| Internet Engineering Task Force | M. Baushke |
| Internet-Draft | Juniper Networks, Inc. |
| Updates: 4250, 4253 (if approved) | March 27, 2017 |
| Intended status: Standards Track | |
| Expires: September 28, 2017 |
Key Exchange (KEX) Method Updates and Recommendations for Secure Shell (SSH)
draft-ietf-curdle-ssh-kex-sha2-07
This document is intended to update the recommended set of key exchange methods for use in the Secure Shell (SSH) protocol to meet evolving needs for stronger security. This RFC updates [RFC4253] MUST algorithms. This RFC also notes that the [IANASSH] has replaced [RFC4250] as the primary reference document for SSH Protocol Assigned Numbers.
This document adds recommendations for adoption of Key Exchange Methods which MUST, SHOULD+, SHOULD, SHOULD-, MAY, SHOULD NOT, and MUST NOT be implemented. New key exchange methods will use the SHA-2 family of hashes and are drawn from these from [I-D.ietf-curdle-ssh-curves] and new-modp from the [I-D.ietf-curdle-ssh-modp-dh-sha2] and gss-keyex [NEWGSSAPI].
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Secure Shell (SSH) is a common protocol for secure communication on the Internet. In [RFC4253], SSH originally defined two Key Exchange Method Names that MUST be implemented. Over time, what was once considered secure, is no longer considered secure. The purpose of this RFC is to recommend that some published key exchanges be adopted or reclassified and others retired.
[TO BE REMOVED: Please send comments on this draft to curdle@ietf.org.]
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].
When used in the tables in this document, these terms indicate that the listed algorithm MUST, MUST NOT, SHOULD, SHOULD NOT or MAY be implemented as part of a Secure Shell implementation. Additional terms used in this document are:
| SHOULD+ | This term means the same as SHOULD. However, it is likely that an algorithm marked as SHOULD+ will be promoted at some future time to be a MUST. |
| SHOULD- | This term means the same as SHOULD. However, an algorithm marked as SHOULD- may be deprecated to a MAY in a future version of this document. |
This memo adopts the style and conventions of [RFC4253] in specifying how the use of data key exchange is indicated in SSH.
This RFC also collects Key Exchange Method Names in various existing RFCs [RFC4253], [RFC4419], [RFC4432], [RFC4462], [RFC5656], [I-D.ietf-curdle-ssh-modp-dh-sha2], [NEWGSSAPI], and [I-D.ietf-curdle-ssh-curves] and provides a suggested suitability for implementation of MUST, SHOULD+, SHOULD, SHOULD-, SHOULD NOT, and MUST NOT.
This document is intended to provide guidance as to what Key Exchange Algorithms are to be considered for new or updated SSH implementations. This document will be superseded when one or more of the listed algorithms are considered too weak to continue to use securely, or when newer methods have been analyzed and found to be secure with wide enough adoption to upgrade their recommendation from MAY to SHOULD or MUST.
The Curve25519 provides strong security and is efficient on a wide range of architectures with properties that allow better implementation properties compared to traditional elliptic curves. The use of SHA2-256 for integrity is a reasonable one for this method. This Key Exchange Method has a few implementations and SHOULD+ be implemented in any SSH interested in using elliptic curve based key exchanges.
This set of ephemerally generated key exchange groups uses SHA-1 which has security concerns [RFC6194]. It is recommended that these key exchange groups NOT be used. This key exchange MUST NOT be implemented.
This method uses [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], this method is considered insecure. This method is being moved from MUST to MUST NOT.
This generated key exchange groups uses SHA-1 which has security concerns [RFC6194]. However, this group is still strong enough and is widely deployed. This method is being moved from MUST to SHOULD- to aid in transition to stronger SHA-2 based hashes. This method will transition to MUST NOT when SHA-2 alternatives are more generally available.
This generated key exchange uses a 2048-bit sized MODP group along with a SHA-2 (SHA2-256) hash. This represents the smallest Finite Field cryptography Diffie-Hellman key exchange method. It is a reasonably simple transition to move from SHA-1 to SHA-2. This method MUST be implemented.
The use of FFC DH is well understood and trusted. Adding larger modulus sizes and protecting with SHA2-512 should give enough head room to be ready for the next scare that someone has pre-computed. This modulus is larger than that required by [CNSA-SUITE] and should be sufficient to inter-operate with more paranoid nation-states. This method SHOULD+ be implemented.
