draft-ietf-dnssec-rsa-01.txt   rfc2537.txt 
INTERNET-DRAFT RSA/MD5 KEYs and SIGs in the DNS Network Working Group D. Eastlake
October 1998 Request for Comments: 2537 IBM
Expires April 1999 Category: Standards Track March 1999
RSA/MD5 KEYs and SIGs in the Domain Name System (DNS) RSA/MD5 KEYs and SIGs in the Domain Name System (DNS)
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Donald E. Eastlake 3rd Status of this Memo
Status of This Document
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[Changes from the previous draft: change date, update author info, Copyright (C) The Internet Society (1999). All Rights Reserved.
add RFC 2119 reference]
Abstract Abstract
A standard method for storing RSA keys and and RSA/MD5 based A standard method for storing RSA keys and and RSA/MD5 based
signatures in the Domain Name System is described which utilizes DNS signatures in the Domain Name System is described which utilizes DNS
KEY and SIG resource records. KEY and SIG resource records.
INTERNET-DRAFT RSA/MD5 in the DNS
Table of Contents Table of Contents
Status of This Document....................................1 Abstract...................................................1
Abstract...................................................1 1. Introduction............................................1
2. RSA Public KEY Resource Records.........................2
Table of Contents..........................................2 3. RSA/MD5 SIG Resource Records............................2
4. Performance Considerations..............................3
1. Introduction............................................3 5. Security Considerations.................................4
2. RSA Public KEY Resource Records.........................3 References.................................................4
Author's Address...........................................5
3. RSA/MD5 SIG Resource Records............................4 Full Copyright Statement...................................6
4. Performance Considerations..............................5
5. Security Considerations.................................5
References.................................................6
Author's Address...........................................6
Expiration and File Name...................................6
INTERNET-DRAFT RSA/MD5 in the DNS
1. Introduction 1. Introduction
The Domain Name System (DNS) is the global hierarchical replicated The Domain Name System (DNS) is the global hierarchical replicated
distributed database system for Internet addressing, mail proxy, and distributed database system for Internet addressing, mail proxy, and
other information. The DNS has been extended to include digital other information. The DNS has been extended to include digital
signatures and cryptographic keys as described in [draft-ietf- signatures and cryptographic keys as described in [RFC 2535]. Thus
dnssec-secext2-*]. Thus the DNS can now be secured and used for the DNS can now be secured and used for secure key distribution.
secure key distribution.
This document describes how to store RSA keys and and RSA/MD5 based This document describes how to store RSA keys and and RSA/MD5 based
signatures in the DNS. Familiarity with the RSA algorithm is assumed signatures in the DNS. Familiarity with the RSA algorithm is assumed
[Schneier]. Implementation of the RSA algorithm in DNS is [Schneier]. Implementation of the RSA algorithm in DNS is
recommended. recommended.
The key words "MUST", "REQUIRED", "SHOULD", "RECOMMENDED", and "MAY" The key words "MUST", "REQUIRED", "SHOULD", "RECOMMENDED", and "MAY"
in this document are to be interpreted as described in RFC 2119. in this document are to be interpreted as described in RFC 2119.
2. RSA Public KEY Resource Records 2. RSA Public KEY Resource Records
RSA public keys are stored in the DNS as KEY RRs using algorithm RSA public keys are stored in the DNS as KEY RRs using algorithm
number 1 [draft-ietf-dnssec-secext2-*]. The structure of the number 1 [RFC 2535]. The structure of the algorithm specific portion
algorithm specific portion of the RDATA part of such RRs is as shown of the RDATA part of such RRs is as shown below.
below.
Field Size Field Size
----- ---- ----- ----
exponent length 1 or 3 octets (see text) exponent length 1 or 3 octets (see text)
exponent as specified by length field exponent as specified by length field
modulus remaining space modulus remaining space
For interoperability, the exponent and modulus are each currently For interoperability, the exponent and modulus are each currently
limited to 4096 bits in length. The public key exponent is a limited to 4096 bits in length. The public key exponent is a
variable length unsigned integer. Its length in octets is 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 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 zero octet followed by a two octet unsigned length if it is longer
than 255 bytes. The public key modulus field is a multiprecision than 255 bytes. The public key modulus field is a multiprecision
unsigned integer. The length of the modulus can be determined from unsigned integer. The length of the modulus can be determined from
the RDLENGTH and the preceding RDATA fields including the exponent. the RDLENGTH and the preceding RDATA fields including the exponent.
