Use of SHA-256 Algorithm with RSA, DSA and ECDSA in SSHFP Resource Records

The Domain Name System (DNS) is the global, hierarchical distributed database for Internet Naming. The Secure Shell (SSH) is a protocol for secure remote login and other secure network services over an insecure network. RFC 4253 defines Public Key Algorithms for the Secure Shell server public keys. The DNS has been extended to store fingerprints in a DNS resource record named SSHFP , which provides out-of-band verification by looking up a fingerprint of the server public key in the DNS , and using Domain Name System Security Extensions (DNSSEC) , , to verify the lookup. RFC 4255 describes how to store the cryptographic fingerprint of SSH public keys in SSHFP resource records. SSHFP records contain the fingerprint and two index numbers identifying the cryptographic algorithms used: to link the fingerprinted public key with the corresponding private key, and to derive the message digest stored as the fingerprint in the record. RFC 4255 then specifies lists of cryptographic algorithms and the corresponding index numbers used to identify them in SSHFP records. This document updates the IANA registry "SSHFP RR Types for public key algorithms" and "SSHFP RR types for fingerprint types" by adding a new option in each list: the Elliptic Curve Digital Signature Algorithm (ECDSA) which has been added to the Secure Shell Public Key list by RFC 5656 in the public key algorithms list; the SHA-256 algorithm in the SSHFP Fingerprint Type list. Familiarity with DNSSEC, SSH Protocol , , , SSHFP , and the SHA-2 family of algorithms is assumed in this document.

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 RFC 2119.

The format of the SSHFP RR can be found in RFC 4255.

The fingerprint type octet identifies the message-digest algorithm used to calculate the fingerprint of the public key.

SHA-256 fingerprints of the public keys are stored in SSHFP Resource Record with the fingerprint type 2.

The SSHFP Resource Record algorithm number octet describes the algorithm of the public key.

ECDSA public keys are stored in SSHFP Resource Records with the algorithm number 3.

SSHFP-aware Secure Shell implementations SHOULD support the SHA-256 fingerprints for verification of the public key. Secure Shell implementations which support SHA-256 fingerprints MUST prefer a SHA-256 fingerprint over SHA-1 if both are available for a server. If the SHA-256 fingerprint is tested and does not match the key SSH public key received from the SSH server, then the key MUST be rejected rather than testing the alternative SHA-1 fingerprint.

SSHFP-aware Secure Shell implementations which also implement ECDSA algorithm for the public key SHOULD support SSHFP fingerprints for ECDSA public keys.

The following examples provide reference for both the newly defined ECDSA algorithm number and the use of the SHA-256 fingerprint combined with both the new and the existing algorithm numbers.

Given a public key with the following value in OpenSSH format : ---- BEGIN SSH2 PUBLIC KEY ---- AAAAB3NzaC1yc2EAAAADAQABAAABAQDCUR4JOhxTinzq7QO3bQXW4jmPCCulFsnh 8Yi7MKwpMnd96+T7uV7nEwy+6+GWYu98IxFJByIjFXX/a6BXDp3878wezH1DZ2tN D/tu/eudz6ErpTFYmnVLyEDARYSzVBNQuIK1UDqvvB6KffJcyt78FpwW27euGkqE kam7GaurPRAgwXehDB/gMwRtXVRZ+13zYWkAmAY+5OAWVmdXuQVm5kjlvcNzto2H 3m3nqJtD4J9L1lKPuSVVqwJr4/6hibXJkQEvWpUvdOAUw3frKpNwa932fXFk3ke4 rsDjQ/W8GyleMtK3Tx8tE4z1wuowXtYe6Ba8q3LAPs/m2S4pUscx ---- END SSH2 PUBLIC KEY ----

The SSHFP Resource Record for this key would be: server.example.net IN SSHFP 1 1 ( dd465c09cfa51fb45020cc83316fff 21b9ec74ac )

