Roughtime
draft-ietf-ntp-roughtime-19

Network Time Protocols                                           W. Ladd
Internet-Draft                                       Akamai Technologies
Intended status: Experimental                                M. Dansarie
Expires: 18 September 2026                                        Netnod
                                                           17 March 2026

                               Roughtime
                      draft-ietf-ntp-roughtime-19

Abstract

   This document describes Roughtime, an experimental protocol that aims
   to achieve two things: secure rough time synchronization even for
   clients without any idea of what time it is, and give clients a
   format for reporting any inconsistencies they observe between
   timeservers.  This document specifies the on-wire protocol required
   for these goals, and discusses aspects of the ecosystem needed for it
   to work.

About This Document

   This note is to be removed before publishing as an RFC.

   Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-ietf-ntp-roughtime/.

   Source for this draft and an issue tracker can be found at
   https://github.com/ietf-wg-ntp/draft-roughtime.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at 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
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 18 September 2026.

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Copyright Notice

   Copyright (c) 2026 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 Revised BSD License text as
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   4
   3.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Single Server Mode  . . . . . . . . . . . . . . . . . . .   5
     3.2.  Multi Server Mode . . . . . . . . . . . . . . . . . . . .   5
   4.  Message Format  . . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Data types  . . . . . . . . . . . . . . . . . . . . . . .   6
       4.1.1.  uint32  . . . . . . . . . . . . . . . . . . . . . . .   6
       4.1.2.  uint64  . . . . . . . . . . . . . . . . . . . . . . .   6
       4.1.3.  Tag . . . . . . . . . . . . . . . . . . . . . . . . .   7
       4.1.4.  Timestamp . . . . . . . . . . . . . . . . . . . . . .   7
     4.2.  Header  . . . . . . . . . . . . . . . . . . . . . . . . .   7
   5.  Protocol Details  . . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Requests  . . . . . . . . . . . . . . . . . . . . . . . .   9
       5.1.1.  VER . . . . . . . . . . . . . . . . . . . . . . . . .  10
       5.1.2.  NONC  . . . . . . . . . . . . . . . . . . . . . . . .  10
       5.1.3.  TYPE  . . . . . . . . . . . . . . . . . . . . . . . .  10
       5.1.4.  SRV . . . . . . . . . . . . . . . . . . . . . . . . .  10
       5.1.5.  ZZZZ  . . . . . . . . . . . . . . . . . . . . . . . .  10
     5.2.  Responses . . . . . . . . . . . . . . . . . . . . . . . .  11
       5.2.1.  SIG . . . . . . . . . . . . . . . . . . . . . . . . .  12
       5.2.2.  NONC  . . . . . . . . . . . . . . . . . . . . . . . .  12
       5.2.3.  TYPE  . . . . . . . . . . . . . . . . . . . . . . . .  12
       5.2.4.  PATH  . . . . . . . . . . . . . . . . . . . . . . . .  12
       5.2.5.  SREP  . . . . . . . . . . . . . . . . . . . . . . . .  13
       5.2.6.  CERT  . . . . . . . . . . . . . . . . . . . . . . . .  13
       5.2.7.  INDX  . . . . . . . . . . . . . . . . . . . . . . . .  14
     5.3.  The Merkle Tree . . . . . . . . . . . . . . . . . . . . .  14
       5.3.1.  Root Value Validity Check Algorithm . . . . . . . . .  15
     5.4.  Validity of Response  . . . . . . . . . . . . . . . . . .  15
   6.  Integration into NTP  . . . . . . . . . . . . . . . . . . . .  15
   7.  Grease  . . . . . . . . . . . . . . . . . . . . . . . . . . .  16

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   8.  Roughtime Clients . . . . . . . . . . . . . . . . . . . . . .  16
     8.1.  Necessary configuration . . . . . . . . . . . . . . . . .  16
     8.2.  Measurement Sequence  . . . . . . . . . . . . . . . . . .  17
     8.3.  Server Lists  . . . . . . . . . . . . . . . . . . . . . .  17
     8.4.  Malfeasance Reporting . . . . . . . . . . . . . . . . . .  19
       8.4.1.  Malfeasance Report Format . . . . . . . . . . . . . .  19
       8.4.2.  Reporting . . . . . . . . . . . . . . . . . . . . . .  19
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  20
     9.1.  Confidentiality . . . . . . . . . . . . . . . . . . . . .  20
     9.2.  Integrity and Authenticity  . . . . . . . . . . . . . . .  20
     9.3.  Generating Private Keys . . . . . . . . . . . . . . . . .  20
     9.4.  Private Key Compromise  . . . . . . . . . . . . . . . . .  20
     9.5.  Quantum Resistance  . . . . . . . . . . . . . . . . . . .  20
     9.6.  Maintaining Lists of Servers  . . . . . . . . . . . . . .  21
     9.7.  Amplification Attacks . . . . . . . . . . . . . . . . . .  21
   10. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  21
   11. Operational Considerations  . . . . . . . . . . . . . . . . .  21
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
     12.1.  Service Name and Transport Protocol Port Number
            Registry . . . . . . . . . . . . . . . . . . . . . . . .  21
     12.2.  Roughtime Versions Registry  . . . . . . . . . . . . . .  22
     12.3.  Roughtime Tags Registry  . . . . . . . . . . . . . . . .  23
     12.4.  Media Type Registry  . . . . . . . . . . . . . . . . . .  24
       12.4.1.  Roughtime Server List MIME type  . . . . . . . . . .  24
       12.4.2.  Roughtime Malfeasance MIME type  . . . . . . . . . .  25
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  26
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  26
     13.2.  Informative References . . . . . . . . . . . . . . . . .  28
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  29
   Appendix A.  Example Server List  . . . . . . . . . . . . . . . .  29
   Appendix B.  Example Malfeasance Report . . . . . . . . . . . . .  31
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  33

1.  Introduction

   Time synchronization is essential to Internet security as many
   security protocols and other applications require it [RFC738].
   Unfortunately, widely deployed protocols such as the Network Time
   Protocol (NTP) [RFC5905] lack essential security features, and even
   newer protocols like Network Time Security (NTS) [RFC8915] lack
   mechanisms to observe that the servers behave correctly.
   Furthermore, clients may lack even a basic idea of the time, creating
   bootstrapping problems as time is required for X.509 certificate
   validation.

   The primary design goal of Roughtime is to permit devices to obtain a
   rough idea of the current time from a fairly static configuration and
   to enable them to report any inconsistencies they observe between

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   servers.  The configuration consists of a list of servers and their
   associated long-term keys, which ideally remain unchanged throughout
   a server's lifetime.  This makes the long-term public keys the roots
   of trust in Roughtime.  With a sufficiently long list of trusted
   servers and keys, a client will be able to acquire authenticated time
   with high probability, even after long periods of inactivity.  Proofs
   of malfeasance constructed by chaining together responses from
   different trusted servers can be used to prove misbehavior by a
   server and, after analysis, result in revoking trust in that
   particular key.

