draft-ietf-dnsext-rsa-03.txt   rfc3110.txt 
INTERNET-DRAFT RSA SIGs and KEYs in the DNS
Expires October 2001
RSA/SHA-1 SIGs and RSA KEYs in the Domain Name System (DNS)
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<draft-ietf-dnsext-rsa-03.txt>
Donald Eastlake
Status of This Document Network Working Group D. Eastlake 3rd
Request for Comments: 3110 Motorola
Obsoletes: 2537 May 2001
Category: Standards Track
This draft is intended to be become a Proposed Standard RFC. RSA/SHA-1 SIGs and RSA KEYs in the Domain Name System (DNS)
Distribution of this document is unlimited. Comments should be sent
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Abstract Abstract
Since the adoption of a Proposed Standard for RSA signatures in the This document describes how to produce RSA/SHA1 SIG resource records
DNS [RFC 2537], advances in hashing have been made. A new DNS (RRs) in Section 3 and, so as to completely replace RFC 2537,
signature algorithm is defined to make these advances available in describes how to produce RSA KEY RRs in Section 2.
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.
INTERNET-DRAFT RSA/SHA1 in the DNS 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 Acknowledgements
Material and comments from the following have been incorporated and Material and comments from the following have been incorporated and
are gratefully acknowledged: are gratefully acknowledged:
Olafur Gudmundsson Olafur Gudmundsson
The IESG The IESG
Charlie Kaufman Charlie Kaufman
Steve Wang Steve Wang
Table of Contents Table of Contents
Status of This Document....................................1 1. Introduction................................................... 2
Abstract...................................................1 2. RSA Public KEY Resource Records................................ 3
3. RSA/SHA1 SIG Resource Records.................................. 3
Acknowledgements...........................................2 4. Performance Considerations..................................... 4
Table of Contents..........................................2 5. IANA Considerations............................................ 5
6. Security Considerations........................................ 5
1. Introduction............................................3 References........................................................ 5
2. RSA Public KEY Resource Records.........................3 Author's Address.................................................. 6
3. RSA/SHA1 SIG Resource Records...........................4 Full Copyright Statement.......................................... 7
4. Performance Considerations..............................5
5. IANA Considerations.....................................6
6. Security Considerations.................................6
References.................................................7
Author's Address...........................................8
Expiration and File Name...................................8
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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 [RFC 1034, 1035, etc.]. The DNS has been extended other information [RFC1034, 1035, etc.]. The DNS has been extended
to include digital signatures and cryptographic keys as described in to include digital signatures and cryptographic keys as described in
[RFC 2535]. Thus the DNS can now be secured and used for secure key [RFC2535]. Thus the DNS can now be secured and used for secure key
distribution. distribution.
Familiarity with the RSA and SHA-1 algorithms is assumed [Schneier, Familiarity with the RSA and SHA-1 algorithms is assumed [Schneier,
FIP180] in this document. FIP180] in this document.
[RFC 2537] described how to store RSA keys and RSA/MD5 based RFC 2537 described how to store RSA keys and RSA/MD5 based signatures
signatures in the DNS. However, since the adoption of [RFC 2537], in the DNS. However, since the adoption of RFC 2537, continued
continued cryptographic research has revealed hints of weakness in cryptographic research has revealed hints of weakness in the MD5
the MD5 [RFC 1321] algorithm used in [RFC 2537]. The SHA1 Secure Hash [RFC1321] algorithm used in RFC 2537. The SHA1 Secure Hash Algorithm
Algorithm [FIP180], which produces a larger hash, has been developed. [FIP180], which produces a larger hash, has been developed. By now
By now there has been sufficient experience with SHA1 that it is there has been sufficient experience with SHA1 that it is generally
generally acknowledged to be stronger than MD5. While this stronger acknowledged to be stronger than MD5. While this stronger hash is
hash is probably not needed today in most secure DNS zones, critical probably not needed today in most secure DNS zones, critical zones
zones such a root, most top level domains, and some second and third such a root, most top level domains, and some second and third level
level domains, are sufficiently valuable targets that it would be domains, are sufficiently valuable targets that it would be negligent
negligent not to provide what are generally agreed to be stronger not to provide what are generally agreed to be stronger mechanisms.
