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Jansen 3 Internet-Draft NLnet Labs 4 Intended status: Standards Track January 08, 2009 5 Expires: July 12, 2009 7 Use of SHA-2 algorithms with RSA in DNSKEY and RRSIG Resource Records 8 for DNSSEC 9 draft-ietf-dnsext-dnssec-rsasha256-10 11 Status of this Memo 13 This Internet-Draft is submitted to IETF in full conformance with the 14 provisions of BCP 78 and 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 12, 2009. 34 Copyright Notice 36 Copyright (c) 2009 IETF Trust and the persons identified as the 37 document authors. All rights reserved. 39 This document is subject to BCP 78 and the IETF Trust's Legal 40 Provisions Relating to IETF Documents 41 (http://trustee.ietf.org/license-info) in effect on the date of 42 publication of this document. Please review these documents 43 carefully, as they describe your rights and restrictions with respect 44 to this document. 46 Abstract 48 This document describes how to produce RSA/SHA-256 and RSA/SHA-512 49 DNSKEY and RRSIG resource records for use in the Domain Name System 50 Security Extensions (DNSSEC, RFC 4033, RFC 4034, and RFC 4035). 52 Table of Contents 54 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 55 2. DNSKEY Resource Records . . . . . . . . . . . . . . . . . . . . 3 56 2.1. RSA/SHA-256 DNSKEY Resource Records . . . . . . . . . . . . 3 57 2.2. RSA/SHA-512 DNSKEY Resource Records . . . . . . . . . . . . 3 58 3. RRSIG Resource Records . . . . . . . . . . . . . . . . . . . . 4 59 3.1. RSA/SHA-256 RRSIG Resource Records . . . . . . . . . . . . 4 60 3.2. RSA/SHA-512 RRSIG Resource Records . . . . . . . . . . . . 4 61 4. Deployment Considerations . . . . . . . . . . . . . . . . . . . 5 62 4.1. Key Sizes . . . . . . . . . . . . . . . . . . . . . . . . . 5 63 4.2. Signature Sizes . . . . . . . . . . . . . . . . . . . . . . 5 64 5. Implementation Considerations . . . . . . . . . . . . . . . . . 5 65 5.1. Support for SHA-2 signatures . . . . . . . . . . . . . . . 5 66 5.2. Support for NSEC3 Denial of Existence . . . . . . . . . . . 5 67 5.2.1. NSEC3 in Authoritative servers . . . . . . . . . . . . 5 68 5.2.2. NSEC3 in Validators . . . . . . . . . . . . . . . . . . 5 69 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6 70 7. Security Considerations . . . . . . . . . . . . . . . . . . . . 6 71 7.1. SHA-1 versus SHA-2 Considerations for RRSIG Resource 72 Records . . . . . . . . . . . . . . . . . . . . . . . . . . 6 73 7.2. Signature Type Downgrade Attacks . . . . . . . . . . . . . 6 74 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 7 75 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7 76 9.1. Normative References . . . . . . . . . . . . . . . . . . . 7 77 9.2. Informative References . . . . . . . . . . . . . . . . . . 7 78 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8 80 1. Introduction 82 The Domain Name System (DNS) is the global hierarchical distributed 83 database for Internet Naming. The DNS has been extended to use 84 cryptographic keys and digital signatures for the verification of the 85 authenticity and integrity of its data. RFC 4033 [RFC4033], RFC 4034 86 [RFC4034], and RFC 4035 [RFC4035] describe these DNS Security 87 Extensions, called DNSSEC. 89 RFC 4034 describes how to store DNSKEY and RRSIG resource records, 90 and specifies a list of cryptographic algorithms to use. This 91 document extends that list with the algorithms RSA/SHA-256 and RSA/ 92 SHA-512, and specifies how to store DNSKEY data and how to produce 93 RRSIG resource records with these hash algorithms. 95 Familiarity with DNSSEC, RSA and the SHA-2 [FIPS.180-3.2008] family 96 of algorithms is assumed in this document. 98 To refer to both SHA-256 and SHA-512, this document will use the name 99 SHA-2. This is done to improve readability. When a part of text is 100 specific for either SHA-256 or SHA-512, their specific names are 101 used. The same goes for RSA/SHA-256 and RSA/SHA-512, which will be 102 grouped using the name RSA/SHA-2. 104 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 105 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 106 document are to be interpreted as described in [RFC2119]. 