Leading zero octets are prohibited in the exponent and modulus. Leading zero octets are prohibited in the exponent and modulus.
INTERNET-DRAFT RSA/MD5 in the DNS
3. RSA/MD5 SIG Resource Records 3. RSA/MD5 SIG Resource Records
The signature portion of the SIG RR RDATA area, when using the The signature portion of the SIG RR RDATA area, when using the
RSA/MD5 algorithm, is calculated as shown below. The data signed is RSA/MD5 algorithm, is calculated as shown below. The data signed is
determined as specified in [draft-ietf-dnssec-secext2-*]. See determined as specified in [RFC 2535]. See [RFC 2535] for fields in
[draft-ietf-dnssec-secext2-*] for fields in the SIG RR RDATA which the SIG RR RDATA which precede the signature itself.
precede the signature itself.
hash = MD5 ( data ) hash = MD5 ( data )
signature = ( 01 | FF* | 00 | prefix | hash ) ** e (mod n) signature = ( 00 | 01 | FF* | 00 | prefix | hash ) ** e (mod n)
where MD5 is the message digest algorithm documented in [RFC 1321], where MD5 is the message digest algorithm documented in [RFC 1321],
"|" is concatenation, "e" is the private key exponent of the signer, "|" 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 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 are fixed octets of the corresponding hexadecimal value. "prefix" is
the ASN.1 BER MD5 algorithm designator prefix specified in PKCS1, the ASN.1 BER MD5 algorithm designator prefix specified in [RFC
that is, 2437], that is,
hex 3020300c06082a864886f70d020505000410 [NETSEC]. hex 3020300c06082a864886f70d020505000410 [NETSEC].
This prefix is included to make it easier to use RSAREF (or similar 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 packages such as EuroRef). The FF octet MUST be repeated the maximum
number of times such that the value of the quantity being number of times such that the value of the quantity being
exponentiated is one octet shorter than the value of n. exponentiated is the same length in octets as the value of n.
(The above specifications are identical to the corresponding part of (The above specifications are identical to the corresponding part of
Public Key Cryptographic Standard #1 [PKCS1].) Public Key Cryptographic Standard #1 [RFC 2437].)
The size of n, including most and least significant bits (which will 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 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. and e SHOULD be chosen such that the public exponent is small.
Leading zero bytes are permitted in the RSA/MD5 algorithm signature. Leading zero bytes are permitted in the RSA/MD5 algorithm signature.
A public exponent of 3 minimizes the effort needed to verify a A public exponent of 3 minimizes the effort needed to verify a
signature. Use of 3 as the public exponent is weak for signature. Use of 3 as the public exponent is weak for
confidentiality uses since, if the same data can be collected confidentiality uses since, if the same data can be collected
encrypted under three different keys with an exponent of 3 then, encrypted under three different keys with an exponent of 3 then,
using the Chinese Remainder Theorem [NETSEC], the original plain text using the Chinese Remainder Theorem [NETSEC], the original plain text
can be easily recovered. This weakness is not significant for DNS can be easily recovered. This weakness is not significant for DNS
security because we seek only authentication, not confidentiality. security because we seek only authentication, not confidentiality.
INTERNET-DRAFT RSA/MD5 in the DNS
4. Performance Considerations 4. Performance Considerations
General signature generation speeds are roughly the same for RSA and General signature generation speeds are roughly the same for RSA and
DSA [RFC xDSA]. With sufficient pre-computation, signature DSA [RFC 2536]. With sufficient pre-computation, signature
generation with DSA is faster than RSA. Key generation is also generation with DSA is faster than RSA. Key generation is also
faster for DSA. However, signature verification is an order of faster for DSA. However, signature verification is an order of
magnitude slower with DSA when the RSA public exponent is chosen to 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 be small as is recommended for KEY RRs used in domain name system
(DNS) data authentication. (DNS) data authentication.
Current DNS implementations are optimized for small transfers, Current DNS implementations are optimized for small transfers,
typically less than 512 bytes including overhead. While larger typically less than 512 bytes including overhead. While larger
transfers will perform correctly and work is underway to make larger transfers will perform correctly and work is underway to make larger
transfers more efficient, it is still advisable at this time to make transfers more efficient, it is still advisable at this time to make
reasonable efforts to minimize the size of KEY RR sets stored within reasonable efforts to minimize the size of KEY RR sets stored within
the DNS consistent with adequate security. Keep in mind that in a the DNS consistent with adequate security. Keep in mind that in a
secure zone, at least one authenticating SIG RR will also be secure zone, at least one authenticating SIG RR will also be
returned. returned.
