Internet-Draft                                                  D. Bider
Expires: June 17, 2016                                   Bitvise Limited
                                                       December 17, 2015

               Extension Negotiation in Secure Shell (SSH)


  This memo defines a mechanism for SSH clients and servers to exchange
  information about supported protocol extensions confidentially after
  completed key exchange.


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

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1.  Overview and Rationale

  Secure Shell (SSH) is a common protocol for secure communication on
  the Internet. The original design of the SSH transport layer [RFC4253]
  lacks proper extension negotiation. Meanwhile, diverse implementations
  take steps to ensure that known message types contain no unrecognized
  information. This makes it difficult for implementations to signal
  capabilities and negotiate extensions without risking disconnection.

  This obstacle has been recognized in relationship with [SSH-RSA-SHA2],
  where the need arises for a client to efficiently discover signature
  algorithms a server accepts, to avoid round-trips of trial and error.

1.1.  Requirements Terminology

  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
  document are to be interpreted as described in [RFC2119].

2.  Extension Negotiation Mechanism

2.1.  Signaling of Extension Negotiation in KEXINIT

  Applications implementing this mechanism MUST add to the field
  "kex_algorithms", in their KEXINIT packet sent for the first key
  exchange, one of the following indicator names:

  - When acting as server: "ext-info-s"
  - When acting as client: "ext-info-c"

  The indicator name is added without quotes, and MAY be added at any
  position in the name-list, subject to proper separation from other
  names as per name-list conventions.

  The names are added to the "kex_algorithms" field because this is one
  of two name-list fields in KEXINIT that do not have a separate copy
  for each data direction.

  The indicator names inserted by the client and server are different to
  ensure that these names will not produce a match, and will be neutral
  with respect to key exchange algorithm negotiation.

  The inclusion of textual indicator names is intended to provide a clue
  for implementers to discover this mechanism.

2.2.  Enabling Criteria

  If a client or server offers "ext-info-c" or "ext-info-s"
  respectively, it must be prepared to accept a SSH_MSG_EXT_INFO message
  from the peer.

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  Thus a server only needs to send "ext-info-s" if it intends to process
  SSH_MSG_EXT_INFO from the client.

  If a server receives an "ext-info-c", it MAY send an SSH_MSG_EXT_INFO
  message, but is not required to do so.

  If a SSH_MSG_EXT_INFO message is sent, then it MUST be the first
  message after the initial SSH_MSG_NEWKEYS.

  Implementations MUST NOT send an incorrect indicator name for their
  role. Implementations MAY disconnect if the counter-party sends an
  incorrect indicator. If "ext-info-c" or "ext-info-s" ends up being
  negotiated as a key exchange method, the parties MUST disconnect.

2.3.  SSH_MSG_EXT_INFO Message

 A party that received the "ext-info-c" or "ext-info-s" indicator
 can send the the following message:

    byte       SSH_MSG_EXT_INFO (value 7)
    uint32     nr-extensions
    repeat "nr-extensions" times:
      string   extension-name
      string   extension-value

  This message is sent without delay, and immediately after

2.4.  Server's Secondary SSH_MSG_EXT_INFO

  If the client sent "ext-info-c", the server MAY send, but is not
  obligated to send, an SSH_MSG_EXT_INFO message immediately before
  SSH_MSG_USERAUTH_SUCCESS, as defined in [RFC4252]. The server MAY send
  this message whether or not it sent EXT_INFO after SSH_MSG_NEWKEYS.

  This allows a server to reveal support for additional extensions that
  it was unwilling to reveal to an unauthenticated client. If a server
  sends a subsequent SSH_MSG_EXT_INFO, this replaces any initial one,
  and both the client and the server re-evaluate extensions in effect.
  The server's last EXT_INFO is matched against the client's original.

2.5.  Interpretation of Extension Names and Values

  Each extension is identified by its extension-name, and defines the
  conditions under which the extension is considered to be in effect.
  Applications MUST ignore unrecognized extension-names.

