.

8.4.1.  Coverage

   This implementation covers the complete specification.






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8.4.2.  Licensing

   The source code is available at: https://gitlab.com/NTPsec/ntpsec.
   Licensing details in LICENSE.

8.4.3.  Contact Information

   Contact Hal Murray: hmurray@megapathdsl.net, devel@ntpsec.org

8.4.4.  Last Update

   The implementation was updated 2019-Apr-10.

8.5.  Implementation 5

   Organization: Cloudflare

   Implementor: Watson Ladd

   Maturity:

   This implementation was used to verify consistency and to ensure
   completeness of this specification.

8.5.1.  Coverage

   This implementation covers the server side of the NTS specification.

8.5.2.  Licensing

   The source code is available at: https://github.com/wbl/nts-rust

   Licensing is ISC (details see LICENSE.txt file).

8.5.3.  Contact Information

   Contact Watson Ladd: watson@cloudflare.com

8.5.4.  Last Update

   The implementation was updated 21.  March 2019.

8.6.  Implementation 6

   Organization: Hacklunch, independent

   Implementor: Michael Cardell Widerkrantz, Daniel Lublin, Martin
   Samuelsson et. al.



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   Maturity: interoperable client, immature server

8.6.1.  Coverage

   NTS-KE client and server.

8.6.2.  Licensing

   Licensing is ISC (details in LICENSE file).

   Source code is available at: https://gitlab.com/hacklunch/ntsclient

8.6.3.  Contact Information

   Contact Michael Cardell Widerkrantz: mc@netnod.se

8.6.4.  Last Update

   The implementation was updated 6.  February 2020.

8.7.  Interoperability

   The Interoperability tests distinguished between NTS key
   establishment protocol and NTS time exchange messages.  For the
   implementations 1, 2, 3, and 4 pairwise interoperability of the NTS
   key establishment protocol and exchange of NTS protected NTP messages
   have been verified successfully.  The implementation 2 was able to
   successfully perform the key establishment protocol against the
   server side of the implementation 5.

   These tests successfully demonstrate that there are at least four
   running implementations of this draft which are able to interoperate.

9.  Security Considerations

9.1.  Protected Modes

   NTP provides many different operating modes in order to support
   different network topologies and to adapt to various requirements.
   This memo only specifies NTS for NTP modes 3 (client) and 4 (server)
   (see Section 1.2).  The best current practice for authenticating the
   other NTP modes is using the symmetric message authentication code
   feature as described in RFC 5905 [RFC5905] and RFC 8573 [RFC8573].








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9.2.  Sensitivity to DDoS Attacks

   The introduction of NTS brings with it the introduction of asymmetric
   cryptography to NTP.  Asymmetric cryptography is necessary for
   initial server authentication and AEAD key extraction.  Asymmetric
   cryptosystems are generally orders of magnitude slower than their
   symmetric counterparts.  This makes it much harder to build systems
   that can serve requests at a rate corresponding to the full line
   speed of the network connection.  This, in turn, opens up a new
   possibility for DDoS attacks on NTP services.

   The main protection against these attacks in NTS lies in that the use
   of asymmetric cryptosystems is only necessary in the initial NTS-KE
   phase of the protocol.  Since the protocol design enables separation
   of the NTS-KE and NTP servers, a successful DDoS attack on an NTS-KE
   server separated from the NTP service it supports will not affect NTP
   users that have already performed initial authentication, AEAD key
   extraction, and cookie exchange.

   NTS users should also consider that they are not fully protected
   against DoS attacks by on-path adversaries.  In addition to dropping
   packets and attacks such as those described in Section 9.5, an on-
   path attacker can send spoofed kiss-o'-death replies, which are not
   authenticated, in response to NTP requests.  This could result in
   significantly increased load on the NTS-KE server.  Implementers have
   to weigh the user's need for unlinkability against the added
   resilience that comes with cookie reuse in cases of NTS-KE server
   unavailability.

9.3.  Avoiding DDoS Amplification

   Certain non-standard and/or deprecated features of the Network Time
   Protocol enable clients to send a request to a server which causes
   the server to send a response much larger than the request.  Servers
   which enable these features can be abused in order to amplify traffic
   volume in DDoS attacks by sending them a request with a spoofed
   source IP.  In recent years, attacks of this nature have become an
   endemic nuisance.