The SSHFP Resource Record for this key would be: server.example.net IN SSHFP 1 2 ( b049f950d1397b8fee6a61e4d14a9a cdc4721e084eff5460bbed80cfaa2c e2cb )

Given a public key with the following value in OpenSSH format: ---- BEGIN SSH2 PUBLIC KEY ---- AAAAB3NzaC1kc3MAAACBAPVFrc0U36gWaywbfJzjcv8ef13qAX4EJl8Na6xqvXh1 t+aCJEdS7soRjtvK4KsNhk78DjdtnfhEhyFKHHNz3i6/c/s9lP0UjV7mRAo6nA7A 3Gs6iQElb6O9Fqm6iVSC6bYWilTSB0tYencEEJUoaAua8YQF/uxRzPrReXxGqHnj AAAAFQDC9M/pli8VIVmEGOO0wC1TeUTN4wAAAIEAgA2Fbkbbeo0+u/qw8mQFOFWZ pTaqNo7d7jov3majbh5LqEVD7yT3MS1GSGhjgvvhus/ehMTqzYbjTc0szUM9JnwT 7xq15P2ZYDK98IVxrw31jMtsUUEmBqB4DUjTurtcaWmJ9LNaP1/k4bMo0/hotnOc OVnIPsTLBFWVvdNRxUAAAACAOZcDcK01NTM1qIIYbBqCffrwjQ+9PmsuSKI6nUzf S4NysXHkdbW5u5VxeXLcwWj5PGbRfoS2P3vwYAmakqgq502wigam18u9nAczUYl+ 2kOeOiIRrtSmLfpV7thLOAb8k1ESjIlkbn35jKmTcoMFRXbFmkKRTK8OEnWQ8AVg 6w8= ---- END SSH2 PUBLIC KEY ----

The SSHFP Resource Record for this key would be: server.example.net IN SSHFP 2 1 ( 3b6ba6110f5ffcd29469fc1ec2ee25 d61718badd )

The SSHFP Resource Record for this key would be: server.example.net IN SSHFP 2 2 ( f9b8a6a460639306f1b38910456a6a e1018a253c47ecec12db77d7a0878b 4d83 )

Given a public key with the following value in OpenSSH format: ---- BEGIN SSH2 PUBLIC KEY ---- AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBAD+9COUiX7W YgcvIOdI8+djdoFDVUTxNrcog8sSYdbIzeG+bYdsssvcyy/nRfVhXC5QBCk8IThq s7D4/lFxX5g= ---- END SSH2 PUBLIC KEY ----

The SSHFP Resource Record for this key would be: server.example.net IN SSHFP 3 1 ( c64607a28c5300fec1180b6e417b92 2943cffcdd )

The SSHFP Resource Record for this key would be: server.example.net IN SSHFP 3 2 ( 821eb6c1c98d9cc827ab7f456304c0 f14785b7008d9e8646a8519de80849 afc7 )

This document updates the IANA registry "SSHFP RR Types for public key algorithms" and "SSHFP RR types for fingerprint types" .

The following entries are added to the "SSHFP RR Types for public key algorithms" registry: Value Description Reference 3 ECDSA [This doc]

The following entries are added to the "SSHFP RR types for fingerprint types" registry: Value Description Reference 2 SHA-256 [This doc]

Please see the security considerations in for SSHFP record and for ECDSA algorithm. Users of SSHFP are encouraged to deploy SHA-256 as soon as implementations allow for it. SHA-2 family of algorithms is widely believed to be more resilient to attack than SHA-1, and confidence in SHA-1's strength is being eroded by recently announced attacks . Regardless of whether or not the attacks on SHA-1 will affect SSHFP, it is believed (at the time of this writing) that SHA-256 is the better choice for use in SSHFP records. SHA-256 is considered sufficiently strong for the immediate future, but predictions about future development in cryptography and cryptanalysis are beyond the scope of this document.