Elliptic Curve Diffie-Hellman (ECDH) are often implemented because they are smaller and faster than using large FFC primes with traditional Diffie-Hellman (DH). However, given [CNSA-SUITE] and [safe-curves], this curve may not be as useful and strong as desired. The SSH development community is divided on this and many implementations do exist. However, there are good implementations of this along with a constant-time SHA2-256 implementation. If an implementer does not have a constant-time SHA2-384 implementation (which helps avoid side-channel attacks), then this is the correct ECDH to implement. This method SHOULD- be implemented.
This ECDH method should be implemented because it is smaller and faster than using large FFC primes with traditional Diffie-Hellman (DH). Given [CNSA-SUITE], it is considered good enough for TOP SECRET for now. This really needs a constant-time implementation of SHA2-384 to be useful. This method SHOULD+ be implemented.
This set of ephemerally generated key exchange groups uses SHA-1 which has security concerns [RFC6194]. It is recommended that these key exchange groups NOT be used. This key exchange MUST NOT be implemented.
This method suffers from the same problems of diffie-hellman-group1-sha1. It uses [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], this method is considered insecure. This method MUST NOT be implemented.
This generated key exchange groups uses SHA-1 which has security concerns [RFC6194]. If GSS-API key exchange methods are being used, then this one SHOULD- be implemented until such time as SHA-2 variants may be implemented and deployed.
If the GSS-API is to be used, then this method SHOULD be implemented.
If the GSS-API is to be used, then this method SHOULD+ be implemented.
The security of RSA 1024-bit modulus keys is not good enough any longer. A minimum bit size should be 2048-bit groups. This generated key exchange groups uses SHA-1 which has security concerns [RFC6194]. This method MUST NOT be implemented.
The full set of official [IANASSH] key algorithm method names not otherwise mentioned in this RFC MAY be implemented.
The Implement column is the current recommendations of this RFC. Key Exchange Method Names are listed alphabetically.
| Key Exchange Method Name | Reference | Implement |
|---|---|---|
| curve25519-sha256 | ssh-curves | SHOULD+ |
| diffie-hellman-group-exchange-sha1 | RFC4419 | MUST NOT |
| diffie-hellman-group1-sha1 | RFC4253 | MUST NOT |
| diffie-hellman-group14-sha1 | RFC4253 | SHOULD- |
| diffie-hellman-group14-sha256 | new-modp | MUST |
| diffie-hellman-group16-sha512 | new-modp | SHOULD+ |
| ecdh-sha2-nistp256 | RFC5656 | SHOULD- |
| ecdh-sha2-nistp384 | RFC5656 | SHOULD+ |
| gss-gex-sha1-* | RFC4462 | MUST NOT |
| gss-group1-sha1-* | RFC4462 | MUST NOT |
| gss-group14-sha1-* | RFC4462 | SHOULD- |
| gss-group14-sha256-* | gss-keyex | SHOULD |
| gss-group16-sha512-* | gss-keyex | SHOULD+ |
| rsa1024-sha1 | RFC4432 | MUST NOT |
The guidance of this RFC is that the SHA-1 algorithm hashing MUST NOT be used. If it is used in implementations, 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 MUST 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 MUST 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.
[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>]
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, Anna Johnston.
Thanks to the following people for code to implement inter-operable 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.
The security considerations of [RFC4253] apply to this RFC.
It is desirable to deprecate or remove key exchange method name that are considered weak. A key exchange method may be weak because too few bits are used, or the hashing algorithm is considered too weak.
The diffie-hellman-group1-sha1 is being moved from MUST to MUST NOT. This method 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], this method is no longer considered secure.
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 MUST to MAY 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 [RFC3526] is explicitly mentioned as the minimum modulus to protect Top Secret communications.
It has been observed in [safe-curves] that the NIST 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 mandatory to 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.
In a Post-Quantum Computing (PQC) world, it will be desirable to use larger cyclic subgroups. To do this using Elliptic Curve Cryptography will require much larger prime base fields, greatly reducing their efficiency. Finite Field based Cryptography already requires large enough base fields to accommodate larger cyclic subgroups.
| [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. |
| [RFC4250] | Lehtinen, S. and C. Lonvick, "The Secure Shell (SSH) Protocol Assigned Numbers", RFC 4250, DOI 10.17487/RFC4250, January 2006. |
| [RFC4253] | Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253, January 2006. |