Extension Negotiation in Secure Shell (SSH)
draft-ietf-curdle-ssh-ext-info-02

Document Type Active Internet-Draft (curdle WG)
Last updated 2017-03-27 (latest revision 2017-02-27)
Replaces draft-ssh-ext-info
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Internet-Draft                                                  D. Bider
Updates: 4252, 4253, 4254 (if approved)                  Bitvise Limited
Intended status: Standards Track                       February 27, 2017
Expires: August 27, 2017

               Extension Negotiation in Secure Shell (SSH)
                  draft-ietf-curdle-ssh-ext-info-02.txt

Abstract

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

Status

  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
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Copyright

  Copyright (c) 2017 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
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  described in the Simplified BSD License.

Bider                                                           [Page 1]
Internet-Draft        Extension Negotiation in SSH         February 2017

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 discover signature algorithms a
  server accepts, to avoid authentication penalties and trial-and-error.

1.1.  Requirements Terminology

  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
  "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
  document are to be interpreted as described in [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 an SSH_MSG_EXT_INFO
  message from the peer.

Bider                                                           [Page 2]
Internet-Draft        Extension Negotiation in SSH         February 2017

  
  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
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