INTERNET-DRAFT RSA SIGs and KEYs in the DNS
OBSOLETES RFC 2537 December 2000
Expires June 2001
D. Eastlake
RSA/SHA-1 SIGs and RSA KEYs in the Domain Name System (DNS)
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Status of This Document
This draft is intended to be become a Proposed Standard RFC.
Distribution of this document is unlimited. Comments should be sent
to the DNS extensions mailing list or to
the author.
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026. Internet-Drafts are
working documents of the Internet Engineering Task Force (IETF), its
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The list of current Internet-Drafts can be accessed at
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Abstract
Since the adoption of a Proposed Standard for RSA signatures in the
DNS [RFC 2537], advances in hashing have been made. A new DNS
signature algorithm is defined to make these advances available in
SIG resource records (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.
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Acknowledgements
Material and comments from the following have been incorporated and
are gratefully acknowledged:
Olafur Gudmundsson
Charlie Kaufman
Steve Wang
Table of Contents
Status of This Document....................................1
Abstract...................................................1
Acknowledgements...........................................2
Table of Contents..........................................2
1. Introduction............................................3
2. RSA Public KEY Resource Records.........................3
3. RSA/SHA1 SIG Resource Records...........................4
4. Performance Considerations..............................5
5. IANA Considerations.....................................5
6. Security Considerations.................................6
References.................................................7
Author's Address...........................................8
Expiration and File Name...................................8
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1. Introduction
The Domain Name System (DNS) is the global hierarchical replicated
distributed database system for Internet addressing, mail proxy, and
other information [RFC 1034, 1035, etc.]. The DNS has been extended
to include digital signatures and cryptographic keys as described in
[RFC 2535]. Thus the DNS can now be secured and used for secure key
distribution.
Familiarity with the RSA and SHA-1 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 [RFC 1321] 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 and most TLDs 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.
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 (TBD, suggest 5) [RFC 2535]. The structure of the algorithm
specific portion of the RDATA part of such RRs is as shown below.
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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 (TBD, 5 suggested).
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)
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 [RFC
2437], 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
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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 part of
Public Key Cryptographic Standard #1 [RFC 2437].)
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.
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. This weakness is not significant for DNS
security because we seek only authentication, not confidentiality.
4. Performance Considerations
General signature generation speeds are roughly the same for RSA and
DSA [RFC 2536]. With sufficient pre-computation, 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.
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 [RFC 2671] 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 (TBD, 5 suggested) is allocated for
RSA/SHA1 SIG RRs and RSA KEY RRs.
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6. Security Considerations
Many of the general security consideration 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".
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References
[FIP180] - U.S. Department of Commerce, "Secure Hash Standard", FIPS
PUB 180-1, 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.
[RFC 1034] - P. Mockapetris, "Domain names - concepts and
facilities", 11/01/1987.
[RFC 1035] - P. Mockapetris, "Domain names - implementation and
specification", 11/01/1987.
[RFC 1321] - R. Rivest, "The MD5 Message-Digest Algorithm", April
1992.
[RFC 2119] - S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", March 1997.
[RFC 2437] - B. Kaliski, J. Staddon, "PKCS #1: RSA Cryptography
Specifications Version 2.0", October 1998.
[RFC 2535] - D. Eastlake, "Domain Name System Security Extensions",
March 1999.
[RFC 2536] - D. Eastlake, "DSA KEYs and SIGs in the Domain Name
System (DNS)", March 1999.
[RFC 2537] - D. Eastlake, "RSA/MD5 KEYs and SIGs in the Domain Name
System (DNS)", March 1999.
[RDC 2541] - D. Eastlake, "DNS Security Operational Considerations",
March 1999.
[RFC 2671] - P. Vixie, "Extension Mechanisms for DNS (EDNS0)", August
1999.
[Schneier] - Bruce Schneier, "Applied Cryptography Second Edition:
protocols, algorithms, and source code in C", 1996, John Wiley and
Sons, ISBN 0-471-11709-9.
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Author's Address
Donald E. Eastlake 3rd
Motorola
155 Beaver Street
Milford, MA 01757 USA
Telephone: +1-508-261-5434 (w)
+1-508-634-2066 (h)
FAX: +1-508-261-4777 (w)
EMail: Donald.Eastlake@motorola.com
Expiration and File Name
This draft expires in June 2001.
Its file name is draft-ietf-dnsext-rsa-02.txt.
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