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RFC 8268 PROPOSED STANDARD SSH · KEY EXCHANGE · CRYPTOGRAPHY

More MODP Groups for SSH: More Modular Exponentiation (MODP) Diffie-Hellman (DH) Key Exchange (KEX) Groups for Secure Shell (SSH)

Five new Diffie-Hellman key exchange methods for SSH, all with SHA-2 hashes, plus a fix to how peers' public keys are range-checked.

On rfc-editor.org ↗
PUBLISHED
December 2017
AUTHORS
M. Baushke
STREAM
IETF
BUILDS ON
RFC 4250 — SSH numbers, RFC 4253 — SSH Transport
NEW METHODS
5
GROUP SIZES
3072, 4096, 6144 and 8192 bits (groups 15 to 18)
HASH FOR GROUPS 15 TO 18
SHA512
HASH FOR GROUP14 METHOD
SHA256
SECURITY STRENGTHS (RFC 3526)
110 to 310 bits
CORRECTED RANGE CHECK
1 < e < p-1 and 1 < f < p-1
DOI
10.17487/RFC8268
LINEAGE · FROM THE RFC INDEXOpen full graph →
'06RFC 4250SSH numbers'06RFC 4253SSH Transport'17RFC 8268More MODP Groups for SSH

RFC text

Contents
Original text · RFC Editor View on rfc-editor.org ↗ 
Internet Engineering Task Force (IETF)                        M. Baushke
Request for Comments: 8268                        Juniper Networks, Inc.
Updates: 4250, 4253                                        December 2017
Category: Standards Track
ISSN: 2070-1721


         More Modular Exponentiation (MODP) Diffie-Hellman (DH)
            Key Exchange (KEX) Groups for Secure Shell (SSH)

Abstract

   This document defines added Modular Exponentiation (MODP) groups for
   the Secure Shell (SSH) protocol using SHA-2 hashes.  This document
   updates RFC 4250.  This document updates RFC 4253 by correcting an
   error regarding checking the Peer's DH Public Key.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8268.

Copyright Notice

   Copyright (c) 2017 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
   (https://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.

Table of Contents

   1.  Overview and Rationale  . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   4
   3.  Key Exchange Algorithms . . . . . . . . . . . . . . . . . . .   4
   4.  Checking the Peer's DH Public Key . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Overview and Rationale

   Secure Shell (SSH) is a common protocol for secure communication on
   the Internet.  Security protocols and primitives are an active area
   for research and help to suggest updates to SSH.

   Section 8 of [RFC4253] contains a small error in point 3 regarding
   checking the Peer's DH Public Key.  Section 4 of this document
   provides the correction.

   Due to security concerns with SHA-1 [RFC6194] and with MODP groups
   with less than 2048 bits [NIST-SP-800-131Ar1], implementers and users
   should request support for larger Diffie-Hellman (DH) MODP group
   sizes with data-integrity verification by using the SHA-2 family of
   secure hash algorithms and by having MODP groups provide more
   security.  The use of larger MODP groups and the move to the SHA-2
   family of hashes are important features to strengthen the key
   exchange algorithms available to the SSH client and server.

   DH primes being adopted by this document are all "safe primes" such
   that p = 2q + 1 where q is also a prime.  New MODP groups are being
   introduced starting with the MODP 3072-bit group15.  All use SHA512
   as the hash algorithm.

   The DH 2048-bit MODP group14 is already present in most SSH
   implementations and most implementations already have a SHA256
   implementation, so "diffie-hellman-group14-sha256" is provided as
   easy to implement.

   It is intended that these new MODP groups with SHA-2-based hashes
   update Section 6.4 of [RFC4253] and Section 4.10 of [RFC4250].

   The United States Information Assurance Directorate (IAD) at the
   National Security Agency (NSA) has published "Commercial National
   Security Algorithm Suite and Quantum Computing Frequently Asked
   Questions".  [MFQ-U-OO-815099-15] is addressed to organizations that
   run classified or unclassified national security systems (NSS) and
   vendors that build products used in NSS.

   This FAQ document indicates that NSS should no longer use:

   o  Elliptic Curve Diffie-Hellman (ECDH) and Elliptic Curve Digital
      Signature Algorithm (ECDSA) with NIST P-256.  (For SSH, this would
      suggest avoiding [RFC5656] Key Exchange Algorithm
      "ecdh-sha2-nistp256" and Public Key Algorithm
      "ecdsa-sha2-nistp256".)

   o  SHA-256 (For SSH, this would suggest avoiding any Key Exchange
      Method using SHA1, SHA224, or SHA256 in favor of using SHA384 or
      SHA512.)

   o  AES-128 (For SSH, this would suggest avoiding Encryption
      Algorithms [RFC4253] "aes128-cbc" and [RFC4344] "aes128-ctr".)

   o  RSA with 2048-bit keys (For SSH, this would suggest avoiding
      [RFC4253] "ssh-rsa" using RSA with SHA1 as well as [RFC6187]
      "x509v3-rsa2048-sha256" as well as any other RSA key that has a
      length less than 3072-bits or uses a hash less than SHA384.)

   o  Diffie-Hellman with 2048-bit keys (For SSH, this would suggest
      avoiding use of [RFC4253] both of "diffie-hellman-group1-sha1" and
      "diffie-hellman-group14-sha1" as well as avoiding
      "diffie-hellman-group14-sha256" added by this document.)

