Use of SHA-2 algorithms with RSA in DNSKEY and RRSIG Resource Records for DNSSEC
draft-ietf-dnsext-dnssec-rsasha256-10

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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
2.  DNSKEY Resource Records
    2.1.  RSA/SHA-256 DNSKEY Resource Records
    2.2.  RSA/SHA-512 DNSKEY Resource Records
3.  RRSIG Resource Records
    3.1.  RSA/SHA-256 RRSIG Resource Records
    3.2.  RSA/SHA-512 RRSIG Resource Records
4.  Deployment Considerations
    4.1.  Key Sizes
    4.2.  Signature Sizes
5.  Implementation Considerations
    5.1.  Support for SHA-2 signatures
    5.2.  Support for NSEC3 Denial of Existence
        5.2.1.  NSEC3 in Authoritative servers
        5.2.2.  NSEC3 in Validators
6.  IANA Considerations
7.  Security Considerations
    7.1.  SHA-1 versus SHA-2 Considerations for RRSIG Resource Records
    7.2.  Signature Type Downgrade Attacks
8.  Acknowledgments
9.  References
    9.1.  Normative References
    9.2.  Informative References
§  Author's Address

1.  Introduction

The Domain Name System (DNS) is the global hierarchical distributed database for Internet Naming. The DNS has been extended to use cryptographic keys and digital signatures for the verification of the authenticity and integrity of its data. RFC 4033 [RFC4033] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “DNS Security Introduction and Requirements,” March 2005.), RFC 4034 [RFC4034] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Resource Records for the DNS Security Extensions,” March 2005.), and RFC 4035 [RFC4035] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Protocol Modifications for the DNS Security Extensions,” March 2005.) 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 and the SHA-2 [FIPS.180‑3.2008] (National Institute of Standards and Technology, “Secure Hash Standard,” October 2008.) 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.

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 [RFC2119] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

2.  DNSKEY Resource Records

The format of the DNSKEY RR can be found in RFC 4034 [RFC4034] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Resource Records for the DNS Security Extensions,” March 2005.). RFC 3110 [RFC3110] (Eastlake, D., “RSA/SHA-1 SIGs and RSA KEYs in the Domain Name System (DNS),” May 2001.) describes the use of RSA/SHA-1 for DNSSEC signatures.

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 interoperability, as in RFC 3110 [RFC3110] (Eastlake, D., “RSA/SHA-1 SIGs and RSA KEYs in the Domain Name System (DNS),” May 2001.), the key size of 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 {TBA2}.

The key size of 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 follows 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 [RFC4034] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Resource Records for the DNS Security Extensions,” March 2005.).

hash = SHA-XXX(data)

Here XXX is either 256 or 512, depending on the algorithm used, as specified in FIPS PUB 180-3 [FIPS.180‑3.2008] (National Institute of Standards and Technology, “Secure Hash Standard,” October 2008.), and "data" is the wire format data of the resource record set that is signed, as specified in RFC 4034 [RFC4034] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Resource Records for the DNS Security Extensions,” March 2005.).

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

Here "|" 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 exact number of times so that the total length of the concatenated term in parentheses equals the length of the modulus of the signer's public key ("n").

The "prefix" is intended to make the use of standard cryptographic libraries easier. These specifications are taken directly from the specifications of RSASSA-PKCS1-v1_5 in PKCS #1 v2.1 section 8.2 [RFC3447] (Jonsson, J. and B. Kaliski, “Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1,” February 2003.), and EMSA-PKCS1-v1_5 encoding in PKCS #1 v2.1 section 9.2 [RFC3447] (Jonsson, J. and B. Kaliski, “Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1,” February 2003.). 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}.

The prefix is the ASN.1 DER SHA-256 algorithm designator prefix as specified in PKCS #1 v2.1 [RFC3447] (Jonsson, J. and B. Kaliski, “Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1,” February 2003.):

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 {TBA2}.

The prefix is the ASN.1 DER SHA-512 algorithm designator prefix as specified in PKCS #1 v2.1 [RFC3447] (Jonsson, J. and B. Kaliski, “Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1,” February 2003.):

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 the restrictions specified in section 2, this document will not specify what size of keys to use. That is an operational issue and depends largely on the environment and intended use. A good starting point for more information would be NIST SP 800-57 [NIST800‑57] (Barker, E., Barker, W., Burr, W., Polk, W., and M. Smid, “Recommendations for Key Management,” March 2007.).

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 will 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-2 signatures

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

5.2.  Support for NSEC3 Denial of Existence

Note that these algorithms have no aliases to signal NSEC3 [RFC5155] (Laurie, B., Sisson, G., Arends, R., and D. Blacka, “DNS Security (DNSSEC) Hashed Authenticated Denial of Existence,” March 2008.) denial of existence. The aliases mechanism used in RFC 5155 was to protect implementations predating that RFC from encountering records they could not know about.

5.2.1.  NSEC3 in Authoritative servers

An authoritative server that does not implement NSEC3 MAY still serve zones that use RSA/SHA2 with NSEC.

5.2.2.  NSEC3 in Validators

A DNSSEC validator that implements RSA/SHA2 MUST be able to handle both NSEC and NSEC3 [RFC5155] (Laurie, B., Sisson, G., Arends, R., and D. Blacka, “DNS Security (DNSSEC) Hashed Authenticated Denial of Existence,” March 2008.) negative answers. If this is not the case, the validator MUST treat a zone signed with RSA/SHA256 or RSA/SHA512 as signed with an unknown algorithm, and thus as insecure.

6.  IANA Considerations

This document updates the IANA registry "DNS SECURITY ALGORITHM NUMBERS -- per [RFC4035] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Protocol Modifications for the DNS Security Extensions,” March 2005.) " (http://www.iana.org/assignments/dns-sec-alg-numbers). The following entries are added to the registry:

                                                  Zone
Value      Algorithm               Mnemonic    Signing  References
{TBA1}   RSA/SHA-256              RSASHA256          y {this memo}
{TBA2}   RSA/SHA-512              RSASHA512          y {this memo}

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 [RFC4035] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Protocol Modifications for the DNS Security Extensions,” March 2005.) 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. This should provide resilience against algorithm downgrade attacks, if the validator supports RSA/SHA-2.

8.  Acknowledgments

This document is a minor extension to RFC 4034 [RFC4034] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Resource Records for the DNS Security Extensions,” March 2005.). Also, we try to follow the documents RFC 3110 [RFC3110] (Eastlake, D., “RSA/SHA-1 SIGs and RSA KEYs in the Domain Name System (DNS),” May 2001.) and RFC 4509 [RFC4509] (Hardaker, W., “Use of SHA-256 in DNSSEC Delegation Signer (DS) Resource Records (RRs),” May 2006.) 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, Francis Dupont, Miek Gieben, Alfred Hoenes, Paul Hoffman, Peter Koch, Michael St. Johns, Scott Rose and Wouter Wijngaards.

9.  References

9.1. Normative References

9.2. Informative References

Author's Address