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NTP Working Group                                              D. Franke
Internet-Draft                                                    Akamai
Intended status: Standards Track                               D. Sibold
Expires: May 4, 2017                                          K. Teichel
                                                                     PTB
                                                        October 31, 2016


Using the Network Time Security Specification to Secure the Network Time
                                Protocol
                  draft-ietf-ntp-using-nts-for-ntp-07

Abstract

   This document describes how to reach the objectives described in the
   Network Time Security (NTS) specification when securing time
   synchronization with servers using the Network Time Protocol (NTP).

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on May 4, 2017.

Copyright Notice

   Copyright (c) 2016 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



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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Objectives  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Terms and Abbreviations . . . . . . . . . . . . . . . . . . .   4
   4.  Overview of NTS-Secured NTP . . . . . . . . . . . . . . . . .   4
     4.1.  Client-Server Mode  . . . . . . . . . . . . . . . . . . .   5
     4.2.  Symmetric/Peer Mode and Control Modes . . . . . . . . . .   5
   5.  Employing DTLS for NTP Security . . . . . . . . . . . . . . .   5
     5.1.  DTLS profile for Network Time Security  . . . . . . . . .   6
     5.2.  Transport mechanisms for DTLS records . . . . . . . . . .   7
       5.2.1.  Transport via NTS port  . . . . . . . . . . . . . . .   7
       5.2.2.  Transport via NTP extension field . . . . . . . . . .   7
     5.3.  The NTS-encapsulated NTPv4 protocol . . . . . . . . . . .   9
     5.4.  The NTS Key Establishment protocol  . . . . . . . . . . .  10
       5.4.1.  NTS-KE record types . . . . . . . . . . . . . . . . .  11
       5.4.2.  Key Extraction (generally)  . . . . . . . . . . . . .  14
     5.5.  NTS Extensions for NTPv4  . . . . . . . . . . . . . . . .  14
       5.5.1.  Key Extraction (for NTPv4)  . . . . . . . . . . . . .  14
       5.5.2.  Packet structure overview . . . . . . . . . . . . . .  15
       5.5.3.  The Unique Identifier extension . . . . . . . . . . .  16
       5.5.4.  The NTS Cookie extension  . . . . . . . . . . . . . .  16
       5.5.5.  The NTS Cookie Placeholder extension  . . . . . . . .  16
       5.5.6.  The NTS Authenticator and Encrypted Extensions
               extension . . . . . . . . . . . . . . . . . . . . . .  17
       5.5.7.  Protocol details  . . . . . . . . . . . . . . . . . .  18
     5.6.  Recommended format for NTS cookies  . . . . . . . . . . .  20
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
     6.1.  Field Type Registry . . . . . . . . . . . . . . . . . . .  21
     6.2.  SMI Security for S/MIME CMS Content Type Registry . . . .  21
     6.3.  DTLS-Based Key Exchange . . . . . . . . . . . . . . . . .  21
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  25
     7.1.  Usage of NTP Pools  . . . . . . . . . . . . . . . . . . .  26
     7.2.  Initial Verification of the Server Certificates . . . . .  26
     7.3.  Treatment of Initial Messages . . . . . . . . . . . . . .  26
     7.4.  DTLS-Related Issues . . . . . . . . . . . . . . . . . . .  26
     7.5.  Delay Attack  . . . . . . . . . . . . . . . . . . . . . .  26
   8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  27
     8.1.  Confidentiality . . . . . . . . . . . . . . . . . . . . .  27
     8.2.  Unlinkability . . . . . . . . . . . . . . . . . . . . . .  27



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   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  28
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  28
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  28
     10.2.  Informative References . . . . . . . . . . . . . . . . .  30
   Appendix A.  Flow Diagrams of Client Behaviour  . . . . . . . . .  31
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  32

1.  Introduction

   The Network Time Security (NTS) draft
   [I-D.ietf-ntp-network-time-security] specifies security measures
   which can be used to enable time synchronization protocols to verify
   authenticity of the time server and integrity of the time
   synchronization protocol packets.

   This document provides detail on how to specifically use those
   measures to secure time synchronization between NTP clients and
   servers.  In particular, it describes a mechanism for using Datagram
   Transport Layer Security [RFC6347] (DTLS) to provide cryptographic
   security for NTP.  Certain sections, are not inherently NTP-specific
   and can be taken as guidance on how future work may apply the
   described techniques to other time synchronization protocols such as
   the Precision Time Protocol [IEC.61588_2009].

2.  Objectives

   The specific objectives for applying the NTS specification to the NTP
   are as follows:

   o  Authenticity: NTS enables an NTP client to authenticate its time
      server(s).

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

   o  Authorization: NTS optionally enables the server to verify the
      client's authorization.

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

   o  Privacy: NTS preserves unlinkability, i. e. it does not leak data
      that would allow a passive attacker to track mobile NTP clients
      when they move between networks.

   o  Request-Response-Consistency: NTS enables a client to match an
      incoming response to a request it has sent.  NTS also enables the




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      client to deduce from the response whether its request to the
      server has arrived without alteration.

   o  Modes of operation: Both the client-server mode and the symmetric
      peer mode of NTP are supported.  The broadcast mode of NTP can NOT
      be secured with measures within this document.

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

   o  Compatibility:

      *  NTP associations which are not secured by NTS are not affected
         by NTS-secured communication.