   The FAQ also states that NSS users should select DH groups based upon
   well-established and validated parameter sets that comply with the
   minimum required sizes.  Some specific examples include:

   o  Elliptic Curves are currently restricted to the NIST P-384 group
      only for both ECDH and ECDSA, in accordance with existing NIST and
      National Information Assurance Partnership (NIAP) standards.  (For
      SSH, this means using [RFC5656] "ecdh-sha2-nistp384" for key
      exchange and "ecdsa-sha2-nistp384" for Public Key Algorithm
      Names.)

   o  RSA moduli should have a minimum size of 3072 bits (other than the
      noted PKI exception), and keys should be generated in accordance
      with all relevant NIST standards.

   o  For Diffie-Hellman, use a Diffie-Hellman prime modulus of at least
      3072 bits.  (For bit sizes as specified in [RFC3526], this would
      allow for any of group15, group16, group17, group18 to be used.)

   Although SSH may not always be used to protect Top Secret
   communications, this document adopts the use of the DH groups
   provided as an example in the FAQ as well as the use of SHA512 rather
   than SHA256 for the new DH groups.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Key Exchange Algorithms

   This document adds some new Key Exchange Algorithm Method Names to
   what originally appeared in [RFC4253] and [RFC4250].

   This document 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 method algorithms are defined:

   o  diffie-hellman-group14-sha256

   o  diffie-hellman-group15-sha512

   o  diffie-hellman-group16-sha512

   o  diffie-hellman-group17-sha512

   o  diffie-hellman-group18-sha512

   The SHA-2 family of secure hash algorithms is defined in [RFC6234].

   The method of key exchange used for the name "diffie-hellman-
   group14-sha256" is the same as that for "diffie-hellman-group14-sha1"
   except that the SHA256 hash algorithm is used.  It is recommended
   that "diffie-hellman-group14-sha256" SHOULD be supported to smooth
   the transition to newer group sizes.

   The group15 through group18 names are the same as those specified in
   [RFC3526]: 3072-bit MODP group15, 4096-bit MODP group16, 6144-bit
   MODP group17, and 8192-bit MODP group18.

   The SHA512 algorithm is to be used when "sha512" is specified as a
   part of the key exchange method name.

4.  Checking the Peer's DH Public Key

   Section 8 of [RFC4253] contains a small error in point 3.  When
   checking e (client Public Key) and f (server Public Key) values, an
   incorrect range is provided.  The erroneous text is:

      Values of 'e' or 'f' that are not in the range [1, p-1] MUST NOT
      be sent or accepted by either side.  If this condition is
      violated, the key exchange fails.

   The problem is that the range should have been an open interval
   excluding the endpoint values. (i.e., "(1, p-1)").  This document
   amends that document text as follows:

      DH Public Key values MUST be checked and both conditions:

         1 < e < p-1

         1 < f < p-1

      MUST be true.  Values not within these bounds MUST NOT be sent or
      accepted by either side.  If either one of these conditions is
      violated, then the key exchange fails.

   This simple check ensures that:

   o  The remote peer behaves properly.

   o  The local system is not forced into the two-element subgroup.

5.  IANA Considerations

   IANA has added the following entries to the "Key Exchange Method
   Names" registry [IANA-KEX]:

                  Method Name                   Reference
                  ----------------------------- ---------
                  diffie-hellman-group14-sha256 RFC 8268
                  diffie-hellman-group15-sha512 RFC 8268
                  diffie-hellman-group16-sha512 RFC 8268
                  diffie-hellman-group17-sha512 RFC 8268
                  diffie-hellman-group18-sha512 RFC 8268

6.  Security Considerations

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

   The security considerations of [RFC3526] suggest that MODP group14
   through group18 have security strengths that range between 110 bits
   of security through 310 bits of security.  They are based on
   "Determining Strengths For Public Keys Used For Exchanging Symmetric
   Keys" [RFC3766].  Care should be taken to use sufficient entropy and/
   or deterministic random-bit generator (DRBG) algorithms to maximize
   the true security strength of the key exchange and ciphers selected.