   NTS is designed to avoid contributing any further to this problem by
   ensuring that NTS-related extension fields included in server
   responses will be the same size as the NTS-related extension fields
   sent by the client.  In particular, this is why the client is
   required to send a separate and appropriately padded-out NTS Cookie
   Placeholder extension field for every cookie it wants to get back,
   rather than being permitted simply to specify a desired quantity.





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   Due to the RFC 7822 [RFC7822] requirement that extensions be padded
   and aligned to four-octet boundaries, response size may still in some
   cases exceed request size by up to three octets.  This is
   sufficiently inconsequential that we have declined to address it.

9.4.  Initial Verification of Server Certificates

   NTS's security goals are undermined if the client fails to verify
   that the X.509 certificate chain presented by the NTS-KE server is
   valid and rooted in a trusted certificate authority.  RFC 5280
   [RFC5280] and RFC 6125 [RFC6125] specify how such verification is to
   be performed in general.  However, the expectation that the client
   does not yet have a correctly-set system clock at the time of
   certificate verification presents difficulties with verifying that
   the certificate is within its validity period, i.e., that the current
   time lies between the times specified in the certificate's notBefore
   and notAfter fields.  It may be operationally necessary in some cases
   for a client to accept a certificate which appears to be expired or
   not yet valid.  While there is no perfect solution to this problem,
   there are several mitigations the client can implement to make it
   more difficult for an adversary to successfully present an expired
   certificate:

      Check whether the system time is in fact unreliable.  If the
      system clock has previously been synchronized since last boot,
      then on operating systems which implement a kernel-based phase-
      locked-loop API, a call to ntp_gettime() should show a maximum
      error less than NTP_PHASE_MAX.  In this case, the clock SHOULD be
      considered reliable and certificates can be strictly validated.

      Allow the system administrator to specify that certificates should
      *always* be strictly validated.  Such a configuration is
      appropriate on systems which have a battery-backed clock and which
      can reasonably prompt the user to manually set an approximately-
      correct time if it appears to be needed.

      Once the clock has been synchronized, periodically write the
      current system time to persistent storage.  Do not accept any
      certificate whose notAfter field is earlier than the last recorded
      time.

      NTP time replies are expected to be consistent with the NTS-KE TLS
      certificate validity period, i.e. time replies received
      immediately after an NTS-KE handshake are expected to lie within
      the certificate validity period.  Implementations are recommended
      to check that this is the case.  Performing a new NTS-KE handshake
      based solely on the fact that the certificate used by the NTS-KE
      server in a previous handshake has expired is normally not



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      necessary.  Clients that still wish to do this must take care not
      to cause an inadvertent denial-of-service attack on the NTS-KE
      server, for example by picking a random time in the week preceding
      certificate expiry to perform the new handshake.

      Use multiple time sources.  The ability to pass off an expired
      certificate is only useful to an adversary who has compromised the
      corresponding private key.  If the adversary has compromised only
      a minority of servers, NTP's selection algorithm (RFC 5905 section 
      11.2.1 [RFC5905]) will protect the client from accepting bad time
      from the adversary-controlled servers.

9.5.  Delay Attacks

   In a packet delay attack, an adversary with the ability to act as a
   man-in-the-middle delays time synchronization packets between client
   and server asymmetrically [RFC7384].  Since NTP's formula for
   computing time offset relies on the assumption that network latency
   is roughly symmetrical, this leads to the client to compute an
   inaccurate value [Mizrahi].  The delay attack does not reorder or
   modify the content of the exchanged synchronization packets.
   Therefore, cryptographic means do not provide a feasible way to
   mitigate this attack.  However, the maximum error that an adversary
   can introduce is bounded by half of the round trip delay.

   RFC 5905 [RFC5905] specifies a parameter called MAXDIST which denotes
   the maximum round-trip latency (including not only the immediate
   round trip between client and server, but the whole distance back to
   the reference clock as reported in the Root Delay field) that a
   client will tolerate before concluding that the server is unsuitable
   for synchronization.  The standard value for MAXDIST is one second,
   although some implementations use larger values.  Whatever value a
   client chooses, the maximum error which can be introduced by a delay
   attack is MAXDIST/2.

   Usage of multiple time sources, or multiple network paths to a given
   time source [Shpiner], may also serve to mitigate delay attacks if
   the adversary is in control of only some of the paths.