   Unlike Khronos [RFC9523], Roughtime produces external evidence that
   timeservers are reporting incompatible times.  This requires changes
   to the format of the timestamps and hence cannot be a mere extension
   to NTP.

   Operational experience is needed to evaluate the viability of using
   Roughtime for secure time bootstrapping in Internet-connected
   systems.  This includes the need for experience with maintaining a
   Roughtime ecosystem with services that maintain and distribute lists
   of trusted servers and process malfeasance reports.  To facilitate
   the experiments necessary to gain that experience, this document is
   limited to describing the Roughtime on-wire protocol.  Apart from
   describing the server list and malfeasance report formats, this
   document does not describe the ecosystem, nor the means by which the
   server list is maintained and distributed or the policies to apply to
   such a list.

2.  Conventions and Definitions

   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.  Protocol Overview

   Roughtime is a protocol for authenticated rough time synchronization
   that enables clients to provide cryptographic proof of server
   malfeasance.  It does so by having responses from servers include a
   signature over a value derived from the client's request, which
   includes a nonce.  This provides cryptographic proof that the
   response was issued after the server received the client's request.
   The derived value included in the server's response is the root of a
   Merkle tree [Merkle] which includes the hash value of the client's
   request as the value of one of its leaf nodes.  This enables the
   server to amortize the relatively costly signing operation over a

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   number of client requests.

3.1.  Single Server Mode

   At its most basic level, Roughtime is a one-round protocol in which a
   completely fresh client requests the current time and the server
   sends a signed response.  The response includes a timestamp and a
   radius used to indicate the server's certainty about the reported
   time.

   The client's request contains a nonce which the server incorporates
   into its signed response.  The client can verify the server's
   signatures and—provided that the nonce has sufficient entropy—this
   proves that the signed response could only have been generated after
   the nonce.

3.2.  Multi Server Mode

   When using multiple servers, a client can detect, cryptographically
   prove, and report inconsistencies between different servers.

   A Roughtime server guarantees that the timestamp included in the
   response to a request is generated after the reception of the request
   and prior to the transmission of the associated response.  If the
   time response from a server is not consistent with time responses
   from other servers, this indicates server error or intentional
   malfeasance that can be reported and potentially used to impeach the
   server.

   Proofs of malfeasance are constructed by chaining requests to
   different Roughtime servers.  Details on proofs and malfeasance
   reporting are provided in Section 8.  For the reporting to result in
   impeachment, an additional mechanism is required that provides a
   review and impeachment process.  Defining such a mechanism is beyond
   the scope of this document.  A simple option could be an online forum
   where a court of human observers evaluate cases after reviewing input
   reports.

4.  Message Format

   Roughtime messages are maps consisting of one or more (tag, value)
   pairs.  They start with a header, which contains the number of pairs,
   the value offsets, and the tags.  The header is followed by a message
   values section which contains the values associated with the tags in
   the header.  Messages are formatted according to Figure 1 as
   described in the following sections.

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   In some cases, messages are recursive, i.e. the value of a tag can
   itself be a Roughtime message.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Number of pairs, N (uint32)                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                                                               .
   .                     N-1 offsets (uint32)                      .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                                                               .
   .                        N tags (uint32)                        .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                                                               .
   .                           N values                            .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 1: Roughtime Message

4.1.  Data types

4.1.1.  uint32

   A uint32 is a 32-bit unsigned integer.  It is serialized with the
   least significant byte first.

4.1.2.  uint64

   A uint64 is a 64-bit unsigned integer.  It is serialized with the
   least significant byte first.

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4.1.3.  Tag

   Tags are used to identify values in Roughtime messages.  A tag is a
   sequence of four octets.  Each tag sequence starts with one to four
   capital ASCII letters (A-Z) [RFC20] followed by zero to three padding
   zero octets.  Throughout this document, tags are referred to by their
   ASCII string representation.  However, they are registered and sorted
   as uint32 values, where the least significant byte is the first octet
   in the sequence.

   For example, the ASCII string "NONC" would correspond to the uint32
   0x434e4f4e which is serialized as {0x4e, 0x4f, 0x4e, 0x43}. "VER"
   would correspond to 0x00524556 and be serialized as {0x56, 0x45,
   0x52, 0x00}.

4.1.4.  Timestamp

   A timestamp is a representation of UTC time as a uint64 count of
   seconds since 00:00:00 on 1 January 1970 (the Unix epoch), assuming
   every day has 86400 seconds.  This is a constant offset from the NTP
   timestamp in seconds.  Leap seconds do not have an unambiguous
   representation in a timestamp, and this has implications for the
   attainable accuracy and setting of the RADI tag (see Section 5.2.5).

4.2.  Header

   As illustrated in Figure 1, the first four bytes of the header is the
   uint32 number of tags N, and hence of (tag, value) pairs.  The
   following 4*(N-1) bytes are offsets, each a uint32, and the last 4*N
   bytes in the header are tags.

   The offsets array is considered to have an implicitly encoded value
   of 0 as its zeroth entry.  Its members refer to the positions of the
   tag values in the message values section.  All offsets are multiples
   of four.

   The members of the offsets and tags arrays, as well as the message
   values section are sorted in ascending order by the tag's uint32
   value.  As a consequence, the offset array is also sorted in
   ascending order.  A tag MUST NOT appear more than once in a header.

   The first post-header byte, i.e. the first byte of the message values
   section, is at offset 0.  The value associated with the ith tag
   begins at offset[i] and ends at offset[i+1]-1, with the exception of
   the last value which ends at the end of the message.  Values MAY have
   zero length.  All lengths and offsets are in bytes.

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5.  Protocol Details

   As described in Section 3, clients initiate time synchronization by
   sending requests containing a nonce to servers who send signed time
   responses in return.  Roughtime packets can be sent between clients
   and servers either as UDP datagrams or via TCP streams.  Servers
   SHOULD support both the UDP and TCP transport modes.

   Roughtime packets are formatted according to Figure 2 and as
   described here.  The first field is a uint64 with the value
   0x4d49544847554f52 ("ROUGHTIM" in ASCII).  The second field is a
   uint32 and contains the length of the third field.  The third and
   last field contains a Roughtime message as specified in Section 4.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  0x4d49544847554f52 (uint64)                  |
   |                        ("ROUGHTIM")                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Message length (uint32)                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                                                               .
   .                      Roughtime message                        .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 2: Roughtime packet

   Roughtime request and response packets MUST be transmitted in a
   single datagram when the UDP transport mode is used.  Setting the
   packet's Don't Fragment bit [RFC791] is OPTIONAL in IPv4 networks.
   Setting it may cause packets to get dropped, but not setting it could
   lead to long delays due to reconstruction and dropped fragments.