      *  An NTP server that does not support NTS is not affected by NTS-
         secured authentication requests.

3.  Terms and Abbreviations

   MAC  Message Authentication Code

   NTP    Network Time Protocol (RFC 5905 [RFC5905])

   NTS    Network Time Security

   DTLS  Datagram Transport Layer Security

   AEAD  Authenticated Encryption with Associated Data (RFC 5116
      [RFC5116])

4.  Overview of NTS-Secured NTP

   The Network Time Protocol includes many different operating modes to
   support various network topologies.  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 contradictory requirements for security and
   performance.  Symmetric and control modes demand mutual
   authentication and mutual replay protection, and 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 servers to process replayed packets.



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   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 document does not discuss how to add security to NTP's broadcast
   mode.

4.1.  Client-Server Mode

   The server does not keep a long-term state of the client.  NTS
   initially verifies the authenticity of the time server and exchanges
   one or more symmetric keys.  The DTLS-based key exchange procedure
   described in Section 5 can be used for this exchange.  An
   implementation MUST support the use of this procedure.  It MAY
   additionally support the use of any alternative secure communication
   for this purpose, as long as it fulfills the preconditions given in
   [I-D.ietf-ntp-network-time-security], Section 6.1.1.

   After the keys have been exchanged, the participants then use them to
   protect the authenticity and the integrity of subsequent unicast-type
   time synchronization packets.  In order to do this, the server
   attaches a Message Authentication Code (MAC) to each time
   synchronization packet.  The calculation of the MAC includes the
   whole time synchronization packet and the symmetric key which is
   stored on the client side.  Therefore, the client can perform a
   validity check for this MAC on reception of a time synchronization
   packet.

4.2.  Symmetric/Peer Mode and Control Modes

   In the symmetric ("peer") mode as well as in control modes, there is
   no requirement for statelessness on either side.  Both sides exchange
   and memorize one or more shared secrets.  The shared secrets
   exchanged are then used to secure NTP peer mode or control packets by
   providing at least authenticity and integrity protection and possibly
   also confidentiality.  The DTLS-based key exchange procedure
   described in Section 5.3 can be used for such communication.  An
   implementation MUST support the use of this procedure.

5.  Employing DTLS for NTP Security

   Since (as discussed in Section 4.1) no single approach can
   simultaneously satisfy the needs of all modes, this specification
   consists of not one protocol but a suite of them:





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   o  The "NTS-encapsulated NTPv4" protocol is little more than "NTP
      over DTLS": the two endpoints perform a DTLS handshake and then
      exchange NTP packets encapsulated as DTLS Application Data.  It is
      suitable for symmetric and control modes, and is also secure for
      client/server mode but relatively wasteful of server resources.

   o  The "NTS Key Establishment" protocol (NTS-KE) uses DTLS to
      establish key material and negotiate some additional protocol
      options, but then quickly closes the DTLS channel and does not use
      it for the exchange of time packets.  NTS-KE is designed to be
      extensible, and might be extended to support key establishment for
      other protocols such as PTP.

   o  The "NTS extensions for NTPv4" are a collection of NTP extension
      fields for cryptographically securing NTPv4 using key material
      previously negotiated using NTS-KE.  They are suitable for
      securing client/server mode because the server can implement them
      without retaining per-client state, but on the other hand are
      suitable *only* for client/server mode because only the client,
      and not the server, is protected from replay.

5.1.  DTLS profile for Network Time Security

   Since securing time protocols is (as of 2016) a novel application of
   DTLS, no backward-compatibility concerns exist to justify using
   obsolete, insecure, or otherwise broken DTLS features or versions.
   We therefore put forward the following requirements and guidelines,
   roughly representing 2016's best practices.

   Implementations MUST NOT negotiate DTLS versions earlier than 1.2.

   Implementations willing to negotiate more than one possible version
   of DTLS SHOULD NOT respond to handshake failures by retrying with a
   downgraded protocol version.  If they do, they MUST implement
   [RFC7507].

   DTLS clients MUST NOT offer, and DTLS servers MUST not select, RC4
   cipher suites.  [RFC7465]

   DTLS clients SHOULD offer, and DTLS servers SHOULD accept, the TLS
   Renegotiation Indication Extension [RFC5746].  Regardless, they MUST
   NOT initiate or permit insecure renegotiation.  (*)

   DTLS clients SHOULD offer, and DTLS servers SHOULD accept, the TLS
   Session Hash and Extended Master Secret Extension [RFC7627]. (*)






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

   (*): Note that DTLS 1.3 or beyond may render the indicated
   recommendations inapplicable.

5.2.  Transport mechanisms for DTLS records

   This section specifies two mechanisms, one REQUIRED and one OPTIONAL,
   for exchanging NTS-related DTLS records.  It is intended that the
   choice of transport mechanism be orthogonal to any concerns at the
   application layer: DTLS records SHOULD receive identical disposition
   regardless of which mechanism they arrive by.

5.2.1.  Transport via NTS port

   In this transport mechanism, DTLS records, formatted according to RFC
   6347 [RFC6347] or a subsequent revision thereof, are exchanged
   directly on UDP port [[TBD]], with one DTLS record per UDP packet and
   no additional layer of encapsulation between the UDP header and the
   DTLS record.  Servers which implement NTS MUST support this
   mechanism.

