1 Network Working Group W. Hardaker
2 Request for Comments: 4509 Sparta
3 Category: Standards Track May 2006
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6 Use of SHA-256 in DNSSEC Delegation Signer (DS) Resource Records (RRs)
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9 Status of This Memo
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11 This document specifies an Internet standards track protocol for the
12 Internet community, and requests discussion and suggestions for
13 improvements. Please refer to the current edition of the "Internet
14 Official Protocol Standards" (STD 1) for the standardization state
15 and status of this protocol. Distribution of this memo is unlimited.
16
17 Copyright Notice
18
19 Copyright (C) The Internet Society (2006).
20
21 Abstract
22
23 This document specifies how to use the SHA-256 digest type in DNS
24 Delegation Signer (DS) Resource Records (RRs). DS records, when
25 stored in a parent zone, point to DNSKEYs in a child zone.
26
27 Table of Contents
28
29 1. Introduction ....................................................2
30 2. Implementing the SHA-256 Algorithm for DS Record Support ........2
31 2.1. DS Record Field Values .....................................2
32 2.2. DS Record with SHA-256 Wire Format .........................3
33 2.3. Example DS Record Using SHA-256 ............................3
34 3. Implementation Requirements .....................................3
35 4. Deployment Considerations .......................................4
36 5. IANA Considerations .............................................4
37 6. Security Considerations .........................................4
38 6.1. Potential Digest Type Downgrade Attacks ....................4
39 6.2. SHA-1 vs SHA-256 Considerations for DS Records .............5
40 7. Acknowledgements ................................................5
41 8. References ......................................................6
42 8.1. Normative References .......................................6
43 8.2. Informative References .....................................6
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52 Hardaker Standards Track [Page 1]
53 RFC 4509 Use of SHA-256 in DNSSEC DS RRs May 2006
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56 1. Introduction
57
58 The DNSSEC [RFC4033] [RFC4034] [RFC4035] DS RR is published in parent
59 zones to distribute a cryptographic digest of one key in a child's
60 DNSKEY RRset. The DS RRset is signed by at least one of the parent
61 zone's private zone data signing keys for each algorithm in use by
62 the parent. Each signature is published in an RRSIG resource record,
63 owned by the same domain as the DS RRset, with a type covered of DS.
64
65 In this document, the key words "MUST", "MUST NOT", "REQUIRED",
66 "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
67 and "OPTIONAL" are to be interpreted as described in [RFC2119].
68
69 2. Implementing the SHA-256 Algorithm for DS Record Support
70
71 This document specifies that the digest type code 2 has been assigned
72 to SHA-256 [SHA256] [SHA256CODE] for use within DS records. The
73 results of the digest algorithm MUST NOT be truncated, and the entire
74 32 byte digest result is to be published in the DS record.
75
76 2.1. DS Record Field Values
77
78 Using the SHA-256 digest algorithm within a DS record will make use
79 of the following DS-record fields:
80
81 Digest type: 2
82
83 Digest: A SHA-256 bit digest value calculated by using the following
84 formula ("|" denotes concatenation). The resulting value is not
85 truncated, and the entire 32 byte result is to be used in the
86 resulting DS record and related calculations.
87
88 digest = SHA_256(DNSKEY owner name | DNSKEY RDATA)
89
90 where DNSKEY RDATA is defined by [RFC4034] as:
91
92 DNSKEY RDATA = Flags | Protocol | Algorithm | Public Key
93
94 The Key Tag field and Algorithm fields remain unchanged by this
95 document and are specified in the [RFC4034] specification.
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107 Hardaker Standards Track [Page 2]
108 RFC 4509 Use of SHA-256 in DNSSEC DS RRs May 2006
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111 2.2. DS Record with SHA-256 Wire Format
112
113 The resulting on-the-wire format for the resulting DS record will be
114 as follows:
115
116 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
117 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
118 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
119 | Key Tag | Algorithm | DigestType=2 |
120 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
121 / /
122 / Digest (length for SHA-256 is 32 bytes) /
123 / /
124 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
125
126 2.3. Example DS Record Using SHA-256
127
128 The following is an example DNSKEY and matching DS record. This
129 DNSKEY record comes from the example DNSKEY/DS records found in
130 section 5.4 of [RFC4034].
131
132 The DNSKEY record:
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134 dskey.example.com. 86400 IN DNSKEY 256 3 5 ( AQOeiiR0GOMYkDshWoSKz9Xz
135 fwJr1AYtsmx3TGkJaNXVbfi/
136 2pHm822aJ5iI9BMzNXxeYCmZ
137 DRD99WYwYqUSdjMmmAphXdvx
138 egXd/M5+X7OrzKBaMbCVdFLU
139 Uh6DhweJBjEVv5f2wwjM9Xzc
140 nOf+EPbtG9DMBmADjFDc2w/r
141 ljwvFw==
142 ) ; key id = 60485
143
144 The resulting DS record covering the above DNSKEY record using a
145 SHA-256 digest:
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147 dskey.example.com. 86400 IN DS 60485 5 2 ( D4B7D520E7BB5F0F67674A0C
148 CEB1E3E0614B93C4F9E99B83
149 83F6A1E4469DA50A )
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151 3. Implementation Requirements
152
153 Implementations MUST support the use of the SHA-256 algorithm in DS
154 RRs. Validator implementations SHOULD ignore DS RRs containing SHA-1
155 digests if DS RRs with SHA-256 digests are present in the DS RRset.