   A Roughtime packet could exceed the maximum deliverable length of a
   packet on a particular path, making Roughtime queries over UDP
   impossible on that path.  A client SHOULD attempt to use the TCP
   transport mode for Roughtime queries to a server if it does not
   receive responses to its UDP queries.

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   Clients MUST implement exponential backoff in establishing TCP
   connections and making requests over UDP.  It is RECOMMENDED that
   clients use an initial retry interval of 1 second, a maximum interval
   of 24 hours, and a base of 1.5.  Therefore, the minimum interval, in
   seconds, before retrying after n failures is min(1.5^{n-1}, 86400).
   Guidance for implementers considering other values can be found in
   Section 3.1.3 of [RFC8085].

   Clients MUST NOT reset the retry interval until they receive a
   properly signed response.

   Multiple requests and responses can be exchanged over an established
   TCP connection.  Clients MAY send multiple outstanding requests and
   servers MAY send responses out of order.  The connection SHOULD be
   closed by the client when it has no more requests to send and has
   received all expected responses.  Either side SHOULD close the
   connection in response to synchronization, format, implementation-
   defined timeouts, or other errors.

   All requests and responses contain the VER tag.  It contains a list
   of one or more uint32 version numbers.  The version of Roughtime
   specified by this document has version number 1.

   NOTE TO RFC EDITOR: remove this paragraph before publication.  For
   testing this draft of the document, a version number of 0x8000000c is
   used.

5.1.  Requests

   A request contains the tags VER, NONC, and TYPE.  It SHOULD include
   the tag SRV.  Unknown tags MUST be ignored by the server.  Requests
   not containing the three mandatory tags MUST be ignored.  A future
   version of this protocol may mandate additional tags in the message
   and assign them semantic meaning.

   The size of the request message SHOULD be at least 1024 bytes when
   the UDP transport mode is used.  To attain this size, the ZZZZ tag is
   added to the message.  A reason for sending request messages smaller
   could be to use the UDP transport mode over paths with low maximum
   deliverable length.  However, responding to request messages shorter
   than 1024 bytes is OPTIONAL and servers MUST NOT send responses
   larger than the request messages they are replying to; see
   Section 9.7.

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5.1.1.  VER

   In a request, the VER tag contains a list of uint32 version numbers.
   The VER tag MUST include at least one Roughtime version supported by
   the client and MUST NOT contain more than 32 version numbers.  The
   version numbers and tags included in the request MUST be compatible
   with each other and the packet contents.

   The version numbers MUST NOT repeat and MUST be sorted in ascending
   numerical order.

   Servers MUST ignore any unknown version numbers in the list supplied
   by the client.  If the list contains no version numbers supported by
   the server, it MAY respond with another version or ignore the request
   entirely, see Section 5.2.5.

5.1.2.  NONC

   The value of the NONC tag is a 32-byte nonce.  It SHOULD be generated
   in a manner indistinguishable from random.  BCP 106 [RFC4086]
   contains specific guidelines regarding this.  Section 8.2 describes
   how to securely generate nonces when querying multiple servers in
   sequence.

5.1.3.  TYPE

   The TYPE tag is used to unambiguously distinguish between request and
   response messages.  In a request, it MUST contain a uint32 with value
   0.  Requests containing a TYPE tag with any other value MUST be
   ignored by servers.

5.1.4.  SRV

   The SRV tag is used by the client to indicate which long-term public
   key it expects to verify the response with.  The value of the SRV tag
   is H(0xff || public_key) where public_key is the server's long-term,
   32-byte Ed25519 public key and H is SHA-512 truncated to the first 32
   bytes.

5.1.5.  ZZZZ

   The ZZZZ tag is used to expand the request to the minimum required
   length.  Its value is all zero bytes.

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5.2.  Responses

   The server begins the request handling process with a set of long-
   term keys.  It resolves which long-term key to use with the following
   procedure:

   1.  If the request contains a SRV tag, then the server looks up the
       long-term key indicated by the SRV value.  If no such key exists,
       then the server MUST ignore the request.

   2.  If the request contains no SRV tag, but the server has just one
       long-term key, it SHOULD select that key.  Otherwise, if the
       server has multiple long-term keys, then it MUST ignore the
       request.

   A response contains the tags SIG, NONC, TYPE, PATH, SREP, CERT, and
   INDX.  The structure of a response message is illustrated in
   Figure 3.

   |--SIG
   |--NONC
   |--TYPE
   |--PATH
   |--SREP
   |  |--VER
   |  |--RADI
   |  |--MIDP
   |  |--VERS
   |  |--ROOT
   |--CERT
   |  |--SIG
   |  |--DELE
   |  |  |--PUBK
   |  |  |--MINT
   |  |  |--MAXT
   |--INDX

              Figure 3: Roughtime response message structure.

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   No mechanism for reporting errors—such as wrong request format,
   unsupported version, or unknown SRV value—back to the client is
   provided.  This is based on experience from the NTP protocol, where
   Kiss-o'-Death packets (see Section 7.4 of [RFC5905]) are used to
   indicate errors.  The existence of this unauthenticated protocol
   feature in NTP makes it possible for on-path attackers to make a
   client stop using authenticated modes or certain servers altogether
   (see Section 5.4 of [RFC8633] and Sections 8.3 and 8.7 of [RFC8915]).
   Considering the protocol's dependence on multiple independent servers
   for security, error reporting functionality has been excluded from
   this version of Roughtime.

5.2.1.  SIG

   In general, a SIG tag value is a 64-byte Ed25519 signature [RFC8032]
   over a concatenation of a signature context ASCII string and the
   entire value of a tag.  All context strings include a terminating
   zero byte.

   The SIG tag in the root of a response is a signature over the SREP
   value using the public key contained in CERT and the context string
   "RoughTime v1 response signature".

5.2.2.  NONC

   The NONC tag contains the nonce of the message being responded to.

5.2.3.  TYPE

   In a response, the TYPE tag MUST contain a uint32 with value 1.
   Responses containing a TYPE tag with any other value MUST be ignored
   by clients.

5.2.4.  PATH

   The PATH tag value is a multiple of 32 bytes long and represents a
   path of 32-byte hash values in the Merkle tree used to generate the
   ROOT value as described in Section 5.3.  In the case where a response
   is prepared for a single request and the Merkle tree contains only
   the root node, the size of PATH is zero.

   The PATH MUST NOT contain more than 32 hash values.  The maximum
   length of PATH is normally limited by the maximum size of the
   response message, see Section 5.1 and Section 9.7.  Server
   implementations MUST select a maximum Merkle tree height (see
   Section 5.3) that ensures this.

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5.2.5.  SREP

   The SREP tag contains a signed response.  Its value is a Roughtime
   message with the tags VER, RADI, MIDP, VERS, and ROOT.

   The VER tag, when used in a response, contains a single uint32
   version number.  It SHOULD be one of the version numbers supplied by
   the client in its request; see Section 5.1.1.  The server MUST ensure
   that the version number corresponds with the rest of the packet
   contents.

