Internet DRAFT - draft-ietf-curdle-rsa-sha2
draft-ietf-curdle-rsa-sha2
Internet-Draft D. Bider
Updates: 4252, 4253 (if approved) Bitvise Limited
Intended status: Standards Track October 12, 2017
Expires: April 12, 2018
Use of RSA Keys with SHA-256 and SHA-512 in Secure Shell (SSH)
draft-ietf-curdle-rsa-sha2-12.txt
Abstract
This memo updates RFC 4252 and RFC 4253 to define new public key
algorithms for use of RSA keys with SHA-256 and SHA-512 for server and
client authentication in SSH connections.
Status
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Copyright
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November 10,
2008. The person(s) controlling the copyright in some of this material
may not have granted the IETF Trust the right to allow modifications
of such material outside the IETF Standards Process. Without obtaining
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Standards Process, and derivative works of it may not be created
outside the IETF Standards Process, except to format it for
publication as an RFC or to translate it into languages other than
English.
1. Overview and Rationale
Secure Shell (SSH) is a common protocol for secure communication on
the Internet. In [RFC4253], SSH originally defined the public key
algorithms "ssh-rsa" for server and client authentication using RSA
with SHA-1, and "ssh-dss" using 1024-bit DSA and SHA-1. These
algorithms are now considered deficient. For US government use, NIST
has disallowed 1024-bit RSA and DSA, and use of SHA-1 for signing
[800-131A].
This memo updates RFC 4252 and RFC 4253 to define new public key
algorithms allowing for interoperable use of existing and new RSA keys
with SHA-256 and SHA-512.
1.1. Requirements Terminology
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].
1.2. Wire Encoding Terminology
The wire encoding types in this document - "boolean", "byte",
"string", "mpint" - have meanings as described in [RFC4251].
2. Public Key Format vs. Public Key Algorithm
In [RFC4252], the concept "public key algorithm" is used to establish
a relationship between one algorithm name, and:
A. Procedures used to generate and validate a private/public keypair.
B. A format used to encode a public key.
C. Procedures used to calculate, encode, and verify a signature.
This document uses the term "public key format" to identify only A and
B in isolation. The term "public key algorithm" continues to identify
all three aspects A, B, and C.
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3. New RSA Public Key Algorithms
This memo adopts the style and conventions of [RFC4253] in specifying
how use of a public key algorithm is indicated in SSH.
The following new public key algorithms are defined:
rsa-sha2-256 RECOMMENDED sign Raw RSA key
rsa-sha2-512 OPTIONAL sign Raw RSA key
These algorithms are suitable for use both in the SSH transport layer
[RFC4253] for server authentication, and in the authentication layer
[RFC4252] for client authentication.
Since RSA keys are not dependent on the choice of hash function, the
new public key algorithms reuse the "ssh-rsa" public key format as
defined in [RFC4253]:
string "ssh-rsa"
mpint e
mpint n
All aspects of the "ssh-rsa" format are kept, including the encoded
string "ssh-rsa". This allows existing RSA keys to be used with the
new public key algorithms, without requiring re-encoding, or affecting
already trusted key fingerprints.
Signing and verifying using these algorithms is performed according to
the RSASSA-PKCS1-v1_5 scheme in [RFC8017] using SHA-2 [SHS] as hash.
For the algorithm "rsa-sha2-256", the hash used is SHA-256.
For the algorithm "rsa-sha2-512", the hash used is SHA-512.
The resulting signature is encoded as follows:
string "rsa-sha2-256" / "rsa-sha2-512"
string rsa_signature_blob
The value for 'rsa_signature_blob' is encoded as a string containing
S - an octet string which is the output of RSASSA-PKCS1-v1_5, of
length equal to the length in octets of the RSA modulus.
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3.1. Use for server authentication
To express support and preference for one or both of these algorithms
for server authentication, the SSH client or server includes one or
both algorithm names, "rsa-sha2-256" and/or "rsa-sha2-512", in the
name-list field "server_host_key_algorithms" in the SSH_MSG_KEXINIT
packet [RFC4253]. If one of the two host key algorithms is negotiated,
the server sends an "ssh-rsa" public key as part of the negotiated key
exchange method (e.g. in SSH_MSG_KEXDH_REPLY), and encodes a signature
with the appropriate signature algorithm name - either "rsa-sha2-256",
or "rsa-sha2-512".
3.2. Use for client authentication
To use this algorithm for client authentication, the SSH client sends
an SSH_MSG_USERAUTH_REQUEST message [RFC4252] encoding the "publickey"
method, and encoding the string field "public key algorithm name" with
the value "rsa-sha2-256" or "rsa-sha2-512". The "public key blob"
field encodes the RSA public key using the "ssh-rsa" public key
format.
For example, as defined in [RFC4252] and [RFC4253], an SSH "publickey"
authentication request using an "rsa-sha2-512" signature would be
properly encoded as follows:
byte SSH_MSG_USERAUTH_REQUEST
string user name
string service name
string "publickey"
boolean TRUE
string "rsa-sha2-512"
string public key blob:
string "ssh-rsa"
mpint e
mpint n
string signature:
string "rsa-sha2-512"
string rsa_signature_blob
If the client includes the signature field, the client MUST encode the
same algorithm name in the signature as in SSH_MSG_USERAUTH_REQUEST -
either "rsa-sha2-256", or "rsa-sha2-512". If a server receives a
mismatching request, it MAY apply arbitrary authentication penalties,
including but not limited to authentication failure or disconnect.