9.6.  NTS Stripping

   Implementers must be aware of the possibility of "NTS stripping"
   attacks, where an attacker tricks clients into reverting to plain
   NTP.  Naive client implementations might, for example, revert
   automatically to plain NTP if the NTS-KE handshake fails.  A man-in-
   the-middle attacker can easily cause this to happen.  Even clients
   that already hold valid cookies can be vulnerable, since an attacker
   can force a client to repeat the NTS-KE handshake by sending faked



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   NTP mode 4 replies with the NTS NAK kiss code.  Forcing a client to
   repeat the NTS-KE handshake can also be the first step in more
   advanced attacks.

   For the reasons described here, implementations SHOULD NOT revert
   from NTS-protected to unprotected NTP with any server without
   explicit user action.

10.  Privacy Considerations

10.1.  Unlinkability

   Unlinkability prevents a device from being tracked when it changes
   network addresses (e.g. because said device moved between different
   networks).  In other words, unlinkability thwarts an attacker that
   seeks to link a new network address used by a device with a network
   address that it was formerly using, because of recognizable data that
   the device persistently sends as part of an NTS-secured NTP
   association.  This is the justification for continually supplying the
   client with fresh cookies, so that a cookie never represents
   recognizable data in the sense outlined above.

   NTS's unlinkability objective is merely to not leak any additional
   data that could be used to link a device's network address.  NTS does
   not rectify legacy linkability issues that are already present in
   NTP.  Thus, a client that requires unlinkability must also minimize
   information transmitted in a client query (mode 3) packet as
   described in the draft [I-D.ietf-ntp-data-minimization].

   The unlinkability objective only holds for time synchronization
   traffic, as opposed to key exchange traffic.  This implies that it
   cannot be guaranteed for devices that function not only as time
   clients, but also as time servers (because the latter can be
   externally triggered to send linkable data, such as the TLS
   certificate).

   It should also be noted that it could be possible to link devices
   that operate as time servers from their time synchronization traffic,
   using information exposed in (mode 4) server response packets (e.g.
   reference ID, reference time, stratum, poll).  Also, devices that
   respond to NTP control queries could be linked using the information
   revealed by control queries.

   Note that the unlinkability objective does not prevent a client
   device to be tracked by its time servers.






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10.2.  Confidentiality

   NTS does not protect the confidentiality of information in NTP's
   header fields.  When clients implement
   [I-D.ietf-ntp-data-minimization], client packet headers do not
   contain any information which the client could conceivably wish to
   keep secret: one field is random, and all others are fixed.
   Information in server packet headers is likewise public: the origin
   timestamp is copied from the client's (random) transmit timestamp,
   and all other fields are set the same regardless of the identity of
   the client making the request.

   Future extension fields could hypothetically contain sensitive
   information, in which case NTS provides a mechanism for encrypting
   them.

11.  Acknowledgements

   The authors would like to thank Richard Barnes, Steven Bellovin,
   Patrik Faeltstroem (Faltstrom), Scott Fluhrer, Sharon Goldberg, Russ
   Housley, Benjamin Kaduk, Suresh Krishnan, Martin Langer, Barry Leiba,
   Miroslav Lichvar, Aanchal Malhotra, Dave Mills, Sandra Murphy, Danny
   Mayer, Karen O'Donoghue, Eric K.  Rescorla, Stephen Roettger, Kurt
   Roeckx, Dan Romascanu, Kyle Rose, Rich Salz, Brian Sniffen, Susan
   Sons, Douglas Stebila, Harlan Stenn, Joachim Stroembergsson
   (Strombergsson), Martin Thomson, Eric (Eric) Vyncke, Richard Welty,
   Christer Weinigel, and Magnus Westerlund for contributions to this
   document and comments on the design of NTS.

12.  References

12.1.  Normative References

   [IANA-AEAD]
              IANA, "Authenticated Encryption with Associated Data
              (AEAD) Parameters",
              <https://www.iana.org/assignments/aead-parameters/>.

   [RFC0020]  Cerf, V., "ASCII format for network interchange", STD 80,
              RFC 20, DOI 10.17487/RFC0020, October 1969,
              <https://www.rfc-editor.org/info/rfc20>.

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





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   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <https://www.rfc-editor.org/info/rfc4291>.

   [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
              <https://www.rfc-editor.org/info/rfc5116>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/info/rfc5280>.

