Key Exchange (KEX) Method Updates and Recommendations for Secure Shell (SSH)
draft-ietf-curdle-ssh-kex-sha2-06

Abstract

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 hashes.

Status of This Memo

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 Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on September 28, 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 (http://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.

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 communications.

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.

Please send comments on this draft to curdle@ietf.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.

A new set of Elliptic Curve Diffie-Hellman ssh-curves exist. The curve25519-sha256 MUST be adopted where possible.

As a hedge against uncertainty raised by the NSA IAD FAQ publication, new MODP Diffie-Hellman based key exchanges are proposed for inclusion in the set of key exchange method names as well as the curve448-sha512 curve.

The following new key exchange algorithms are defined:

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

4. IANA Considerations

This RFC augments the Key Exchange Method Names in [RFC4253]. It downgrades the use of SHA-1 hashing for key exchange methods in [RFC4419], [RFC4432], and [RFC4462]. It also moves from MUST to SHOULD the ecdh-sha2-nistp256 given in [RFC5656].

It adds a new set of named "gss-*" methods to [RFC4462] with a MAY recommendation.

It is desirable to also include the new-modp from the [I-D.ietf-curdle-ssh-modp-dh-sha2] in this list.

It is desirable to also include the ssh-curves from the [I-D.ietf-curdle-ssh-curves] in this list. The "curve25519-sha256" is currently available in some Secure Shell implementations under the name "curve25519-sha256@libssh.org" and is the best candidate for a fast, safe, and secure key exchange method.

IANA is requested to update the SSH algorithm registry to ensure that all of the listed Key Exchange Method Name and References exist in the following table. However, the Implement column is just the current recommendations of this RFC.

The Implement column in the above table is a suggestion/recommendation for the listed key exchange method to be implemented in the default list of key exchange methods. It is up to the end-user as to what algorithms they choose to be able to negotiate, so the KEX algorithms should be configurable by the administrator of the server as well as the user of the client. This RFC is intended to provide IANA defined names for these groups for interoperability. The Note column of the IANA table should probably continue to point to the implementation detail sections of the Reference RFCs where appropriate.

The guidance of his RFC is that the SHA-1 algorithm hashing SHOULD NOT be used. If it is used, it should only be provided for backwards compatibility, should not be used in new designs, and should be phased out of existing key exchanges as quickly as possible because of its known weaknesses. Any key exchange using SHA-1 SHOULD NOT be in a default key exchange list if at all possible. If they are needed for backward compatibility, they SHOULD be listed after all of the SHA-2 based key exchanges.

The RFC4253 REQUIRED diffie-hellman-group14-sha1 method SHOULD be retained for compatibility with older Secure Shell implementations. It is intended that this key exchange method be phased out as soon as possible.

It is believed that all current SSH implementations should be able to achieve an implementation of the "diffie-hellman-group14-sha256" method. To that end, this is one method that MUST be implemented.

If GSS-API methods are available, then the RFC4462 REQUIRED gss-group14-sha1-* method SHOULD be retained for compatibility with older Secure Shell implementations and the gss-groups14-sha256-* method SHOULD be added as for "sha1".

[TO BE REMOVED: This registration should take place at the following location: <http://www.iana.org/assignments/ssh-parameters/ssh-parameters.xhtml#ssh-parameters-16>]

5. Acknowledgements

Thanks to the following people for review and comments: Denis Bider, Peter Gutmann, Damien Miller, Niels Moeller, Matt Johnston, Iwamoto Kouichi, Simon Josefsson, Dave Dugal, Daniel Migault.

Thanks to the following people for code to implement interoperable exchanges using some of these groups as found in an this draft: Darren Tucker for OpenSSH and Matt Johnston for Dropbear. And thanks to Iwamoto Kouichi for information about RLogin, Tera Term (ttssh) and Poderosa implementations also adopting new Diffie-Hellman groups based on this draft.

6. Security Considerations

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

The security considerations of [RFC3526] suggest that these MODP groups have security strengths given in this table. They are based on [RFC3766] Determining Strengths For Public Keys Used For Exchanging Symmetric Keys.

+--------+----------+---------------------+---------------------+
| 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- |
|  17    | 6144-bit |      170 |     340- |      270 |     540- |
|  18    | 8192-bit |      190 |     380- |      310 |     620- |
+--------+----------+---------------------+---------------------+