Internet DRAFT - draft-baushke-ssh-dh-group-sha2
draft-baushke-ssh-dh-group-sha2
Internet Engineering Task Force M. Baushke
Internet-Draft Juniper Networks, Inc.
Updates: 4253, 4419, 4432, 4462, 5656 March 1, 2016
(if approved)
Intended status: Standards Track
Expires: September 2, 2016
More Modular Exponential (MODP) Diffie-Hellman Key Exchange Groups for
Secure Shell (SSH)
draft-baushke-ssh-dh-group-sha2-06
Abstract
This document defines two added Modular Exponential (MODP) Groups 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 including two new Diffie-Hellman
key exchange algorithms using SHA-2 hashes.
Status of This Memo
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This Internet-Draft will expire on September 2, 2016.
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to this document. Code Components extracted from this document must
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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 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] 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 at the National
Security Agency has published a FAQ [MFQ-U-OO-815099-15] suggesting
that the use of 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
introduced are using SHA2-512 as the hash algorithm. 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 for small embedded applications.
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 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 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].
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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-group14-sha256 MAY/OPTIONAL
diffie-hellman-group16-sha512 SHOULD/RECOMMENDED
diffie-hellman-group18-sha512 MAY/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"
except that the SHA2-256 hash algorithm is used.
The group16 and group18 names are the same as those specified in
[RFC3526] 4096-bit MODP Group 16 and 8192-bit MODP Group 18.
The SHA2-512 algorithm is to be used when "sha512" is specified as a
part of the key exchange method name.
4. IANA Considerations
This document 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 MAY
the ecdh-sha2-nistp256 given in [RFC5656].
It is desirable to also include the ssh-curves from the
[I-D.josefsson-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.
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IANA is requested to update the SSH algorithm registry with the
following entries:
Key Exchange Method Name Reference Note
diffie-hellman-group-exchange-sha1 RFC4419 SHOULD NOT
diffie-hellman-group-exchange-sha256 RFC4419 MAY
diffie-hellman-group1-sha1 RFC4253 SHOULD NOT
diffie-hellman-group14-sha1 RFC4253 SHOULD
ecdh-sha2-nistp256 RFC5656 MAY
ecdh-sha2-nistp384 RFC5656 SHOULD
ecdh-sha2-nistp521 RFC5656 SHOULD
ecdh-sha2-* RFC5656 MAY
ecmqv-sha2 RFC5656 MAY
gss-gex-sha1-* RFC4462 SHOULD NOT
gss-group1-sha1-* RFC4462 SHOULD NOT
gss-group14-sha1-* RFC4462 MAY
gss-* RFC4462 MAY
rsa1024-sha1 RFC4432 SHOULD NOT
rsa2048-sha256 RFC4432 MAY
diffie-hellman-group14-sha256 This Draft MAY
diffie-hellman-group16-sha512 This Draft SHOULD
diffie-hellman-group18-sha512 This Draft MAY
curve25519-sha256 ssh-curves MUST
curve448-sha512 ssh-curves MAY
Figure 2
The Note in the above table is an implementation suggestion/
recommendation for the listed key exchange method. It is up to the
end-user as to what algorithms they choose to be able to negotiate.
The guidance of his document 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 be phased out as soon as
possible.
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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.
Thanks to the following people for code to implement interoperable
exchanges using some of these groups as found in an the -01 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 the -01 draft.
6. 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. They are based
on [RFC3766] Determining Strengths For Public Keys Used For
Exchanging Symmetric Keys.
Group modulus security strength estimates (RFC3526)
+--------+----------+---------------------+---------------------+
| 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- |
+--------+----------+---------------------+---------------------+
Figure 3
Many users seem to be interested in the perceived safety of using
larger MODP groups and hashing with SHA2-based algorithms.
7. References
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7.1. Normative References
[FIPS-180-4]
National Institute of Standards and Technology, "Secure
Hash Standard (SHS)", FIPS PUB 180-4, August 2015,
<http://nvlpubs.nist.gov/nistpubs/FIPS/
NIST.FIPS.180-4.pdf>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://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,
<http://www.rfc-editor.org/info/rfc3526>.
[RFC4253] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253,
January 2006, <http://www.rfc-editor.org/info/rfc4253>.
7.2. Informative References
[I-D.josefsson-ssh-curves]
Adamantiadis, A. and S. Josefsson, "Secure Shell (SSH) Key
Exchange Method using Curve25519 and Curve448", draft-
josefsson-ssh-curves-04 (work in progress), March 2016.
[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>.
[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, DOI 10.17487/RFC2409, November 1998,
<http://www.rfc-editor.org/info/rfc2409>.
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[RFC3174] Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm 1
(SHA1)", RFC 3174, DOI 10.17487/RFC3174, September 2001,
<http://www.rfc-editor.org/info/rfc3174>.
[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,
<http://www.rfc-editor.org/info/rfc3766>.
[RFC4419] Friedl, M., Provos, N., and W. Simpson, "Diffie-Hellman
Group Exchange for the Secure Shell (SSH) Transport Layer
Protocol", RFC 4419, DOI 10.17487/RFC4419, March 2006,
<http://www.rfc-editor.org/info/rfc4419>.
[RFC4432] Harris, B., "RSA Key Exchange for the Secure Shell (SSH)
Transport Layer Protocol", RFC 4432, DOI 10.17487/RFC4432,
March 2006, <http://www.rfc-editor.org/info/rfc4432>.
[RFC4462] Hutzelman, J., Salowey, J., Galbraith, J., and V. Welch,
"Generic Security Service Application Program Interface
(GSS-API) Authentication and Key Exchange for the Secure
Shell (SSH) Protocol", RFC 4462, DOI 10.17487/RFC4462, May
2006, <http://www.rfc-editor.org/info/rfc4462>.
[RFC5656] Stebila, D. and J. Green, "Elliptic Curve Algorithm
Integration in the Secure Shell Transport Layer",
RFC 5656, DOI 10.17487/RFC5656, December 2009,
<http://www.rfc-editor.org/info/rfc5656>.
[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,
<http://www.rfc-editor.org/info/rfc6194>.
[safe-curves]
Bernstein, and Lange, "SafeCurves: choosing safe curves
for elliptic-curve cryptography.", February 2016,
<https://safecurves.cr.yp.to/>.
Author's Address
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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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