   The RADI tag value is a uint32 representing the server's estimate of
   the accuracy of MIDP in seconds.  Servers MUST ensure that the true
   time is within (MIDP-RADI, MIDP+RADI) at the moment of processing.
   The value of RADI MUST NOT be zero.  Since leap seconds cannot be
   unambiguously represented by Roughtime timestamps, servers MUST take
   this into account when setting the RADI value during leap second
   events.  Servers that do not have any leap second information SHOULD
   set the value of RADI to at least 3.  Failure to do so will impact
   the observed correctness of Roughtime servers and can lead to
   malfeasance reports.

   The MIDP tag value is the timestamp of the moment of processing.

   The VERS tag value contains a list of uint32 version numbers
   supported by the server, sorted in ascending numerical order.  It
   MUST contain the version number specified in the VER tag.  It MUST
   NOT contain more than 32 version numbers.

   The ROOT tag contains a 32-byte value of a Merkle tree root as
   described in Section 5.3.

5.2.6.  CERT

   The CERT tag contains a public-key certificate signed with the
   server's private long-term key.  Its value is a Roughtime message
   with the tags SIG and DELE, where SIG is a signature over the DELE
   value with the context string "RoughTime v1 delegation signature".

   The DELE tag contains a delegated public-key certificate used by the
   server to sign the SREP tag.  Its value is a Roughtime message with
   the tags PUBK, MINT, and MAXT.  The purpose of the DELE tag is to
   enable separation of a long-term public key from keys on devices
   exposed to the public Internet.

   The PUBK tag contains a temporary 32-byte Ed25519 public key which is
   used to sign the SREP tag.

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   The MINT tag is the minimum timestamp for which the key in PUBK is
   trusted to sign responses.  MIDP MUST be more than or equal to MINT
   for a response to be considered valid.

   The MAXT tag is the maximum timestamp for which the key in PUBK is
   trusted to sign responses.  MIDP MUST be less than or equal to MAXT
   for a response to be considered valid.

5.2.7.  INDX

   The INDX tag value is a uint32 determining the position of NONC in
   the Merkle tree used to generate the ROOT value as described in
   Section 5.3.

5.3.  The Merkle Tree

   A Merkle tree [Merkle] is a binary tree where the value of each non-
   leaf node is a hash value derived from its two children.  The root of
   the tree is thus dependent on all leaf nodes.

   In Roughtime, each leaf node in the Merkle tree represents one
   request.  Leaf nodes are indexed left to right, beginning with zero.

   The values of all nodes are calculated from the leaf nodes and up
   towards the root node using the first 32 bytes of the output of the
   SHA-512 hash algorithm [RFC6234].  For leaf nodes, the byte 0x00 is
   prepended to the full value of the client's request packet, including
   the "ROUGHTIM" header, before applying the hash function.  For all
   other nodes, the byte 0x01 is concatenated with first the left and
   then the right child node value before applying the hash function.

   The value of the Merkle tree's root node is included in the ROOT tag
   of the response.

   The index of a request leaf node is included in the INDX tag of the
   response.

   The values of all sibling nodes in the path between a request leaf
   node and the root node are stored in the PATH tag so that the client
   can reconstruct and validate the value in the ROOT tag using its
   request packet.  These values are each 32 bytes and are stored one
   after the other with no additional padding or structure.  The order
   in which they are stored is described in the next section.

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5.3.1.  Root Value Validity Check Algorithm

   This section describes how to compute the value of the root of the
   Merkle tree from the values in the tags PATH, INDX, and NONC.  The
   bits of INDX are ordered from least to most significant.  H(x)
   denotes the first 32 bytes of the SHA-512 hash digest of x and ||
   denotes concatenation.

   The algorithm maintains a current value h.  At initialization, h is
   set to H(0x00 || request_packet).  For each step of the algorithm,
   let node be the next 32 bytes in PATH.  If the current bit in INDX is
   0 then h = H(0x01 || h || node), else h = H(0x01 || node || h).  When
   no more entries remain in PATH, h is compared to the value of the
   root of the Merkle tree contained in ROOT.  If they are equal, the
   algorithm succeeds.  If they are not, or if any of the remaining bits
   of INDX is non-zero, the algorithm fails.

5.4.  Validity of Response

   A client MUST check the following properties when it receives a
   response.  We assume the long-term server public key is known to the
   client through other means.

   The signature in CERT was made with the long-term key of the server.

   The MIDP timestamp lies in the interval specified by the MINT and
   MAXT timestamps.

   The INDX and PATH values prove a hash value derived from the request
   packet was included in the Merkle tree with value ROOT using the
   algorithm in Section 5.3.1.

   The signature of SREP in SIG validates with the public key in DELE.

   A response that passes these checks is said to be valid.  Validity of
   a response does not prove that the timestamp's value in the response
   is correct, but merely that the server guarantees that it signed the
   timestamp and computed its signature during the time interval (MIDP-
   RADI, MIDP+RADI).

6.  Integration into NTP

   We assume that there is a bound phi on the frequency error in the
   clock on the machine.  Let delta be the time difference between the
   clock on the client and the clock on the server and let sigma
   represent the error in the measured value of delta introduced by the
   measurement process.  Given a measurement taken at a local time t, we
   know the true time is in (t-delta-sigma, t-delta+sigma).  After d

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   seconds have elapsed we know the true time is within (t-delta-sigma-
   d*phi, t-delta+sigma+d*phi).

   This bound can be used as a simple and effective means to limit the
   error an attacker can introduce into NTP or Precision Time Protocol
   (PTP) measurements.  For example, an NTP client can ensure that its
   observation intervals fall entirely within this range or reject
   measurements that fall outside.

   An application that needs to verify X.509 certificates (which
   requires knowledge of the current time), but lacks an accurate and
   trusted time source can use Roughtime to obtain a time estimate.  In
   particular, securely establishing NTS-protected NTP time
   synchronization requires verification of the NTS-KE server's
   certificate, which is not possible if the client has no idea of the
   current time (see Section 8.5 of [RFC8915]).  In that case, a
   Roughtime time estimate can be used for certificate validation.

   If an NTP server uses a Roughtime server as a time source for
   synchronization (and not only for filtering its NTP measurements),
   the root dispersion SHOULD include the server's RADI value and root
   delay SHOULD include the interval between sending the Roughtime
   request and receiving the response.

7.  Grease

   The primary purpose of grease is to prevent protocol ossification,
   which could prohibit future protocol extensions and development
   [RFC9170].  In Roughtime, grease is also intended to ensure that
   clients validate signatures.  To grease the Roughtime protocol,
   servers SHOULD send back a fraction of responses with any of the
   following: lack of mandatory tags, version numbers not in the
   request, undefined tags, or invalid signatures together with
   incorrect times.  Clients MUST properly ignore undefined tags and
   reject invalid responses.  Servers MUST NOT send back responses with
   incorrect times and valid signatures.  Either signature MAY be
   invalid for this application.

