draft-ietf-curdle-ssh-modp-dh-sha2-09.txt   rfc8268.txt 
Internet Engineering Task Force M. Baushke Internet Engineering Task Force (IETF) M. Baushke
Internet-Draft Juniper Networks, Inc. Request for Comments: 8268 Juniper Networks, Inc.
Updates: 4250, 4253 (if approved) September 15, 2017 Updates: 4250, 4253 December 2017
Intended status: Standards Track Category: Standards Track
Expires: March 19, 2018 ISSN: 2070-1721
More Modular Exponential (MODP) Diffie-Hellman (DH) Key Exchange (KEX) More Modular Exponentiation (MODP) Diffie-Hellman (DH)
Groups for Secure Shell (SSH) Key Exchange (KEX) Groups for Secure Shell (SSH)
draft-ietf-curdle-ssh-modp-dh-sha2-09
Abstract Abstract
This document defines added Modular Exponential (MODP) Groups for the This document defines added Modular Exponentiation (MODP) groups for
Secure Shell (SSH) protocol using SHA-2 hashes. This document the Secure Shell (SSH) protocol using SHA-2 hashes. This document
updates RFC 4250. This document updates RFC 4253 including an errata updates RFC 4250. This document updates RFC 4253 by correcting an
fix for checking the Peer's DH Public Key. error regarding checking the Peer's DH Public Key.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This is an Internet Standards Track document.
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-
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Internet-Drafts are draft documents valid for a maximum of six months This document is a product of the Internet Engineering Task Force
and may be updated, replaced, or obsoleted by other documents at any (IETF). It represents the consensus of the IETF community. It has
time. It is inappropriate to use Internet-Drafts as reference received public review and has been approved for publication by the
material or to cite them other than as "work in progress." Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
This Internet-Draft will expire on March 19, 2018. 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 Notice
Copyright (c) 2017 IETF Trust and the persons identified as the Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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described in the Simplified BSD License. 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 1. Overview and Rationale
Secure Shell (SSH) is a common protocol for secure communication on Secure Shell (SSH) is a common protocol for secure communication on
the Internet. Security protocols and primitives are an active area the Internet. Security protocols and primitives are an active area
for research and help to suggest updates to SSH. for research and help to suggest updates to SSH.
Section 3 of the [RFC4253] contains a small errata for checking the Section 8 of [RFC4253] contains a small error in point 3 regarding
Peer's DH Public key. Section 4 of this document provides the checking the Peer's DH Public Key. Section 4 of this document
correction. provides the correction.
Due to security concerns with SHA-1 [RFC6194] and with MODP groups Due to security concerns with SHA-1 [RFC6194] and with MODP groups
with less than 2048 bits [NIST-SP-800-131Ar1] implementer and users with less than 2048 bits [NIST-SP-800-131Ar1], implementers and users
request support for larger Diffie Hellman (DH) MODP group sizes with should request support for larger Diffie-Hellman (DH) MODP group
data integrity verification using the SHA-2 family of secure hash sizes with data-integrity verification by using the SHA-2 family of
algorithms as well as MODP groups providing more security. The use secure hash algorithms and by having MODP groups provide more
of larger MODP groups and the move to the SHA-2 family of hashes are security. The use of larger MODP groups and the move to the SHA-2
important features to strengthen the key exchange algorithms family of hashes are important features to strengthen the key
available to the SSH client and server. exchange algorithms available to the SSH client and server.
DH primes being adopted by this document are all "safe primes" such 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 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 introduced starting with the MODP 3072-bit group15. All use SHA512
as the hash algorithm. as the hash algorithm.
The DH 2048-bit MODP group 14 is already present in most SSH The DH 2048-bit MODP group14 is already present in most SSH
implementations and most implementations already have a SHA256 implementations and most implementations already have a SHA256
implementation, so diffie-hellman-group14-sha256 is provided as easy implementation, so "diffie-hellman-group14-sha256" is provided as
to implement. easy to implement.
It is intended that these new MODP groups with SHA-2 based hashes It is intended that these new MODP groups with SHA-2-based hashes
update the [RFC4253] section 6.4 and [RFC4250] section 4.10 update Section 6.4 of [RFC4253] and Section 4.10 of [RFC4250].
standards.
