--- 1/draft-ietf-curdle-rsa-sha2-11.txt 2017-10-12 01:13:44.470820511 -0700 +++ 2/draft-ietf-curdle-rsa-sha2-12.txt 2017-10-12 01:13:44.486820991 -0700 @@ -1,23 +1,23 @@ Internet-Draft D. Bider Updates: 4252, 4253 (if approved) Bitvise Limited -Intended status: Standards Track October 6, 2017 -Expires: April 6, 2018 +Intended status: Standards Track October 12, 2017 +Expires: April 12, 2018 - Use of RSA Keys with SHA-2 256 and 512 in Secure Shell (SSH) - draft-ietf-curdle-rsa-sha2-11.txt + 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-2 hashing for server and + 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. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. @@ -65,21 +65,21 @@ 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-2 hashing. + 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", @@ -121,22 +121,22 @@ 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-2 256. - For the algorithm "rsa-sha2-512", the hash used is SHA-2 512. + 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. @@ -152,42 +152,53 @@ 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. The signature field, if present, encodes a signature using an - algorithm name that MUST match the SSH authentication request - either - "rsa-sha2-256", or "rsa-sha2-512". + 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. + 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. @@ -220,20 +231,22 @@ 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. + 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 @@ -268,56 +281,53 @@ [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, + . + 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, . [RFC4250] Lehtinen, S. and C. Lonvick, Ed., "The Secure Shell (SSH) Protocol Assigned Numbers", RFC 4250, January 2006. - [RFC6979] Pornin, T., "Deterministic Usage of the Digital - Signature Algorithm (DSA) and Elliptic Curve Digital - Signature Algorithm (ECDSA)", RFC 6979, August 2013. - [RFC8017] Moriarty, K., Kaliski, B., Jonsson, J. and Rusch, A., "PKCS #1: RSA Cryptography Specifications Version 2.2", RFC 8017, November 2016. - [EXT-INFO] Bider, D., "Extension Negotiation in Secure Shell (SSH)", - draft-ietf-curdle-ssh-ext-info-15.txt, September 2017, - . - [IANA-PKA] "Secure Shell (SSH) Protocol Parameters", . 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, and Eric Rescorla for reviews, - comments, and suggestions. + Berchtold, Roumen Petrov, Daniel Migault, Eric Rescorla, Russ Housley, + Alissa Cooper, Adam Roach, and Ben Campbell for reviews, comments, and + suggestions.