8.  Roughtime Clients

8.1.  Necessary configuration

   To carry out a Roughtime measurement, a client needs a list of
   servers, a minimum of three of which are operational and not run by
   the same parties.  Roughtime clients SHOULD regularly update their
   view of which servers are trustworthy in order to benefit from the
   detection of misbehavior (see Section 8.3).  Clients SHOULD also have
   a means of reporting to the provider of such a list, such as an

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   operating system or software vendor, a malfeasance report as
   described in Section 8.4.

8.2.  Measurement Sequence

   The client randomly selects at least three servers from the list, and
   sequentially queries them.  To ensure that all possible
   inconsistencies can be detected, it is necessary for clients to
   repeat the query sequence twice with the servers in the same order.

   The first probe uses a nonce that is randomly generated.  The second
   query uses H(resp || rand) where rand is a random 32-byte value and
   resp is the entire response to the first probe, including the
   "ROUGHTIM" header.  Each subsequent query uses H(resp || rand) for
   the previous response and a different 32-byte rand value.  H(x)
   and || are defined as in Section 5.3.1.

   For each pair of responses (i, j), where i was received before j, the
   client MUST check that MIDP_i-RADI_i is less than or equal to
   MIDP_j+RADI_j.  If these checks pass, the times are consistent with
   causal ordering.  The measurement succeeds if the validity checks
   described in Section 5.4 are successful, the times reported are
   consistent with causal ordering, and the delay between request and
   response is within an implementation-dependent maximum value.

   If the validity checks are successful, but at least one of the
   responses is not consistent with causal ordering, there has been a
   malfeasance.  In case of detected malfeasance, clients SHOULD, if it
   is technically possible, generate a malfeasance report (see
   Section 8.4), alert the user, and make another measurement.  See
   Section 5 for guidance on backoff when making repeated measurements.

8.3.  Server Lists

   To facilitate regular updates of lists of trusted servers, a common
   server list format is specified here.  Support for the common server
   list format is OPTIONAL and clients MAY instead implement their own
   mechanisms for configuring server lists.

   A server list is a JSON [RFC8259] object that contains the key
   "servers".  Server list objects MAY also contain the keys "sources"
   and "reports".  Appendix A contains an example server list in the
   format described here.

   Server lists have the "application/roughtime-server+json" media type.

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   The value of the "servers" key is a list of server objects, each
   containing the keys "name", "version", "publicKeyType", "publicKey",
   and "addresses".

   The value of "name" is a string that contains a server name suitable
   for display to a user.

   The value of "version" is an integer that indicates the highest
   Roughtime version number supported by the server.

   NOTE TO RFC EDITOR: remove this paragraph before publication.  To
   indicate compatibility with drafts of this document, a decimal
   representation of the version number indicated in Section 5 SHOULD be
   used.  For indicating compatibility with pre-IETF specifications of
   Roughtime, the version number 3000600613 SHOULD be used.

   The value of "publicKeyType" is a string indicating the signature
   scheme used by the server.  The value for servers supporting version
   1 of Roughtime is "ed25519".

   The value of "publicKey" is a base64-encoded [RFC4648] string
   representing the long-term public key of the server in a format
   consistent with the value of "publicKeyType".

   The value of "addresses" is a list of address objects.  An address
   object contains the keys "protocol" and "address".  The value of
   "protocol" is either "tcp" or "udp", indicating the transport mode to
   use.  The value of "address" is a string indicating a host and a port
   number, separated by a colon character, for example
   "roughtime.example.com:2002".  The host part is either an IPv4
   address, an IPv6 address, or a fully qualified domain name (FQDN).
   IPv4 addresses are specified in dotted decimal notation.  IPv6
   addresses MUST conform to the "Text Representation of Addresses"
   [RFC4291] and MUST NOT include zone identifiers [RFC9844].  To
   disambiguate IPv6 addresses from ports when zero compression happens,
   IPv6 addresses are encapsulated within [].  The port part is a
   decimal integer representing a valid port number, i.e. in the range
   0-65535.

   The value of "sources", if present, is a list of strings indicating
   where updated versions of the list may be acquired using the HTTP GET
   method [RFC9110].  Each string is a URL [RFC3986] pointing to a list
   in the format specified here.  The URI scheme MUST be HTTPS
   [RFC9110].

   The value of "reports", if present, is a string indicating a URL
   [RFC3986] where malfeasance reports can be sent by clients using the
   HTTP POST method.  The URI scheme MUST be HTTPS [RFC9110].

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8.4.  Malfeasance Reporting

   A malfeasance report is cryptographic proof that a sequence of
   responses arrived in that order.  It can be used to demonstrate that
   at least one server sent the wrong time.

8.4.1.  Malfeasance Report Format

   A malfeasance report is a JSON [RFC8259] object that contains the key
   "responses".  Its value is a list of response objects, sorted in the
   order received.  Each response object contains the keys "rand",
   "publicKey", "request", and "response".  The values of all four keys
   are represented as base64-encoded [RFC4648] strings.  Appendix B
   contains an example malfeasance report in the format described here.

   Malfeasance reports have the "application/roughtime-malfeasance+json"
   media type.

   The "rand" key MAY be omitted from the first response object in the
   list.  In all other cases, its value is the 32-byte value used to
   generate the request nonce value from the previous response packet.

   The value of "publicKey" is the long-term key that the server was
   expected to use for deriving the response signature.

   The value of "request" is the transmitted request packet, including
   the "ROUGHTIM" header.

   The value of "response" is the received response packet, including
   the "ROUGHTIM" header.

8.4.2.  Reporting

   When the client's list of servers has an associated URL for
   malfeasance reports, it SHOULD send a malfeasance report to that URL
   when malfeasance is detected (see Section 8.2) and it is technically
   feasible to do so.  Malfeasance reports are sent using the HTTP POST
   method [RFC9110].

   Since the failure of a popular Roughtime server can cause numerous
   clients to send malfeasance reports at the same time, clients MUST
   use exponential backoff to prevent overloading the server receiving
   the reports.  It is RECOMMENDED that clients use an initial retry
   interval of 10 seconds, a maximum interval of 24 hours, and a base of
   1.5.  Therefore, the minimum interval, in seconds, before retrying
   after n failures is min(10 * 1.5^(n-1), 86400).

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   Clients MUST NOT send malfeasance reports in response to signature
   verification failures or any other protocol errors.

   As described in Section 1, the operational rules for acceptance or
   rejection of a particular malfeasance report are beyond the scope of
   this document.

9.  Security Considerations

9.1.  Confidentiality

   This protocol does not provide any confidentiality.  Given the nature
   of timestamps, such impact is minor.

9.2.  Integrity and Authenticity

   The Roughtime protocol only provides integrity and authenticity
   protection for data contained in the SREP tag.  Accordingly, new tags
   SHOULD be added to the SREP tag whenever possible.