The United States Information Assurance Directorate (IAD) at the The United States Information Assurance Directorate (IAD) at the
National Security Agency (NSA) has published "Commercial National National Security Agency (NSA) has published "Commercial National
Security Algorithm (CNSA) Suite and Quantum Computing Frequently Security Algorithm Suite and Quantum Computing Frequently Asked
Asked Questions (FAQ)" [MFQ-U-OO-815099-15] addressed to Questions". [MFQ-U-OO-815099-15] is addressed to organizations that
organizations that run classified or unclassified national security run classified or unclassified national security systems (NSS) and
systems (NSS) and vendors that build products used in NSS. vendors that build products used in NSS.
This FAQ document indicates that NSS should no longer use: This FAQ document indicates that NSS should no longer use:
o ECDH and ECDSA with NIST P-256 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 o SHA-256 (For SSH, this would suggest avoiding any Key Exchange
o AES-128 Method using SHA1, SHA224, or SHA256 in favor of using SHA384 or
SHA512.)
o RSA with 2048-bit keys o AES-128 (For SSH, this would suggest avoiding Encryption
Algorithms [RFC4253] "aes128-cbc" and [RFC4344] "aes128-ctr".)
o Diffie-Hellman with 2048-bit keys 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 The FAQ also states that NSS users should select DH groups based upon
well established and validated parameter sets that comply with the well-established and validated parameter sets that comply with the
minimum required sizes. Some specific examples include: minimum required sizes. Some specific examples include:
o Elliptic Curves are currently restricted to the NIST P-384 group o Elliptic Curves are currently restricted to the NIST P-384 group
only for both ECDH and ECDSA, in accordance with existing NIST and only for both ECDH and ECDSA, in accordance with existing NIST and
NIAP standards. 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 o RSA moduli should have a minimum size of 3072 bits (other than the
noted PKI exception), and keys should be generated in accordance noted PKI exception), and keys should be generated in accordance
with all relevant NIST standards. with all relevant NIST standards.
o For Diffie-Hellman use a Diffie-Hellman prime modulus of at least 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). 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 Although SSH may not always be used to protect Top Secret
communications, this document adopts the use of the DH groups 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 provided as an example in the FAQ as well as the use of SHA512 rather
than SHA256 for the new DH groups. than SHA256 for the new DH groups.
[TO BE REMOVED: Please send comments on this draft to
curdle@ietf.org.]
2. Requirements Language 2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
document are to be interpreted as described in RFC 2119 [RFC2119]. "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 3. Key Exchange Algorithms
This document adds some new Key Exchange Algorithm Method Names in This document adds some new Key Exchange Algorithm Method Names to
[RFC4253] and [RFC4250]. what originally appeared in [RFC4253] and [RFC4250].
This document adopts the style and conventions of [RFC4253] in This document adopts the style and conventions of [RFC4253] in
specifying how the use of new data key exchange is indicated in SSH. specifying how the use of new data key exchange is indicated in SSH.
The following new key exchange method algorithms are defined: The following new key exchange method algorithms are defined:
o diffie-hellman-group14-sha256 o diffie-hellman-group14-sha256
o diffie-hellman-group15-sha512 o diffie-hellman-group15-sha512
o diffie-hellman-group16-sha512 o diffie-hellman-group16-sha512
o diffie-hellman-group17-sha512 o diffie-hellman-group17-sha512
o diffie-hellman-group18-sha512 o diffie-hellman-group18-sha512
The SHA-2 family of secure hash algorithms are defined in [RFC6234]. The SHA-2 family of secure hash algorithms is defined in [RFC6234].
The method of key exchange used for the name "diffie-hellman- The method of key exchange used for the name "diffie-hellman-
group14-sha256" is the same as that for "diffie-hellman-group14-sha1" group14-sha256" is the same as that for "diffie-hellman-group14-sha1"
except that the SHA256 hash algorithm is used. It is recommended except that the SHA256 hash algorithm is used. It is recommended
that diffie-hellman-group14-sha256 SHOULD be supported to smooth the that "diffie-hellman-group14-sha256" SHOULD be supported to smooth
transition to newer group sizes. the transition to newer group sizes.
