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Hardaker 3 Internet-Draft Sparta 4 Expires: July 17, 2006 January 13, 2006 6 Use of SHA-256 in DNSSEC Delegation Signer (DS) Resource Records (RRs) 7 draft-ietf-dnsext-ds-sha256-04.txt 9 Status of this Memo 11 By submitting this Internet-Draft, each author represents that any 12 applicable patent or other IPR claims of which he or she is aware 13 have been or will be disclosed, and any of which he or she becomes 14 aware will be disclosed, in accordance with Section 6 of BCP 79. 16 Internet-Drafts are working documents of the Internet Engineering 17 Task Force (IETF), its areas, and its working groups. Note that 18 other groups may also distribute working documents as Internet- 19 Drafts. 21 Internet-Drafts are draft documents valid for a maximum of six months 22 and may be updated, replaced, or obsoleted by other documents at any 23 time. It is inappropriate to use Internet-Drafts as reference 24 material or to cite them other than as "work in progress." 26 The list of current Internet-Drafts can be accessed at 27 http://www.ietf.org/ietf/1id-abstracts.txt. 29 The list of Internet-Draft Shadow Directories can be accessed at 30 http://www.ietf.org/shadow.html. 32 This Internet-Draft will expire on July 17, 2006. 34 Copyright Notice 36 Copyright (C) The Internet Society (2006). 38 Abstract 40 This document specifies how to use the SHA-256 digest type in DNS 41 Delegation Signer (DS) Resource Records (RRs). DS records, when 42 stored in a parent zone, point to key signing DNSKEY key(s) in a 43 child zone. 45 Table of Contents 47 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 48 2. Implementing the SHA-256 algorithm for DS record support . . . 3 49 2.1. DS record field values . . . . . . . . . . . . . . . . . . 3 50 2.2. DS Record with SHA-256 Wire Format . . . . . . . . . . . . 3 51 2.3. Example DS Record Using SHA-256 . . . . . . . . . . . . . . 4 52 3. Implementation Requirements . . . . . . . . . . . . . . . . . . 4 53 4. Deployment Considerations . . . . . . . . . . . . . . . . . . . 4 54 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5 55 6. Security Considerations . . . . . . . . . . . . . . . . . . . . 5 56 6.1. Potential Digest Type Downgrade Attacks . . . . . . . . . . 5 57 6.2. SHA-1 vs SHA-256 Considerations for DS Records . . . . . . 6 58 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 6 59 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6 60 8.1. Normative References . . . . . . . . . . . . . . . . . . . 6 61 8.2. Informative References . . . . . . . . . . . . . . . . . . 7 62 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8 63 Intellectual Property and Copyright Statements . . . . . . . . . . 9 65 1. Introduction 67 The DNSSEC [RFC4033] [RFC4034] [RFC4035] DS RR is published in parent 68 zones to distribute a cryptographic digest of a child's Key Signing 69 Key (KSK) DNSKEY RR. The DS RRset is signed by at least one of the 70 parent zone's private zone data signing keys for each algorithm in 71 use by the parent. Each signature is published in an RRSIG resource 72 record, owned by the same domain as the DS RRset and with a type 73 covered of DS. 75 2. Implementing the SHA-256 algorithm for DS record support 77 This document specifies that the digest type code [XXX: To be 78 assigned by IANA; likely 2] is to be assigned to SHA-256 [SHA256] for 79 use within DS records. The results of the digest algorithm MUST NOT 80 be truncated and the entire 32 byte digest result is to be published 81 in the DS record. 83 2.1. DS record field values 85 Using the SHA-256 digest algorithm within a DS record will make use 86 of the following DS-record fields: 88 Digest type: [XXX: To be assigned by IANA; likely 2] 90 Digest: A SHA-256 bit digest value calculated by using the following 91 formula ("|" denotes concatenation). The resulting value is not 92 truncated and the entire 32 byte result is to used in the 93 resulting DS record and related calculations. 95 digest = SHA_256(DNSKEY owner name | DNSKEY RDATA) 97 where DNSKEY RDATA is defined by [RFC4034] as: 99 DNSKEY RDATA = Flags | Protocol | Algorithm | Public Key 101 The Key Tag field and Algorithm fields remain unchanged by this 102 document and are specified in the [RFC4034] specification. 104 2.2. DS Record with SHA-256 Wire Format 106 The resulting on-the-wire format for the resulting DS record will be 107 [XXX: IANA assignment should replace the 2 below]: 109 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 110 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 111 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 112 | Key Tag | Algorithm | DigestType=2 | 113 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 114 / / 115 / Digest (length for SHA-256 is 32 bytes) / 116 / / 117 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| 119 2.3. Example DS Record Using SHA-256 121 The following is an example DNSKEY and matching DS record. This 122 DNSKEY record comes from the example DNSKEY/DS records found in 123 section 5.4 of [RFC4034]. 