Network Working Group D. Eastlake 3rd
Request for Comments: 3110 Motorola
Obsoletes: 2537 May 2001
Category: Standards Track
RSA/SHA1 SIGs and RSA KEYs in the Domain Name System (DNS)
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) The Internet Society (2001). All Rights Reserved.
Abstract
This document describes how to produce RSA/SHA1 SIG resource records
(RRs) in Section 3 and, so as to completely replace RFC 2537,
describes how to produce RSA KEY RRs in Section 2.
Since the adoption of a Proposed Standard for RSA signatures in the
DNS (Domain Name Space), advances in hashing have been made. A new
DNS signature algorithm is defined to make these advances available
in SIG RRs. The use of the previously specified weaker mechanism is
deprecated. The algorithm number of the RSA KEY RR is changed to
correspond to this new SIG algorithm. No other changes are made to
DNS security.
Acknowledgements
Material and comments from the following have been incorporated and
are gratefully acknowledged:
Olafur Gudmundsson
The IESG
Charlie Kaufman
Steve Wang
D. Eastlake 3rd Standards Track [Page 1]
RFC 3110 RSA SIGs and KEYs in the DNS May 2001
Table of Contents
1. Introduction................................................... 2
2. RSA Public KEY Resource Records................................ 3
3. RSA/SHA1 SIG Resource Records.................................. 3
4. Performance Considerations..................................... 4
5. IANA Considerations............................................ 5
6. Security Considerations........................................ 5
References........................................................ 5
Author's Address.................................................. 6
Full Copyright Statement.......................................... 7
1. Introduction
The Domain Name System (DNS) is the global hierarchical replicated
distributed database system for Internet addressing, mail proxy, and
other information [RFC1034, 1035, etc.]. The DNS has been extended
to include digital signatures and cryptographic keys as described in
[RFC2535]. Thus the DNS can now be secured and used for secure key
distribution.
Familiarity with the RSA and SHA1 algorithms is assumed [Schneier,
FIP180] in this document.
RFC 2537 described how to store RSA keys and RSA/MD5 based signatures
in the DNS. However, since the adoption of RFC 2537, continued
cryptographic research has revealed hints of weakness in the MD5
[RFC1321] algorithm used in RFC 2537. The SHA1 Secure Hash Algorithm
[FIP180], which produces a larger hash, has been developed. By now
there has been sufficient experience with SHA1 that it is generally
acknowledged to be stronger than MD5. While this stronger hash is
probably not needed today in most secure DNS zones, critical zones
such a root, most top level domains, and some second and third level
domains, are sufficiently valuable targets that it would be negligent
not to provide what are generally agreed to be stronger mechanisms.
Furthermore, future advances in cryptanalysis and/or computer speeds
may require a stronger hash everywhere. In addition, the additional
computation required by SHA1 above that required by MD5 is
insignificant compared with the computational effort required by the
RSA modular exponentiation.
This document describes how to produce RSA/SHA1 SIG RRs in Section 3
and, so as to completely replace RFC 2537, describes how to produce
RSA KEY RRs in Section 2.
Implementation of the RSA algorithm in DNS with SHA1 is MANDATORY for
DNSSEC. The generation of RSA/MD5 SIG RRs as described in RFC 2537
is NOT RECOMMENDED.
D. Eastlake 3rd Standards Track [Page 2]
RFC 3110 RSA SIGs and KEYs in the DNS May 2001
The key words "MUST", "REQUIRED", "SHOULD", "RECOMMENDED", "NOT
RECOMMENDED", and "MAY" in this document are to be interpreted as
described in RFC 2119.
2. RSA Public KEY Resource Records
RSA public keys are stored in the DNS as KEY RRs using algorithm
number 5 [RFC2535]. The structure of the algorithm specific portion
of the RDATA part of such RRs is as shown below.
Field Size
 
exponent length 1 or 3 octets (see text)
exponent as specified by length field
modulus remaining space
For interoperability, the exponent and modulus are each limited to
4096 bits in length. The public key exponent is a variable length
unsigned integer. Its length in octets is represented as one octet
if it is in the range of 1 to 255 and by a zero octet followed by a
two octet unsigned length if it is longer than 255 bytes. The public
key modulus field is a multiprecision unsigned integer. The length
of the modulus can be determined from the RDLENGTH and the preceding
RDATA fields including the exponent. Leading zero octets are
prohibited in the exponent and modulus.
Note: KEY RRs for use with RSA/SHA1 DNS signatures MUST use this
algorithm number (rather than the algorithm number specified in the
obsoleted RFC 2537).
Note: This changes the algorithm number for RSA KEY RRs to be the
same as the new algorithm number for RSA/SHA1 SIGs.
3. RSA/SHA1 SIG Resource Records
RSA/SHA1 signatures are stored in the DNS using SIG resource records
(RRs) with algorithm number 5.