5. Security Considerations 5. Security Considerations
Many of the general security consideration in [draft-ietf-dnssec- Many of the general security consideration in [RFC 2535] apply. Keys
secext2-*] apply. Keys retrieved from the DNS should not be trusted retrieved from the DNS should not be trusted unless (1) they have
unless (1) they have been securely obtained from a secure resolver or been securely obtained from a secure resolver or independently
independently verified by the user and (2) this secure resolver and verified by the user and (2) this secure resolver and secure
secure obtainment or independent verification conform to security obtainment or independent verification conform to security policies
policies acceptable to the user. As with all cryptographic acceptable to the user. As with all cryptographic algorithms,
algorithms, evaluating the necessary strength of the key is essential evaluating the necessary strength of the key is essential and
and dependent on local policy. dependent on local policy.
For interoperability, the RSA key size is limited to 4096 bits. For For interoperability, the RSA key size is limited to 4096 bits. For
particularly critical applications, implementors are encouraged to particularly critical applications, implementors are encouraged to
consider the range of available algorithms and key sizes. consider the range of available algorithms and key sizes.
INTERNET-DRAFT RSA/MD5 in the DNS
References References
[NETSEC] - Network Security: PRIVATE Communications in a PUBLIC [NETSEC] Kaufman, C., Perlman, R. and M. Speciner, "Network
World, Charlie Kaufman, Radia Perlman, & Mike Speciner, Prentice Hall Security: PRIVATE Communications in a PUBLIC World",
Series in Computer Networking and Distributed Communications, 1995. Series in Computer Networking and Distributed
Communications, 1995.
[PKCS1] - PKCS #1: RSA Encryption Standard, RSA Data Security, Inc., [RFC 2437] Kaliski, B. and J. Staddon, "PKCS #1: RSA Cryptography
3 June 1991, Version 1.4. [there is an ID on this and any resulting Specifications Version 2.0", RFC 2437, October 1998.
RFC could be substitutes if available in time]
[RFC 1034] - P. Mockapetris, "Domain names - concepts and [RFC 1034] Mockapetris, P., "Domain Names - Concepts and
facilities", 11/01/1987. Facilities", STD 13, RFC 1034, November 1987.
[RFC 1035] - P. Mockapetris, "Domain names - implementation and [RFC 1035] Mockapetris, P., "Domain Names - Implementation and
specification", 11/01/1987. Specification", STD 13, RFC 1035, November 1987.
[RFC 1321] - R. Rivest, "The MD5 Message-Digest Algorithm", April [RFC 1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321
1992. April 1992.
[draft-ietf-dnssec-secext2-*] - Domain Name System Security [RFC 2535] Eastlake, D., "Domain Name System Security Extensions",
Extensions, D. Eastlake, C. Kaufman, January 1997. RFC 2535, March 1999.
[RFC xDSA] - draft-ietf-dnssec-dss-*.txt [RFC 2536] EastLake, D., "DSA KEYs and SIGs in the Domain Name
System (DNS)", RFC 2536, March 1999.
[Schneier] - Bruce Schneier, "Applied Cryptography Second Edition: [Schneier] Bruce Schneier, "Applied Cryptography Second Edition:
protocols, algorithms, and source code in C", 1996, John Wiley and protocols, algorithms, and source code in C", 1996, John
Sons, ISBN 0-471-11709-9. Wiley and Sons, ISBN 0-471-11709-9.
Author's Address Author's Address
Donald E. Eastlake 3rd Donald E. Eastlake 3rd
IBM IBM
318 Acton Street 65 Shindegan Hill Road, RR #1
Carlisle, MA 01741 USA Carmel, NY 10512
Telephone: +1-978-287-4877 Phone: +1-914-276-2668(h)
+1-914-784-7913 +1-914-784-7913(w)
FAX: +1-978-371-7148 Fax: +1-914-784-3833(w)
EMail: dee3@us.ibm.com EMail: dee3@us.ibm.com
Expiration and File Name Full Copyright Statement
This draft expires in April 1999. Copyright (C) The Internet Society (1999). All Rights Reserved.
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