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RFC2536 - DSA KEYs and SIGs in the Domain Name System (DNS)

王朝other·作者佚名  2008-05-31
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Network Working Group D. EastLake

Request for Comments: 2536 IBM

Category: Standards Track March 1999

DSA 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 US Government Digital Signature

Algorithm keys and 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. DSA KEY Resource Records................................2

3. DSA SIG Resource Records................................3

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

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

6. IANA Considerations.....................................4

References.................................................5

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 can be used for secure key

distribution.

This document describes how to store US Government Digital Signature

Algorithm (DSA) keys and signatures in the DNS. Familiarity with the

US Digital Signature Algorithm is assumed [Schneier]. Implementation

of DSA is mandatory for DNS security.

2. DSA KEY Resource Records

DSA public keys are stored in the DNS as KEY RRs using algorithm

number 3 [RFC2535]. The structure of the algorithm specific portion

of the RDATA part of this RR is as shown below. These fields, from Q

through Y are the "public key" part of the DSA KEY RR.

The period of key validity is not in the KEY RR but is indicated by

the SIG RR(s) which signs and authenticates the KEY RR(s) at that

domain name.

Field Size

----- ----

T 1 octet

Q 20 octets

P 64 + T*8 octets

G 64 + T*8 octets

Y 64 + T*8 octets

As described in [FIPS 186] and [Schneier]: T is a key size parameter

chosen such that 0 <= T <= 8. (The meaning for algorithm 3 if the T

octet is greater than 8 is reserved and the remainder of the RDATA

portion may have a different format in that case.) Q is a prime

number selected at key generation time such that 2**159 < Q < 2**160

so Q is always 20 octets long and, as with all other fields, is

stored in "big-endian" network order. P, G, and Y are calculated as

directed by the FIPS 186 key generation algorithm [Schneier]. P is

in the range 2**(511+64T) < P < 2**(512+64T) and so is 64 + 8*T

octets long. G and Y are quantities modulus P and so can be up to

the same length as P and are allocated fixed size fields with the

same number of octets as P.

During the key generation process, a random number X must be

generated such that 1 <= X <= Q-1. X is the private key and is used

in the final step of public key generation where Y is computed as

Y = G**X mod P

3. DSA SIG Resource Records

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

Digital Signature Algorithm, is shown below with fields in the order

they occur. See [RFC2535] for fields in the SIG RR RDATA which

precede the signature itself.

Field Size

----- ----

T 1 octet

R 20 octets

S 20 octets

The data signed is determined as specified in [RFC2535]. Then the

following steps are taken, as specified in [FIPS 186], where Q, P, G,

and Y are as specified in the public key [Schneier]:

hash = SHA-1 ( data )

Generate a random K such that 0 < K < Q.

R = ( G**K mod P ) mod Q

S = ( K**(-1) * (hash + X*R) ) mod Q

Since Q is 160 bits long, R and S can not be larger than 20 octets,

which is the space allocated.

T is copied from the public key. It is not logically necessary in

the SIG but is present so that values of T > 8 can more conveniently

be used as an escape for extended versions of DSA or other algorithms

as later specified.

4. Performance Considerations

General signature generation speeds are roughly the same for RSA [RFC

2537] and DSA. 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 than

RSA 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.

The key size limitation of a maximum of 1024 bits ( T = 8 ) in the

current DSA standard may limit the security of DSA. For particularly

critical applications, implementors are encouraged to consider the

range of available algorithms and key sizes.

DSA assumes the ability to frequently generate high quality random

numbers. See [RFC1750] for guidance. DSA is designed so that if

manipulated rather than random numbers are used, very high bandwidth

covert channels are possible. See [Schneier] and more recent

research. The leakage of an entire DSA private key in only two DSA

signatures has been demonstrated. DSA provides security only if

trusted implementations, including trusted random number generation,

are used.

6. IANA Considerations

Allocation of meaning to values of the T parameter that are not

defined herein requires an IETF standards actions. It is intended

that values unallocated herein be used to cover future extensions of

the DSS standard.

References

[FIPS 186] U.S. Federal Information Processing Standard: Digital

Signature Standard.

[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.

[RFC1750] Eastlake, D., Crocker, S. and J. Schiller, "Randomness

Recommendations for Security", RFC1750, December 1994.

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

RFC2535, March 1999.

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

System (DNS)", RFC2537, March 1999.

[Schneier] Schneier, B., "Applied Cryptography Second Edition:

protocols, algorithms, and source code in C", 1996.

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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