9.3.  Generating Private Keys

   Although any random 256-bit string can be used as a private Ed25519
   key, it has a high risk of being vulnerable to small-subgroup attacks
   and timing side-channel leaks.  For this reason, all private keys
   used in Roughtime MUST be generated following the procedure described
   in Section 5.1.5 of [RFC8032].

9.4.  Private Key Compromise

   The compromise of a PUBK's private key, even past MAXT, is a problem
   as the private key can be used to sign invalid times that are in the
   range MINT to MAXT, and thus violate the good-behavior guarantee of
   the server.  To protect against this, it is necessary for clients to
   query multiple servers in accordance with the procedure described in
   Section 8.2.

9.5.  Quantum Resistance

   Since the only supported signature scheme, Ed25519, is not quantum
   resistant, the Roughtime version described in this document will not
   survive the advent of quantum computers.  A later version will have
   to be devised and implemented before then.  The use of a single
   version number as negotiation point rather than defining a suite of
   acceptable signatures is intended to prevent fragmentation and
   misconfiguration.

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9.6.  Maintaining Lists of Servers

   The infrastructure and procedures for maintaining a list of trusted
   servers and adjudicating violations of the rules by servers is not
   discussed in this document and is essential for security.

9.7.  Amplification Attacks

   UDP protocols that send responses significantly larger than requests,
   such as NTP, have previously been leveraged for amplification
   attacks.  To prevent Roughtime from being used for such attacks,
   servers MUST NOT send response packets larger than the request
   packets sent by clients.

10.  Privacy Considerations

   This protocol is designed to obscure all client identifiers.  Servers
   necessarily have persistent long-term identities essential to
   enforcing correct behavior.  Generating nonces in a nonrandom manner
   can cause leaks of private data or enable tracking of clients as they
   move between networks.

11.  Operational Considerations

   It is expected that clients identify a server by its long-term public
   key.  In multi-tenancy environments, where multiple servers may be
   listening on the same IP or port space, the protocol is designed so
   that the client indicates which server it expects to respond.  This
   is done with the SRV tag.  Additional recommendations for clients are
   listed in Section 8.

12.  IANA Considerations

12.1.  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:

   Service Name: roughtime

   Transport Protocol: tcp,udp

   Assignee: IESG iesg@ietf.org (mailto:iesg@ietf.org)

   Contact: IETF Chair chair@ietf.org (mailto:chair@ietf.org)

   Description: Roughtime time synchronization

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   Reference: [[this document]]

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

12.2.  Roughtime Versions Registry

   IANA is requested to create a new registry titled "Roughtime
   Versions" in a new "Roughtime" registry group.  Entries will have the
   following fields:

   Version ID (REQUIRED): a 32-bit unsigned integer

   Version name (REQUIRED): A short text string naming the version being
   identified.

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

   The policy for allocation of new entries is IETF Review [RFC8126].

   The initial contents of this registry are specified in Table 1.

    +=======================+=====================+===================+
    | Version ID            | Version name        | Reference         |
    +=======================+=====================+===================+
    | 0x0                   | Reserved            | [[this document]] |
    +-----------------------+---------------------+-------------------+
    | 0x1                   | Roughtime version 1 | [[this document]] |
    +-----------------------+---------------------+-------------------+
    | 0x2-0x7fffffff        | Unassigned          |                   |
    +-----------------------+---------------------+-------------------+
    | 0x80000000-0xbfffffff | Reserved for        | [[this document]] |
    |                       | experimental use    |                   |
    +-----------------------+---------------------+-------------------+
    | 0xc0000000-0xffffffff | Reserved for        | [[this document]] |
    |                       | private use         |                   |
    +-----------------------+---------------------+-------------------+

       Table 1: Initial contents of the Roughtime Versions registry.

   Private and experimental use are defined in [RFC8126].  The
   experimental range is intended for testing and evaluating new
   versions of the Roughtime protocol.  Such tests may be conducted over
   the open Internet.

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12.3.  Roughtime Tags Registry

   IANA is requested to create a new registry titled "Roughtime Tags" in
   a new "Roughtime" registry group.  Entries will have the following
   fields:

   Tag (REQUIRED): A 32-bit unsigned integer in hexadecimal format.

   ASCII Representation (REQUIRED): The ASCII representation of the tag
   in accordance with Section 4.1.3 of this document.

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

   The policy for allocation of new entries in this registry is
   Specification Required [RFC8126].

   The initial contents of this registry are specified in Table 2.

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         +============+======================+===================+
         |        Tag | ASCII Representation | Reference         |
         +============+======================+===================+
         | 0x00474953 | SIG                  | [[this document]] |
         +------------+----------------------+-------------------+
         | 0x00524556 | VER                  | [[this document]] |
         +------------+----------------------+-------------------+
         | 0x00565253 | SRV                  | [[this document]] |
         +------------+----------------------+-------------------+
         | 0x434e4f4e | NONC                 | [[this document]] |
         +------------+----------------------+-------------------+
         | 0x454c4544 | DELE                 | [[this document]] |
         +------------+----------------------+-------------------+
         | 0x45505954 | TYPE                 | [[this document]] |
         +------------+----------------------+-------------------+
         | 0x48544150 | PATH                 | [[this document]] |
         +------------+----------------------+-------------------+
         | 0x49444152 | RADI                 | [[this document]] |
         +------------+----------------------+-------------------+
         | 0x4b425550 | PUBK                 | [[this document]] |
         +------------+----------------------+-------------------+
         | 0x5044494d | MIDP                 | [[this document]] |
         +------------+----------------------+-------------------+
         | 0x50455253 | SREP                 | [[this document]] |
         +------------+----------------------+-------------------+
         | 0x53524556 | VERS                 | [[this document]] |
         +------------+----------------------+-------------------+
         | 0x544e494d | MINT                 | [[this document]] |
         +------------+----------------------+-------------------+
         | 0x544f4f52 | ROOT                 | [[this document]] |
         +------------+----------------------+-------------------+
         | 0x54524543 | CERT                 | [[this document]] |
         +------------+----------------------+-------------------+
         | 0x5458414d | MAXT                 | [[this document]] |
         +------------+----------------------+-------------------+
         | 0x58444e49 | INDX                 | [[this document]] |
         +------------+----------------------+-------------------+
         | 0x5a5a5a5a | ZZZZ                 | [[this document]] |
         +------------+----------------------+-------------------+

         Table 2: Initial contents of the Roughtime Tags registry.

12.4.  Media Type Registry

12.4.1.  Roughtime Server List MIME type

   IANA is requested to allocate the following entry in the Media Type
   registry.

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   Type name: application

   Subtype name: roughtime-server+json

   Required parameters: N/A

   Optional parameters: N/A

   Encoding considerations: Encoding considerations are identical to
   those specified for the "application/json" media type, see [RFC8259].

   Security considerations: Section 9 of [[this document]].

