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RFC2537 - RSA/MD5 KEYs and SIGs in the Domain Name System (DNS)

王朝other·作者佚名  2008-05-31
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Request for Comments: 2537 IBM

Category: Standards Track March 1999

RSA/MD5 KEYs and SIGs 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 (1999). All Rights Reserved.

Abstract

A standard method for storing RSA keys and and RSA/MD5 based

signatures in the Domain Name System is described which utilizes DNS

KEY and SIG resource records.

Table of Contents

Abstract...................................................1

1. IntrodUCtion............................................1

2. RSA Public KEY Resource Records.........................2

3. RSA/MD5 SIG Resource Records............................2

4. Performance Considerations..............................3

5. Security Considerations.................................4

References.................................................4

Author's Address...........................................5

Full Copyright Statement...................................6

1. Introduction

The Domain Name System (DNS) is the global hierarchical replicated

distributed database system for Internet addressing, mail proxy, and

other information. 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.

This document describes how to store RSA keys and and RSA/MD5 based

signatures in the DNS. Familiarity with the RSA algorithm is assumed

[Schneier]. Implementation of the RSA algorithm in DNS is

recommended.

The key Words "MUST", "REQUIRED", "SHOULD", "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 1 [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 currently

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.

3. RSA/MD5 SIG Resource Records

The signature portion of the SIG RR RDATA area, when using the

RSA/MD5 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 = MD5 ( data )

signature = ( 00 01 FF* 00 prefix hash ) ** e (mod n)

where MD5 is the message digest algorithm documented in [RFC1321],

"" 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 MD5 algorithm designator prefix specified in [RFC

2437], that is,

hex 3020300c06082a864886f70d020505000410 [NETSEC].

This prefix is included to make it easier to use RSAREF (or similar

packages such as EuroRef). The FF octet MUST be repeated the maximum

number of times such that the value of the quantity being

exponentiated is the same length in octets as the value of n.

(The above specifications are identical to the corresponding part 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.

Leading zero bytes are permitted in the RSA/MD5 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 [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.

Current DNS implementations are optimized for small transfers,

typically less than 512 bytes including overhead. While larger

transfers will perform correctly and work is underway 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. Security Considerations

Many of the general security consideration 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 interoperability, the RSA key size is limited to 4096 bits. For

particularly critical applications, implementors are encouraged to

consider the range of available algorithms and key sizes.

References

[NETSEC] Kaufman, C., Perlman, R. and M. Speciner, "Network

Security: PRIVATE Communications in a PUBLIC World",

Series in Computer Networking and Distributed

Communications, 1995.

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

Specifications Version 2.0", RFC2437, October 1998.

[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.

[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.

[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

IBM

65 Shindegan Hill Road, RR #1

Carmel, NY 10512

Phone: +1-914-276-2668(h)

+1-914-784-7913(w)

Fax: +1-914-784-3833(w)

EMail: dee3@us.ibm.com

Full Copyright Statement

Copyright (C) The Internet Society (1999). All Rights Reserved.

This document and translations of it may be copied and furnished to

others, and derivative works that comment on or otherwise explain it

or assist in its implementation may be prepared, copied, published

and distributed, in whole or in part, without restriction of any

kind, provided that the above copyright notice and this paragraph are

included on all such copies and derivative works. However, this

document itself may not be modified in any way, such as by removing

the copyright notice or references to the Internet Society or other

Internet organizations, except as needed for the purpose of

developing Internet standards in which case the procedures for

copyrights defined in the Internet Standards process must be

followed, or as required to translate it into languages other than

English.

The limited permissions granted above are perpetual and will not be

revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an

"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING

TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING

BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION

HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF

MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

 
 
 
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