Use of SHA-2 algorithms with RSA in DNSKEY and RRSIG Resource Records for DNSSECNLnet LabsKruislaan 419Amsterdam1098VANLjelte@NLnetLabs.nlhttp://www.nlnetlabs.nl/
General
DNS Extensions working groupRFCRequest for CommentsI-DInternet-DraftDNSSECRSASHA-256SHA-512
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).
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 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 use with NSEC3 ,
the algorithm number for RSA/SHA-256 will be {TBA2}.
The use of a different algorithm number to differentiate
between the use of NSEC and NSEC3 is in keeping with the
approach adopted in RFC5155.
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 {TBA3}.
For use with NSEC3, the algorithm number for
RSA/SHA-512 will be {TBA4}.
The use of a different algorithm number to differentiate
between the use of NSEC and NSEC3 is in keeping with the
approach adopted in RFC5155.
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 180-2
, 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} for use with NSEC, or {TBA2} for use with
NSEC3.
The prefix is the ASN.1 BER 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
{TBA3} for use with NSEC, or {TBA4} for use with
NSEC3.
The prefix is the ASN.1 BER 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 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
.
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.
DNSSEC aware implementations SHOULD be able to support
RRSIG resource records with the RSA/SHA-2 algorithms.
IANA has assigned DNS Security Algorithm Numbers
{TBA1} for RSA/SHA-256 with NSEC,
{TBA2} for RSA/SHA-256 with NSEC3,
{TBA3} for RSA/SHA-512 with NSEC, and
{TBA4} for RSA/SHA-512 with NSEC3.
The algorithm list from RFC 4034 Appendix A.1 is extended with the following
entries:
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, Roy Arends, Rob Austein,
Francis Dupont, Miek Gieben, Alfred Hoenes,
Paul Hoffman, Peter Koch, Michael St. Johns,
Scott Rose and Wouter Wijngaards.
Secure Hash StandardNational Institute of Standards and TechnologyRSA/SHA-1 SIGs and RSA KEYs in the Domain Name System (DNS)DNS Security Introduction and RequirementsResource Records for the DNS Security ExtensionsProtocol Modifications for the DNS Security ExtensionsRecommendations for Key ManagementPublic-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1 Use of SHA-256 in DNSSEC Delegation Signer (DS) Resource Records (RRs)DNS Security (DNSSEC) Hashed Authenticated Denial of Existence