DNS Extensions working group                                   J. Jansen
Internet-Draft                                                NLnet Labs
Expires: August 19,
Intended status: Standards Track                          April 11, 2008                               February 16,
Expires: October 13, 2008

 Use of SHA-2 algorithms with RSA in DNSKEY and RRSIG Resource Records
                               for DNSSEC
                 draft-ietf-dnsext-dnssec-rsasha256-03

                 draft-ietf-dnsext-dnssec-rsasha256-04

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Copyright Notice

   Copyright (C) The IETF Trust (2008).

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, RFC 4033, RFC 4034, and RFC 4035).

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.  Deployment Considerations . . . . . . . . . . . . . . . . . . . 5
     4.1.  Key Sizes . . . . . . . . . . . . . . . . . . . . . . . . . 5
     4.2.  Signature Sizes . . . . . . . . . . . . . . . . . . . . . . 5
   5.  Implementation Considerations . . . . . . . . . . . . . . . . . 5
     5.1.  Support for SHA-1 and SHA-2 signatures  . . . . . . . . . . 5
     5.2.  Support for NSEC3 denial of existence . . . . . . . . . . . 5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
     7.1.  SHA-1 versus SHA-2 Considerations for RRSIG Resource
           Records . . . . . . . . . . . . . . . . . . . . . . . . . . 6
     7.2.  Signature Type Downgrade Attacks  . . . . . . . . . . . . . 6
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 6
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     9.1.  Normative References  . . . . . . . . . . . . . . . . . . . 7
     9.2.  Informative References  . . . . . . . . . . . . . . . . . . 7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 8
   Intellectual Property and Copyright Statements  . . . . . . . . . . 9

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.  RFC 4033 [1], RFC 4034 [2], and RFC 4035 [3]
   describe these DNS Security Extensions, called DNSSEC.

   RFC 4034 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 algorithms 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] [8] 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 RFC 4034 [2] and RFC 3110
   [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 {TBA1}.

   For use with NSEC3, the algorithm number of RSA/SHA-256 will be
   {TBA2}.

   The key size for RSA/SHA-256 keys MUST NOT be less than 512 bits, and
   MUST NOT be more than 4096 bits.

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 {TBA3}.

   For use with NSEC3, the algorithm number of RSA/SHA-512 will be
   {TBA4}. {TBA2}.

   The key size for RSA/SHA-512 keys MUST NOT be less than 1024 bits,
   and MUST NOT be more than 4096 bits.

3.  RRSIG Resource Records

   The value of the signature field in the RRSIG RR follow the RSASSA-
   PKCS1-v1_5 signature scheme, and is calculated as follows.  The
   values for the RDATA fields that precede the signature data are
   specified in RFC 4034 [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
   PUB 180-2 [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 RFC 4034 [2].

   The "prefix" is intended to make the use of standard cryptographic
   libraries easier.  These specifications are taken directly from the
   specification of EMSA-PKCS1-v1_5 encoding in 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 {TBA1} for use with NSEC, or
   {TBA2} for use with NSEC3. {TBA1}.

   The prefix is the ASN.1 BER SHA-256 algorithm designator prefix as
   specified in PKCS #1 v2.1 [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 {TBA3} for use with NSEC, or
   {TBA4} for use with NSEC3. {TBA2}.

   The prefix is the ASN.1 BER SHA-512 algorithm designator prefix as
   specified in PKCS #1 v2.1 [4]:

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

4.  Deployment Considerations

4.1.  Key Sizes

   Apart from prohibiting RSA/SHA-512 signatures smaller than 1024
   bytes, this document will not specify what size of keys to use.  That
   is more an operational issue and depends largely on the environment and
   intended use.  Some good starting points for more information might
   be DNSSEC Operational Practises [9], [10], section 3.5, and NIST SP 800-57
   Part 1
   [10] [11] and Part 3 [11]. [12].

4.2.  Signature Sizes

   In this family of signing algorithms, the size of signatures is
   related to the size of the key, and not the hashing algorithm used in
   the signing process.  Therefore, RRSIG resource records produced with
   RSA/SHA256 or RSA/SHA512 shall have the same size as those produced
   with RSA/SHA1, if the keys have the same length.

5.  Implementation Considerations

5.1.  Support for SHA-1 and SHA-2 signatures

   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.

5.2.  Support for NSEC3 denial of existence

   Implementations that have support for RSA/SHA-2 MUST also have
   support for NSEC3 denial of existence, as specified in RFC 5155 [7].

6.  IANA Considerations

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

   The algorithm list from RFC 4034 Appendix A.1 [2] is extended with
   the following entries:

                                               Zone
 Value Algorithm          [Mnemonic]        Signing  References   Status
 ----- -----------        -----------       ------- ----------- --------
 {TBA1} RSA/SHA-256       RSASHA256               y {this memo} OPTIONAL
 {TBA2} RSA/SHA-256-NSEC3 RSASHA256NSEC3          y {this memo} OPTIONAL
 {TBA3} RSA/SHA-512       RSASHA512               y {this memo} OPTIONAL
 {TBA4} RSA/SHA-512-NSEC3 RSASHA512NSEC3          y {this memo} OPTIONAL

7.  Security Considerations

7.1.  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.

   The signature scheme RSASSA-PKCS1-v1_5 is chosen to match the one
   used for RSA/SHA-1 signatures.  This should ease implementation of
   the new hashing algorithms in DNSSEC software.

7.2.  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 5 of
   this document, this provides resilience against algorithm downgrade
   attacks, if the validator supports RSA/SHA-2.

8.  Acknowledgments

   This document is a minor extension to RFC 4034 [2].  Also, we try to
   follow the documents RFC 3110 [6] and RFC 4509 [8] [9] 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, Roy Arends, Rob Austein, Miek Gieben, Alfred Hoenes,
   Michael St. Johns, Scott Rose and Wouter Wijngaards.

9.  References

9.1.  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.

   [7]   Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
         Security (DNSSEC) Hashed Authenticated Denial of Existence",
         RFC 5155, March 2008.

9.2.  Informative References

   [7]

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

   [8]

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

   [9]

   [10]  Kolkman, O. and R. Gieben, "DNSSEC Operational Practices",
         RFC 4641, September 2006.

   [10]

   [11]  Barker, E., Barker, W., Burr, W., Polk, W., and M. Smid,
         "Recommendations for Key Management Part 1: General", NIST
         SP 800-57 Part 1, March 2007.

   [11]

   [12]  Barker, E., Barker, W., Burr, W., Jones, A., Polk, W., Smid,
         M., and S. Rose, "Recommendations for Key Management Part 3:

         Application-Specific Key Guidance", NIST SP 800-57 Part 3,
         March 2007.

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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