125 The DNSKEY record: 127 dskey.example.com. 86400 IN DNSKEY 256 3 5 ( AQOeiiR0GOMYkDshWoSKz9Xz 128 fwJr1AYtsmx3TGkJaNXVbfi/ 129 2pHm822aJ5iI9BMzNXxeYCmZ 130 DRD99WYwYqUSdjMmmAphXdvx 131 egXd/M5+X7OrzKBaMbCVdFLU 132 Uh6DhweJBjEVv5f2wwjM9Xzc 133 nOf+EPbtG9DMBmADjFDc2w/r 134 ljwvFw== 135 ) ; key id = 60485 137 The resulting DS record covering the above DNSKEY record using a SHA- 138 256 digest: [RFC Editor: please replace XXX with the assigned digest 139 type (likely 2):] 141 dskey.example.com. 86400 IN DS 60485 5 XXX ( D4B7D520E7BB5F0F67674A0C 142 CEB1E3E0614B93C4F9E99B83 143 83F6A1E4469DA50A ) 145 3. Implementation Requirements 147 Implementations MUST support the use of the SHA-256 algorithm in DS 148 RRs. Validator implementations SHOULD ignore DS RRs containing SHA-1 149 digests if DS RRs with SHA-256 digests are present in the DS RRset. 151 4. Deployment Considerations 153 If a validator does not support the SHA-256 digest type and no other 154 DS RR exists in a zone's DS RRset with a supported digest type, then 155 the validator has no supported authentication path leading from the 156 parent to the child. The resolver should treat this case as it would 157 the case of an authenticated NSEC RRset proving that no DS RRset 158 exists, as described in [RFC4035], section 5.2. 160 Because zone administrators can not control the deployment speed of 161 support for SHA-256 in validators that may be referencing any of 162 their zones, zone operators should consider deploying both SHA-1 and 163 SHA-256 based DS records. This should be done for every DNSKEY for 164 which DS records are being generated. Whether to make use of both 165 digest types and for how long is a policy decision that extends 166 beyond the scope of this document. 168 5. IANA Considerations 170 The Digest Type to be used for supporting SHA-256 within DS records 171 needs to be assigned by IANA. This document requests that the Digest 172 Type value of 2 be assigned to the SHA-256 digest algorithm. 174 At the time of this writing, the current digest types assigned for 175 use in DS records are as follows: 177 VALUE Digest Type Status 178 0 Reserved - 179 1 SHA-1 MANDATORY 180 2 SHA-256 MANDATORY 181 3-255 Unassigned - 183 6. Security Considerations 185 6.1. Potential Digest Type Downgrade Attacks 187 A downgrade attack from a stronger digest type to a weaker one is 188 possible if all of the following are true: 190 o A zone includes multiple DS records for a given child's DNSKEY, 191 each of which use a different digest type. 193 o A validator accepts a weaker digest even if a stronger one is 194 present but invalid. 196 For example, if the following conditions are all true: 198 o Both SHA-1 and SHA-256 based digests are published in DS records 199 within a parent zone for a given child zone's DNSKEY. 201 o The DS record with the SHA-1 digest matches the digest computed 202 using the child zone's DNSKEY. 204 o The DS record with the SHA-256 digest fails to match the digest 205 computed using the child zone's DNSKEY. 207 Then if the validator accepts the above situation as secure then this 208 can be used as a downgrade attack since the stronger SHA-256 digest 209 is ignored. 211 6.2. SHA-1 vs SHA-256 Considerations for DS Records 213 Because of the weaknesses recently discovered within the SHA-1 214 algorithm, users of DNSSEC are encouraged to deploy the use of SHA- 215 256 as soon as the software implementations in use allow for it. 217 At the time of this publication, the SHA-256 digest algorithm is 218 considered sufficiently strong for the immediate future. It is also 219 considered sufficient for use in DNSSEC DS RRs for the immediate 220 future. However, future published attacks may weaken the usability 221 of this algorithm within the DS RRs. It is beyond the scope of this 222 document to speculate extensively on the cryptographic strength of 223 the SHA-256 digest algorithm. 225 Likewise, it is also beyond the scope of this document to specify 226 whether or for how long SHA-1 based DS records should be 227 simultaneously published alongside SHA-256 based DS records. 229 7. Acknowledgments 231 This document is a minor extension to the existing DNSSEC documents 232 and those authors are gratefully appreciated for the hard work that 233 went into the base documents. 235 The following people contributed to portions of this document in some 236 fashion: Mark Andrews, Roy Arends, Olafur Gudmundsson, Olaf M. 237 Kolkman, Edward Lewis, Scott Rose, Stuart E. Schechter, Sam Weiler. 239 8. References 241 8.1. Normative References 243 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 244 Rose, "DNS Security Introduction and Requirements", 245 RFC 4033, March 2005. 247 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. 248 Rose, "Resource Records for the DNS Security Extensions", 249 RFC 4034, March 2005. 251 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. 252 Rose, "Protocol Modifications for the DNS Security 253 Extensions", RFC 4035, March 2005. 255 [SHA256] National Institute of Standards and Technology, "Secure 256 Hash Algorithm. NIST FIPS 180-2", August 2002. 258 8.2. Informative References 260 [SHA256CODE] 261 Motorola Labs, "US Secure Hash Algorithms (SHA)", 262 June 2005. 264 Author's Address 266 Wes Hardaker 267 Sparta 268 P.O. Box 382 269 Davis 95617 270 US 272 Email: hardaker@tislabs.com 274 Intellectual Property Statement 276 The IETF takes no position regarding the validity or scope of any 277 Intellectual Property Rights or other rights that might be claimed to 278 pertain to the implementation or use of the technology described in 279 this document or the extent to which any license under such rights 280 might or might not be available; nor does it represent that it has 281 made any independent effort to identify any such rights. 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