RFC3110 - RSA/SHA-1 SIGs and RSA KEYs in the Domain Name System (DNS)

Network Working Group D. Eastlake 3rd

Request for Comments: 3110 Motorola

Obsoletes: 2537 May 2001

Category: Standards Track

RSA/SHA-1 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.

Abstract

This document describes how to prodUCe RSA/SHA1 SIG resource records

(RRs) in Section 3 and, so as to completely replace RFC2537,

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

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 SHA-1 algorithms is assumed [Schneier,

FIP180] in this document.

RFC2537 described how to store RSA keys and RSA/MD5 based signatures

in the DNS. However, since the adoption of RFC2537, continued

cryptographic research has revealed hints of weakness in the MD5

[RFC1321] algorithm used in RFC2537. 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 RFC2537, 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 RFC2537

is NOT RECOMMENDED.

The key Words "MUST", "REQUIRED", "SHOULD", "RECOMMENDED", "NOT

RECOMMENDED", and "MAY" in this document are to be interpreted as

described in RFC2119.

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 RFC2537).

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 RFC2535. See RFC2535 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

[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

RFC2541.

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 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.

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).

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 RFC2535 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 RFC2541, "DNS Security

Operational Considerations".

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.

[RFC1034] Mockapetris, P., "Domain Names - Concepts and Facilities",

STD 13, RFC1034, November 1987.

[RFC1035] Mockapetris, P., "Domain Names - Implementation and

Specification", STD 13, RFC1035, November 1987.

[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC1321,

April 1992.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate

Requirement Levels", BCP 14, RFC2119, March 1997.

[RFC2437] Kaliski, B. and J. Staddon, "PKCS #1: RSA Cryptography

Specifications Version 2.0", RFC2437, October 1998.

[RFC2535] Eastlake, D., "Domain Name System Security Extensions",

RFC2535, March 1999.

[RFC2536] Eastlake, D., "DSA KEYs and SIGs in the Domain Name System

(DNS)", RFC2536, March 1999.

[RFC2537] Eastlake, D., "RSA/MD5 KEYs and SIGs in the Domain Name

System (DNS)", RFC2537, March 1999.

[RFC2541] Eastlake, D., "DNS Security Operational Considerations",

RFC2541, 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 0-471-11709-9.

Author's Address

Donald E. Eastlake 3rd

Motorola

155 Beaver Street

Milford, MA 01757 USA

Phone: +1-508-261-5434 (w)

+1-508-634-2066 (h)

Fax +1-508-261-4777 (w)

EMail: Donald.Eastlake@motorola.com

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