5.2.2.  Transport via NTP extension field

   In this transport mechanism, DTLS records are exchanged within
   extension fields of specially-formed NTP packets, which are
   themselves exchanged via the usual NTP service port (123/udp).  NTP
   packets conveying DTLS records SHALL be formatted as in Figure 1.
   They MUST NOT contain any other extensions or a legacy MAC field.




















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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                                                               .
   .                     NTP Header (48 octets)                    .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Extension Type         |       Extension Length      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                                                               .
   .                     DTLS Record (variable)                    .
   .                                                               .
   |                                                               |
   +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               |                                               |
   +-+-+-+-+-+-+-+-+                                               +
   |                                                               |
   .                                                               .
   .                    Padding (1-24 octets)                      .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 1: Format of NTP packets conveying DTLS records

   Within the NTP header,

   o  The Leap Indicator field SHALL be set to 3 (unsynchronized).

   o  The Version Number field SHALL be set to 4.

   o  DTLS clients SHALL set the Mode field to 3, and DTLS servers SHALL
      set the Mode field to 4, even if the DTLS record is being used (in
      the full-encapsulation protocol) to protect some NTP mode other
      than client/server.

   o  The Stratum field SHALL be set to 0 (unspecified or invalid).

   o  The Reference ID field (conveying a kiss code) SHALL be set to
      "DTLS"

   o  DTLS servers SHALL set the origin timestamp field from the
      transmit timestamp field of the packet most recently received from
      the client.




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   o  All other header fields MUST be ignored by the receiver, and MAY
      contain arbitrary or bogus values.

   The Extension Type field SHALL be set to [[TBD]].  The Extension
   Length field SHALL be computed and set as per RFC 7822 [RFC7822].

   The DTLS Record field SHALL contain a DTLS Record formatted as per
   RFC 6347 [RFC6347] or a subsequent revision thereof.

   The Padding field SHALL contain between 1 and 24 octets of padding,
   with every octet set to the number of padding octets included, e.g.,
   "01", "02 02", or "03 03 03".  The number of padding bytes should be
   chosen in order to comply with the RFC 7822 [RFC7822] requirement
   that (in the absence of a legacy MAC) extensions have a total length
   in octets (including the four octets for the type and length fields)
   which is at least 28 and divisible by 4.  Furthermore, since future
   revisions of DTLS may employ record formats that are not self-
   delimiting, at least one octet of padding MUST be included so that
   receivers can unambiguously determine where the DTLS record ends and
   the padding begins.  If the length of the DTLS record is already at
   least 24 and a multiple of 4, then the correct amount of padding to
   include is 4 octets.

   The NTP header values specified above are selected such that NTP
   implementations which do not understand NTS will interpret the packet
   as an innocuous no-op and not attempt to use it for time
   synchronization.  To NTS-aware implementations, however, these
   packets are best understood as not being NTP packets at all, but
   simply a means of "smuggling" arbitrary DTLS records across port 123/
   udp.  Indeed, these records need not be pertinent to NTP at all --
   for example, they could be NTS-KE messages eventually intended for
   securing PTP traffic.

   This transport mechanism is intended for use as a fallback in
   situations where firewalls or other middleboxes are preventing
   communication on the NTS port.  Support for it is OPTIONAL.

5.3.  The NTS-encapsulated NTPv4 protocol

   The NTS-encapsulated NTPv4 protocol proceeds in two parts.  First,
   DTLS handshake records are exchanged using one of the two transport
   mechanisms specified in Section 5.2.  The two endpoints carry out a
   DTLS handshake in conformance with Section 5.1, with the client
   offering (via an ALPN [RFC7301] extension), and the server accepting,
   an application-layer protocol of "ntp/4".  Second, once the handshake
   is successfully completed, the two endpoints use the established
   channel to exchange arbitrary NTPv4 packets as DTLS-protected
   Application Data.



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   In addition to the requirements specified in Section 5.1,
   implementations MUST enforce the anti-replay mechanism specified in
   Section 4.1.2.6 of RFC 6347 [RFC6347] (or an equivalent mechanism
   specified in a subsequent revision of DTLS).  Servers wishing to
   enforce access control SHOULD either demand a client certificate or
   use a PSK-based handshake in order to establish the client's
   identity.

   The NTS-encapsulated NTPv4 protocol is the RECOMMENDED mechanism for
   cryptographically securing mode 1 (symmetric active), 2 (symmetric
   passive), and 6 (control) NTPv4 traffic.  It is equally safe for mode
   3/4 (client/server) traffic, but is NOT RECOMMENDED for this purpose
   because it scales poorly compared to using NTS Extensions for NTPv4
   (Section 5.5).

5.4.  The NTS Key Establishment protocol

   The NTS Key Establishment (NTS-KE) protocol is carried out by
   exchanging DTLS records using one of the two transport mechanisms
   specified in Section 5.2.  The two endpoints carry out a DTLS
   handshake in conformance with Section 5.1, 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 DTLS-protected channel), then
   the server SHALL send a single response followed by a "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 sequence
   SHALL be terminated by a "End of Message" record, which has a Record
   Type of zero and a zero-length body.  Furthermore, requests and non-
   error responses each SHALL include exactly one NTS Next Protocol
   Negotiation record.