108 2. DNSKEY Resource Records 110 The format of the DNSKEY RR can be found in RFC 4034 [RFC4034]. RFC 111 3110 [RFC3110] describes the use of RSA/SHA-1 for DNSSEC signatures. 113 2.1. RSA/SHA-256 DNSKEY Resource Records 115 RSA public keys for use with RSA/SHA-256 are stored in DNSKEY 116 resource records (RRs) with the algorithm number {TBA1}. 118 For interoperability, as in RFC 3110 [RFC3110], the key size of RSA/ 119 SHA-256 keys MUST NOT be less than 512 bits, and MUST NOT be more 120 than 4096 bits. 122 2.2. RSA/SHA-512 DNSKEY Resource Records 124 RSA public keys for use with RSA/SHA-512 are stored in DNSKEY 125 resource records (RRs) with the algorithm number {TBA2}. 127 The key size of RSA/SHA-512 keys MUST NOT be less than 1024 bits, and 128 MUST NOT be more than 4096 bits. 130 3. RRSIG Resource Records 132 The value of the signature field in the RRSIG RR follows the RSASSA- 133 PKCS1-v1_5 signature scheme, and is calculated as follows. The 134 values for the RDATA fields that precede the signature data are 135 specified in RFC 4034 [RFC4034]. 137 hash = SHA-XXX(data) 139 Here XXX is either 256 or 512, depending on the algorithm used, as 140 specified in FIPS PUB 180-3 [FIPS.180-3.2008], and "data" is the wire 141 format data of the resource record set that is signed, as specified 142 in RFC 4034 [RFC4034]. 144 signature = ( 00 | 01 | FF* | 00 | prefix | hash ) ** e (mod n) 146 Here "|" is concatenation, "00", "01", "FF" and "00" are fixed octets 147 of corresponding hexadecimal value, "e" is the private exponent of 148 the signing RSA key, and "n" is the public modulus of the signing 149 key. The FF octet MUST be repeated the exact number of times so that 150 the total length of the concatenated term in parentheses equals the 151 length of the modulus of the signer's public key ("n"). 153 The "prefix" is intended to make the use of standard cryptographic 154 libraries easier. These specifications are taken directly from the 155 specifications of RSASSA-PKCS1-v1_5 in PKCS #1 v2.1 section 8.2 156 [RFC3447], and EMSA-PKCS1-v1_5 encoding in PKCS #1 v2.1 section 9.2 157 [RFC3447]. The prefixes for the different algorithms are specified 158 below. 160 3.1. RSA/SHA-256 RRSIG Resource Records 162 RSA/SHA-256 signatures are stored in the DNS using RRSIG resource 163 records (RRs) with algorithm number {TBA1}. 165 The prefix is the ASN.1 DER SHA-256 algorithm designator prefix as 166 specified in PKCS #1 v2.1 [RFC3447]: 168 hex 30 31 30 0d 06 09 60 86 48 01 65 03 04 02 01 05 00 04 20 170 3.2. RSA/SHA-512 RRSIG Resource Records 172 RSA/SHA-512 signatures are stored in the DNS using RRSIG resource 173 records (RRs) with algorithm number {TBA2}. 175 The prefix is the ASN.1 DER SHA-512 algorithm designator prefix as 176 specified in PKCS #1 v2.1 [RFC3447]: 178 hex 30 51 30 0d 06 09 60 86 48 01 65 03 04 02 03 05 00 04 40 180 4. Deployment Considerations 182 4.1. Key Sizes 184 Apart from the restrictions specified in section 2, this document 185 will not specify what size of keys to use. That is an operational 186 issue and depends largely on the environment and intended use. A 187 good starting point for more information would be NIST SP 800-57 188 [NIST800-57]. 190 4.2. Signature Sizes 192 In this family of signing algorithms, the size of signatures is 193 related to the size of the key, and not the hashing algorithm used in 194 the signing process. Therefore, RRSIG resource records produced with 195 RSA/SHA256 or RSA/SHA512 will have the same size as those produced 196 with RSA/SHA1, if the keys have the same length. 198 5. Implementation Considerations 200 5.1. Support for SHA-2 signatures 202 DNSSEC aware implementations SHOULD be able to support RRSIG resource 203 records with the RSA/SHA-2 algorithms. 205 5.2. Support for NSEC3 Denial of Existence 207 Note that these algorithms have no aliases to signal NSEC3 [RFC5155] 208 denial of existence. The aliases mechanism used in RFC 5155 was to 209 protect implementations predating that RFC from encountering records 210 they could not know about. 