   Interoperability considerations: N/A

   Published specification: Section 8.3 of [[this document]].

   Applications that use this media type: Roughtime clients [[this
   document]] that update their lists of Roughtime servers.

   Fragment identifier considerations: N/A

   Additional information:
   Deprecated alias names for this type: N/A
   Magic number(s): N/A
   File extension(s): N/A
   Macintosh file type code(s): N/A

   Person & email address to contact for further information: See
   Authors' Addresses section of [[this document]].

   Intended usage: COMMON

   Restrictions on usage: N/A

   Author: See Authors' Addresses section of [[this document]].

   Change controller: Internet Engineering Task Force

12.4.2.  Roughtime Malfeasance MIME type

   IANA is requested to allocate the following entry in the Media Type
   registry.

   Type name: application

   Subtype name: roughtime-malfeasance+json

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   Required parameters: N/A

   Optional parameters: N/A

   Encoding considerations: Encoding considerations are identical to
   those specified for the "application/json" media type, see [RFC8259].

   Security considerations: Section 9 of [[this document]].

   Interoperability considerations: N/A

   Published specification: Section 8.4.1 of [[this document]].

   Applications that use this media type: Roughtime clients [[this
   document]] use this media type to report cryptographic proof that a
   Roughtime server has sent the wrong time.

   Fragment identifier considerations: N/A

   Additional information:
   Deprecated alias names for this type: N/A
   Magic number(s): N/A
   File extension(s): N/A
   Macintosh file type code(s): N/A

   Person & email address to contact for further information: See
   Authors' Addresses section of [[this document]].

   Intended usage: COMMON

   Restrictions on usage: N/A

   Author: See Authors' Addresses section of [[this document]].

   Change controller: Internet Engineering Task Force

13.  References

13.1.  Normative References

   [RFC20]    Cerf, V., "ASCII format for network interchange", STD 80,
              RFC 20, DOI 10.17487/RFC0020, October 1969,
              <https://www.rfc-editor.org/rfc/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/rfc/rfc2119>.

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   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/rfc/rfc3986>.

   [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/rfc/rfc4086>.

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

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/rfc/rfc4648>.

   [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234,
              DOI 10.17487/RFC6234, May 2011,
              <https://www.rfc-editor.org/rfc/rfc6234>.

   [RFC791]   Postel, J., "Internet Protocol", STD 5, RFC 791,
              DOI 10.17487/RFC0791, September 1981,
              <https://www.rfc-editor.org/rfc/rfc791>.

   [RFC8032]  Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
              Signature Algorithm (EdDSA)", RFC 8032,
              DOI 10.17487/RFC8032, January 2017,
              <https://www.rfc-editor.org/rfc/rfc8032>.

   [RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
              Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
              March 2017, <https://www.rfc-editor.org/rfc/rfc8085>.

   [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/rfc/rfc8174>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/rfc/rfc8259>.

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   [RFC9110]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,
              <https://www.rfc-editor.org/rfc/rfc9110>.

13.2.  Informative References

   [Merkle]   Merkle, R. C., "A Digital Signature Based on a
              Conventional Encryption Function", in Pomerance, C. (eds)
              Advances in Cryptology, Lecture Notes in Computer
              Science vol 293, DOI 10.1007/3-540-48184-2_32, 1988,
              <https://doi.org/10.1007/3-540-48184-2_32>.

   [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/rfc/rfc5905>.

   [RFC738]   Harrenstien, K., "Time server", RFC 738,
              DOI 10.17487/RFC0738, October 1977,
              <https://www.rfc-editor.org/rfc/rfc738>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/rfc/rfc8126>.

   [RFC8633]  Reilly, D., Stenn, H., and D. Sibold, "Network Time
              Protocol Best Current Practices", BCP 223, RFC 8633,
              DOI 10.17487/RFC8633, July 2019,
              <https://www.rfc-editor.org/rfc/rfc8633>.

   [RFC8915]  Franke, D., Sibold, D., Teichel, K., Dansarie, M., and R.
              Sundblad, "Network Time Security for the Network Time
              Protocol", RFC 8915, DOI 10.17487/RFC8915, September 2020,
              <https://www.rfc-editor.org/rfc/rfc8915>.

   [RFC9170]  Thomson, M. and T. Pauly, "Long-Term Viability of Protocol
              Extension Mechanisms", RFC 9170, DOI 10.17487/RFC9170,
              December 2021, <https://www.rfc-editor.org/rfc/rfc9170>.

   [RFC9523]  Rozen-Schiff, N., Dolev, D., Mizrahi, T., and M. Schapira,
              "A Secure Selection and Filtering Mechanism for the
              Network Time Protocol with Khronos", RFC 9523,
              DOI 10.17487/RFC9523, February 2024,
              <https://www.rfc-editor.org/rfc/rfc9523>.

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   [RFC9844]  Carpenter, B. and R. Hinden, "Entering IPv6 Zone
              Identifiers in User Interfaces", RFC 9844,
              DOI 10.17487/RFC9844, August 2025,
              <https://www.rfc-editor.org/rfc/rfc9844>.

Acknowledgments

   Aanchal Malhotra and Adam Langley authored early drafts of this
   document.  Daniel Franke, Sarah Grant, Erik Kline, Martin Langer, Ben
   Laurie, Peter Löthberg, Michael McCourt, Hal Murray, Tal Mizrahi,
   Ruben Nijveld, Christopher Patton, Thomas Peterson, Rich Salz, Dieter
   Sibold, Ragnar Sundblad, Kristof Teichel, Luke Valenta, David
   Venhoek, Ulrich Windl, and the other members of the NTP working group
   contributed comments and suggestions as well as pointed out errors.
   We also acknowledge the helpful comments and suggestions provided by
   the last call reviewers and members of the IESG.

Appendix A.  Example Server List

   This appendix presents an example Roughtime server list in the format
   described by Section 8.3.

   NOTE TO RFC EDITOR: replace all occurrences of the port number 2002
   below with the port number assigned for Roughtime by IANA.

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   {
     "servers": [
       {
         "name": "example.com Roughtime server",
         "version": 1,
         "publicKeyType": "ed25519",
         "publicKey": "2O3mkkheDExCuhG+ZNIoWmO/IdCdLzADgUn8SnC4hME=",
         "addresses": [
           {
             "protocol": "udp",
             "address": "roughtime.example.com:2002"
           },
           {
             "protocol": "tcp",
             "address": "roughtime.example.com:2002"
           }
         ]
       },
       {
         "name": "A UDP-only server specified with IP addresses",
         "version": 1,
         "publicKeyType": "ed25519",
         "publicKey": "ZYfeGa94YuG1IZrV3kR9+8/nmZ2lX2XyHmiSb+wI0OY=",
         "addresses": [
           {
             "protocol": "udp",
             "address": "192.0.2.33:2002"
           },
           {
             "protocol": "udp",
             "address": "[2001:db8::2:33]:2002"
           }
         ]
       }
     ],
     "sources": [
       "https://www.example.net/roughtime/ecosystem.json",
       "https://www.example.org/roughtime/ecosystem.json"
     ],
     "reports":  "https://www.example.net/roughtime/malfeasance"
   }

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Appendix B.  Example Malfeasance Report

   This appendix presents an example Roughtime malfeasance report in the
   format described by Section 8.4.1.  The report provides sufficient
   information to prove that the server with the public key
   lRhHag6fn2wZQ6idy10ChgpRgks3gvdMM2hWNeJNgXg= responded with a time
   that is inconsistent with the times reported by the two other
   servers.