The group15 through group18 names are the same as those specified in 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 [RFC3526]: 3072-bit MODP group15, 4096-bit MODP group16, 6144-bit
MODP Group 17, and 8192-bit MODP Group 18. MODP group17, and 8192-bit MODP group18.
The SHA512 algorithm is to be used when "sha512" is specified as a The SHA512 algorithm is to be used when "sha512" is specified as a
part of the key exchange method name. part of the key exchange method name.
4. Checking the Peer's DH Public Key 4. Checking the Peer's DH Public Key
Section 3 of [RFC4253] contains a small errata. When checking e Section 8 of [RFC4253] contains a small error in point 3. When
(client public key) and f (server public key) values, an incorrect checking e (client Public Key) and f (server Public Key) values, an
range is provided. The erroneous text is: incorrect range is provided. The erroneous text is:
Values of 'e' or 'f' that are not in the range [1, p-1] MUST NOT 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 be sent or accepted by either side. If this condition is
violated, the key exchange fails. violated, the key exchange fails.
The errata is that the range should have been an open interval The problem is that the range should have been an open interval
excluding the end point values. (i.e "(1, p-1)"). This document excluding the endpoint values. (i.e., "(1, p-1)"). This document
amends that document text as follows: amends that document text as follows:
DH Public key values MUST be checked and both conditions: DH Public Key values MUST be checked and both conditions:
1 < e < p-1 1 < e < p-1
1 < f < p-1 1 < f < p-1
MUST be true. Values not within these bounds MUST NOT be sent or MUST be true. Values not within these bounds MUST NOT be sent or
accepted by either side. If either one of these condition is accepted by either side. If either one of these conditions is
violated, then the key exchange fails. violated, then the key exchange fails.
This simple check ensures: This simple check ensures that:
o The remote peer behaves properly. o The remote peer behaves properly.
o The local system is not forced into the two-element subgroup. o The local system is not forced into the two-element subgroup.
5. IANA Considerations 5. IANA Considerations
IANA is requested to add to the Key Exchange Method Names algorithm IANA has added the following entries to the "Key Exchange Method
registry [IANA-KEX] with the following entries: Names" registry [IANA-KEX]:
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, Method Name Reference
Peter Gutmann, Damien Miller, Niels Moeller, Matt Johnston, Iwamoto ----------------------------- ---------
Kouichi, Dave Dugal, Daniel Migault, Anna Johnston, Ron Frederick, diffie-hellman-group14-sha256 RFC 8268
Rich Salz, Travis Finkenauer, Eric Rescorla. diffie-hellman-group15-sha512 RFC 8268
diffie-hellman-group16-sha512 RFC 8268
diffie-hellman-group17-sha512 RFC 8268
diffie-hellman-group18-sha512 RFC 8268
7. Security Considerations 6. Security Considerations
The security considerations of [RFC4253] apply to this document. The security considerations of [RFC4253] apply to this document.
The security considerations of [RFC3526] suggest that MODP group14 The security considerations of [RFC3526] suggest that MODP group14
through group18 have security strengths that range between 110 bits through group18 have security strengths that range between 110 bits
of security through 310 bits of security. They are based on of security through 310 bits of security. They are based on
[RFC3766] Determining Strengths For Public Keys Used For Exchanging "Determining Strengths For Public Keys Used For Exchanging Symmetric
Symmetric Keys. Care should be taken to use sufficient entropy and/ Keys" [RFC3766]. Care should be taken to use sufficient entropy and/
or DRBG algorithms to maximize the true security strength of the key or deterministic random-bit generator (DRBG) algorithms to maximize
exchange and ciphers selected. the true security strength of the key exchange and ciphers selected.
Using a fixed set of Diffie-Hellman parameters makes them a high Using a fixed set of Diffie-Hellman parameters makes them a high
value target for pre-computation. Generating additional sets of value target for pre-computation. Generating additional sets of
primes to be used, or moving to larger values is a mitigation against primes to be used, or moving to larger values mitigates this issue.
this issue.
