Key Exchange Method Updates for Secure Shell (SSH)

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Document Type Active Internet-Draft (curdle WG)
Author Mark Baushke 
Last updated 2016-03-08
Replaces draft-baushke-ssh-dh-group-sha2
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Internet Engineering Task Force                               M. Baushke
Internet-Draft                                    Juniper Networks, Inc.
Updates: 4253, 4419, 4432, 4462, 5656                      March 7, 2016
         (if approved)
Intended status: Standards Track
Expires: September 8, 2016

           Key Exchange Method Updates for Secure Shell (SSH)


   This document deprecates some previously specified Key Exchange
   Method algorithm names as well as defining a few added Modular
   Exponential (MODP) Groups for the Secure Shell (SSH) protocol.  It
   also updates [RFC4253], [RFC4419], [RFC4462], and [RFC5656] by
   specifying the set key exchange algorithms that currently exist and
   which ones MUST, SHOULD, MAY, and SHOULD NOT be implemented.  New key
   exchange methods use the SHA-2 family of hashes.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on September 8, 2016.

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   Copyright (c) 2016 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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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
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   described in the Simplified BSD License.

1.  Overview and Rationale

   Secure Shell (SSH) is a common protocol for secure communication on
   the Internet.  In [RFC4253], SSH originally defined the Key Exchange
   Method Name diffie-hellman-group1-sha1 which used [RFC2409] Oakley
   Group 1 (a MODP group with 768 bits) and SHA-1 [RFC3174].  Due to
   recent security concerns with SHA-1 [RFC6194] and with MODP groups
   with less than 2048 bits [NIST-SP-800-131Ar1] implementer and users
   request support for larger MODP group sizes with data integrity
   verification using the SHA-2 family of secure hash algorithms as well
   as MODP groups providing more security.

   The United States Information Assurance Directorate at the National
   Security Agency has published a FAQ [MFQ-U-OO-815099-15] suggesting
   that the use of ECDH using the nistp256 curve and SHA-2 based hashes
   less than SHA2-384 are no longer sufficient for transport of Top
   Secret information.  It is for this reason that this draft moves
   ecdh-sha2-nistp256 from a REQUIRED to OPTIONAL as a key exchange
   method.  This is the same reason that the stronger MODP groups being
   introduced are using SHA2-512 as the hash algorithm.  Group14 is
   already present in most SSH implementations and most implementations
   already have a SHA2-256 implementation, so diffie-hellman-
   group14-sha256 is provided as an easy to implement and faster to use
   key exchange for small embedded applications.

   It has been observed in [safe-curves] that the NIST recommended
   Elliptic Curve Prime Curves (P-256, P-384, and P-521) are perhaps not
   the best available for Elliptic Curve Cryptography Security.  For
   this reason, none of the [RFC5656] curves are marked as a MUST
   implement.  However, the requirement that "every compliant SSH ECC
   implementation MUST implement ECDH key exchange" is now taken to mean
   that if ecdsa-sha2-[identifier] is implemented, then ecdh-
   sha2-[identifier] MUST be implemented.

   Please send comments on this draft to

2.  Requirements Language

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

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3.  Key Exchange Algorithms

   This memo adopts the style and conventions of [RFC4253] in specifying
   how the use of new data key exchange is indicated in SSH.
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