Network Working Group J. Jansen

Network Working Group J. Jansen
Request for Comments: 5702 NLnet Labs
Category: Standards Track October 2009

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

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

Status of This Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the BSD License.

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RFC 5702 DNSSEC RSA/SHA-2 October 2009

Table of Contents

1. Introduction ....................................................2
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 ..........................................3
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-2 Signatures ...............................5
5.2. Support for NSEC3 Denial of Existence ......................5
6. Examples ........................................................6
6.1. RSA/SHA-256 Key and Signature ..............................6
6.2. RSA/SHA-512 Key and Signature ..............................7
7. IANA Considerations .............................................8
8. Security Considerations .........................................8
8.1. SHA-1 versus SHA-2 Considerations for RRSIG
Resource Records ...........................................8
8.2. Signature Type Downgrade Attacks ...........................8
9. Acknowledgments .................................................9
10. References .....................................................9
10.1. Normative References ......................................9
10.2. Informative References ....................................9

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. [RFC4033], [RFC4034], and
[RFC4035] 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] family
of algorithms is assumed in this document.

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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 [RFC2119].

2. DNSKEY Resource Records

The format of the DNSKEY RR can be found in [RFC4034]. [RFC3110]
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 8.

For interoperability, as in [RFC3110], 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 10.

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
[RFC4034].

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hash = SHA-XXX(data)

Here XXX is either 256 or 512, depending on the algorithm used, as
specified in FIPS PUB 180-3; "data" is the wire format data of the
resource record set that is signed, as specified in [RFC4034].

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 of
[RFC3447]), and EMSA-PKCS1-v1_5 encoding in PKCS #1 v2.1 (Section 9.2
of [RFC3447]). 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 8.

The prefix is the ASN.1 DER SHA-256 algorithm designator prefix, as
specified in PKCS #1 v2.1 [RFC3447]:

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 10.

The prefix is the ASN.1 DER SHA-512 algorithm designator prefix, as
specified in PKCS #1 v2.1 [RFC3447]:

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

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4. Deployment Considerations

4.1. Key Sizes

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 [NIST800-57].

4.2. Signature Sizes

In this family of signing algorithms, the size of signatures is
related to the size of the key and not to 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.

5. Implementation Considerations

5.1. Support for SHA-2 Signatures

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.

5.2. Support for NSEC3 Denial of Existence

[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 about which
they could not know. 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 [RFC5155]. An authoritative server that
does not implement NSEC3 MAY still serve zones that use RSA/SHA-2
with NSEC denial of existence.

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6. Examples

6.1. RSA/SHA-256 Key and Signature

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

Private-key-format: v1.2
Algorithm: 8 (RSASHA256)
Modulus: wVwaxrHF2CK64aYKRUibLiH30KpPuPBjel7E8ZydQW1HYWHfoGm
idzC2RnhwCC293hCzw+TFR2nqn8OVSY5t2Q==
PublicExponent: AQAB
PrivateExponent: UR44xX6zB3eaeyvTRzmskHADrPCmPWnr8dxsNwiDGHzrMKLN+i/
HAam+97HxIKVWNDH2ba9Mf1SA8xu9dcHZAQ==
Prime1: 4c8IvFu1AVXGWeFLLFh5vs7fbdzdC6U82fduE6KkSWk=
Prime2: 2zZpBE8ZXVnL74QjG4zINlDfH+EOEtjJJ3RtaYDugvE=
Exponent1: G2xAPFfK0KGxGANDVNxd1K1c9wOmmJ51mGbzKFFNMFk=
Exponent2: GYxP1Pa7CAwtHm8SAGX594qZVofOMhgd6YFCNyeVpKE=
Coefficient: icQdNRjlZGPmuJm2TIadubcO8X7V4y07aVhX464tx8Q=

The DNSKEY record for this key would be:

example.net. 3600 IN DNSKEY (256 3 8 AwEAAcFcGsaxxdgiuuGmCkVI
my4h99CqT7jwY3pexPGcnUFtR2Fh36BponcwtkZ4cAgtvd4Qs8P
kxUdp6p/DlUmObdk= );{id = 9033 (zsk), size = 512b}

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

www.example.net. 3600 IN A 192.0.2.91

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:

www.example.net. 3600 IN RRSIG (A 8 3 3600 20300101000000
20000101000000 9033 example.net. kRCOH6u7l0QGy9qpC9
l1sLncJcOKFLJ7GhiUOibu4teYp5VE9RncriShZNz85mwlMgNEa
cFYK/lPtPiVYP4bwg==);{id = 9033}

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6.2. RSA/SHA-512 Key and Signature