  In general, if an extension requires both the client and the server
  to include it in order for the extension to take effect, the relative
  position of the extension-name in each EXT_INFO message is irrelevant.

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  Extension-value fields are interpreted as defined by their respective
  extension. An extension-value field MAY be empty if so permitted by
  the extension. Applications that do not implement or recognize a
  particular extension MUST ignore the associated extension-value field,
  regardless of its size or content.

  The cumulative size of an SSH_MSG_EXT_INFO message is limited only by
  the maximum packet length that an implementation may apply in
  accordance with [RFC4253]. Implementations MUST accept well-formed
  SSH_MSG_EXT_INFO messages up to the maximum packet length they accept.

3. Initially Defined Extensions

3.1. "server-sig-algs"

  This extension is sent with the following extension name and value:

    string      "server-sig-algs"
    name-list   signature-algorithms-accepted

  Note that the name-list type is a strict subset of the string type,
  and is thus permissible as an extension-value.

  This extension is sent by the server only, and contains a list of
  signature algorithms that the server is able to process as part of a
  "publickey" request.

  A client that wishes to proceed with public key authentication MAY
  wait for the server's SSH_MSG_EXT_INFO so it can send a "publickey"
  authentication request with an appropriate signature algorithm, rather
  than resorting to trial and error.

  Servers that implement public key authentication SHOULD implement this

  If a server does not send this extension, a client SHALL NOT make any
  assumptions about the server's signature algorithm support, and MAY
  proceed with authentication request trial and error.

3.2.  "no-flow-control"

  This extension is sent with the following extension name and value:

    string      "no-flow-control"
    string      (empty)

  This extension MUST be sent by both parties in order to take effect.

  If included by both parties, the effect of this extension is that the
  "initial window size" fields in the messages SSH_MSG_CHANNEL_OPEN and

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  SSH_MSG_CHANNEL_OPEN_CONFIRMATION, as defined in [RFC4254], become
  meaningless. The values of these fields MUST be ignored, and a channel
  behaves as if the window size in either direction is infinite. Neither
  side is required to send any SSH_MSG_CHANNEL_WINDOW_ADJUST messages,
  and if received, such messages MUST be ignored.

  This extension is intended, but not limited to, use by file transfer
  applications that are only going to use one channel, and for which the
  flow control provided by SSH is an impediment, rather than a feature.

  Implementations MUST refuse to open more than one simultaneous channel
  when this extension is in effect. Nevertheless, server implementations
  SHOULD support clients opening more than one non-simultaneous channel.

3.3.  "accept-channels"

  This extension is sent with the following extension name and value:

    string      "accept-channels"
    name-list   channel-types-accepted

  An implementation MAY use this extension to signal to the other party
  a list of channel types it might accept. A server that adapts the list
  of available channel types based on authentication MAY defer sending
  this extension until a subsequent EXT_INFO, just before sending the

  An implementation is not obligated to unconditionally accept open
  requests for channel types advertised in this extension. An open
  request for a listed channel type MAY still fail for another reason.

3.4.  "elevation"

  This extension MAY be sent by the client as follows:

    string      "elevation"
    string      choice of: "y" | "n" | "d"

  A client sends "y" to indicate its preference that the session should
  be elevated; "n" to not be elevated; and "d" for the server to use its
  default behavior. If a client does not send the "elevation" extension,
  the server SHOULD act as if "d" was sent.

  If a client has included this extension, then after authentication, a
  server that supports this extension SHOULD indicate to the client
  whether elevation was done by sending the following global request:

    byte        SSH_MSG_GLOBAL_REQUEST
    string      "elevation"
    boolean     want reply = false
    boolean     elevation performed

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3.5.  "delay-compression"

  This extension MAY be sent by both parties as follows:

    string         "delay-compression"
      name-list    compression_algorithms_client_to_server
      name-list    compression_algorithms_server_to_client

  This extension allows the server and client to renegotiate compression
  algorithm support without having to conduct a key re-exchange, putting
  new algorithms into effect immediately upon successful authentication.

