Key Exchange (KEX) Method Updates and Recommendations for Secure Shell (SSH)

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Document Type Active Internet-Draft (curdle WG)
Author Mark Baushke 
Last updated 2017-03-27
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                 September 20, 2016
         (if approved)
Intended status: Standards Track
Expires: March 24, 2017

 Key Exchange (KEX) Method Updates and Recommendations for Secure Shell


   This document adds recommendations for adoption of ssh-curves from
   the [I-D.ietf-curdle-ssh-curves] and new-modp from the
   [I-D.ietf-curdle-ssh-modp-dh-sha2], and deprecates some previously
   specified Key Exchange Method algorithm names 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

Status of This Memo

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   This Internet-Draft will expire on March 24, 2017.

Copyright Notice

   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
   Provisions Relating to IETF Documents

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Internet-Draft KEX Method Updates/Recommendations for SSH September 2016

   ( in effect on the date of
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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 2 (a 1024-bit MODP group) 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 (IAD) at the
   National Security Agency (NSA) has published a FAQ
   [MFQ-U-OO-815099-15] suggesting that the use of Elliptic Curve
   Diffie-Hellman (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
   adopted.  As the MODP 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.  Small embedded
   applications may find this KEX desirable to use.

   The NSA Information Assurance Directorate (IAD) has also published
   the Commercial National Security Algorithm Suite (CNSA Suite)
   [CNSA-SUITE] in which the 3072-bit MODP Group 15 in RFC 3526 is
   explicitly mentioned as the minimum modulus to protect Top Secret

   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 (ECC) 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.

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