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Internet Engineering Task Force (IETF)
Request for Comments: 6594
Category: Standards Track
ISSN: 2070-1721
O. Sury
CZ.NIC
April 2012

Use of the SHA-256 Algorithm with RSA,

Digital Signature Algorithm (DSA),

and Elliptic Curve DSA (ECDSA) in SSHFP Resource Records

Abstract

This document updates the IANA registries in RFC 4255, which defines SSHFP, a DNS Resource Record (RR) that contains a standard Secure Shell (SSH) key fingerprint used to verify SSH host keys using DNS Security Extensions (DNSSEC). This document defines additional options supporting SSH public keys applying the Elliptic Curve Digital Signature Algorithm (ECDSA) and the implementation of fingerprints computed using the SHA-256 message digest algorithm in SSHFP Resource Records.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the fInternet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6594.

Copyright Notice

Copyright © 2012 IETF Trust and the persons identified as the document authors. All rights reserved.

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.

Table of Contents

   1. Introduction ....................................................3
   2. Requirements Language ...........................................4
   3. SSHFP Resource Records ..........................................4
      3.1. SSHFP Fingerprint Type Specification .......................4
           3.1.1. SHA-256 SSHFP Fingerprint Type Specification ........4
      3.2. SSHFP Algorithm Number Specification .......................4
           3.2.1. ECDSA SSHFP Algorithm Number Specification ..........4
   4. Implementation Considerations ...................................4
      4.1. Support for SHA-256 Fingerprints ...........................4
      4.2. Support for ECDSA ..........................................4
   5. Examples ........................................................5
      5.1. RSA Public Key .............................................5
           5.1.1. RSA Public Key with SHA1 Fingerprint ................5
           5.1.2. RSA Public Key with SHA-256 Fingerprint .............5
      5.2. DSA Public Key .............................................6
           5.2.1. DSA Public Key with SHA1 Fingerprint ................6
           5.2.2. DSA Public Key with SHA-256 Fingerprint .............6
      5.3. ECDSA Public Key ...........................................6
           5.3.1. ECDSA Public Key with SHA1 Fingerprint ..............7
           5.3.2. ECDSA Public Key with SHA-256 Fingerprint ...........7
   6. IANA Considerations .............................................7
      6.1. SSHFP RR Types for Public Key Algorithms ...................7
      6.2. SSHFP RR Types for Fingerprint Types .......................7
   7. Security Considerations .........................................8
   8. References ......................................................8
      8.1. Normative References .......................................8
      8.2. Informative References .....................................9

1. Introduction

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 [RFC4253] 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 [RFC4255], which provides out-of-band verification by looking up a fingerprint of the server public key in the DNS [RFC1034][RFC1035] and using Domain Name System Security Extensions (DNSSEC) [RFC4033][RFC4034][RFC4035] to verify the lookup.

RFC 4255 [RFC4255] describes how to store the cryptographic fingerprint of SSH public keys in SSHFP Resource Records. SSHFP Resource Records contain the fingerprint and two index numbers identifying the cryptographic algorithms used:

  1. to link the fingerprinted public key with the corresponding private key, and
  1. to derive the message digest stored as the fingerprint in the record.

RFC 4255 [RFC4255] then specifies lists of cryptographic algorithms and the corresponding index numbers used to identify them in SSHFP Resource Records.

This document updates the IANA registry "SSHFP RR Types for public key algorithms" and "SSHFP RR types for fingerprint types" [SSHFPVALS] by adding a new option in each list:

  • the Elliptic Curve Digital Signature Algorithm (ECDSA) [RFC6090], which has been added to the Secure Shell Public Key list by RFC 5656 [RFC5656] in the public key algorithms list, and
   o  the SHA-256 algorithm [FIPS.180-3.2008] in the SSHFP fingerprint
      type list.
   
   Familiarity with DNSSEC, SSH Protocol [RFC4251][RFC4253][RFC4250],
   SSHFP [RFC4255], and the SHA-2 [FIPS.180-3.2008] family of algorithms
   is assumed in this document.

2. Requirements Language

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 [RFC2119].

3. SSHFP Resource Records

The format of the SSHFP RR can be found in RFC 4255 [RFC4255].

3.1. SSHFP Fingerprint Type Specification

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

3.1.1. SHA-256 SSHFP Fingerprint Type Specification

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

3.2. SSHFP Algorithm Number Specification

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

3.2.1. ECDSA SSHFP Algorithm Number Specification

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

4. Implementation Considerations

4.1. Support for SHA-256 Fingerprints

SSHFP-aware Secure Shell implementations SHOULD support the SHA-256 fingerprints for verification of the public key. Secure Shell implementations that 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 SSH public key received from the SSH server, then the key MUST be rejected rather than testing the alternative SHA-1 fingerprint.

