Internet Engineering Task Force M. Baushke
Internet-Draft Juniper Networks, Inc.
Updates: 4253 (if approved) December 10, 2015
Intended status: Standards Track
Expires: June 12, 2016

More Modular Exponential (MODP) Diffie-Hellman Groups for SSH
draft-baushke-ssh-dh-group-sha2-01

Abstract

This document defines two added Modular Exponential (MODP) Groups for the Secure Shell (SSH) protocol. It also updates [RFC4253] by specifying new RECOMMENDED and new OPTIONAL Diffie-Hellman key exchange algorithms using SHA-2 hashes.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on June 12, 2016.

Copyright Notice

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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 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] implementors 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.

Please send comments on this draft to ietf-ssh@NetBSD.org.

2. Requirements Language

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].

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.

The following new key exchange algorithms are defined:

Key Exchange Method Name          Note
diffie-hellman-group1-sha1        NOT RECOMMENDED
diffie-hellman-group14-sha256     RECOMMENDED
diffie-hellman-group15-sha256     RECOMMENDED
diffie-hellman-group16-sha256     OPTIONAL
       

Figure 1

The SHA-2 family of secure hash algorithms are defined in [FIPS-180-4].

The method of key exchange used for the name "diffie-hellman-group14-sha256" is the same as that for "diffie-hellman-group14-sha1" escept that the SHA2-256 hash algorith is used.

The group15 and group16 names are the same as those specified in [RFC3526] as 3072-bit MODP Group 14 and 4096-bit MODP Group 15.

4. IANA Considerations

This document augments the Key Exchange Method Names in [RFC4253].

IANA is requested to update the SSH algorithm registry with the following entries:

Key Exchange Method Name       Reference   Note
diffie-hellman-group1-sha1     RFC4253     NOT RECOMMENDED
diffie-hellman-group14-sha256  This draft  RECOMMENDED
diffie-hellman-group15-sha256  This draft  RECOMMENDED
diffie-hellman-group16-sha256  This draft  OPTIONAL
       

Figure 2

It is RECOMMENDED that the new diffie-hellman-group14-sha256 method be proposed before the diffie-hellman-group14-sha1 method.

5. Security Considerations

The security considerations of [RFC4253] apply to this document.

The security considerations of [RFC3526] suggest that these MODP groups have security strengths given in this table.

Group modulus security strength estimates

+--------+----------+---------------------+---------------------+
| Group  | Modulus  | Strength Estimate 1 | Strength Estimate 2 |
|        |          +----------+----------+----------+----------+
|        |          |          | exponent |          | exponent |
|        |          | in bits  | size     | in bits  | size     |
+--------+----------+----------+----------+----------+----------+
|  14    | 2048-bit |      110 |     220- |      160 |     320- |
|  15    | 3072-bit |      130 |     260- |      210 |     420- |
|  16    | 4096-bit |      150 |     300- |      240 |     480- |
+--------+----------+---------------------+---------------------+
       

Figure 3

Many users seem to be interested in the perceived safety of using the SHA2-based algorithms for hashing.

6. References

6.1. Normative References

[FIPS-180-4] National Institute of Standards and Technology, "Secure Hash Standard (SHS)", FIPS PUB 180-4, August 2015.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC3526] Kivinen, T. and M. Kojo, "More Modular Exponential (MODP) Diffie-Hellman groups for Internet Key Exchange (IKE)", RFC 3526, DOI 10.17487/RFC3526, May 2003.
[RFC4253] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253, January 2006.

6.2. Informative References

[NIST-SP-800-131Ar1] Barker, and Roginsky, "Transitions: Recommendation for the Transitioning of the Use of Cryptographic Algorithms and Key Lengths", NIST Special Publication 800-131A Revision 1, November 2015.
[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)", RFC 2409, DOI 10.17487/RFC2409, November 1998.
[RFC3174] Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm 1 (SHA1)", RFC 3174, DOI 10.17487/RFC3174, September 2001.
[RFC6194] Polk, T., Chen, L., Turner, S. and P. Hoffman, "Security Considerations for the SHA-0 and SHA-1 Message-Digest Algorithms", RFC 6194, DOI 10.17487/RFC6194, March 2011.

Author's Address

Mark D. Baushke Juniper Networks, Inc. 1133 Innovation Way Sunnyvale, CA 94089-1228 US Phone: +1 408 745 2952 EMail: mdb@juniper.net URI: http://www.juniper.net/

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