Use of SHA-2 algorithms with RSA in DNSKEY and RRSIG Resource Records for DNSSEC

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 , RFC 4034 , and RFC 4035 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 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 term "SHA-2" is not officially defined, but is usually used to refer to the collection of the algorithms SHA-224, SHA-256, SHA-384 and SHA-512. Since SHA-224 and SHA-384 are not used in DNSSEC, SHA-2 will only refer to SHA-256 and SHA-512 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 .

The format of the DNSKEY RR can be found in RFC 4034 . RFC 3110 describes the use of RSA/SHA-1 for DNSSEC signatures.

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

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.

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 . hash = SHA-XXX(data) Here XXX is either 256 or 512, depending on the algorithm used, as specified in FIPS PUB 180-3 , and "data" is the wire format data of the resource record set that is signed, as specified in RFC 4034 . 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 , and EMSA-PKCS1-v1_5 encoding in PKCS #1 v2.1 section 9.2 . The prefixes for the different algorithms are specified below.

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

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

Apart from the restrictions 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 .

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/SHA-256 or RSA/SHA-512 will have the same size as those produced with RSA/SHA-1, if the keys have the same length.

DNSSEC aware implementations SHOULD be able to support RRSIG and DNSKEY resource records created with the RSA/SHA-2 algorithms as defined in this document.

RFC 5155 defines new algorithm identifiers for existing signing algorithms, to indicate that zones signed with these algorithm identifiers can use NSEC3 as well as NSEC records to provide denial of existence. That mechanism was chosen to protect implementations predating RFC5155 from encountering resource records they could not know about. This document does not define such algorithm aliases. A DNSSEC validator that implements RSA/SHA-2 MUST be able to validate negative answers in the form of both NSEC and NSEC3 with hash algorithm 1, as defined in . An authoritative server that does not implement NSEC3 MAY still serve zones that use RSA/SHA-2 with NSEC denial of existence.

Given a private key with the following values (in Base64):

The DNSKEY record for this key would be:

With this key, sign the following RRSet, consisting of 1 A record:

If the inception date is set at 00:00 hours on January 1st, 2000, and the expiration date at 00:00 hours on January 1st, 2030, the following signature should be created:

Given a private key with the following values (in Base64):

The DNSKEY record for this key would be:

With this key, sign the following RRSet, consisting of 1 A record:

If the inception date is set at 00:00 hours on January 1st, 2000, and the expiration date at 00:00 hours on January 1st, 2030, the following signature should be created:

This document updates the IANA registry "DNS SECURITY ALGORITHM NUMBERS -- per " (http://www.iana.org/assignments/dns-sec-alg-numbers). The following entries are added to the registry:

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.

Since each RRSet MUST be signed with each algorithm present in the DNSKEY RRSet at the zone apex (see 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.

This document is a minor extension to RFC 4034 . Also, we try to follow the documents RFC 3110 and RFC 4509 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, Mark Andrews, Roy Arends, Rob Austein, Francis Dupont, Miek Gieben, Alfred Hoenes, Paul Hoffman, Peter Koch, Michael St. Johns, Scott Rose and Wouter Wijngaards.