mechanisms. Furthermore, future advances in cryptanalysis and/or Furthermore, future advances in cryptanalysis and/or computer speeds
computer speeds may require a stronger hash everywhere. In addition, may require a stronger hash everywhere. In addition, the additional
the additional computation required by SHA1 above that required by computation required by SHA1 above that required by MD5 is
MD5 is insignificant compared with the computational effort required insignificant compared with the computational effort required by the
by the RSA modular exponentiation. RSA modular exponentiation.
This document describes how to produce RSA/SHA1 SIG RRs in Section 3 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 and, so as to completely replace RFC 2537, describes how to produce
RSA KEY RRs in Section 2. RSA KEY RRs in Section 2.
Implementation of the RSA algorithm in DNS with SHA1 is MANDATORY for 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] DNSSEC. The generation of RSA/MD5 SIG RRs as described in RFC 2537
is NOT RECOMMENDED. is NOT RECOMMENDED.
The key words "MUST", "REQUIRED", "SHOULD", "RECOMMENDED", "NOT The key words "MUST", "REQUIRED", "SHOULD", "RECOMMENDED", "NOT
RECOMMENDED", and "MAY" in this document are to be interpreted as RECOMMENDED", and "MAY" in this document are to be interpreted as
described in [RFC 2119]. 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 (TBD, suggest 5) [RFC 2535]. The structure of the algorithm number 5 [RFC2535]. The structure of the algorithm specific portion
specific portion of the RDATA part of such RRs is as shown below. of the RDATA part of such RRs is as shown below.
INTERNET-DRAFT RSA/SHA1 in the DNS
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 limited to For interoperability, the exponent and modulus are each limited to
4096 bits in length. The public key exponent is a variable length 4096 bits in length. The public key exponent is a variable length
unsigned integer. Its length in octets is represented as one octet 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 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 two octet unsigned length if it is longer than 255 bytes. The public
key modulus field is a multiprecision unsigned integer. The length key modulus field is a multiprecision unsigned integer. The length
of the modulus can be determined from the RDLENGTH and the preceding of the modulus can be determined from the RDLENGTH and the preceding
RDATA fields including the exponent. Leading zero octets are RDATA fields including the exponent. Leading zero octets are
prohibited in the exponent and modulus. prohibited in the exponent and modulus.
Note: KEY RRs for use with RSA/SHA1 DNS signatures MUST use this Note: KEY RRs for use with RSA/SHA1 DNS signatures MUST use this
algorithm number (rather than the algorithm number specified in the algorithm number (rather than the algorithm number specified in the
obsoleted [RFC 2537]). obsoleted RFC 2537).
Note: This changes the algorithm number for RSA KEY RRs to be the Note: This changes the algorithm number for RSA KEY RRs to be the
same as the new algorithm number for RSA/SHA1 SIGs. same as the new algorithm number for RSA/SHA1 SIGs.
3. RSA/SHA1 SIG Resource Records 3. RSA/SHA1 SIG Resource Records
RSA/SHA1 signatures are stored in the DNS using SIG resource records RSA/SHA1 signatures are stored in the DNS using SIG resource records
(RRs) with algorithm number (TBD, 5 suggested). (RRs) with algorithm number 5.
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/SHA1 algorithm, is calculated as shown below. The data signed is RSA/SHA1 algorithm, is calculated as shown below. The data signed is
determined as specified in [RFC 2535]. See [RFC 2535] for fields in determined as specified in RFC 2535. See RFC 2535 for fields in the
the SIG RR RDATA which precede the signature itself. SIG RR RDATA which precede the signature itself.