   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



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   [[Ed.  Note: this ad-hoc binary format should be fine as long as we
   continue to keep things very simple.  However, if we think there's
   any reasonable probability of wanting to include more complex data
   structures, we should consider using some semi-structured data format
   such as JSON, Protocol Buffers, or (ugh) ASN.1]]

   The requirement that all NTS-KE messages be terminated by an End of
   Message record makes them self-delimiting.  One DTLS record MAY, and
   typically will, contain multiple NTS-KE records.  NTS-KE records MAY
   be split across DTLS record boundaries.  If, likely due to packet
   loss, an incomplete NTS-KE message is received, implementations MUST
   treat this an error, which clients SHOULD handle by restarting with a
   fresh DTLS handshake and trying again.

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

   o  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.

   o  Record Type: A 15-bit integer in network byte order (from most-to-
      least significant, its bits are record bits 7-1 and then 15-8).
      The semantics of record types 0-5 are specified in this memo;
      additional type numbers SHALL be tracked through the IANA Network
      Time Security Key Establishment Record Types registry.

   o  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.

   o  Record Body: the syntax and semantics of this field shall be
      determined by the Record Type.

5.4.1.  NTS-KE record types

   The following NTS-KE Record Types are defined.

5.4.1.1.  End of Message

   The End of Message record has a Record Type number of 0 and an 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.








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5.4.1.2.  NTS Next Protocol Negotiation

   The NTS Next Protocol Negotiation record has a record type of 1.  It
   MUST occur exactly once in every NTS-KE request and response.  Its
   body consists of a sequence of 16-octet strings.  Each 16-octet
   string represents a Protocol Name from the IANA Network Time Security
   Next Protocols registry.  The Critical Bit MUST be set.

   The Protocol Names 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.
   The Protocol Names listed in the server's response MUST comprise a
   subset of those listed in the request, and denote those protocols
   which the 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.

5.4.1.3.  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 negotiated key material and MUST NOT proceed to the Next
   Protocol.

   The following error code are defined.

   o  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.

   o  Error code 1 means "Bad Request".  The server MUST respond with
      this error if, upon the expiration of an implementation-defined
      timeout, it has not yet received a complete and syntactically
      well-formed request from the client.  This error is likely to be
      the result of a dropped packet, so the client SHOULD start over
      with a new DTLS handshake and retry its request.








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5.4.1.4.  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 an 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.

5.4.1.5.  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
   registry [RFC5116].  The Critical Bit MAY be set.

   If the NTS Next Protocol Negotiation record offers "ntp/4",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, or 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 extensions for NTPv4 (Section 5.5) MUST
   support AEAD_AES_128_GCM (Numeric Identifier 1).  That is, if the
   client includes AEAD_AES_128_GCM in its AEAD Algorithm Negotiation
   record, and the server accepts the "ntp/4" protocol in its NTS Next
   Protocol Negotiation record, then the server's AEAD Algorithm
   Negotiation record MUST NOT be empty.








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5.4.1.6.  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 [[TODO]] for a RECOMMENDED
   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 they
   accept "ntp/4" as a Next Protocol.  The Critical Bit SHOULD NOT be
   set.

5.4.2.  Key Extraction (generally)

   Following a successful run of the NTS-KE protocol, key material SHALL
   be extracted according to RFC 5705 [RFC5705].  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:

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

   o  The per-association context value MUST be provided, and MUST begin
      with the 16-octet Protocol Name which was negotiated as a Next
      Protocol.

5.5.  NTS Extensions for NTPv4

5.5.1.  Key Extraction (for NTPv4)

   Following a successful run of the NTS-KE protocol wherein "ntp/4" 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 according to RFC 5705 [RFC5705], using the
   following inputs.

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

      The per-association context value SHALL consist of the following
      19 octets:

         The first 16 octets SHALL be (in hexadecimal):

         6E 74 70 2F 34 00 00 00 00 00 00 00 00 00 00 00





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         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 provides
   that disambiguating label strings beginning with "EXPERIMENTAL" MAY
   be used without IANA registration.

5.5.2.  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 extensions which are authenticated but not encrypted.

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

      Possibly, some additional extensions which are neither encrypted
      nor authenticated.  These are discarded by the receiver.  [[Ed.
      Note: right now there's no good reason for the sender to include
      anything here, but eventually there might be.  We've seen Checksum
      Complement [RFC7821] and LAST-EF as two examples of semantically-
      void extensions that are included to satsify constraints imposed
      lower on the protocol stack, and while there's no reason to use
      either of these on NTS-protected packets, I think we could see
      similar examples in the future.  So, rejecting packets with
      unauthenticated extensions could cause interoperability problems,
      while accepting and processing those extensions would of course be
      a security risk.  Thus, I think "allow and discard" is the correct
      policy.]]

   Always included among the authenticated or authenticated-and-
   encrypted extensions are a cookie extension and a unique-identifier
   extension.  The purpose of the cookie extension is to enable the
   server to offload storage of session state onto the client.  The
   purpose of the unique-identifier extension is to protect the client
   from replay attacks.




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5.5.3.  The Unique Identifier extension

   The Unique Identifier extension has a Field Type of [[TBD]].  When
   the extension is included in a client packet (mode 3), its body SHALL
   consist of a string of octets generated uniformly at random.  The
   string SHOULD be 32 octets long.  When the extension 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.  Its use in modes other than client/server is not
   defined.

   The Unique Identifier extension provides the client with a
   cryptographically strong means of detecting replayed packets.  It 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 enables a degree of unpredictability and
   collision-resistance more consistent with cryptographic best
   practice.

