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Hardaker 3 Internet-Draft Sparta 4 Expires: August 25, 2006 February 21, 2006 6 Use of SHA-256 in DNSSEC Delegation Signer (DS) Resource Records (RRs) 7 draft-ietf-dnsext-ds-sha256-05.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 August 25, 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 . . . . . . . . . . . . . . . . . . . . . . . . . . 7 60 8.1. Normative References . . . . . . . . . . . . . . . . . . . 7 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 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 76 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 77 document are to be interpreted as described in [RFC2119]. 79 2. Implementing the SHA-256 algorithm for DS record support 81 This document specifies that the digest type code [XXX: To be 82 assigned by IANA; likely 2] is to be assigned to SHA-256 [SHA256] 83 [SHA256CODE] for use within DS records. The results of the digest 84 algorithm MUST NOT be truncated and the entire 32 byte digest result 85 is to be published in the DS record. 87 2.1. DS record field values 89 Using the SHA-256 digest algorithm within a DS record will make use 90 of the following DS-record fields: 92 Digest type: [XXX: To be assigned by IANA; likely 2] 94 Digest: A SHA-256 bit digest value calculated by using the following 95 formula ("|" denotes concatenation). The resulting value is not 96 truncated and the entire 32 byte result is to used in the 97 resulting DS record and related calculations. 99 digest = SHA_256(DNSKEY owner name | DNSKEY RDATA) 101 where DNSKEY RDATA is defined by [RFC4034] as: 103 DNSKEY RDATA = Flags | Protocol | Algorithm | Public Key 105 The Key Tag field and Algorithm fields remain unchanged by this 106 document and are specified in the [RFC4034] specification. 108 2.2. DS Record with SHA-256 Wire Format 110 The resulting on-the-wire format for the resulting DS record will be 111 [XXX: IANA assignment should replace the 2 below]: 113 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 114 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 1 115 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 116 | Key Tag | Algorithm | DigestType=2 | 117 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 118 / / 119 / Digest (length for SHA-256 is 32 bytes) / 120 / / 121 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| 123 2.3. Example DS Record Using SHA-256 125 The following is an example DNSKEY and matching DS record. This 126 DNSKEY record comes from the example DNSKEY/DS records found in 127 section 5.4 of [RFC4034]. 129 The DNSKEY record: 131 dskey.example.com. 86400 IN DNSKEY 256 3 5 ( AQOeiiR0GOMYkDshWoSKz9Xz 132 fwJr1AYtsmx3TGkJaNXVbfi/ 133 2pHm822aJ5iI9BMzNXxeYCmZ 134 DRD99WYwYqUSdjMmmAphXdvx 135 egXd/M5+X7OrzKBaMbCVdFLU 136 Uh6DhweJBjEVv5f2wwjM9Xzc 137 nOf+EPbtG9DMBmADjFDc2w/r 138 ljwvFw== 139 ) ; key id = 60485 141 The resulting DS record covering the above DNSKEY record using a SHA- 142 256 digest: [RFC Editor: please replace XXX with the assigned digest 143 type (likely 2):] 145 dskey.example.com. 86400 IN DS 60485 5 XXX ( D4B7D520E7BB5F0F67674A0C 146 CEB1E3E0614B93C4F9E99B83 147 83F6A1E4469DA50A ) 149 3. Implementation Requirements 151 Implementations MUST support the use of the SHA-256 algorithm in DS 152 RRs. Validator implementations SHOULD ignore DS RRs containing SHA-1 153 digests if DS RRs with SHA-256 digests are present in the DS RRset. 155 4. Deployment Considerations 157 If a validator does not support the SHA-256 digest type and no other 158 DS RR exists in a zone's DS RRset with a supported digest type, then 159 the validator has no supported authentication path leading from the 160 parent to the child. The resolver should treat this case as it would 161 the case of an authenticated NSEC RRset proving that no DS RRset 162 exists, as described in [RFC4035], section 5.2. 164 Because zone administrators can not control the deployment speed of 165 support for SHA-256 in validators that may be referencing any of 166 their zones, zone operators should consider deploying both SHA-1 and 167 SHA-256 based DS records. This should be done for every DNSKEY for 168 which DS records are being generated. Whether to make use of both 169 digest types and for how long is a policy decision that extends 170 beyond the scope of this document. 