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Extension Negotiation in Secure Shell (SSH)

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This is an older version of an Internet-Draft whose latest revision state is "Replaced".
Author denis bider
Last updated 2015-11-07
Replaced by draft-ietf-curdle-ssh-ext-info, draft-ietf-curdle-ssh-ext-info
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Internet-Draft                                                  D. Bider
Expires: May 7, 2016                                     Bitvise Limited
                                                        November 7, 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 is done while optimizing protocol startup
  to eliminate the round-trip the original protocol requires on messages


  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), its areas, and its working groups.  Note that other
  groups may also distribute working documents as Internet-Drafts.

  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."

  The list of current Internet-Drafts can be accessed at
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  Copyright (c) 2015 IETF Trust and the persons identified as the
  document authors.  All rights reserved.

  This document is subject to BCP 78 and the IETF Trust's Legal
  Provisions Relating to IETF Documents
  ( 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.

Bider                                                           [Page 1]
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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.
  The design of SSH as-is also spends an unnecessary roundtrip on the
  client sending an SSH_MSG_SERVICE_REQUEST and awaiting an
  SSH_MSG_SERVICE_ACCEPT in reply. This was designed envisioning a
  possible SSH service other than "ssh-userauth"; but a decade later,
  there is no such service. (If it were to arise, negotiation for such
  a service can also be supported using the mechanism in this document.)
  This memo describes an extension negotiation mechanism that replaces
  SSH_MSG_SERVICE_REQUEST and ACCEPT, and meets the above needs.

1.1.  Requirements Terminology

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

2.  Discussion of Extension Options

  The SSH protocol contains multiple potential avenues for extension:

a)  By adding fields to existing packets.

  If existing implementations permitted, it would have been possible to
  add additional fields to the currently not-most-useful SERVICE_REQUEST
  and SERVICE_ACCEPT messages. Such additional fields could serve for
  extension negotiation.
  However, a number of implementations, including OpenSSH, have chosen a
  restrictive view of packet formats, and will disconnect if additional
  fields are included in known packet types.
b)  By designating a use for the "reserved" field in SSH_MSG_KEXINIT.
  [RFC4253] defines the following field at the end of KEXINIT:
      uint32       0 (reserved for future extension)  

  A logical interpretation of this definition would be that applications
  must send this field as zero, but must accept values other than zero.
  Otherwise, this field could not be useful for "future extension".

Bider                                                           [Page 2]
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  Unfortunately, current non-use of this field makes it likely that
  there will be implementations that do not handle unexpected values
  correctly. This is the case with current latest versions of libssh,
  which do not decode this field when it is received, but instead assume
  it is zero without reading it. Key exchange then fails if the field is
  non-zero, because the parties are hashing different KEXINIT values.
c)  By incrementing the protocol version number.
  [RFC4253] includes the following statement:
    Since the protocol being defined in this set of documents is
    version 2.0, the 'protoversion' MUST be "2.0".
  The RFC then goes on to discuss 1.xx compatibility cases when the
  version does not have to be "2.0", after all.
  The author has investigated the source code of several implementations
  (OpenSSH 7.1; PuTTY 0.64; libssh 0.7.2; the author's own Bitvise SSH)
  and has not found evidence that any of these implementations would
  reject a counterparty that advertises a protocol version number
  incremented from "2.0" to e.g. "2.1".
  However, the author cannot prove there are not other, more restrictive
  implementations. Applications that could have trouble with a changed
  protocol version may include not only SSH clients and servers, but
  also applications that interact with SSH, including security scanners;
  load balancers; or deep packet inspection firewalls.
  The author perceives that the protocol version number is too central
  and fundamental to be incremented for a change that does not affect
  security, and does not involve a change in KEXINIT itself.

d) By defining special names to be used with algorithm negotiation.

  This is the only extension method known to the author which appears to
  have guaranteed compatibility with existing applications; without side
  effects; and has the added bonus of providing clues to implementers
  about what to search for, to learn about the extension.
  The author has chosen this method for the mechanism described herein.

Bider                                                           [Page 3]
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3.  Extension Negotiation Mechanism
3.1.  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 suggested position is last in
  the list, but implementations MUST recognize these indicator names
  when included at any position.
  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; "server_host_key_algorithms" being the other.
  Of the two fields, "server_host_key_algorithms" has special cases for
  negotiation that the "kex_algorithms" field does not. This makes
  "kex_algorithms" a more dependable candidate for this mechanism.
  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.
3.2.  Mechanism Enabling Criteria
  The mechanism described in this memo is enabled when all of the
  following conditions are true:
  - The server and client have completed one or more key exchanges.
  - The KEXINIT packet sent by the server during the LAST key exchange
    included the indicator name "ext-info-s" at any position in the
    "kex_algorithms" field (subject to name-list conventions).
  - The KEXINIT packet sent by the client during the SAME key exchange
    included the indicator name "ext-info-c" at any position in the
    "kex_algorithms" field (subject to name-list conventions).
  Once the mechanism has been enabled, it remains enabled for the rest
  of the SSH session. Once enabled, both parties MUST continue to send
  their respective indicator name in subsequent key exchanges. However,
  both parties MUST ignore the presence or absence of these indicator
  names in subsequent key exchanges once the mechanism has been enabled.

