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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 RFC 5702

DNS Extensions working group                                   J. Jansen
Internet-Draft                                                NLnet Labs
Expires: June 13, 2008                                 December 11, 2007


 Use of SHA-2 algorithms with RSA in DNSKEY and RRSIG Resource Records
                               for DNSSEC
                 draft-ietf-dnsext-dnssec-rsasha256-02

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Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This document describes how to produce RSA/SHA-256 and RSA/SHA-512
   DNSKEY and RRSIG resource records for use in the Domain Name System
   Security Extensions (DNSSEC, RFC4033, RFC4034, and RFC4035).








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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  DNSKEY Resource Records . . . . . . . . . . . . . . . . . . . . 3
     2.1.  RSA/SHA-256 DNSKEY Resource Records . . . . . . . . . . . . 3
     2.2.  RSA/SHA-512 DNSKEY Resource Records . . . . . . . . . . . . 3
   3.  RRSIG Resource Records  . . . . . . . . . . . . . . . . . . . . 4
     3.1.  RSA/SHA-256 RRSIG Resource Records  . . . . . . . . . . . . 4
     3.2.  RSA/SHA-512 RRSIG Resource Records  . . . . . . . . . . . . 4
   4.  Implementation Considerations . . . . . . . . . . . . . . . . . 5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
     6.1.  SHA-1 versus SHA-2 Considerations for RRSIG resource
           records . . . . . . . . . . . . . . . . . . . . . . . . . . 5
     6.2.  Signature Type Downgrade Attacks  . . . . . . . . . . . . . 6
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 6
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 6
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 6
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 7
   Intellectual Property and Copyright Statements  . . . . . . . . . . 8






























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1.  Introduction

   The Domain Name System (DNS) is the global hierarchical distributed
   database for Internet Addressing.  The DNS has been extended to use
   cryptographic keys and digital signatures for the verification of the
   integrity of its data.  RFC4033 [1], RFC4034 [2], and RFC4035 [3]
   describe these DNS Security Extensions, called DNSSEC.

   RFC4034 describes how to store DNSKEY and RRSIG resource records, and
   specifies a list of cryptographic algorithms to use.  This document
   extends that list with the algorithm RSA/SHA-256 and RSA/SHA-512, and
   specifies how to store DNSKEY data and how to produce RRSIG resource
   records with these hash algorithms.

   Familiarity with DNSSEC, RSA [7] and the SHA-2 [5] family of
   algorithms is assumed in this document.

   To refer to both SHA-256 and SHA-512, this document will use the name
   SHA-2.  This is done to improve readability.  When a part of text is
   specific for either SHA-256 or SHA-512, their specific names are
   used.  The same goes for RSA/SHA-256 and RSA/SHA-512, which will be
   grouped using the name RSA/SHA-2.


2.  DNSKEY Resource Records

   The format of the DNSKEY RR can be found in RFC4034 [2] and RFC3110
   [6].

2.1.  RSA/SHA-256 DNSKEY Resource Records

   RSA public keys for use with RSA/SHA-256 are stored in DNSKEY
   resource records (RRs) with the algorithm number [TBA].

   For use with NSEC3, the algorithm number of RSA/SHA-256 will be
   [TBA].

2.2.  RSA/SHA-512 DNSKEY Resource Records

   RSA public keys for use with RSA/SHA-512 are stored in DNSKEY
   resource records (RRs) with the algorithm number [TBA].

   For use with NSEC3, the algorithm number of RSA/SHA-512 will be
   [TBA].







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3.  RRSIG Resource Records

   The value of the signature field in the RRSIG RR is calculated as
   follows.  The values for the fields that precede the signature data
   are specified in RFC4034 [2].

   hash = SHA-XXX(data)

   Where XXX is either 256 or 512, depending on the algorithm used.

   signature = ( 00 | 01 | FF* | 00 | prefix | hash ) ** e (mod n)

   Where SHA-XXX is the message digest algorithm as specified in FIPS
   180 [5], | is concatenation, 00, 01, FF and 00 are fixed octets of
   corresponding hexadecimal value, "e" is the private exponent of the
   signing RSA key, and "n" is the public modulus of the signing key.
   The FF octet MUST be repeated the maximum number of times so that the
   total length of the signature equals the length of the modulus of the
   signer's public key ("n"). "data" is the data of the resource record
   set that is signed, as specified in RFC4034 [2].

   The prefix should make the use of standard cryptographic libraries
   easier.  These specifications are taken directly from PKCS #1 v2.1
   section 9.2 [4].  The prefixes for the different algorithms are
   specified below.

3.1.  RSA/SHA-256 RRSIG Resource Records

   RSA/SHA-256 signatures are stored in the DNS using RRSIG resource
   records (RRs) with algorithm number [TBA].