hash = SHA1 ( data ) hash = SHA1 ( data )
signature = ( 01 | FF* | 00 | prefix | hash ) ** e (mod n) signature = ( 01 | FF* | 00 | prefix | hash ) ** e (mod n)
where SHA1 is the message digest algorithm documented in [FIP180], where SHA1 is the message digest algorithm documented in [FIP180],
"|" 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 SHA1 algorithm designator prefix required in PKCS1 [RFC the ASN.1 BER SHA1 algorithm designator prefix required in PKCS1
2437], that is, [RFC2437], that is,
hex 30 21 30 09 06 05 2B 0E 03 02 1A 05 00 04 14 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 This prefix is included to make it easier to use standard
cryptographic libraries. The FF octet MUST be repeated the maximum cryptographic libraries. The FF octet MUST be repeated the maximum
INTERNET-DRAFT RSA/SHA1 in the DNS
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 one octet shorter than the value of n.
(The above specifications are identical to the corresponding parts of (The above specifications are identical to the corresponding parts of
Public Key Cryptographic Standard #1 [RFC 2437].) Public Key Cryptographic Standard #1 [RFC2437].)
The size of "n", including most and least significant bits (which 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 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 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 small. These are protocol limits. For a discussion of key size see
[RFC 2541]. RFC 2541.
Leading zero bytes are permitted in the RSA/SHA1 algorithm signature. Leading zero bytes are permitted in the RSA/SHA1 algorithm signature.
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 2536]. With sufficient pre-computation, signature DSA [RFC2536]. With sufficient pre-computation, signature generation
generation with DSA is faster than RSA. Key generation is also with DSA is faster than RSA. Key generation is also faster for DSA.
faster for DSA. However, signature verification is an order of However, signature verification is an order of magnitude slower with
magnitude slower with DSA when the RSA public exponent is chosen to DSA when the RSA public exponent is chosen to be small as is
be small as is recommended for KEY RRs used in domain name system recommended for KEY RRs used in domain name system (DNS) data
(DNS) data authentication. authentication.
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. If a key is known to be used only for 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 authentication, as is the case with DNSSEC, then an exponent of 3 is
acceptable. However other applications in the future may wish to acceptable. However other applications in the future may wish to
leverage DNS distributed keys for applications that do require leverage DNS distributed keys for applications that do require
confidentiality. For keys which might have such other uses, a more confidentiality. For keys which might have such other uses, a more
conservative choice would be 65537 (F4, the fourth fermat number). conservative choice would be 65537 (F4, the fourth fermat number).
Current DNS implementations are optimized for small transfers, Current DNS implementations are optimized for small transfers,
typically less than 512 bytes including DNS overhead. Larger typically less than 512 bytes including DNS overhead. Larger
transfers will perform correctly and extensions have been transfers will perform correctly and extensions have been
standardized [RFC 2671] to make larger transfers more efficient, it standardized [RFC2671] to make larger transfers more efficient, it is
is still advisable at this time to make reasonable efforts to still advisable at this time to make reasonable efforts to minimize
minimize the size of KEY RR sets stored within the DNS consistent the size of KEY RR sets stored within the DNS consistent with
with adequate security. Keep in mind that in a secure zone, at least adequate security. Keep in mind that in a secure zone, at least one
one authenticating SIG RR will also be returned. authenticating SIG RR will also be returned.
INTERNET-DRAFT RSA/SHA1 in the DNS
5. IANA Considerations 5. IANA Considerations
The DNSSEC algorithm number 5 is allocated for RSA/SHA1 SIG RRs and The DNSSEC algorithm number 5 is allocated for RSA/SHA1 SIG RRs and
RSA KEY RRs. RSA KEY RRs.
6. Security Considerations 6. Security Considerations
Many of the general security consideration in [RFC 2535] apply. Keys Many of the general security considerations in RFC 2535 apply. Keys
retrieved from the DNS should not be trusted unless (1) they have retrieved from the DNS should not be trusted unless (1) they have
been securely obtained from a secure resolver or independently been securely obtained from a secure resolver or independently
verified by the user and (2) this secure resolver and secure verified by the user and (2) this secure resolver and secure
obtainment or independent verification conform to security policies obtainment or independent verification conform to security policies
acceptable to the user. As with all cryptographic algorithms, acceptable to the user. As with all cryptographic algorithms,
evaluating the necessary strength of the key is essential and evaluating the necessary strength of the key is essential and
dependent on local policy. For particularly critical applications, dependent on local policy. For particularly critical applications,
implementers are encouraged to consider the range of available implementers are encouraged to consider the range of available
algorithms and key sizes. See also [RFC 2541], "DNS Security algorithms and key sizes. See also RFC 2541, "DNS Security
Operational Considerations". Operational Considerations".