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163 RFC 4509 Use of SHA-256 in DNSSEC DS RRs May 2006
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166 4. Deployment Considerations
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168 If a validator does not support the SHA-256 digest type and no other
169 DS RR exists in a zone's DS RRset with a supported digest type, then
170 the validator has no supported authentication path leading from the
171 parent to the child. The resolver should treat this case as it would
172 the case of an authenticated NSEC RRset proving that no DS RRset
173 exists, as described in [RFC4035], Section 5.2.
174
175 Because zone administrators cannot control the deployment speed of
176 support for SHA-256 in validators that may be referencing any of
177 their zones, zone operators should consider deploying both SHA-1 and
178 SHA-256 based DS records. This should be done for every DNSKEY for
179 which DS records are being generated. Whether to make use of both
180 digest types and for how long is a policy decision that extends
181 beyond the scope of this document.
182
183 5. IANA Considerations
184
185 Only one IANA action is required by this document:
186
187 The Digest Type to be used for supporting SHA-256 within DS records
188 has been assigned by IANA.
189
190 At the time of this writing, the current digest types assigned for
191 use in DS records are as follows:
192
193 VALUE Digest Type Status
194 0 Reserved -
195 1 SHA-1 MANDATORY
196 2 SHA-256 MANDATORY
197 3-255 Unassigned -
198
199 6. Security Considerations
200
201 6.1. Potential Digest Type Downgrade Attacks
202
203 A downgrade attack from a stronger digest type to a weaker one is
204 possible if all of the following are true:
205
206 o A zone includes multiple DS records for a given child's DNSKEY,
207 each of which uses a different digest type.
208
209 o A validator accepts a weaker digest even if a stronger one is
210 present but invalid.
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218 RFC 4509 Use of SHA-256 in DNSSEC DS RRs May 2006
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221 For example, if the following conditions are all true:
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223 o Both SHA-1 and SHA-256 based digests are published in DS records
224 within a parent zone for a given child zone's DNSKEY.
225
226 o The DS record with the SHA-1 digest matches the digest computed
227 using the child zone's DNSKEY.
228
229 o The DS record with the SHA-256 digest fails to match the digest
230 computed using the child zone's DNSKEY.
231
232 Then, if the validator accepts the above situation as secure, then
233 this can be used as a downgrade attack since the stronger SHA-256
234 digest is ignored.
235
236 6.2. SHA-1 vs. SHA-256 Considerations for DS Records
237
238 Users of DNSSEC are encouraged to deploy SHA-256 as soon as software
239 implementations allow for it. SHA-256 is widely believed to be more
240 resilient to attack than SHA-1, and confidence in SHA-1's strength is
241 being eroded by recently announced attacks. Regardless of whether
242 the attacks on SHA-1 will affect DNSSEC, it is believed (at the time
243 of this writing) that SHA-256 is the better choice for use in DS
244 records.
245
246 At the time of this publication, the SHA-256 digest algorithm is
247 considered sufficiently strong for the immediate future. It is also
248 considered sufficient for use in DNSSEC DS RRs for the immediate
249 future. However, future published attacks may weaken the usability
250 of this algorithm within the DS RRs. It is beyond the scope of this
251 document to speculate extensively on the cryptographic strength of
252 the SHA-256 digest algorithm.
253
254 Likewise, it is also beyond the scope of this document to specify
255 whether or for how long SHA-1 based DS records should be
256 simultaneously published alongside SHA-256 based DS records.
257
258 7. Acknowledgements
259
260 This document is a minor extension to the existing DNSSEC documents
261 and those authors are gratefully appreciated for the hard work that
262 went into the base documents.
263
264 The following people contributed to portions of this document in some
265 fashion: Mark Andrews, Roy Arends, Olafur Gudmundsson, Paul Hoffman,
266 Olaf M. Kolkman, Edward Lewis, Scott Rose, Stuart E. Schechter, Sam
267 Weiler.
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273 RFC 4509 Use of SHA-256 in DNSSEC DS RRs May 2006
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276 8. References
277
278 8.1. Normative References
279
280 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
281 Requirement Levels", BCP 14, RFC 2119, March 1997.
282
283 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
284 Rose, "DNS Security Introduction and Requirements", RFC
285 4033, March 2005.
286
287 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
288 Rose, "Resource Records for the DNS Security
289 Extensions", RFC 4034, March 2005.
290
291 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
292 Rose, "Protocol Modifications for the DNS Security
293 Extensions", RFC 4035, March 2005.
294
295 [SHA256] National Institute of Standards and Technology, "Secure
296 Hash Algorithm. NIST FIPS 180-2", August 2002.
297
298 8.2. Informative References
299
300 [SHA256CODE] Eastlake, D., "US Secure Hash Algorithms (SHA)", Work in
301 Progress.
302
303 Author's Address
304
305 Wes Hardaker
306 Sparta
307 P.O. Box 382
308 Davis, CA 95617
309 USA
310
311 EMail: hardaker@tislabs.com
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328 RFC 4509 Use of SHA-256 in DNSSEC DS RRs May 2006
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331 Full Copyright Statement
332
333 Copyright (C) The Internet Society (2006).
334
335 This document is subject to the rights, licenses and restrictions
336 contained in BCP 78, and except as set forth therein, the authors
337 retain all their rights.
338
339 This document and the information contained herein are provided on an
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341 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
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370
371 Acknowledgement
372
373 Funding for the RFC Editor function is provided by the IETF
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Updates: 4035