   [[TODO: consider using separate extension types for request and
   response, thus allowing for use in symmetric mode.  But proper
   handling in the presence of dropped packets needs to be documented
   and involves a lot of subtlety.]]

5.5.4.  The NTS Cookie extension

   The NTS Cookie extension has a Field Type of [[TBD]].  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 [[TODO]] for a RECOMMENDED
   construction.  The NTS Cookie extension MUST NOT be included in NTP
   packets whose mode is other than 3 (client) or 4 (server).

5.5.5.  The NTS Cookie Placeholder extension

   The NTS Cookie Placeholder extension has a Field Type of [[TBD]].
   When this extension 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 MUST NOT be
   included in NTP packets whose mode is other than 3.

   Whenever an NTS Cookie Placeholder extension is present, it MUST be
   accompanied by an NTS Cookie extension, and the body length of the



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   NTS Cookie Placeholder extension MUST be the same as the body length
   of the NTS Cookie Extension.  (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's body are undefined and, aside from checking its length,
   MUST be ignored by the server.

5.5.6.  The NTS Authenticator and Encrypted Extensions extension

   The NTS Authenticator and Encrypted Extensions extension is the
   central cryptographic element of an NTS-protected NTP packet.  Its
   Field Type is [[TBD]] and the format of its body SHALL be as follows:

      Nonce length: two octets in network byte order, giving the length
      of the Nonce field.

      Nonce: a nonce as required by the negotiated AEAD Algorithm.

      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.

      Padding: between 1 and 24 octets of padding, with every octet set
      to the number of padding octets included, e.g., "01", "02 02", or
      "03 03 03".  The number of padding bytes should be chosen in order
      to comply with the RFC 7822 [RFC7822] requirement that (in the
      absence of a legacy MAC) extensions have a total length in octets
      (including the four octets for the type and length fields) which
      is at least 28 and divisible by 4.  At least one octet of padding
      MUST be included, so that implementations can unambiguously
      delimit the end of the ciphertext from the start of the padding.

   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 which precedes the NTS Authenticator
      and Encrypted Extensions extension.

      P: The plaintext SHALL consist of all (if any) extensions to be
      encrypted.



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

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

5.5.7.  Protocol details

   A client sending an NTS-protected request SHALL include the following
   extensions:

      Exactly one Unique Identifier extension, which MUST be
      authenticated and MUST NOT be encrypted [[Ed.  Note: so that if
      the server can't decrypt the request, it can still echo back the
      Unique Identifier in the NTS NAK it sends]].  MUST NOT duplicate
      those of any previous request.

      Exactly one NTS Cookie extension, which MUST be authenticated and
      MUST NOT be encrypted.  The cookie MUST be one which the server
      previously provided the client; it may have been provided during
      the NTS-KE handshake or in response to a previous NTS-protected
      NTP request.  To protect client's privacy, the same cookie SHOULD
      NOT be included in multiple requests.  If the client does not have
      any cookies that it has not already sent, it SHOULD re-run the
      NTS-KE protocol before continuing.

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

   The client MAY include one or more NTS Cookie Placeholder extensions,
   which MUST be authenticated and MAY be encrypted.  The number of NTS
   Cookie Placeholder extensions 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.  When both the client and server adhere to all cookie-
   management guidance provided in this memo, the number of placeholder
   extensions will equal the number of dropped packets since the last
   successful volley.

   The client MAY include additional (non-NTS-related) extensions, which
   MAY appear prior to the NTS Authenticator and Encrypted Extensions
   extension (therefore authenticated but not encrypted), within it
   (therefore encrypted and authenticated), or after it (therefore
   neither encrypted nor authenticated).  In general, however, the
   server MUST discard any unauthenticated extensions and process the
   packet as though they were not present.  Servers MAY implement



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   exceptions to this requirement for particular extensions if their
   specification explicitly provides for such.

   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, then the server SHALL include the following extensions in
   its response:

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

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

      One or more NTS Cookie extensions, which MUST be authenticated and
      encrypted.  The number of NTS Cookie extensions included SHOULD be
      equal to, and MUST NOT exceed, one plus the number of valid NTS
      Cookie Placeholder extensions included in the request.

   The server MAY include additional (non-NTS-related) extensions, which
   MAY appear prior to the NTS Authenticator and Encrypted Extensions
   extension (therefore authenticated but not encrypted), within it
   (therefore encrypted and authenticated), or after it (therefore
   neither encrypted nor authenticated).  In general, however, the
   client MUST discard any unauthenticated extensions and process the
   packet as though they were not present.  Clients MAY implement
   exceptions to this requirement for particular extensions if their
   specification explicitly provides for such.

   If the server is unable to validate the cookie or authenticate the
   request, it SHOULD respond with a Kiss-o'-Death packet (see RFC 5905,
   Section 7.4) [RFC5905]) with kiss code "NTSN" (meaning "NTS NAK").
   Such a response MUST include exactly one Unique Identifier extension
   whose contents SHALL echo those provided by the client.  It MUST NOT
   include any NTS Cookie or NTS Authenticator and Encrypted Extensions
   extension.  [[Ed.  Note: RFC 5905 already provides the kiss code
   "CRYP" meaning "Cryptographic authentication or identification
   failed" but I think this is meant to be Autokey-specific.]]