4.2. Support for ECDSA

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

5. Examples

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

5.1. RSA Public Key

A public key with the following value in OpenSSH format [RFC4716] would appear as follows: 

       ---- 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 ----

5.1.1. RSA Public Key with SHA1 Fingerprint

The SSHFP Resource Record for this key would be: 

       server.example.net IN SSHFP 1 1 ( dd465c09cfa51fb45020cc83316fff
                                         21b9ec74ac )

5.1.2. RSA Public Key with SHA-256 Fingerprint

The SSHFP Resource Record for this key would be: 

       server.example.net IN SSHFP 1 2 ( b049f950d1397b8fee6a61e4d14a9a
                                         cdc4721e084eff5460bbed80cfaa2c
                                         e2cb )

5.2. DSA Public Key

A public key with the following value in OpenSSH format would appear as follows: 

       ---- 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 ----

5.2.1. DSA Public Key with SHA1 Fingerprint

The SSHFP Resource Record for this key would be: 

       server.example.net IN SSHFP 2 1 ( 3b6ba6110f5ffcd29469fc1ec2ee25
                                         d61718badd )

5.2.2. DSA Public Key with SHA-256 Fingerprint

The SSHFP Resource Record for this key would be: 

       server.example.net IN SSHFP 2 2 ( f9b8a6a460639306f1b38910456a6a
                                         e1018a253c47ecec12db77d7a0878b
                                         4d83 )

5.3. ECDSA Public Key

A public key with the following value in OpenSSH format would appear as follows: 

       ---- BEGIN SSH2 PUBLIC KEY ----
       AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBAD+9COUiX7W
       YgcvIOdI8+djdoFDVUTxNrcog8sSYdbIzeG+bYdsssvcyy/nRfVhXC5QBCk8IThq
       s7D4/lFxX5g=
       ---- END SSH2 PUBLIC KEY ----

5.3.1. ECDSA Public Key with SHA1 Fingerprint

The SSHFP Resource Record for this key would be: 

       server.example.net IN SSHFP 3 1 ( c64607a28c5300fec1180b6e417b92
                                         2943cffcdd )

5.3.2. ECDSA Public Key with SHA-256 Fingerprint

The SSHFP Resource Record for this key would be: 

       server.example.net IN SSHFP 3 2 ( 821eb6c1c98d9cc827ab7f456304c0
                                         f14785b7008d9e8646a8519de80849
                                         afc7 )

6. IANA Considerations

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

6.1. SSHFP RR Types for Public Key Algorithms

The following entries have been added to the "SSHFP RR Types for public key algorithms" registry: 

                   +-------+-------------+------------+
                   | Value | Description |  Reference |
                   +-------+-------------+------------+
                   |   3   |    ECDSA    | [This doc] |
                   +-------+-------------+------------+

6.2. SSHFP RR Types for Fingerprint Types

The following entries have been added to the "SSHFP RR types for fingerprint types" registry: 

                   +-------+-------------+------------+
                   | Value | Description |  Reference |
                   +-------+-------------+------------+
                   |   2   |   SHA-256   | [This doc] |
                   +-------+-------------+------------+

7. Security Considerations

Please see the security considerations in [RFC4255] for SSHFP Resource Records and [RFC5656] for the ECDSA.

Users of SSHFP are encouraged to deploy SHA-256 as soon as implementations allow for it. The 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 [IACR2007/474]. 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.

8. References

8.1. Normative References

[FIPS.180-3.2008] 

               National Institute of Standards and Technology, "Secure
               Hash Standard (SHS)", FIPS PUB 180-3, October 2008,
               <http://csrc.nist.gov/publications/fips/fips180-3/
               fips180-3_final.pdf>.
   
   [RFC1034]   Mockapetris, P., "Domain names - concepts and
               facilities", STD 13, RFC 1034, November 1987.
   
   [RFC1035]   Mockapetris, P., "Domain names - implementation and
               specification", STD 13, RFC 1035, November 1987.
   
   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.
   
   [RFC4250]   Lehtinen, S. and C. Lonvick, "The Secure Shell (SSH)
               Protocol Assigned Numbers", RFC 4250, January 2006.
   
   [RFC4251]   Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
               Protocol Architecture", RFC 4251, January 2006.
   
   [RFC4253]   Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
               Transport Layer Protocol", RFC 4253, January 2006.
   
   [RFC4255]   Schlyter, J. and W. Griffin, "Using DNS to Securely
               Publish Secure Shell (SSH) Key Fingerprints", RFC 4255,
               January 2006.
   
   [RFC5656]   Stebila, D. and J. Green, "Elliptic Curve Algorithm
               Integration in the Secure Shell Transport Layer",
               RFC 5656, December 2009.

8.2. Informative References

[IACR2007/474] 

               Cochran, M., "Notes on the Wang et al. 2^63 SHA-1
               Differential Path", IACR 2007/474,
               <http://eprint.iacr.org/2007/474.pdf>.
   
   [RFC4033]   Arends, R., Austein, R., Larson, M., Massey, D., and S.
               Rose, "DNS Security Introduction and Requirements",
               RFC 4033, March 2005.
   
   [RFC4034]   Arends, R., Austein, R., Larson, M., Massey, D., and S.
               Rose, "Resource Records for the DNS Security Extensions",
               RFC 4034, March 2005.
   
   [RFC4035]   Arends, R., Austein, R., Larson, M., Massey, D., and S.
               Rose, "Protocol Modifications for the DNS Security
               Extensions", RFC 4035, March 2005.
   
   [RFC4716]   Galbraith, J. and R. Thayer, "The Secure Shell (SSH)
               Public Key File Format", RFC 4716, November 2006.
   
   [RFC6090]   McGrew, D., Igoe, K., and M. Salter, "Fundamental
               Elliptic Curve Cryptography Algorithms", RFC 6090,
               February 2011.

[SSHFPVALS] IANA, "DNS SSHFP Resource Records Parameters", 

               <http://www.iana.org/assignments/
               dns-sshfp-rr-parameters>.

Author's Address

Ondrej Sury
CZ.NIC
Americka 23
120 00 Praha 2
Czech Republic 

   Phone: +420 222 745 110
   EMail: ondrej.sury@nic.cz