OpenSSH 7.2 (but not 7.2p2) incorrectly encodes the algorithm in the
signature as "ssh-rsa" when the algorithm in SSH_MSG_USERAUTH_REQUEST
is "rsa-sha2-256" or "rsa-sha2-512". In this case, the signature does
actually use either SHA-256 or SHA-512. A server MAY, but is not
required to, accept this variant, or another variant that corresponds
to a good-faith implementation, and is decided to be safe to accept.
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3.3. Discovery of public key algorithms supported by servers
Implementation experience has shown that there are servers which apply
authentication penalties to clients attempting public key algorithms
which the SSH server does not support.
Servers that accept rsa-sha2-* signatures for client authentication
SHOULD implement the extension negotiation mechanism defined in
[EXT-INFO], including especially the "server-sig-algs" extension.
When authenticating with an RSA key against a server that does not
implement the "server-sig-algs" extension, clients MAY default to an
"ssh-rsa" signature to avoid authentication penalties. When the new
rsa-sha2-* algorithms have been sufficiently widely adopted to warrant
disabling "ssh-rsa", clients MAY default to one of the new algorithms.
4. IANA Considerations
IANA is requested to update the "Secure Shell (SSH) Protocol
Parameters" registry established with [RFC4250], to extend the table
Public Key Algorithm Names [IANA-PKA]:
- To the immediate right of the column Public Key Algorithm Name,
a new column is to be added, titled Public Key Format. For existing
entries, the column Public Key Format should be assigned the same
value found under Public Key Algorithm Name.
- Immediately following the existing entry for "ssh-rsa", two sibling
entries are to be added:
P. K. Alg. Name P. K. Format Reference Note
rsa-sha2-256 ssh-rsa [this document] Section 3
rsa-sha2-512 ssh-rsa [this document] Section 3
5. Security Considerations
The security considerations of [RFC4251] apply to this document.
5.1. Key Size and Signature Hash
The National Institute of Standards and Technology (NIST) Special
Publication 800-131A, Revision 1 [800-131A], disallows the use of RSA
and DSA keys shorter than 2048 bits for US government use. The same
document disallows the SHA-1 hash function for digital signature
generation, except under NIST's protocol-specific guidance.
It is prudent to follow this advice also outside of US government use.
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5.2. Transition
This document is based on the premise that RSA is used in environments
where a gradual, compatible transition to improved algorithms will be
better received than one that is abrupt and incompatible. It advises
that SSH implementations add support for new RSA public key algorithms
along with SSH_MSG_EXT_INFO and the "server-sig-algs" extension to
allow coexistence of new deployments with older versions that support
only "ssh-rsa". Nevertheless, implementations SHOULD start to disable
"ssh-rsa" in their default configurations as soon as they have reason
to believe that new RSA signature algorithms have been widely adopted.
5.3. PKCS#1 v1.5 Padding and Signature Verification
This document prescribes RSASSA-PKCS1-v1_5 signature padding because:
(1) RSASSA-PSS is not universally available to all implementations;
(2) PKCS#1 v1.5 is widely supported in existing SSH implementations;
(3) PKCS#1 v1.5 is not known to be insecure for use in this scheme.
Implementers are advised that a signature with PKCS#1 v1.5 padding
MUST NOT be verified by applying the RSA key to the signature, and
then parsing the output to extract the hash. This may give an attacker
opportunities to exploit flaws in the parsing and vary the encoding.
Verifiers MUST instead apply PKCS#1 v1.5 padding to the expected hash,
then compare the encoded bytes with the output of the RSA operation.
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6. References
6.1. Normative References
[SHS] National Institute of Standards and Technology (NIST),
United States of America, "Secure Hash Standard (SHS)",
FIPS Publication 180-4, August 2015,
<http://dx.doi.org/10.6028/NIST.FIPS.180-4>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4251] Lehtinen, S. and C. Lonvick, Ed., "The Secure Shell (SSH)
Protocol Architecture", RFC 4251, January 2006.
[RFC4252] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Authentication Protocol", RFC 4252, January 2006.
[RFC4253] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Transport Layer Protocol", RFC 4253, January 2006.
[EXT-INFO] Bider, D., "Extension Negotiation in Secure Shell (SSH)",
draft-ietf-curdle-ssh-ext-info-15.txt, September 2017,
<https://tools.ietf.org/html/
draft-ietf-curdle-ssh-ext-info-15>.
6.2. Informative References
[800-131A] National Institute of Standards and Technology (NIST),
"Transitions: Recommendation for Transitioning 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>.
[RFC4250] Lehtinen, S. and C. Lonvick, Ed., "The Secure Shell (SSH)
Protocol Assigned Numbers", RFC 4250, January 2006.
[RFC8017] Moriarty, K., Kaliski, B., Jonsson, J. and Rusch, A.,
"PKCS #1: RSA Cryptography Specifications Version 2.2",
RFC 8017, November 2016.
[IANA-PKA] "Secure Shell (SSH) Protocol Parameters",
<https://www.iana.org/assignments/ssh-parameters/
ssh-parameters.xhtml#ssh-parameters-19>.
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Author's Address
Denis Bider
Bitvise Limited
4105 Lombardy Court
Colleyville, Texas 76034
United States of America
Email: ietf-ssh3@denisbider.com
URI: https://www.bitvise.com/
Acknowledgments
Thanks to Jon Bright, Niels Moeller, Stephen Farrell, Mark D. Baushke,
Jeffrey Hutzelman, Hanno Boeck, Peter Gutmann, Damien Miller, Mat
Berchtold, Roumen Petrov, Daniel Migault, Eric Rescorla, Russ Housley,
Alissa Cooper, Adam Roach, and Ben Campbell for reviews, comments, and
suggestions.
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