   Upon receiving an NTS-protected response, the client MUST verify that
   the Unique Identifier matches that of an outstanding request, and
   that the packet is authentic under the S2C key associated with that




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   request.  If either of these checks fails, the packet MUST be
   discarded without further processing.

   Upon receiving an NTS NAK, the client MUST verify 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 SHOULD discard all cookies and AEAD
   keys associated with the server which sent the NAK and initiate a
   fresh NTS-KE handshake.

5.6.  Recommended format for NTS cookies

   This section provides a RECOMMENDED way for servers to construct NTS
   cookies.  Clients MUST NOT examine the cookie under the assumption
   that it is constructed according to this section.

   The role of cookies in NTS is closely analagous 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.

   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 accidential
   nonce reuse, and 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.  Immediately following each such key rotation,
   servers should securely erase any keys generated 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.

   [[TODO: discuss key management considerations for load-balanced
   servers]]

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




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      The AEAD algorithm negotiated during NTS-KE

      The S2C key

      The C2S key

   Servers should the 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)`.

   [[TODO: explicitly specify how to verify and decrypt a cookie, not
   just how to form one]]

6.  IANA Considerations

6.1.  Field Type Registry

   Within the "NTP Extensions Field Types" registry table, add the field
   types:

   Field Type  Meaning                              References
   ----------  ------------------------------------ ----------
   TBD1        NTS-Related Content                  [this doc]
   TBD2        NTS-Related Content                  [this doc]
   TBD3        NTS-Related Content                  [this doc]

6.2.  SMI Security for S/MIME CMS Content Type Registry

   Within the "SMI Security for S/MIME CMS Content Type
   (1.2.840.113549.1.9.16.1)" table, add one content type identifier:

   Decimal  Description                                   References
   -------  --------------------------------------------  ----------
   TBD4     id-ct-nts-ntsForNtpMessageAuthenticationCode  [this doc]

6.3.  DTLS-Based Key Exchange

   IANA is requested to allocate an entry in the Service Name and
   Transport Protocol Port Number Registry as follows:

      Service Name: nts

      Transport Protocol: udp

      Assignee: IESG <iesg@ietf.org>




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      Contact: IETF Chair <chair@ietf.org>

      Description: Network Time Security

      Reference: [[this memo]]

      Port Number: selected by IANA from the user port range

   IANA is requested to allocate the following two entries in the
   Application-Layer Protocol Negotiation (ALPN) Protocol IDs registry:

      Protocol: Network Time Security Key Establishment, version 1

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

      Reference: [[this memo]]

      Protocol: Network Time Protocol, version 4

      Identification Sequence:
      0x6E 0x74 0x70 0x2F 0x34 ("ntp/4")

      Reference: [[this memo]]

   IANA is requested to allocate the following entry in the TLS Exporter
   Label Registry:

   +----------------------------------+---------+---------------+------+
   | Value                            | DTLS-OK | Reference     | Note |
   +----------------------------------+---------+---------------+------+
   | EXPORTER-network-time-security/1 | Y       | [[this memo]] |      |
   +----------------------------------+---------+---------------+------+

   IANA is requested to allocate the following entries in the registry
   of NTP Kiss-o'-Death codes:

                  +------+------------------------------+
                  | Code | Meaning                      |
                  +------+------------------------------+
                  | DTLS | Packet conveys a DTLS record |
                  | NTSN | NTS NAK                      |
                  +------+------------------------------+

   IANA is requested to allocate the following entries in the NTP
   Extensions Field Types registry:





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   +------------+---------------------------------------+--------------+
   | Field Type | Meaning                               | Reference    |
   +------------+---------------------------------------+--------------+
   | [[TBD]]    | DTLS Record                           | [[this       |
   |            |                                       | memo]]       |
   | [[TBD]]    | Unique Identifier                     | [[this       |
   |            |                                       | memo]]       |
   | [[TBD]]    | NTS Cookie                            | [[this       |
   |            |                                       | memo]]       |
   | [[TBD]]    | NTS Authenticator and Encrypted       | [[this       |
   |            | Extensions                            | memo]]       |
   +------------+---------------------------------------+--------------+

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

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

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

      Set Critical Bit (REQUIRED): One of "MUST", "SHOULD", "MAY",
      "SHOULD NOT", or "MUST NOT"

      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 Type Number, as follows:

      0-1023: Standards Action

      1024-16383: Specification Required

      16384-32767: Private and Experimental Use

   Applications for new entries SHALL specify the contents of the
   Description, Set Critical Bit and Reference fields and which of the
   above ranges the Type Number should be allocated from.  Applicants
   MAY request a specific 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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   +-------------+-----------------------------+----------+------------+
   | Field       | Description                 | Critical | Reference  |
   | Number      |                             |          |            |
   +-------------+-----------------------------+----------+------------+
   | 0           | End of message              | MUST     | [[this     |
   |             |                             |          | memo]]     |
   | 1           | NTS next protocol           | MUST     | [[this     |
   |             | negotiation                 |          | memo]]     |
   | 2           | Error                       | MUST     | [[this     |
   |             |                             |          | memo]]     |
   | 3           | Warning                     | MUST     | [[this     |
   |             |                             |          | memo]]     |
   | 4           | AEAD algorithm negotation   | MAY      | [[this     |
   |             |                             |          | memo]]     |
   | 5           | New cookie for NTPv4        | SHOULD   | [[this     |
   |             |                             | NOT      | memo]]     |
   | 16384-32767 | Reserved for Private &      | MAY      | [[this     |
   |             | Experimental Use            |          | memo]]     |
   +-------------+-----------------------------+----------+------------+

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

      Protocol Name (REQUIRED): a sequence of 16 octets.  Shorter
      sequences SHALL implicitly be right-padded with null octets
      (0x00).