Bider                                                           [Page 4]
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  A server MUST include its indicator name in the first key exchange.
  A client MAY omit its indicator name in the first key exchange. After
  this key exchange, the client MAY immediately initiate a subsequent
  key exchange, in which it includes its indicator. If the server also
  includes its indicator in this subsequent exchange, the mechanism is
  enabled starting from completion of that key exchange.
  Implementations MUST NOT send an incorrect indicator name for their
  role. Implementations MAY disconnect if the counter-party does so.
  If an indicator name ("ext-info-c" or "ext-info-s") ends up being
  negotiated as a key exchange method, the parties MUST disconnect.
3.3.  Actions after Mechanism is Enabled
  If a party has already sent SERVICE_REQUEST or SERVICE_ACCEPT at the
  time this mechanism has become enabled, the state of the mechanism
  is undefined, and the party MUST disconnect.
  Otherwise, immediately after sending SSH_MSG_NEWKEYS for the key 
  exchange in which the mechanism was first enabled, each party sends
  the following message, replacing SERVICE_REQUEST and SERVICE_ACCEPT:
    byte       SSH_MSG_EXT_INFO (value 7)
    uint32     nr-extensions
    repeat "nr-extensions" times:
      string   extension-name
      string   extension-value
  Each party sends this message without delay, and immediately after
  SSH_MSG_NEWKEYS; but allowing for any intervening SSH_MSG_IGNORE or
  DEBUG, which MUST be either handled appropriately or discarded. This
  EXT_INFO message is sent by the client at most once in an SSH session,
  and by the server one or more times before SSH_MSG_USERAUTH_SUCCESS.
  The SSH_MSG_EXT_INFO message replaces both the SERVICE_REQUEST and
  SERVICE_ACCEPT messages that might have otherwise been sent by either
  party. Enabling this mechanism implies the same effects as if the
  client had sent a SERVICE_REQUEST for the service "ssh-userauth", and
  as if the server had responded with SERVICE_ACCEPT.
  Since SSH_MSG_EXT_INFO is sent immediately by both parties, and
  client MAY follow immediately with SSH_MSG_USERAUTH_REQUEST; or it MAY
  wait for the server's SSH_MSG_EXT_INFO before starting authentication.
3.4.  Server's Secondary SSH_MSG_EXT_INFO

  The client sends its SSH_MSG_EXT_INFO only once per session,
  immediately after the key exchange that enabled this mechanism.

Bider                                                           [Page 5]
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  The server MUST send its first SSH_MSG_EXT_INFO at the same logical
  time as the client, immediately after the key exchange that first
  enabled this mechanism.
  In addition, the server MAY send, but is not obligated to send, one or
  more additional SSH_MSG_EXT_INFO messages at any time before sending
  the message SSH_MSG_USERAUTH_SUCCESS, as defined in [RFC4252].
  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 previous ones,
  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.
  A server MUST NOT send another SSH_MSG_EXT_INFO message after sending
3.5.  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.

  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.

Bider                                                           [Page 6]
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4. Initially Defined Extensions

4.1. "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" authentication 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.
  If a server sends this extension with an empty algorithm list, the
  client SHOULD take this to mean that the server will not accept any
  public key authentication request; even if the "publickey" method is
  included by the server in the "authentications that can continue"
  field in the message SSH_MSG_USERAUTH_FAILURE.
  Note that the server's list of accepted signature algorithms MAY
  change in subsequent EXT_INFO messages sent before the end of user

4.2.  "client-req-ok"

  This extension is sent with the following extension name and value:
    string      "client-req-ok"
    string      (empty)

  This extension is sent by the client only, and indicates that the
  client implementation will gracefully handle a global request;
  especially an unrecognized global request.

Bider                                                           [Page 7]
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  All clients SHOULD send this extension, and SHOULD gracefully handle
  unrecognized global requests. This allows the server to use an active
  keep-alive strategy whereby a global request is periodically sent to
  the client, to which the client is expected to reply to indicate the
  connection is active.
  In the absence of this extension, a server cannot assume that the
  client won't disconnect when receiving a global request, since there
  are existing deployed clients that do so.

4.3.  "no-handbrake"

  This extension is sent with the following extension name and value:
    string      "no-handbrake"
    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
  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 MAY refuse to open more than one simultaneous channel
  when this extension is in effect. However, server implementations
  SHOULD support clients opening more than one consecutive channel.

Bider                                                           [Page 8]
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5.  IANA Considerations

5.1.  Additions to existing tables

  IANA is requested to insert the following entry into the table Message
  Numbers under Secure Shell (SSH) Protocol Parameters [RFC4250]:
    Value    Message ID          Reference
    7        SSH_MSG_EXT_INFO    [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

5.2.  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
    client-req-ok      [this document]    Section 3.2
    no-handbrake       [this document]    Section 3.3

5.2.1.  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].

Bider                                                           [Page 9]
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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,

Bider                                                          [Page 10]
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Author's Address

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

  Phone: +506 8315 6519


Bider                                                          [Page 11]