The signature portion of the SIG RR RDATA area, when using the
RSA/SHA1 algorithm, is calculated as shown below. The data signed is
determined as specified in RFC 2535. See RFC 2535 for fields in the
SIG RR RDATA which precede the signature itself.
hash = SHA1 ( data )
signature = ( 01  FF*  00  prefix  hash ) ** e (mod n)
D. Eastlake 3rd Standards Track [Page 3]
RFC 3110 RSA SIGs and KEYs in the DNS May 2001
where SHA1 is the message digest algorithm documented in [FIP180],
"" is concatenation, "e" is the private key exponent of the signer,
and "n" is the modulus of the signer's public key. 01, FF, and 00
are fixed octets of the corresponding hexadecimal value. "prefix" is
the ASN.1 BER SHA1 algorithm designator prefix required in PKCS1
[RFC2437], that is,
hex 30 21 30 09 06 05 2B 0E 03 02 1A 05 00 04 14
This prefix is included to make it easier to use standard
cryptographic libraries. The FF octet MUST be repeated the maximum
number of times such that the value of the quantity being
exponentiated is one octet shorter than the value of n.
(The above specifications are identical to the corresponding parts of
Public Key Cryptographic Standard #1 [RFC2437].)
The size of "n", including most and least significant bits (which
will be 1) MUST be not less than 512 bits and not more than 4096
bits. "n" and "e" SHOULD be chosen such that the public exponent is
small. These are protocol limits. For a discussion of key size see
RFC 2541.
Leading zero bytes are permitted in the RSA/SHA1 algorithm signature.
4. Performance Considerations
General signature generation speeds are roughly the same for RSA and
DSA [RFC2536]. With sufficient precomputation, signature generation
with DSA is faster than RSA. Key generation is also faster for DSA.
However, signature verification is an order of magnitude slower with
DSA when the RSA public exponent is chosen to be small as is
recommended for KEY RRs used in domain name system (DNS) data
authentication.
A public exponent of 3 minimizes the effort needed to verify a
signature. Use of 3 as the public exponent is weak for
confidentiality uses since, if the same data can be collected
encrypted under three different keys with an exponent of 3 then,
using the Chinese Remainder Theorem [NETSEC], the original plain text
can be easily recovered. If a key is known to be used only for
authentication, as is the case with DNSSEC, then an exponent of 3 is
acceptable. However other applications in the future may wish to
leverage DNS distributed keys for applications that do require
confidentiality. For keys which might have such other uses, a more
conservative choice would be 65537 (F4, the fourth fermat number).
D. Eastlake 3rd Standards Track [Page 4]
RFC 3110 RSA SIGs and KEYs in the DNS May 2001
Current DNS implementations are optimized for small transfers,
typically less than 512 bytes including DNS overhead. Larger
transfers will perform correctly and extensions have been
standardized [RFC2671] to make larger transfers more efficient, it is
still advisable at this time to make reasonable efforts to minimize
the size of KEY RR sets stored within the DNS consistent with
adequate security. Keep in mind that in a secure zone, at least one
authenticating SIG RR will also be returned.
5. IANA Considerations
The DNSSEC algorithm number 5 is allocated for RSA/SHA1 SIG RRs and
RSA KEY RRs.
6. Security Considerations
Many of the general security considerations in RFC 2535 apply. Keys
retrieved from the DNS should not be trusted unless (1) they have
been securely obtained from a secure resolver or independently
verified by the user and (2) this secure resolver and secure
obtainment or independent verification conform to security policies
acceptable to the user. As with all cryptographic algorithms,
evaluating the necessary strength of the key is essential and
dependent on local policy. For particularly critical applications,
implementers are encouraged to consider the range of available
algorithms and key sizes. See also RFC 2541, "DNS Security
Operational Considerations".
References
[FIP180] U.S. Department of Commerce, "Secure Hash Standard", FIPS
PUB 1801, 17 Apr 1995.
[NETSEC] Network Security: PRIVATE Communications in a PUBLIC
World, Charlie Kaufman, Radia Perlman, & Mike Speciner,
Prentice Hall Series in Computer Networking and
Distributed Communications, 1995.
[RFC1034] Mockapetris, P., "Domain Names  Concepts and Facilities",
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain Names  Implementation and
Specification", STD 13, RFC 1035, November 1987.
[RFC1321] Rivest, R., "The MD5 MessageDigest Algorithm", RFC 1321,
April 1992.
D. Eastlake 3rd Standards Track [Page 5]
RFC 3110 RSA SIGs and KEYs in the DNS May 2001
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2437] Kaliski, B. and J. Staddon, "PKCS #1: RSA Cryptography
Specifications Version 2.0", RFC 2437, October 1998.
[RFC2535] Eastlake, D., "Domain Name System Security Extensions",
RFC 2535, March 1999.
[RFC2536] Eastlake, D., "DSA KEYs and SIGs in the Domain Name System
(DNS)", RFC 2536, March 1999.
[RFC2537] Eastlake, D., "RSA/MD5 KEYs and SIGs in the Domain Name
System (DNS)", RFC 2537, March 1999.
[RFC2541] Eastlake, D., "DNS Security Operational Considerations",
RFC 2541, March 1999.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC
2671, August 1999.
[Schneier] Bruce Schneier, "Applied Cryptography Second Edition:
protocols, algorithms, and source code in C", 1996, John
Wiley and Sons, ISBN 0471117099.
Author's Address
Donald E. Eastlake 3rd
Motorola
155 Beaver Street
Milford, MA 01757 USA
Phone: +15082615434 (w)
+15086342066 (h)
Fax +15082614777 (w)
EMail: Donald.Eastlake@motorola.com
D. Eastlake 3rd Standards Track [Page 6]
RFC 3110 RSA SIGs and KEYs in the DNS May 2001
Full Copyright Statement
Copyright (C) The Internet Society (2001). All Rights Reserved.
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
D. Eastlake 3rd Standards Track [Page 7]