      Human-Readable Name (OPTIONAL): if the sequence of octets making
      up the protocol name intentionally represent a valid UTF-8
      [RFC3629] string, this field SHALL consist of that string.

      Reference (RECOMMENDED): a reference to a relevant specification
      document.  If no relevant document exists, a point-of-contact for
      questions regarding the entry SHOULD be listed here in lieu.

   Applications for new entries in this registry SHALL specify all
   desired fields, and SHALL be granted on a First Come, First Serve
   basis.  Protocol Names beginning with 0x78 0x2D ("x-") SHALL be
   reserved for Private or Experimental Use, and SHALL NOT be
   registered.  The reserved entry "ptp/2" may be updated or released by
   a future Standards Action.

   The initial contents of this registry SHALL be as follows:








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   +---------------------------+-----------------+---------------------+
   | Protocol Name             | Human-Readable  | Reference           |
   |                           | Name            |                     |
   +---------------------------+-----------------+---------------------+
   | 0x6E 0x74 0x70 0x2F 0x34  | ntp/4           | [[this memo]]       |
   | 0x70 0x74 0x70 0x2F 0x32  | ptp/2           | Reserved by [[this  |
   |                           |                 | memo]]              |
   +---------------------------+-----------------+---------------------+

   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): a 16-bit unsigned integer

      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: Standards Action

      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 Extension | [[this memo]] |
       | 1      | Bad Request                     | [[this memo]] |
       +--------+---------------------------------+---------------+

   The Network Time Security Warning Codes Registry SHALL initially be
   empty.

7.  Security Considerations

   All security considerations described in
   [I-D.ietf-ntp-network-time-security] have to be taken into account.
   The application of NTS to NTP requires the following additional
   considerations.



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7.1.  Usage of NTP Pools

   The certification-based authentication scheme described in
   [I-D.ietf-ntp-network-time-security] is not applicable to the concept
   of NTP pools.  Therefore, NTS is unable to provide secure usage of
   NTP pools.

7.2.  Initial Verification of the Server Certificates

   The client may wish to verify the validity of certificates during the
   initial association phase.  Since it generally has no reliable time
   during this initial communication phase, it is impossible to verify
   the period of validity of the certificates.

7.3.  Treatment of Initial Messages

   NTP packets which contains extension fields with key exchange
   messages do not provide integrity and authenticity protection of the
   included time stamps.  Therefore these NTP packets MUST NOT be used
   for clock synchronization.  Otherwise an initial attack on the
   client's clock [attacking-ntp] can potentially circumvent the
   employed security measures of later messages [delorean].

7.4.  DTLS-Related Issues

   ... TBD

7.5.  Delay Attack

   In a packet delay attack, an adversary with the ability to act as a
   MITM delays time synchronization packets between client and server
   asymmetrically [RFC7384].  This prevents the client from accurately
   measuring the network delay, and hence its time offset to the server
   [Mizrahi].  The delay attack does not modify the content of the
   exchanged synchronization packets.  Therefore, cryptographic means do
   not provide a feasible way to mitigate this attack.  However, the
   maximum error that an adversary can introduced is bounded by half of
   the round trip delay.  Also, several non-cryptographic precautions
   can be taken in order to detect this attack.

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

   2.  Multiple communication paths: The client and server utilize
       different paths for packet exchange as described in the I-D



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       [I-D.ietf-tictoc-multi-path-synchronization].  The client can
       detect the attack, provided that the adversary is unable to
       manipulate the majority of the available paths [Shpiner].  Note
       that this approach is not yet available, neither for NTP nor for
       PTP.

   3.  Usage of an encrypted connection: the client exchanges all
       packets with the time server over an encrypted connection (e.g.
       IPsec).  This measure does not mitigate the delay attack, but it
       makes it more difficult for the adversary to identify the time
       synchronization packets.

   4.  Introduction of a threshold value for the delay time of the
       synchronization packets.  The client can discard a time server if
       the packet delay time of this time server is larger than the
       threshold value.

8.  Privacy Considerations

8.1.  Confidentiality

   The actual time synchronization data in NTP packets does not involve
   any information that needs to be kept secret.  There also does not
   seem to be any necessity to disguise the nature of an NTP
   association.  This is why content confidentiality is a non-objective
   for this document.

8.2.  Unlinkability

   The scenario that is to be prevented is one where whenever a new
   network address is associated with a device (e.g. because said device
   moved between different networks), a passive attacker is able to link
   said new address with one that was formerly used by the device,
   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.

   Note that the objective of NTS regarding unlinkability is merely to
   not leak any additional data that would cause linkability.  NTS does
   not rectify legacy linkability issues that are already present in
   NTP.  To minimize the risk of being tracked by a passive adversary
   the NTP client has to minimize the information it transmits within a
   client request (mode 3 packet) as described in the draft "I-D.draft-
   dfranke-ntp-data-minimization".





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   Also note that normal NTP clients should not act as NTP servers.
   Otherwise, an active adversary may be able to abuse the client's
   server responses (mode 4 packets) for its tracking.  This is done by
   [tbd].