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

Private-key-format: v1.2
Algorithm: 10 (RSASHA512)
Modulus: 0eg1M5b563zoq4k5ZEOnWmd2/BvpjzedJVdfIsDcMuuhE5SQ3pf
Q7qmdaeMlC6Nf8DKGoUPGPXe06cP27/WRODtxXquSUytkO0kJDk
8KX8PtA0+yBWwy7UnZDyCkynO00Uuk8HPVtZeMO1pHtlAGVnc8V
jXZlNKdyit99waaE4s=
PublicExponent: AQAB
PrivateExponent: rFS1IPbJllFFgFc33B5DDlC1egO8e81P4fFadODbp56V7sphKa6
AZQCx8NYAew6VXFFPAKTw41QdHnK5kIYOwxvfFDjDcUGza88qbj
yrDPSJenkeZbISMUSSqy7AMFzEolkk6WSn6k3thUVRgSlqDoOV3
SEIAsrB043XzGrKIVE=
Prime1: 8mbtsu9Tl9v7tKSHdCIeprLIQXQLzxlSZun5T1n/OjvXSUtvD7x
nZJ+LHqaBj1dIgMbCq2U8O04QVcK3TS9GiQ==
Prime2: 3a6gkfs74d0Jb7yL4j4adAif4fcp7ZrGt7G5NRVDDY/Mv4TERAK
Ma0TKN3okKE0A7X+Rv2K84mhT4QLDlllEcw==
Exponent1: v3D5A9uuCn5rgVR7wgV8ba0/KSpsdSiLgsoA42GxiB1gvvs7gJM
MmVTDu/ZG1p1ZnpLbhh/S/Qd/MSwyNlxC+Q==
Exponent2: m+ezf9dsDvYQK+gzjOLWYeKq5xWYBEYFGa3BLocMiF4oxkzOZ3J
PZSWU/h1Fjp5RV7aPP0Vmx+hNjYMPIQ8Y5w==
Coefficient: Je5YhYpUron/WdOXjxNAxDubAp3i5X7UOUfhJcyIggqwY86IE0Q
/Bk0Dw4SC9zxnsimmdBXW2Izd8Lwuk8FQcQ==

The DNSKEY record for this key would be:

example.net. 3600 IN DNSKEY (256 3 10 AwEAAdHoNTOW+et86KuJOWRD
p1pndvwb6Y83nSVXXyLA3DLroROUkN6X0O6pnWnjJQujX/AyhqFD
xj13tOnD9u/1kTg7cV6rklMrZDtJCQ5PCl/D7QNPsgVsMu1J2Q8g
pMpztNFLpPBz1bWXjDtaR7ZQBlZ3PFY12ZTSncorffcGmhOL
);{id = 3740 (zsk), size = 1024b}

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

www.example.net. 3600 IN A 192.0.2.91

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:

www.example.net. 3600 IN RRSIG (A 10 3 3600 20300101000000
20000101000000 3740 example.net. tsb4wnjRUDnB1BUi+t
6TMTXThjVnG+eCkWqjvvjhzQL1d0YRoOe0CbxrVDYd0xDtsuJRa
eUw1ep94PzEWzr0iGYgZBWm/zpq+9fOuagYJRfDqfReKBzMweOL
DiNa8iP5g9vMhpuv6OPlvpXwm9Sa9ZXIbNl1MBGk0fthPgxdDLw
=);{id = 3740}

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7. IANA Considerations

This document updates the IANA registry "DNS SECURITY ALGORITHM
NUMBERS -- per [RFC4035]" (http://www.iana.org/protocols). The
following entries are added to the registry:

Zone Trans.
Value Description Mnemonic Signing Sec. References
8 RSA/SHA-256 RSASHA256 Y * RFC 5702
10 RSA/SHA-512 RSASHA512 Y * RFC 5702

* There has been no determination of standardization of the use of
this algorithm with Transaction Security.

8. Security Considerations

8.1. 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.

8.2. Signature Type Downgrade Attacks

Since each RRSet MUST be signed with each algorithm present in the
DNSKEY RRSet at the zone apex (see Section 2.2 of [RFC4035]), 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.

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9. Acknowledgments

This document is a minor extension to [RFC4034]. Also, we try to
follow the documents [RFC3110] and [RFC4509] 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, Scott Rose,
Michael St. Johns, and Wouter Wijngaards.

10. References

10.1. Normative References

[FIPS.180-3.2008]
National Institute of Standards and Technology, "Secure
Hash Standard", FIPS PUB 180-3, October 2008.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC3110] Eastlake, D., "RSA/SHA-1 SIGs and RSA KEYs in the Domain
Name System (DNS)", RFC 3110, May 2001.

[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005.

[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, March 2005.

[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, March 2005.

10.2. Informative References

[NIST800-57]
Barker, E., Barker, W., Burr, W., Polk, W., and M. Smid,
"Recommendations for Key Management", NIST SP 800-57,
March 2007.

[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, February 2003.

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RFC 5702 DNSSEC RSA/SHA-2 October 2009

[RFC4509] Hardaker, W., "Use of SHA-256 in DNSSEC Delegation Signer
(DS) Resource Records (RRs)", RFC 4509, May 2006.

[RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
Security (DNSSEC) Hashed Authenticated Denial of
Existence", RFC 5155, March 2008.

Author's Address

Jelte Jansen
NLnet Labs
Science Park 140
1098 XG Amsterdam
NL

EMail: [email protected]
URI: http://www.nlnetlabs.nl/

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