   [RFC5297]  Harkins, D., "Synthetic Initialization Vector (SIV)
              Authenticated Encryption Using the Advanced Encryption
              Standard (AES)", RFC 5297, DOI 10.17487/RFC5297, October
              2008, <https://www.rfc-editor.org/info/rfc5297>.

   [RFC5705]  Rescorla, E., "Keying Material Exporters for Transport
              Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
              March 2010, <https://www.rfc-editor.org/info/rfc5705>.

   [RFC5869]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
              Key Derivation Function (HKDF)", RFC 5869,
              DOI 10.17487/RFC5869, May 2010,
              <https://www.rfc-editor.org/info/rfc5869>.

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, DOI 10.17487/RFC5890, August 2010,
              <https://www.rfc-editor.org/info/rfc5890>.

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

   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
              2011, <https://www.rfc-editor.org/info/rfc6125>.







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   [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
              Cheshire, "Internet Assigned Numbers Authority (IANA)
              Procedures for the Management of the Service Name and
              Transport Protocol Port Number Registry", BCP 165,
              RFC 6335, DOI 10.17487/RFC6335, August 2011,
              <https://www.rfc-editor.org/info/rfc6335>.

   [RFC6874]  Carpenter, B., Cheshire, S., and R. Hinden, "Representing
              IPv6 Zone Identifiers in Address Literals and Uniform
              Resource Identifiers", RFC 6874, DOI 10.17487/RFC6874,
              February 2013, <https://www.rfc-editor.org/info/rfc6874>.

   [RFC7301]  Friedl, S., Popov, A., Langley, A., and E. Stephan,
              "Transport Layer Security (TLS) Application-Layer Protocol
              Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
              July 2014, <https://www.rfc-editor.org/info/rfc7301>.

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
              2015, <https://www.rfc-editor.org/info/rfc7525>.

   [RFC7822]  Mizrahi, T. and D. Mayer, "Network Time Protocol Version 4
              (NTPv4) Extension Fields", RFC 7822, DOI 10.17487/RFC7822,
              March 2016, <https://www.rfc-editor.org/info/rfc7822>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

12.2.  Informative References

   [I-D.ietf-ntp-data-minimization]
              Franke, D. and A. Malhotra, "NTP Client Data
              Minimization", draft-ietf-ntp-data-minimization-04 (work
              in progress), March 2019.

   [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
              DOI 10.1109/ISPCS.2012.6336612, September 2012.




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

   [RFC5077]  Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
              "Transport Layer Security (TLS) Session Resumption without
              Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
              January 2008, <https://www.rfc-editor.org/info/rfc5077>.

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

   [RFC8573]  Malhotra, A. and S. Goldberg, "Message Authentication Code
              for the Network Time Protocol", RFC 8573,
              DOI 10.17487/RFC8573, June 2019,
              <https://www.rfc-editor.org/info/rfc8573>.

   [Shpiner]  Shpiner, A., Revah, Y., and T. Mizrahi, "Multi-path Time
              Protocols", in Proceedings of IEEE International Symposium
              on Precision Clock Synchronization for Measurement,
              Control and Communication (ISPCS)
              DOI 10.1109/ISPCS.2013.6644754, September 2013.

Appendix A.  Terms and Abbreviations

   AEAD    Authenticated Encryption with Associated Data [RFC5116]

   ALPN    Application-Layer Protocol Negotiation [RFC7301]

   C2S     Client-to-server

   DoS     Denial-of-Service

   DDoS    Distributed Denial-of-Service

   EF      Extension Field [RFC5905]

   HKDF    Hashed Message Authentication Code-based Key Derivation
      Function [RFC5869]

   KoD     Kiss-o'-Death [RFC5905]

   NTP     Network Time Protocol [RFC5905]

   NTS     Network Time Security




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   NTS NAK  NTS negative-acknowledgment

   NTS-KE  Network Time Security Key Exchange

   PRF     Pseudorandom Function

   S2C     Server-to-client

   TLS     Transport Layer Security [RFC8446]

Authors' Addresses

   Daniel Fox Franke
   Akamai Technologies
   145 Broadway
   Cambridge, MA  02142
   United States

   Email: dafranke@akamai.com


   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


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

   Phone: +49-(0)531-592-4471
   Email: kristof.teichel@ptb.de









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   Marcus Dansarie
   Sweden

   Email: marcus@dansarie.se
   URI:   https://orcid.org/0000-0001-9246-0263


   Ragnar Sundblad
   Netnod
   Sweden

   Email: ragge@netnod.se







































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