Internet Engineering Task Force (IETF) P. Hoffman Request for Comments: 6605 VPN Consortium Category: Standards Track W.C.A. Wijngaards ISSN: 2070-1721 NLnet Labs April 2012
Elliptic Curve Digital Signature Algorithm (DSA) for DNSSEC
This document describes how to specify Elliptic Curve Digital Signature Algorithm (DSA) keys and signatures in DNS Security (DNSSEC). It lists curves of different sizes and uses the SHA-2 family of hashes for signatures.
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 Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741.
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DNSSEC, which is broadly defined in RFCs 4033, 4034, and 4035 ([RFC4033], [RFC4034], and [RFC4035]), uses cryptographic keys and digital signatures to provide authentication of DNS data. Currently, the most popular signature algorithm is RSA with SHA-1, using keys that are 1024 or 2048 bits long.
This document defines the DNSKEY and RRSIG resource records (RRs) of two new signing algorithms: ECDSA (Elliptic Curve DSA) with curve P-256 and SHA-256, and ECDSA with curve P-384 and SHA-384. (A description of ECDSA can be found in [FIPS-186-3].) This document also defines the DS RR for the SHA-384 one-way hash algorithm; the associated DS RR for SHA-256 is already defined in RFC 4509 [RFC4509]. [RFC6090] provides a good introduction to implementing ECDSA.
Current estimates are that ECDSA with curve P-256 has an approximate equivalent strength to RSA with 3072-bit keys. Using ECDSA with curve P-256 in DNSSEC has some advantages and disadvantages relative to using RSA with SHA-256 and with 3072-bit keys. ECDSA keys are much shorter than RSA keys; at this size, the difference is 256 versus 3072 bits. Similarly, ECDSA signatures are much shorter than RSA signatures. This is relevant because DNSSEC stores and transmits both keys and signatures.
In the two signing algorithms defined in this document, the size of the key for the elliptic curve is matched with the size of the output of the hash algorithm. This design is based on the widespread belief that the equivalent strength of P-256 and P-384 is half the length of the key, and also that the equivalent strength of SHA-256 and SHA-384 is half the length of the key. Using matched strengths prevents an attacker from choosing the weaker half of a signature algorithm. For example, in a signature that uses RSA with 2048-bit keys and SHA-256, the signing portion is significantly weaker than the hash portion, whereas the two algorithms here are balanced.
Signing with ECDSA is significantly faster than with RSA (over 20 times in some implementations). However, validating RSA signatures is significantly faster than validating ECDSA signatures (about 5 times faster in some implementations).
Some of the material in this document is copied liberally from RFC 6460 [RFC6460].
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].
SHA-384 is defined in FIPS 180-3 [FIPS-180-3] and RFC 6234 [RFC6234], and is similar to SHA-256 in many ways. The implementation of SHA- 384 in DNSSEC follows the implementation of SHA-256 as specified in RFC 4509 except that the underlying algorithm is SHA-384, the digest value is 48 bytes long, and the digest type code is 4.
Verifying ECDSA signatures requires agreement between the signer and the verifier of the parameters used. FIPS 186-3 [FIPS-186-3] lists some NIST-recommended elliptic curves. (Other documents give more detail on ECDSA than is given in FIPS 186-3.) These are the same curves listed in RFC 5114 [RFC5114]. The curves used in this document are:
FIPS 186-3 RFC 5114 ------------------------------------------------------------------ P-256 (Section D.1.2.3) 256-bit Random ECP Group (Section 2.6) P-384 (Section D.1.2.4) 384-bit Random ECP Group (Section 2.7)
ECDSA public keys consist of a single value, called "Q" in FIPS 186-3. In DNSSEC keys, Q is a simple bit string that represents the uncompressed form of a curve point, "x | y".
The ECDSA signature is the combination of two non-negative integers, called "r" and "s" in FIPS 186-3. The two integers, each of which is formatted as a simple octet string, are combined into a single longer octet string for DNSSEC as the concatenation "r | s". (Conversion of the integers to bit strings is described in Section C.2 of FIPS 186-3.) For P-256, each integer MUST be encoded as 32 octets; for P-384, each integer MUST be encoded as 48 octets.
The algorithm numbers associated with the DNSKEY and RRSIG resource records are fully defined in the IANA Considerations section. They are:
o DNSKEY and RRSIG RRs signifying ECDSA with the P-256 curve and SHA-256 use the algorithm number 13.
o DNSKEY and RRSIG RRs signifying ECDSA with the P-384 curve and SHA-384 use the algorithm number 14.
Hoffman & Wijngaards Standards Track [Page 3]
RFC 6605 ECDSA for DNSSEC April 2012
Conformant implementations that create records to be put into the DNS MUST implement signing and verification for both of the above algorithms. Conformant DNSSEC verifiers MUST implement verification for both of the above algorithms.
RFC 5155 [RFC5155] 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 RFC 5155 from encountering resource records they could not know about. This document does not define such algorithm aliases.
A DNSSEC validator that implements the signing algorithms defined in this document MUST be able to validate negative answers in the form of both NSEC and NSEC3 with hash algorithm 1, as defined in RFC 5155. An authoritative server that does not implement NSEC3 MAY still serve zones that use the signing algorithms defined in this document with NSEC denial of existence.
The cryptographic work factor of ECDSA with curve P-256 or P-384 is generally considered to be equivalent to half the size of the key, or 128 bits and 192 bits, respectively. Such an assessment could, of course, change in the future if new attacks that work better than the ones known today are found.
The security considerations listed in RFC 4509 apply here as well.