  {
    "responses": [
      {
        "publicKey": "FnDyLV/68ephhLdFJbdEGCdkVvpXDaVe5PYvRDdlOOY=",
        "request": "Uk9VR0hUSU0ABAAABQAAAAQAAAAkAAAARAAAAEgAAABWRVIAU1JW
  AE5PTkNUWVBFWlpaWgEAAACf4gKLPdPfiNTv93lrhNqYgyehDgMyHFmA1BrAhM1QEDBh9l
  BlN6LUye6zghiqSWMwyNm0IucxQxW3zTMrwj4dAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  ==",
        "response": "Uk9VR0hUSU2UAQAABwAAAEAAAABgAAAAZAAAAGQAAADAAAAAWAE
  AAFNJRwBOT05DVFlQRVBBVEhTUkVQQ0VSVElORFhBWL64CToGs4v/4UtfN/80HLFiA09vG
  IDRP/zTjcTj8/1DlZWCsVja6RlfwaYnc1wfJqThfhcuSDonrTGyKngBMGH2UGU3otTJ7rO
  CGKpJYzDI2bQi5zFDFbfNMyvCPh0BAAAABQAAAAQAAAAIAAAAEAAAABQAAABWRVIAUkFES
  U1JRFBWRVJTUk9PVAEAAAADAAAAQ0u4aQAAAAABAAAAc86AWYB/O3Kxzsx4d5P5cbSOftJ
  UA8bWVtVrQ3tc+b0CAAAAQAAAAFNJRwBERUxFI2B5tbj5ePjVKYE0PAL1NmgZOAsqh/E2f
  rom9Ol5BAnVcLje0C6exbXY8hE3dRvYV01Alru8Ocle+jOZT5r8AwMAAAAgAAAAKAAAAFB
  VQktNSU5UTUFYVKqljhhqi4A54vW20e+slwViPyxybNnqKXzimIiIUHQMaBCvaQAAAADYy
  d9pAAAAAAAAAAA="
      },
      {
        "publicKey": "l9cdSuR8dFxtG9aJo9pWzUXaX8pftNG4UDC45Qk3znc=",
        "rand": "v/DirVBRQLGtictYD7mN3px02UlMT4J3haTRomt1NNM=",
        "request": "Uk9VR0hUSU0ABAAABQAAAAQAAAAkAAAARAAAAEgAAABWRVIAU1JW

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  AE5PTkNUWVBFWlpaWgEAAABFCvadUKPN/U9OXqFalVFnv/EhsAuPPpBYfo5MDULZKv335k
  bNTZcsYLM96o77yLbvvRk1fm2TQB4yKxbP1jzeAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  ==",
        "response": "Uk9VR0hUSU2UAQAABwAAAEAAAABgAAAAZAAAAGQAAADAAAAAWAE
  AAFNJRwBOT05DVFlQRVBBVEhTUkVQQ0VSVElORFj+2I02cCBAfDYzP+8znW6bICqVrAF23
  xNLfM+Qycmkpfp1+BQSb4l/6mRll66l2VIfPSQzigl2V5OJgQzGEBcG/ffmRs1Nlyxgsz3
  qjvvItu+9GTV+bZNAHjIrFs/WPN4BAAAABQAAAAQAAAAIAAAAEAAAABQAAABWRVIAUkFES
  U1JRFBWRVJTUk9PVAEAAAADAAAAw/m2aQAAAAABAAAAS8ROJoXIPSlO9yN+uREb+/UiOJJ
  qCjx3VWZ/vD2FqBMCAAAAQAAAAFNJRwBERUxFw0MFQMiDoHySot8rlnV83Vqaa3qTrAY15
  W1TzqNuAurOvreNw08BfUwxF7BQ/b/JCwCQcqtD5uRYsvikvuyLCwMAAAAgAAAAKAAAAFB
  VQktNSU5UTUFYVCQs/eCxtWjVrXyse0SeojyZdNFkOwe3B3nLHtHJZK/9gRCvaQAAAADxy
  d9pAAAAAAAAAAA="
      },
      {
        "publicKey": "lRhHag6fn2wZQ6idy10ChgpRgks3gvdMM2hWNeJNgXg=",
        "rand": "lMvMVoLsakxc5ZmMzEFQ8hh1FaDo2gCXXIX/L4QPSxQ=",
        "request": "Uk9VR0hUSU0ABAAABQAAAAQAAAAkAAAARAAAAEgAAABWRVIAU1JW
  AE5PTkNUWVBFWlpaWgEAAADmAab6GlTSr3xRmwwwvRjW6EKRhtyWYYWZHZIGnF8w2b4DuI
  ICvoCWTwCaupBfcOEwV+RqBWB6L1JUGn+ot8+9AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

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  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
  ==",
        "response": "Uk9VR0hUSU2UAQAABwAAAEAAAABgAAAAZAAAAGQAAADAAAAAWAE
  AAFNJRwBOT05DVFlQRVBBVEhTUkVQQ0VSVElORFg8MlhdCJeQ2qzN6b9q646W9kB+XmAdS
  g7ToU1gj3AD8AP8eCHfduMBE0E8j4/LqWIr72zWQx8Y1U/1uxs97yMAvgO4ggK+gJZPAJq
  6kF9w4TBX5GoFYHovUlQaf6i3z70BAAAABQAAAAQAAAAIAAAAEAAAABQAAABWRVIAUkFES
  U1JRFBWRVJTUk9PVAEAAAADAAAAw/m2aQAAAAABAAAA6ho4+0Cml+VJbqU7hsF717uusV2
  HYvRbU9CdjJE/Zn0CAAAAQAAAAFNJRwBERUxFM9Fvq8T9kOrxcS7jviCPHe44HX/75Je1h
  afPJ0f8NoASy29EgD7C0c/LMxXiXxEwyuxYTPN9oseAr9XIt68uDwMAAAAgAAAAKAAAAFB
  VQktNSU5UTUFYVMxBDiNG247IO4onsGcjFHsA3vP+arl+s0lBLhXw0c1RlBCvaQAAAAAEy
  t9pAAAAAAAAAAA="
      }
    ]
  }

Authors' Addresses

   Watson Ladd
   Akamai Technologies
   Email: watsonbladd@gmail.com

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

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