   Using a fixed set of Diffie-Hellman parameters makes them a high
   value target for pre-computation.  Generating additional sets of
   primes to be used, or moving to larger values mitigates this issue.

7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [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,
              <https://www.rfc-editor.org/info/rfc3526>.

   [RFC4250]  Lehtinen, S. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Protocol Assigned Numbers", RFC 4250,
              DOI 10.17487/RFC4250, January 2006,
              <https://www.rfc-editor.org/info/rfc4250>.

   [RFC4253]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253,
              January 2006, <https://www.rfc-editor.org/info/rfc4253>.

   [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234,
              DOI 10.17487/RFC6234, May 2011,
              <https://www.rfc-editor.org/info/rfc6234>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

7.2.  Informative References

   [IANA-KEX] IANA, "Secure Shell (SSH) Protocol Parameters",
              <http://www.iana.org/assignments/ssh-parameters/>

   [MFQ-U-OO-815099-15]
              National Security Agency / Central Security Service,
              "Commerical National Security Algorithm Suite and Quantum
              Computing FAQ", MFQ U/OO/815099-15 , January 2016,
              <https://www.iad.gov/iad/library/ia-guidance/
              ia-solutions-for-classified/algorithm-
              guidance/assets/public/upload/
              CNSA-Suite-and-Quantum-Computing-FAQ.pdf>.

   [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, DOI 10.6028/NIST.SP.800-131Ar1, November 2015,
              <http://dx.doi.org/10.6028/NIST.SP.800-131Ar1>.

   [RFC3766]  Orman, H. and P. Hoffman, "Determining Strengths For
              Public Keys Used For Exchanging Symmetric Keys", BCP 86,
              RFC 3766, DOI 10.17487/RFC3766, April 2004,
              <https://www.rfc-editor.org/info/rfc3766>.

   [RFC4344]  Bellare, M., Kohno, T., and C. Namprempre, "The Secure
              Shell (SSH) Transport Layer Encryption Modes", RFC 4344,
              DOI 10.17487/RFC4344, January 2006,
              <https://www.rfc-editor.org/info/rfc4344>.

   [RFC5656]  Stebila, D. and J. Green, "Elliptic Curve Algorithm
              Integration in the Secure Shell Transport Layer",
              RFC 5656, DOI 10.17487/RFC5656, December 2009,
              <https://www.rfc-editor.org/info/rfc5656>.

   [RFC6187]  Igoe, K. and D. Stebila, "X.509v3 Certificates for Secure
              Shell Authentication", RFC 6187, DOI 10.17487/RFC6187,
              March 2011, <https://www.rfc-editor.org/info/rfc6187>.

   [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,
              <https://www.rfc-editor.org/info/rfc6194>.

Acknowledgements

   Thanks to the following people for review and comments: Denis Bider,
   Peter Gutmann, Damien Miller, Niels Moller, Matt Johnston, Iwamoto
   Kouichi, Dave Dugal, Daniel Migault, Anna Johnston, Ron Frederick,
   Rich Salz, Travis Finkenauer, and Eric Rescorla.

Author's Address

   Mark D. Baushke
   Juniper Networks, Inc.
   1133 Innovation Way
   Sunnyvale, CA  94089-1228
   United States of America

   Phone: +1 408 745 2952
   Email: mdb@juniper.net
   URI:   http://www.juniper.net/

AI ELI5

On this page
PROBLEMSSH key exchange relied on SHA-1 and on MODP groups smaller than 2048 bits, which raised security concerns, and RFC 4253 also contained a small error in the range check for the peer's Diffie-Hellman public key.
MECHANISMThe RFC defines five new key exchange method names that pair MODP groups 14 through 18 with SHA-256 or SHA-512, and corrects the public key check to the open interval 1 < value < p-1.
WHY IT MATTERSClients and servers get stronger, standardized key exchange choices, and the corrected check keeps a peer from forcing the local system into a two-element subgroup.

§1Overview and Rationale

RFC 8268 extends Secure Shell (SSH) with new Diffie-Hellman (DH) key exchange methods that use Modular Exponentiation (MODP) groups and SHA-2 hashes. It does two jobs: it adds new method names, and it corrects an error in RFC 4253.

The motivation is security concern about SHA-1 and about MODP groups with fewer than 2048 bits. The RFC asks implementers and users to request larger MODP groups and the SHA-2 family of hash algorithms. It also says the new methods are intended to update Section 6.4 of RFC 4253 and Section 4.10 of RFC 4250.

The DH primes adopted here are all safe primes, meaning p = 2q + 1 where q is also prime.

Some of the old secret-sharing tricks in SSH are getting too easy to crack. This RFC adds stronger ones and fixes one small mistake in the rulebook.

