Internet DRAFT - draft-ietf-curdle-ssh-modp-dh-sha2
draft-ietf-curdle-ssh-modp-dh-sha2
Internet Engineering Task Force M. Baushke
Internet-Draft Juniper Networks, Inc.
Updates: 4250, 4253 (if approved) September 15, 2017
Intended status: Standards Track
Expires: March 19, 2018
More Modular Exponential (MODP) Diffie-Hellman (DH) Key Exchange (KEX)
Groups for Secure Shell (SSH)
draft-ietf-curdle-ssh-modp-dh-sha2-09
Abstract
This document defines added Modular Exponential (MODP) Groups for the
Secure Shell (SSH) protocol using SHA-2 hashes. This document
updates RFC 4250. This document updates RFC 4253 including an errata
fix for checking the Peer's DH Public Key.
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
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 19, 2018.
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
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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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. Security protocols and primitives are an active area
for research and help to suggest updates to SSH.
Section 3 of the [RFC4253] contains a small errata for 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] implementer and users
request support for larger Diffie Hellman (DH) 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 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 group 15. All use SHA512
as the hash algorithm.
The DH 2048-bit MODP group 14 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 the [RFC4253] section 6.4 and [RFC4250] section 4.10
standards.
The United States Information Assurance Directorate (IAD) at the
National Security Agency (NSA) has published "Commercial National
Security Algorithm (CNSA) Suite and Quantum Computing Frequently
Asked Questions (FAQ)" [MFQ-U-OO-815099-15] 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 ECDH and ECDSA with NIST P-256
o SHA-256
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o AES-128
o RSA with 2048-bit keys
o Diffie-Hellman with 2048-bit keys
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
NIAP standards.
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 as specified in IETF RFC 3526 [RFC3526] (Groups 15-18).
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.
[TO BE REMOVED: 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 RFC 2119 [RFC2119].
3. Key Exchange Algorithms
This document adds some new Key Exchange Algorithm Method Names 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
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o diffie-hellman-group16-sha512
o diffie-hellman-group17-sha512
o diffie-hellman-group18-sha512
The SHA-2 family of secure hash algorithms are 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 Group 15, 4096-bit MODP Group 16, 6144-bit
MODP Group 17, and 8192-bit MODP Group 18.
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 3 of [RFC4253] contains a small errata. 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 errata is that the range should have been an open interval
excluding the end point 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 condition is
violated, then the key exchange fails.
This simple check ensures:
o The remote peer behaves properly.
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o The local system is not forced into the two-element subgroup.
5. IANA Considerations
IANA is requested to add to the Key Exchange Method Names algorithm
registry [IANA-KEX] with the following entries:
Key Exchange Method Name Reference
----------------------------- ----------
diffie-hellman-group14-sha256 This Draft
diffie-hellman-group15-sha512 This Draft
diffie-hellman-group16-sha512 This Draft
diffie-hellman-group17-sha512 This Draft
diffie-hellman-group18-sha512 This Draft
[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>]
6. Acknowledgements
Thanks to the following people for review and comments: Denis Bider,
Peter Gutmann, Damien Miller, Niels Moeller, Matt Johnston, Iwamoto
Kouichi, Dave Dugal, Daniel Migault, Anna Johnston, Ron Frederick,
Rich Salz, Travis Finkenauer, Eric Rescorla.
7. 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
[RFC3766] Determining Strengths For Public Keys Used For Exchanging
Symmetric Keys. Care should be taken to use sufficient entropy and/
or 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 is a mitigation against
this issue.
8. References
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8.1. 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>.
8.2. Informative References
[IANA-KEX]
Internet Assigned Numbers Authority (IANA), "Secure Shell
(SSH) Protocol Parameters: Key Exchange Method Names",
March 2017, <http://www.iana.org/assignments/ssh-
parameters/ssh-parameters.xhtml#ssh-parameters-16>.
[MFQ-U-OO-815099-15]
"National Security Agency/Central Security Service", "CNSA
Suite and Quantum Computing FAQ", January 2016,
<https://www.iad.gov/iad/library/ia-guidance/
ia-solutions-for-classified/algorithm-guidance/
cnsa-suite-and-quantum-computing-faq.cfm>.
[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,
<http://nvlpubs.nist.gov/nistpubs/SpecialPublications/
NIST.SP.800-131Ar1.pdf>.
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[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>.
[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>.
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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