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INTERNET-DRAFT DSA Information in the DNS
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OBSOLETES: RFC 2536 Donald E. Eastlake 3rd
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Expires: January 2006 July 2005
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DSA Keying and Signature Information in the DNS
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--- ------ --- --------- ----------- -- --- ---
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<draft-ietf-dnsext-rfc2536bis-dsa-06.txt>
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Donald E. Eastlake 3rd
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Status of This Document
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By submitting this Internet-Draft, each author represents that any
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applicable patent or other IPR claims of which he or she is aware
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have been or will be disclosed, and any of which he or she becomes
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aware will be disclosed, in accordance with Section 6 of BCP 79.
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Distribution of this document is unlimited. Comments should be sent
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to the DNS extensions working group mailing list
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<namedroppers@ops.ietf.org>.
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Internet-Drafts are working documents of the Internet Engineering
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Task Force (IETF), its areas, and its working groups. Note that
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other groups may also distribute working documents as Internet-
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Internet-Drafts are draft documents valid for a maximum of six months
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and may be updated, replaced, or obsoleted by other documents at any
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time. It is inappropriate to use Internet-Drafts as reference
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material or to cite them other than a "work in progress."
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The list of current Internet-Drafts can be accessed at
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http://www.ietf.org/1id-abstracts.html
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The list of Internet-Draft Shadow Directories can be accessed at
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http://www.ietf.org/shadow.html
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The standard method of encoding US Government Digital Signature
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Algorithm keying and signature information for use in the Domain Name
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Copyright (C) The Internet Society 2005. All Rights Reserved.
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D. Eastlake 3rd [Page 1]
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Status of This Document....................................1
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Abstract...................................................1
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Copyright Notice...........................................1
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Table of Contents..........................................2
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1. Introduction............................................3
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2. DSA Keying Information..................................3
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3. DSA Signature Information...............................4
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4. Performance Considerations..............................4
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5. Security Considerations.................................5
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6. IANA Considerations.....................................5
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Copyright and Disclaimer...................................5
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Normative References.......................................7
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Informative References.....................................7
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Authors Address............................................8
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Expiration and File Name...................................8
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D. Eastlake 3rd [Page 2]
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The Domain Name System (DNS) is the global hierarchical replicated
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distributed database system for Internet addressing, mail proxy, and
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other information [RFC 1034, 1035]. The DNS has been extended to
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include digital signatures and cryptographic keys as described in
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[RFC 4033, 4034, 4035] and additional work is underway which would
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require the storage of keying and signature information in the DNS.
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This document describes how to encode US Government Digital Signature
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Algorithm (DSA) keys and signatures in the DNS. Familiarity with the
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US Digital Signature Algorithm is assumed [FIPS 186-2, Schneier].
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2. DSA Keying Information
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When DSA public keys are stored in the DNS, the structure of the
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relevant part of the RDATA part of the RR being used is the fields
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listed below in the order given.
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The period of key validity is not included in this data but is
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indicated separately, for example by an RR such as RRSIG which signs
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and authenticates the RR containing the keying information.
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As described in [FIPS 186-2] and [Schneier], T is a key size
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parameter chosen such that 0 <= T <= 8. (The meaning if the T octet
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is greater than 8 is reserved and the remainder of the data may have
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a different format in that case.) Q is a prime number selected at
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key generation time such that 2**159 < Q < 2**160. Thus Q is always
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20 octets long and, as with all other fields, is stored in "big-
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endian" network order. P, G, and Y are calculated as directed by the
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[FIPS 186-2] key generation algorithm [Schneier]. P is in the range
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2**(511+64T) < P < 2**(512+64T) and thus is 64 + 8*T octets long. G
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and Y are quantities modulo P and so can be up to the same length as
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P and are allocated fixed size fields with the same number of octets
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During the key generation process, a random number X must be
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generated such that 1 <= X <= Q-1. X is the private key and is used
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in the final step of public key generation where Y is computed as
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3. DSA Signature Information
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The portion of the RDATA area used for US Digital Signature Algorithm
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signature information is shown below with fields in the order they
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are listed and the contents of each multi-octet field in "big-endian"
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First, the data signed must be determined. Then the following steps
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are taken, as specified in [FIPS 186-2], where Q, P, G, and Y are as
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specified in the public key [Schneier]:
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hash = SHA-1 ( data )
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Generate a random K such that 0 < K < Q.
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R = ( G**K mod P ) mod Q
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S = ( K**(-1) * (hash + X*R) ) mod Q
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For information on the SHA-1 hash function see [FIPS 180-2] and [RFC
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Since Q is 160 bits long, R and S can not be larger than 20 octets,
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which is the space allocated.