§2Why These Groups and Hashes

The RFC explains its choices by pointing to an FAQ from the United States Information Assurance Directorate at the National Security Agency, written for operators of national security systems. That FAQ says such systems should stop using SHA-256, AES-128, RSA with 2048-bit keys and Diffie-Hellman with 2048-bit keys, and should use a Diffie-Hellman prime modulus of at least 3072 bits.

For SSH, the RFC translates this into avoiding key exchange with SHA1, SHA224 or SHA256 in favor of SHA384 or SHA512. For DH sizes in RFC 3526, groups 15, 16, 17 and 18 meet the 3072-bit minimum.

The RFC adopts those example groups and uses SHA512 rather than SHA256 for the new groups, while admitting that SSH may not always protect Top Secret communications.

A big security agency said that small locks are no longer good enough. So SSH gets some much bigger locks to pick from.

§3The New Key Exchange Methods

Five method names are defined: diffie-hellman-group14-sha256, diffie-hellman-group15-sha512, diffie-hellman-group16-sha512, diffie-hellman-group17-sha512 and diffie-hellman-group18-sha512. They follow the style and conventions of RFC 4253, and the SHA-2 hashes are defined in RFC 6234.

The group14-sha256 method works exactly like diffie-hellman-group14-sha1 except that SHA256 is the hash. It is included because the 2048-bit group14 is already in most SSH implementations, and most already have SHA256, so it is easy to implement. It SHOULD be supported to smooth the transition to newer group sizes.

The group15 through group18 names use the groups from RFC 3526: 3072-bit group15, 4096-bit group16, 6144-bit group17 and 8192-bit group18. Whenever sha512 appears in the method name, SHA512 is the hash.

There are now five new recipes for agreeing on a secret. Each one picks a lock size and a scrambler, and the bigger the lock number, the tougher it is.

NEW METHOD NAMES AND THEIR MODP GROUPS (RFC 8268)
Method nameMODP groupHash
diffie-hellman-group14-sha2562048-bit group14SHA256
diffie-hellman-group15-sha5123072-bit group15SHA512
diffie-hellman-group16-sha5124096-bit group16SHA512
diffie-hellman-group17-sha5126144-bit group17SHA512
diffie-hellman-group18-sha5128192-bit group18SHA512

§4Checking the Peer's DH Public Key

Section 8 of RFC 4253 says values of e (the client's public key) or f (the server's public key) outside the range [1, p-1] must not be sent or accepted. The RFC calls this a small error: the range should have excluded both endpoints, as an open interval.

The corrected rule is that both 1 < e < p-1 and 1 < f < p-1 MUST be true. Values outside these bounds MUST NOT be sent or accepted by either side, and if either condition is violated the key exchange fails.

The RFC says this simple check ensures that the remote peer behaves properly and that the local system is not forced into the two-element subgroup.

When the other side hands you a number, you must check it is not one of the two silly numbers at the very edges. If it is, the whole deal is off.

§5IANA Registrations

IANA added the five new method names to the "Key Exchange Method Names" registry, each with RFC 8268 as its reference. The names are the same five listed in the algorithms section above.

This is what lets SSH clients and servers announce and agree on these methods by name during negotiation.

The five new recipes were written down in an official list, so everyone uses the same names for them.

§6Security Considerations

The security considerations of RFC 4253 apply. RFC 3526 suggests that MODP group14 through group18 have security strengths ranging from 110 bits to 310 bits, based on RFC 3766. The RFC adds that care should be taken to use sufficient entropy or deterministic random-bit generator (DRBG) algorithms to maximize the true security strength of the key exchange and ciphers selected.

It also warns that a fixed set of DH parameters is a high-value target for pre-computation. Generating additional sets of primes, or moving to larger values, mitigates this issue.

Everybody using the same big numbers makes them a juicy target, so bigger numbers or fresh ones help keep the secret safe.

§7References

[RFC3526]RFC 3526Kivinen, T. and M. Kojo, "More Modular Exponential (MODP) Diffie-Hellman groups for Internet Key Exchange (IKE)", RFC 3526, May 2003.May 2003[RFC4250]RFC 4250Lehtinen, S. and C. Lonvick, Ed., "The Secure Shell (SSH) Protocol Assigned Numbers", RFC 4250, January 2006.January 2006[RFC4253]RFC 4253Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH) Transport Layer Protocol", RFC 4253, January 2006.January 2006[RFC6234]RFC 6234Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms (SHA and SHA-based HMAC and HKDF)", RFC 6234, May 2011.May 2011[RFC3766]RFC 3766Orman, H. and P. Hoffman, "Determining Strengths For Public Keys Used For Exchanging Symmetric Keys", BCP 86, RFC 3766, April 2004.April 2004