INTERNET-DRAFT RSA/SHA1 in the DNS
References References
[FIP180] - U.S. Department of Commerce, "Secure Hash Standard", FIPS [FIP180] U.S. Department of Commerce, "Secure Hash Standard", FIPS
PUB 180-1, 17 Apr 1995. 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 [NETSEC] Network Security: PRIVATE Communications in a PUBLIC
facilities", 11/01/1987. World, Charlie Kaufman, Radia Perlman, & Mike Speciner,
Prentice Hall Series in Computer Networking and
Distributed Communications, 1995.
[RFC 1035] - P. Mockapetris, "Domain names - implementation and [RFC1034] Mockapetris, P., "Domain Names - Concepts and Facilities",
specification", 11/01/1987. STD 13, RFC 1034, November 1987.
[RFC 1321] - R. Rivest, "The MD5 Message-Digest Algorithm", April [RFC1035] Mockapetris, P., "Domain Names - Implementation and
1992. Specification", STD 13, RFC 1035, November 1987.
[RFC 2119] - S. Bradner, "Key words for use in RFCs to Indicate [RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
Requirement Levels", March 1997. April 1992.
[RFC 2437] - B. Kaliski, J. Staddon, "PKCS #1: RSA Cryptography [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Specifications Version 2.0", October 1998. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 2535] - D. Eastlake, "Domain Name System Security Extensions", [RFC2437] Kaliski, B. and J. Staddon, "PKCS #1: RSA Cryptography
March 1999. Specifications Version 2.0", RFC 2437, October 1998.
[RFC 2536] - D. Eastlake, "DSA KEYs and SIGs in the Domain Name [RFC2535] Eastlake, D., "Domain Name System Security Extensions",
System (DNS)", March 1999. RFC 2535, March 1999.
[RFC 2537] - D. Eastlake, "RSA/MD5 KEYs and SIGs in the Domain Name [RFC2536] Eastlake, D., "DSA KEYs and SIGs in the Domain Name System
System (DNS)", March 1999. (DNS)", RFC 2536, March 1999.
[RDC 2541] - D. Eastlake, "DNS Security Operational Considerations", [RFC2537] Eastlake, D., "RSA/MD5 KEYs and SIGs in the Domain Name
March 1999. System (DNS)", RFC 2537, March 1999.
[RFC 2671] - P. Vixie, "Extension Mechanisms for DNS (EDNS0)", August [RFC2541] Eastlake, D., "DNS Security Operational Considerations",
1999. RFC 2541, March 1999.
[Schneier] - Bruce Schneier, "Applied Cryptography Second Edition: [RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC
protocols, algorithms, and source code in C", 1996, John Wiley and 2671, August 1999.
Sons, ISBN 0-471-11709-9.
INTERNET-DRAFT RSA/SHA1 in the DNS [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 Author's Address
Donald E. Eastlake 3rd Donald E. Eastlake 3rd
Motorola Motorola
155 Beaver Street 155 Beaver Street
Milford, MA 01757 USA Milford, MA 01757 USA
Telephone: +1-508-261-5434 (w) Phone: +1-508-261-5434 (w)
+1-508-634-2066 (h) +1-508-634-2066 (h)
FAX: +1-508-261-4777 (w) Fax +1-508-261-4777 (w)
EMail: Donald.Eastlake@motorola.com EMail: Donald.Eastlake@motorola.com
Expiration and File Name Full Copyright Statement
This draft expires in October 2001. Copyright (C) The Internet Society (2001). All Rights Reserved.
Its file name is draft-ietf-dnsext-rsa-03.txt. This document and translations of it may be copied and furnished to
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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