Extension Negotiation in the Secure Shell (SSH) Protocol
RFC 8308

Document Type RFC - Proposed Standard (March 2018; No errata)
Last updated 2018-03-19
Replaces draft-ssh-ext-info
Stream IETF
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Stream WG state Submitted to IESG for Publication
Document shepherd Daniel Migault
Shepherd write-up Show (last changed 2017-06-01)
IESG IESG state RFC 8308 (Proposed Standard)
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Responsible AD Eric Rescorla
Send notices to Daniel Migault <daniel.migault@ericsson.com>
IANA IANA review state Version Changed - Review Needed
IANA action state RFC-Ed-Ack
Internet Engineering Task Force (IETF)                          D. Bider
Request for Comments: 8308                               Bitvise Limited
Updates: 4251, 4252, 4253, 4254                               March 2018
Category: Standards Track
ISSN: 2070-1721

        Extension Negotiation in the Secure Shell (SSH) Protocol

Abstract

   This memo updates RFCs 4251, 4252, 4253, and 4254 by defining a
   mechanism for Secure Shell (SSH) clients and servers to exchange
   information about supported protocol extensions confidentially after
   SSH key exchange.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8308.

Copyright Notice

   Copyright (c) 2018 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 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                        Standards Track                    [Page 1]
RFC 8308              Extension Negotiation in SSH            March 2018

Table of Contents

   1. Overview and Rationale ..........................................3
      1.1. Requirements Terminology ...................................3
      1.2. Wire Encoding Terminology ..................................3
   2. Extension Negotiation Mechanism .................................3
      2.1. Signaling of Extension Negotiation in SSH_MSG_KEXINIT ......3
      2.2. Enabling Criteria ..........................................4
      2.3. SSH_MSG_EXT_INFO Message ...................................4
      2.4. Message Order ..............................................5
      2.5. Interpretation of Extension Names and Values ...............6
   3. Initially Defined Extensions ....................................6
      3.1. "server-sig-algs" ..........................................6
      3.2. "delay-compression" ........................................7
           3.2.1. Awkwardly Timed Key Re-Exchange .....................9
           3.2.2. Subsequent Re-Exchange ..............................9
           3.2.3. Compatibility Note: OpenSSH up to Version 7.5 .......9
      3.3. "no-flow-control" .........................................10
           3.3.1. Prior "No Flow Control" Practice ...................10
      3.4. "elevation" ...............................................11
   4. IANA Considerations ............................................12
      4.1. Additions to Existing Registries ..........................12
      4.2. New Registry: Extension Names .............................12
           4.2.1. Future Assignments to Extension Names Registry .....12
   5. Security Considerations ........................................12
   6. References .....................................................13
      6.1. Normative References ......................................13
      6.2. Informative References ....................................13
   Acknowledgments ...................................................14
   Author's Address ..................................................14

Bider                        Standards Track                    [Page 2]
RFC 8308              Extension Negotiation in SSH            March 2018

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
   the process of updating SSH to support RSA signatures using SHA-256
   and SHA-512 [RFC8332].  To avoid trial and error as well as
   authentication penalties, a client must be able to discover public
   key algorithms a server accepts.  This extension mechanism permits
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