212 5.2.1. NSEC3 in Authoritative servers 214 An authoritative server that does not implement NSEC3 MAY still serve 215 zones that use RSA/SHA2 with NSEC. 217 5.2.2. NSEC3 in Validators 219 A DNSSEC validator that implements RSA/SHA2 MUST be able to handle 220 both NSEC and NSEC3 [RFC5155] negative answers. If this is not the 221 case, the validator MUST treat a zone signed with RSA/SHA256 or RSA/ 222 SHA512 as signed with an unknown algorithm, and thus as insecure. 224 6. IANA Considerations 226 This document updates the IANA registry "DNS SECURITY ALGORITHM 227 NUMBERS -- per [RFC4035] " 228 (http://www.iana.org/assignments/dns-sec-alg-numbers). The following 229 entries are added to the registry: 231 Zone 232 Value Algorithm Mnemonic Signing References 233 {TBA1} RSA/SHA-256 RSASHA256 y {this memo} 234 {TBA2} RSA/SHA-512 RSASHA512 y {this memo} 236 7. Security Considerations 238 7.1. SHA-1 versus SHA-2 Considerations for RRSIG Resource Records 240 Users of DNSSEC are encouraged to deploy SHA-2 as soon as software 241 implementations allow for it. SHA-2 is widely believed to be more 242 resilient to attack than SHA-1, and confidence in SHA-1's strength is 243 being eroded by recently-announced attacks. Regardless of whether or 244 not the attacks on SHA-1 will affect DNSSEC, it is believed (at the 245 time of this writing) that SHA-2 is the better choice for use in 246 DNSSEC records. 248 SHA-2 is considered sufficiently strong for the immediate future, but 249 predictions about future development in cryptography and 250 cryptanalysis are beyond the scope of this document. 252 The signature scheme RSASSA-PKCS1-v1_5 is chosen to match the one 253 used for RSA/SHA-1 signatures. This should ease implementation of 254 the new hashing algorithms in DNSSEC software. 256 7.2. Signature Type Downgrade Attacks 258 Since each RRSet MUST be signed with each algorithm present in the 259 DNSKEY RRSet at the zone apex (see [RFC4035] Section 2.2), a 260 malicious party cannot filter out the RSA/SHA-2 RRSIG, and force the 261 validator to use the RSA/SHA-1 signature if both are present in the 262 zone. This should provide resilience against algorithm downgrade 263 attacks, if the validator supports RSA/SHA-2. 265 8. Acknowledgments 267 This document is a minor extension to RFC 4034 [RFC4034]. Also, we 268 try to follow the documents RFC 3110 [RFC3110] and RFC 4509 [RFC4509] 269 for consistency. The authors of and contributors to these documents 270 are gratefully acknowledged for their hard work. 272 The following people provided additional feedback and text: Jaap 273 Akkerhuis, Roy Arends, Rob Austein, Francis Dupont, Miek Gieben, 274 Alfred Hoenes, Paul Hoffman, Peter Koch, Michael St. Johns, Scott 275 Rose and Wouter Wijngaards. 277 9. References 279 9.1. Normative References 281 [FIPS.180-3.2008] 282 National Institute of Standards and Technology, "Secure 283 Hash Standard", FIPS PUB 180-3, October 2008. 285 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 286 Requirement Levels", RFC 2119, March 1997. 288 [RFC3110] Eastlake, D., "RSA/SHA-1 SIGs and RSA KEYs in the Domain 289 Name System (DNS)", RFC 3110, May 2001. 291 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 292 Rose, "DNS Security Introduction and Requirements", 293 RFC 4033, March 2005. 295 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. 296 Rose, "Resource Records for the DNS Security Extensions", 297 RFC 4034, March 2005. 299 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. 300 Rose, "Protocol Modifications for the DNS Security 301 Extensions", RFC 4035, March 2005. 303 9.2. Informative References 305 [NIST800-57] 306 Barker, E., Barker, W., Burr, W., Polk, W., and M. Smid, 307 "Recommendations for Key Management", NIST SP 800-57, 308 March 2007. 310 [RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography 311 Standards (PKCS) #1: RSA Cryptography Specifications 312 Version 2.1", RFC 3447, February 2003. 314 [RFC4509] Hardaker, W., "Use of SHA-256 in DNSSEC Delegation Signer 315 (DS) Resource Records (RRs)", RFC 4509, May 2006. 317 [RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS 318 Security (DNSSEC) Hashed Authenticated Denial of 319 Existence", RFC 5155, March 2008. 321 Author's Address 323 Jelte Jansen 324 NLnet Labs 325 Kruislaan 419 326 Amsterdam 1098VA 327 NL 329 Email: jelte@NLnetLabs.nl 330 URI: http://www.nlnetlabs.nl/