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Network Working Group                                        D. EastLakeRequest for Comments: 2536                                           IBMCategory: Standards Track                                     March 1999DSA 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...................................................11. Introduction............................................12. DSA KEY Resource Records................................23. DSA SIG Resource Records................................34. Performance Considerations..............................35. Security Considerations.................................46. IANA Considerations.....................................4   References.................................................5   Author's Address...........................................5   Full Copyright Statement...................................61. 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 [RFC 2535].  Thus   the DNS can now be secured and can be used for secure key   distribution.Eastlake                    Standards Track                     [Page 1]

RFC 2536                     DSA in the DNS                   March 1999   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 [RFC 2535].  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 PEastlake                    Standards Track                     [Page 2]

RFC 2536                     DSA in the DNS                   March 19993. 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 [RFC 2535] 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 [RFC 2535].  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 withinEastlake                    Standards Track                     [Page 3]

RFC 2536                     DSA in the DNS                   March 1999   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 [RFC 2535] 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 [RFC 1750] 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.Eastlake                    Standards Track                     [Page 4]

RFC 2536                     DSA in the DNS                   March 1999References   [FIPS 186]   U.S. Federal Information Processing Standard: Digital                Signature Standard.   [RFC 1034]   Mockapetris, P., "Domain Names - Concepts and                Facilities", STD 13,RFC 1034, November 1987.   [RFC 1035]   Mockapetris, P., "Domain Names - Implementation and                Specification", STD 13,RFC 1035, November 1987.   [RFC 1750]   Eastlake, D., Crocker, S. and J. Schiller, "Randomness                Recommendations for Security",RFC 1750, December 1994.   [RFC 2535]   Eastlake, D., "Domain Name System Security Extensions",RFC 2535, March 1999.   [RFC 2537]   Eastlake, D., "RSA/MD5 KEYs and SIGs in the Domain Name                System (DNS)",RFC 2537, 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.comEastlake                    Standards Track                     [Page 5]

RFC 2536                     DSA in the DNS                   March 1999Full 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.Eastlake                    Standards Track                     [Page 6]