   The prefix is the ASN.1 BER SHA-256 algorithm designator prefix as
   specified in PKCS 2.1 [4]:

   hex 30 31 30 0d 06 09 60 86 48 01 65 03 04 02 01 05 00 04 20

3.2.  RSA/SHA-512 RRSIG Resource Records

   RSA/SHA-512 signatures are stored in the DNS using RRSIG resource
   records (RRs) with algorithm number [TBA].

   The prefix is the ASN.1 BER SHA-512 algorithm designator prefix as
   specified in PKCS 2.1 [4]:

   hex 30 51 30 0d 06 09 60 86 48 01 65 03 04 02 03 05 00 04 40






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4.  Implementation Considerations

   DNSSEC aware implementations SHOULD be able to support RRSIG resource
   records with the RSA/SHA-2 algorithms.

   If both RSA/SHA-2 and RSA/SHA-1 RRSIG resource records are available
   for a certain rrset, with a secure path to their keys, the validator
   SHOULD ignore the SHA-1 signature.  If the RSA/SHA-2 signature does
   not verify the data, and the RSA/SHA-1 signature does, the validator
   SHOULD mark the data with the security status from the RSA/SHA-2
   signature.


5.  IANA Considerations

   IANA has not yet assigned an algorithm number for RSA/SHA-256 and
   RSA/SHA-512.

   The algorithm list from RFC4034 Appendix A.1 [2] is extended with the
   following entries:

                                   Zone
   Value Algorithm         [Mnemonic]       Signing References   Status
   ----- -----------       -----------      ------- ---------- --------
   [TBA] RSA/SHA-256       [RSASHA256]             y     [TBA] OPTIONAL
   [TBA] RSA/SHA-256-NSEC3 [RSASHA256NSEC3]        y     [TBA] OPTIONAL
   [TBA] RSA/SHA-512       [RSASHA512]             y     [TBA] OPTIONAL
   [TBA] RSA/SHA-512-NSEC3 [RSASHA512NSEC3]        y     [TBA] OPTIONAL


6.  Security Considerations

6.1.  SHA-1 versus SHA-2 Considerations for RRSIG resource records

   Users of DNSSEC are encouraged to deploy SHA-2 as soon as software
   implementations allow for it.  SHA-2 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 DNSSEC, it is believed (at the
   time of this writing) that SHA-2 is the better choice for use in
   DNSSEC records.

   SHA-2 is considered sufficiently strong for the immediate future, but
   predictions about future development in cryptography and
   cryptanalysis are beyond the scope of this document.






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6.2.  Signature Type Downgrade Attacks

   Since each RRset MUST be signed with each algorithm present in the
   DNSKEY RRset at the zone apex (see [3] Section 2.2), a malicious
   party cannot filter out the RSA/SHA-2 RRSIG, and force the validator
   to use the RSA/SHA-1 signature if both are present in the zone.
   Together with the implementation considerations from Section 4 of
   this document, this provides resilience against algorithm downgrade
   attacks, if the validator supports RSA/SHA-2.


7.  Acknowledgments

   This document is a minor extension to RFC4034 [2].  Also, we try to
   follow the documents RFC3110 [6] and RFC4509 [8] for consistency.
   The authors of and contributors to these documents are gratefully
   acknowledged for their hard work.

   The following people provided additional feedback and text: Jaap
   Akkerhuis, Rob Austein, Miek Gieben, Scott Rose and Wouter
   Wijngaards.


8.  References

8.1.  Normative References

   [1]  Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
        "DNS Security Introduction and Requirements", RFC 4033,
        March 2005.

   [2]  Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
        "Resource Records for the DNS Security Extensions", RFC 4034,
        March 2005.

   [3]  Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
        "Protocol Modifications for the DNS Security Extensions",
        RFC 4035, March 2005.

   [4]  Jonsson, J. and B. Kaliski, "Public-Key Cryptography Standards
        (PKCS) #1: RSA Cryptography Specifications Version 2.1",
        RFC 3447, February 2003.

   [5]  National Institute of Standards and Technology, "Secure Hash
        Standard", FIPS PUB 180-2, August 2002.

   [6]  Eastlake, D., "RSA/SHA-1 SIGs and RSA KEYs in the Domain Name
        System (DNS)", RFC 3110, May 2001.



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8.2.  Informative References

   [7]  Schneier, B., "Applied Cryptography Second Edition: protocols,
        algorithms, and source code in C", Wiley and Sons , ISBN 0-471-
        11709-9, 1996.

   [8]  Hardaker, W., "Use of SHA-256 in DNSSEC Delegation Signer (DS)
        Resource Records (RRs)", RFC 4509, May 2006.


Author's Address

   Jelte Jansen
   NLnet Labs
   Kruislaan 419
   Amsterdam  1098VA
   NL

   Email: jelte@NLnetLabs.nl
   URI:   http://www.nlnetlabs.nl/































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Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
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Acknowledgment

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).





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