9.  Acknowledgements

   The authors would like to thank Richard Barnes, Steven Bellovin,
   Sharon Goldberg, Russ Housley, Martin Langer, Miroslav Lichvar,
   Aanchal Malhotra, Dave Mills, Danny Mayer, Karen O'Donoghue, Eric K.
   Rescorla, Stephen Roettger, Kurt Roeckx, Kyle Rose, Rich Salz, Brian
   Sniffen, Susan Sons, Douglas Stebila, Harlan Stenn, Martin Thomson,
   and Richard Welty for contributions to this document. on the design
   of NTS.

10.  References

10.1.  Normative References

   [I-D.ietf-ntp-cms-for-nts-message]
              Sibold, D., Teichel, K., Roettger, S., and R. Housley,
              "Protecting Network Time Security Messages with the
              Cryptographic Message Syntax (CMS)", draft-ietf-ntp-cms-
              for-nts-message-06 (work in progress), February 2016.

   [I-D.ietf-ntp-extension-field]
              Mizrahi, T. and D. Mayer, "The Network Time Protocol
              Version 4 (NTPv4) Extension Fields", draft-ietf-ntp-
              extension-field-07 (work in progress), February 2016.

   [I-D.ietf-ntp-network-time-security]
              Sibold, D., Roettger, S., and K. Teichel, "Network Time
              Security", draft-ietf-ntp-network-time-security-13 (work
              in progress), February 2016.

   [I-D.ietf-tictoc-multi-path-synchronization]
              Shpiner, A., Tse, R., Schelp, C., and T. Mizrahi, "Multi-
              Path Time Synchronization", draft-ietf-tictoc-multi-path-
              synchronization-06 (work in progress), October 2016.

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

   [RFC3394]  Schaad, J. and R. Housley, "Advanced Encryption Standard
              (AES) Key Wrap Algorithm", RFC 3394, DOI 10.17487/RFC3394,
              September 2002, <http://www.rfc-editor.org/info/rfc3394>.



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   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <http://www.rfc-editor.org/info/rfc3629>.

   [RFC4082]  Perrig, A., Song, D., Canetti, R., Tygar, J., and B.
              Briscoe, "Timed Efficient Stream Loss-Tolerant
              Authentication (TESLA): Multicast Source Authentication
              Transform Introduction", RFC 4082, DOI 10.17487/RFC4082,
              June 2005, <http://www.rfc-editor.org/info/rfc4082>.

   [RFC4634]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and HMAC-SHA)", RFC 4634, DOI 10.17487/RFC4634, July
              2006, <http://www.rfc-editor.org/info/rfc4634>.

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

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

   [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, DOI 10.17487/RFC5652, September 2009,
              <http://www.rfc-editor.org/info/rfc5652>.

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

   [RFC5746]  Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
              "Transport Layer Security (TLS) Renegotiation Indication
              Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010,
              <http://www.rfc-editor.org/info/rfc5746>.

   [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,
              <http://www.rfc-editor.org/info/rfc5905>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <http://www.rfc-editor.org/info/rfc6347>.







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   [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, <http://www.rfc-editor.org/info/rfc7301>.

   [RFC7465]  Popov, A., "Prohibiting RC4 Cipher Suites", RFC 7465,
              DOI 10.17487/RFC7465, February 2015,
              <http://www.rfc-editor.org/info/rfc7465>.

   [RFC7507]  Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher
              Suite Value (SCSV) for Preventing Protocol Downgrade
              Attacks", RFC 7507, DOI 10.17487/RFC7507, April 2015,
              <http://www.rfc-editor.org/info/rfc7507>.

   [RFC7627]  Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A.,
              Langley, A., and M. Ray, "Transport Layer Security (TLS)
              Session Hash and Extended Master Secret Extension",
              RFC 7627, DOI 10.17487/RFC7627, September 2015,
              <http://www.rfc-editor.org/info/rfc7627>.

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

10.2.  Informative References

   [attacking-ntp]
              "Attacking the Network Time Protocol", October 2015.

   [delorean]
              "Bypassing HTTP Strict Transport Security", 2014.

   [IEC.61588_2009]
              IEEE/IEC, "Precision clock synchronization protocol for
              networked measurement and control systems",
              IEEE 1588-2008(E), IEC 61588:2009(E),
              DOI 10.1109/IEEESTD.2009.4839002, February 2009,
              <http://ieeexplore.ieee.org/servlet/
              opac?punumber=4839000>.

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







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   [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, <http://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, <http://www.rfc-editor.org/info/rfc7384>.

   [RFC7821]  Mizrahi, T., "UDP Checksum Complement in the Network Time
              Protocol (NTP)", RFC 7821, DOI 10.17487/RFC7821, March
              2016, <http://www.rfc-editor.org/info/rfc7821>.

   [Shpiner]  "Multi-path Time Protocols", in Proceedings of IEEE
              International Symposium on Precision Clock Synchronization
              for Measurement, Control and Communication (ISPCS),
              September 2013.

Appendix A.  Flow Diagrams of Client Behaviour
































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

           Figure 3: The client's behavior in NTS unicast mode.

Authors' Addresses

   Daniel Fox Franke
   Akamai Technologies, Inc.
   150 Broadway
   Cambridge, MA  02142
   United States

   Email: dafranke@akamai.com
   URI:   https://www.dfranke.us






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   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)531-592-8421
   Email: kristof.teichel@ptb.de
































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