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T is copied from the public key. It is not logically necessary in
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the SIG but is present so that values of T > 8 can more conveniently
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be used as an escape for extended versions of DSA or other algorithms
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as later standardized.
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4. Performance Considerations
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General signature generation speeds are roughly the same for RSA [RFC
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3110] and DSA. With sufficient pre-computation, signature generation
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with DSA is faster than RSA. Key generation is also faster for DSA.
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However, signature verification is an order of magnitude slower than
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RSA when the RSA public exponent is chosen to be small, as is
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recommended for some applications.
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Current DNS implementations are optimized for small transfers,
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typically less than 512 bytes including DNS overhead. Larger
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transfers will perform correctly and extensions have been
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standardized [RFC 2671] to make larger transfers more efficient, it
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is still advisable at this time to make reasonable efforts to
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minimize the size of RR sets containing keying and/or signature
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inforamtion consistent with adequate security.
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5. Security Considerations
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Keys retrieved from the DNS should not be trusted unless (1) they
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have been securely obtained from a secure resolver or independently
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verified by the user and (2) this secure resolver and secure
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obtainment or independent verification conform to security policies
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acceptable to the user. As with all cryptographic algorithms,
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evaluating the necessary strength of the key is essential and
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dependent on local policy.
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The key size limitation of a maximum of 1024 bits ( T = 8 ) in the
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current DSA standard may limit the security of DSA. For particular
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applications, implementors are encouraged to consider the range of
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available algorithms and key sizes.
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DSA assumes the ability to frequently generate high quality random
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numbers. See [random] for guidance. DSA is designed so that if
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biased rather than random numbers are used, high bandwidth covert
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channels are possible. See [Schneier] and more recent research. The
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leakage of an entire DSA private key in only two DSA signatures has
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been demonstrated. DSA provides security only if trusted
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implementations, including trusted random number generation, are
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6. IANA Considerations
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Allocation of meaning to values of the T parameter that are not
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defined herein (i.e., > 8 ) requires an IETF standards actions. It
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is intended that values unallocated herein be used to cover future
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extensions of the DSS standard.
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Copyright and Disclaimer
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Copyright (C) The Internet Society (2005). This document is subject to
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the rights, licenses and restrictions contained in BCP 78, and except
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as set forth therein, the authors retain all their rights.
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This document and the information contained herein are provided on an
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"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
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OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
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ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
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INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
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INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
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WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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[FIPS 186-2] - U.S. Federal Information Processing Standard: Digital
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Signature Standard, 27 January 2000.
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[RFC 4034] - Arends, R., Austein, R., Larson, M., Massey, D., and S.
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Rose, "Resource Records for the DNS Security Extensions", RFC 4034,
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Informative References
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[RFC 1034] - "Domain names - concepts and facilities", P.
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Mockapetris, 11/01/1987.
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[RFC 1035] - "Domain names - implementation and specification", P.
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Mockapetris, 11/01/1987.
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[RFC 2671] - "Extension Mechanisms for DNS (EDNS0)", P. Vixie, August
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[RFC 3110] - "RSA/SHA-1 SIGs and RSA KEYs in the Domain Name System
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(DNS)", D. Eastlake 3rd. May 2001.
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[RFC 3174] - "US Secure Hash Algorithm 1 (SHA1)", D. Eastlake, P.
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Jones, September 2001.
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[RFC 4033] - Arends, R., Austein, R., Larson, M., Massey, D., and S.
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Rose, "DNS Security Introduction and Requirements", RFC 4033, March
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[RFC 4035] - Arends, R., Austein, R., Larson, M., Massey, D., and S.
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Rose, "Protocol Modifications for the DNS Security Extensions", RFC
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[RFC 4086] - Eastlake, D., 3rd, Schiller, J., and S. Crocker,
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"Randomness Requirements for Security", BCP 106, RFC 4086, June 2005.
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[Schneier] - "Applied Cryptography Second Edition: protocols,
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algorithms, and source code in C" (second edition), Bruce Schneier,
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1996, John Wiley and Sons, ISBN 0-471-11709-9.
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Donald E. Eastlake 3rd
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Milford, MA 01757 USA
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Telephone: +1-508-786-7554(w)
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EMail: Donald.Eastlake@motorola.com
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Expiration and File Name
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This draft expires in January 2006.
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Its file name is draft-ietf-dnsext-rfc2536bis-dsa-06.txt.
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