--- 1/draft-ietf-curdle-rsa-sha2-04.txt 2017-04-09 00:13:48.777094115 -0700 +++ 2/draft-ietf-curdle-rsa-sha2-05.txt 2017-04-09 00:13:48.797094517 -0700 @@ -1,24 +1,24 @@ Internet-Draft D. Bider Updates: 4252, 4253 (if approved) Bitvise Limited -Intended status: Standards Track March 29, 2017 -Expires: September 29, 2017 +Intended status: Standards Track April 9, 2017 +Expires: October 9, 2017 Use of RSA Keys with SHA-2 256 and 512 in Secure Shell (SSH) - draft-ietf-curdle-rsa-sha2-04.txt + draft-ietf-curdle-rsa-sha2-05.txt Abstract - This memo defines an algorithm name, public key format, and signature - format for use of RSA keys with SHA-2 hashing for server and client - authentication in SSH connections. + This memo updates [RFC4252] and [RFC4253] to define an algorithm name, + public key format, and signature format for use of RSA keys with SHA-2 + hashing 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. @@ -41,20 +41,32 @@ This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. + 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 + an adequate license from the person(s) controlling the copyright in + such materials, this document may not be modified outside the IETF + 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 signature methods "ssh-rsa" for server and client authentication using RSA with SHA-1, and "ssh-dss" using 1024-bit DSA and SHA-1. A decade later, these signature methods are considered deficient. For US government use, NIST has disallowed 1024-bit RSA and DSA, and use of SHA-1 for signing [800-131A]. @@ -137,22 +149,22 @@ 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 signature formats, 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 [RFC3447] using SHA-2 [FIPS-180-4] as - hash; MGF1 as mask function; and salt length equal to hash size. + the RSASSA-PKCS1-v1_5 scheme in [RFC8017] using SHA-2 [SHS] as hash; + MGF1 as mask function; and salt length equal to hash size. 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. 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 @@ -200,120 +212,148 @@ string rsa_signature_blob 3.3. Discovery of signature algorithms supported by servers Implementation experience has shown that there are servers which apply authentication penalties to clients attempting signature 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 - [SSH-EXT-INFO], including especially the "server-sig-algs" extension. + [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. + "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, to extend the table Public Key Algorithm Names: + 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 Signature Algorithm Name. For existing entries, the column Signature Algorithm Name 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 Sig. Alg. Name Reference Note ssh-rsa rsa-sha2-256 [this document] Section 3 ssh-rsa rsa-sha2-512 [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 [800-131A] disallows the use of RSA and DSA keys shorter than 2048 bits for US government use after 2013. The same document disallows the SHA-1 hash function, as used in the "ssh-rsa" and "ssh-dss" algorithms, for digital signature generation after 2013. +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 signature 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 use of PKCS#1 v1.5 signature padding because: + + (1) PSS is not universally available to all SSH 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, + assuming reasonable implementation. + + 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. + Implementations SHOULD apply PKCS#1 v1.5 padding to the expected hash, + THEN compare the encoded bytes with the output of the RSA operation. + 6. Why no DSA? A draft version of this memo also defined an algorithm name for use of 2048-bit and 3072-bit DSA keys with a 256-bit subgroup and SHA-2 256 hashing. It is possible to implement DSA securely by generating "k" deterministically as per [RFC6979]. However, a plurality of reviewers were concerned that implementers would continue to use libraries that generate "k" randomly. This is vulnerable to biased "k" generation, and extremely vulnerable to "k" reuse. This document therefore disrecommends DSA, in favor of RSA and elliptic curve cryptography. 7. References 7.1. Normative References - [FIPS-180-4] - National Institute of Standards and Technology (NIST), + [SHS] National Institute of Standards and Technology (NIST), United States of America, "Secure Hash Standard (SHS)", FIPS Publication 180-4, August 2015, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. - [RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography - Standards (PKCS) #1: RSA Cryptography Specifications - Version 2.1", RFC 3447, February 2003. + [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO + 10646", STD 63, RFC 3629, November 2003. [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. + [RFC8017] Moriarty, K., Kaliski, B., Jonsson, J. and Rusch, A., + "PKCS #1: RSA Cryptography Specifications Version 2.2", + RFC 8017, November 2016. + 7.2. 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, January 2011, . [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. - [SSH-EXT-INFO] - Bider, D., "Extension Negotiation in Secure Shell (SSH)", - draft-ietf-curdle-ssh-ext-info-03.txt, March 2017, + [EXT-INFO] Bider, D., "Extension Negotiation in Secure Shell (SSH)", + draft-ietf-curdle-ssh-ext-info-04.txt, April 2017, . + draft-ietf-curdle-ssh-ext-info-04>. + + [IANA-PKA] "Secure Shell (SSH) Protocol Parameters", + . Author's Address Denis Bider Bitvise Limited Suites 41/42, Victoria House 26 Main Street GI Phone: +506 8315 6519