172 5. IANA Considerations 174 Only one IANA action is required by this document: 176 The Digest Type to be used for supporting SHA-256 within DS records 177 needs to be assigned by IANA. This document requests that the Digest 178 Type value of 2 be assigned to the SHA-256 digest algorithm. 180 At the time of this writing, the current digest types assigned for 181 use in DS records are as follows: 183 VALUE Digest Type Status 184 0 Reserved - 185 1 SHA-1 MANDATORY 186 2 SHA-256 MANDATORY 187 3-255 Unassigned - 189 6. Security Considerations 191 6.1. Potential Digest Type Downgrade Attacks 193 A downgrade attack from a stronger digest type to a weaker one is 194 possible if all of the following are true: 196 o A zone includes multiple DS records for a given child's DNSKEY, 197 each of which use a different digest type. 199 o A validator accepts a weaker digest even if a stronger one is 200 present but invalid. 202 For example, if the following conditions are all true: 204 o Both SHA-1 and SHA-256 based digests are published in DS records 205 within a parent zone for a given child zone's DNSKEY. 207 o The DS record with the SHA-1 digest matches the digest computed 208 using the child zone's DNSKEY. 210 o The DS record with the SHA-256 digest fails to match the digest 211 computed using the child zone's DNSKEY. 213 Then if the validator accepts the above situation as secure then this 214 can be used as a downgrade attack since the stronger SHA-256 digest 215 is ignored. 217 6.2. SHA-1 vs SHA-256 Considerations for DS Records 219 Users of DNSSEC are encouraged to deploy SHA-256 as soon as software 220 implementations allow for it. SHA-256 is widely believed to be more 221 resilient to attack than SHA-1, and confidence in SHA-1's strength is 222 being eroded by recently-announced attacks. Regardless of whether or 223 not the attacks on SHA-1 will affect DNSSEC, it is believed (at the 224 time of this writing) that SHA-256 is the better choice for use in DS 225 records. 227 At the time of this publication, the SHA-256 digest algorithm is 228 considered sufficiently strong for the immediate future. It is also 229 considered sufficient for use in DNSSEC DS RRs for the immediate 230 future. However, future published attacks may weaken the usability 231 of this algorithm within the DS RRs. It is beyond the scope of this 232 document to speculate extensively on the cryptographic strength of 233 the SHA-256 digest algorithm. 235 Likewise, it is also beyond the scope of this document to specify 236 whether or for how long SHA-1 based DS records should be 237 simultaneously published alongside SHA-256 based DS records. 239 7. Acknowledgments 241 This document is a minor extension to the existing DNSSEC documents 242 and those authors are gratefully appreciated for the hard work that 243 went into the base documents. 245 The following people contributed to portions of this document in some 246 fashion: Mark Andrews, Roy Arends, Olafur Gudmundsson, Paul Hoffman, 247 Olaf M. Kolkman, Edward Lewis, Scott Rose, Stuart E. Schechter, Sam 248 Weiler. 250 8. References 252 8.1. Normative References 254 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 255 Requirement Levels", BCP 14, RFC 2119, March 1997. 257 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 258 Rose, "DNS Security Introduction and Requirements", 259 RFC 4033, March 2005. 261 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. 262 Rose, "Resource Records for the DNS Security Extensions", 263 RFC 4034, March 2005. 265 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. 266 Rose, "Protocol Modifications for the DNS Security 267 Extensions", RFC 4035, March 2005. 269 [SHA256] National Institute of Standards and Technology, "Secure 270 Hash Algorithm. NIST FIPS 180-2", August 2002. 272 8.2. Informative References 274 [SHA256CODE] 275 Eastlake, D., "US Secure Hash Algorithms (SHA)", 276 June 2005. 278 Author's Address 280 Wes Hardaker 281 Sparta 282 P.O. Box 382 283 Davis, CA 95617 284 US 286 Email: hardaker@tislabs.com 288 Intellectual Property Statement 290 The IETF takes no position regarding the validity or scope of any 291 Intellectual Property Rights or other rights that might be claimed to 292 pertain to the implementation or use of the technology described in 293 this document or the extent to which any license under such rights 294 might or might not be available; nor does it represent that it has 295 made any independent effort to identify any such rights. 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