  This extension takes effect only if both parties send it. Name-lists
  MAY include any compression algorithm that could have been negotiated
  in SSH_MSG_KEXINIT, except algorithms that define their own delayed
  compression semantics. This means "zlib,none" is a valid algorithm
  list in this context; but "" is not.

  If both parties send this extension, but the name-lists do not contain
  a common algorithm in either direction, the parties MUST disconnect in
  the same way as if negotiation failed as part of SSH_MSG_KEXINIT.

  If this extension takes effect, the renegotiated compression algorithm
  is used as follows:

  - By the server, starting with the very next SSH message after

  - By the client, after sending SSH_MSG_NEWCOMPRESS. If this extension
    takes effect, the client MUST send the following message immediately
    after receiving the server's SSH_MSG_USERAUTH_SUCCESS:

      byte       SSH_MSG_NEWCOMPRESS (value 8)

    The purpose of this message is to avoid a race condition where the
    server cannot reliably know whether a message sent by the client was
    sent before or after receiving the server's USERAUTH_SUCCESS.

  As with all extensions, the server may delay including this extension
  until its secondary SSH_MSG_EXT_INFO, sent before USERAUTH_SUCCESS.
  This allows the server to avoid advertising compression support until
  the client has been authenticated.

  In subsequent key re-exchange, the compression algorithms negotiated
  in re-exchange override the algorithms negotiated with this extension.

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4.  IANA Considerations

4.1.  Additions to existing tables

  IANA is requested to insert the following entries into the table
  Message Numbers under Secure Shell (SSH) Protocol Parameters

    Value    Message ID             Reference
    7        SSH_MSG_EXT_INFO       [this document]
    8        SSH_MSG_NEWCOMPRESS    [this document]

  IANA is requested to insert the following entries into the table Key
  Exchange Method Names:

    Method Name     Reference          Note
    ext-info-s      [this document]    Section 2.2
    ext-info-c      [this document]    Section 2.2

4.2.  New table: Extension Names

  Also under Secure Shell (SSH) Protocol Parameters, IANA is requested
  to create a new table, Extension Names, with initial content:

    Extension Name       Reference          Note
    server-sig-algs      [this document]    Section 3.1
    no-flow-control      [this document]    Section 3.2
    accept-channels      [this document]    Section 3.3
    elevation            [this document]    Section 3.4
    delay-compression    [this document]    Section 3.5

4.2.1.  Future Assignments to Extension Names

  Names in the Extension Names table MUST follow the Conventions for
  Names defined in [RFC4250], Section 4.6.1.

  Requests for assignments of new non-local names in the Extension Names
  table (i.e. names not including the '@' character) MUST be done
  through the IETF CONSENSUS method, as described in [RFC5226].

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6.  References

6.1.  Normative References

  [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

  [RFC4250]   Lehtinen, S. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Protocol Assigned Numbers", RFC 4250, January 2006.

  [RFC4252]   Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Authentication Protocol", RFC 4252, January 2006.

  [RFC4253]   Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Transport Layer Protocol", RFC 4253, January 2006.

  [RFC4254]   Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Connection Protocol", RFC 4254, January 2006.

  [RFC5226]   Narten, T. and Alvestrand, H., "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

6.2.  Informative References

              Bider, D., "Use of RSA Keys with SHA-2 256 and 512 in
              Secure Shell (SSH)", draft-rsa-dsa-sha2-256-02,
              November 2015,

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Author's Address

  Denis Bider
  Bitvise Limited
  Suites 41/42, Victoria House
  26 Main Street

  Phone: +506 8315 6519


  Thanks to Markus Friedl and Damien Miller for comments and initial

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