8. References 7. References
8.1. Normative References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC3526] Kivinen, T. and M. Kojo, "More Modular Exponential (MODP) [RFC3526] Kivinen, T. and M. Kojo, "More Modular Exponential (MODP)
Diffie-Hellman groups for Internet Key Exchange (IKE)", Diffie-Hellman groups for Internet Key Exchange (IKE)",
RFC 3526, DOI 10.17487/RFC3526, May 2003, RFC 3526, DOI 10.17487/RFC3526, May 2003,
<https://www.rfc-editor.org/info/rfc3526>. <https://www.rfc-editor.org/info/rfc3526>.
skipping to change at page 6, line 30 skipping to change at page 6, line 49
[RFC4253] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH) [RFC4253] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253, Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253,
January 2006, <https://www.rfc-editor.org/info/rfc4253>. January 2006, <https://www.rfc-editor.org/info/rfc4253>.
[RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms [RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", RFC 6234, (SHA and SHA-based HMAC and HKDF)", RFC 6234,
DOI 10.17487/RFC6234, May 2011, DOI 10.17487/RFC6234, May 2011,
<https://www.rfc-editor.org/info/rfc6234>. <https://www.rfc-editor.org/info/rfc6234>.
8.2. Informative References [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>.
[IANA-KEX] 7.2. Informative References
Internet Assigned Numbers Authority (IANA), "Secure Shell
(SSH) Protocol Parameters: Key Exchange Method Names", [IANA-KEX] IANA, "Secure Shell (SSH) Protocol Parameters",
March 2017, <http://www.iana.org/assignments/ssh- <http://www.iana.org/assignments/ssh-parameters/>
parameters/ssh-parameters.xhtml#ssh-parameters-16>.
[MFQ-U-OO-815099-15] [MFQ-U-OO-815099-15]
"National Security Agency/Central Security Service", "CNSA National Security Agency / Central Security Service,
Suite and Quantum Computing FAQ", January 2016, "Commerical National Security Algorithm Suite and Quantum
Computing FAQ", MFQ U/OO/815099-15 , January 2016,
<https://www.iad.gov/iad/library/ia-guidance/ <https://www.iad.gov/iad/library/ia-guidance/
ia-solutions-for-classified/algorithm-guidance/ ia-solutions-for-classified/algorithm-
cnsa-suite-and-quantum-computing-faq.cfm>. guidance/assets/public/upload/
CNSA-Suite-and-Quantum-Computing-FAQ.pdf>.
[NIST-SP-800-131Ar1] [NIST-SP-800-131Ar1]
Barker and Roginsky, "Transitions: Recommendation for the Barker and Roginsky, "Transitions: Recommendation for the
Transitioning of the Use of Cryptographic Algorithms and Transitioning of the Use of Cryptographic Algorithms and
Key Lengths", NIST Special Publication 800-131A Revision Key Lengths", NIST Special Publication 800-131A,
1, November 2015, Revision 1, DOI 10.6028/NIST.SP.800-131Ar1, November 2015,
<http://nvlpubs.nist.gov/nistpubs/SpecialPublications/ <http://dx.doi.org/10.6028/NIST.SP.800-131Ar1>.
NIST.SP.800-131Ar1.pdf>.
[RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For [RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For
Public Keys Used For Exchanging Symmetric Keys", BCP 86, Public Keys Used For Exchanging Symmetric Keys", BCP 86,
RFC 3766, DOI 10.17487/RFC3766, April 2004, RFC 3766, DOI 10.17487/RFC3766, April 2004,
<https://www.rfc-editor.org/info/rfc3766>. <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 [RFC6194] Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security
Considerations for the SHA-0 and SHA-1 Message-Digest Considerations for the SHA-0 and SHA-1 Message-Digest
Algorithms", RFC 6194, DOI 10.17487/RFC6194, March 2011, Algorithms", RFC 6194, DOI 10.17487/RFC6194, March 2011,
<https://www.rfc-editor.org/info/rfc6194>. <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 Author's Address
Mark D. Baushke Mark D. Baushke
Juniper Networks, Inc. Juniper Networks, Inc.
1133 Innovation Way 1133 Innovation Way
Sunnyvale, CA 94089-1228 Sunnyvale, CA 94089-1228
US United States of America
Phone: +1 408 745 2952 Phone: +1 408 745 2952
Email: mdb@juniper.net Email: mdb@juniper.net
URI: http://www.juniper.net/ URI: http://www.juniper.net/
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