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Jansen 3 Internet-Draft NLnet Labs 4 Intended status: Standards Track March 23, 2009 5 Expires: September 24, 2009 7 Use of SHA-2 algorithms with RSA in DNSKEY and RRSIG Resource Records 8 for DNSSEC 9 draft-ietf-dnsext-dnssec-rsasha256-12 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 September 24, 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 in effect on the date of 41 publication of this document (http://trustee.ietf.org/license-info). 42 Please review these documents carefully, as they describe your rights 43 and restrictions with respect to this document. 45 Abstract 47 This document describes how to produce RSA/SHA-256 and RSA/SHA-512 48 DNSKEY and RRSIG resource records for use in the Domain Name System 49 Security Extensions (DNSSEC, RFC 4033, RFC 4034, and RFC 4035). 51 Table of Contents 53 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 54 2. DNSKEY Resource Records . . . . . . . . . . . . . . . . . . . . 3 55 2.1. RSA/SHA-256 DNSKEY Resource Records . . . . . . . . . . . . 3 56 2.2. RSA/SHA-512 DNSKEY Resource Records . . . . . . . . . . . . 4 57 3. RRSIG Resource Records . . . . . . . . . . . . . . . . . . . . 4 58 3.1. RSA/SHA-256 RRSIG Resource Records . . . . . . . . . . . . 4 59 3.2. RSA/SHA-512 RRSIG Resource Records . . . . . . . . . . . . 5 60 4. Deployment Considerations . . . . . . . . . . . . . . . . . . . 5 61 4.1. Key Sizes . . . . . . . . . . . . . . . . . . . . . . . . . 5 62 4.2. Signature Sizes . . . . . . . . . . . . . . . . . . . . . . 5 63 5. Implementation Considerations . . . . . . . . . . . . . . . . . 5 64 5.1. Support for SHA-2 signatures . . . . . . . . . . . . . . . 5 65 5.2. Support for NSEC3 Denial of Existence . . . . . . . . . . . 5 66 5.2.1. NSEC3 in Authoritative servers . . . . . . . . . . . . 6 67 5.2.2. NSEC3 in Validators . . . . . . . . . . . . . . . . . . 6 68 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6 69 7. Security Considerations . . . . . . . . . . . . . . . . . . . . 6 70 7.1. SHA-1 versus SHA-2 Considerations for RRSIG Resource 71 Records . . . . . . . . . . . . . . . . . . . . . . . . . . 6 72 7.2. Signature Type Downgrade Attacks . . . . . . . . . . . . . 7 73 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 7 74 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7 75 9.1. Normative References . . . . . . . . . . . . . . . . . . . 7 76 9.2. Informative References . . . . . . . . . . . . . . . . . . 8 77 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8 79 1. Introduction 81 The Domain Name System (DNS) is the global hierarchical distributed 82 database for Internet Naming. The DNS has been extended to use 83 cryptographic keys and digital signatures for the verification of the 84 authenticity and integrity of its data. RFC 4033 [RFC4033], RFC 4034 85 [RFC4034], and RFC 4035 [RFC4035] describe these DNS Security 86 Extensions, called DNSSEC. 88 RFC 4034 describes how to store DNSKEY and RRSIG resource records, 89 and specifies a list of cryptographic algorithms to use. This 90 document extends that list with the algorithms RSA/SHA-256 and RSA/ 91 SHA-512, and specifies how to store DNSKEY data and how to produce 92 RRSIG resource records with these hash algorithms. 94 Familiarity with DNSSEC, RSA and the SHA-2 [FIPS.180-3.2008] family 95 of algorithms is assumed in this document. 97 To refer to both SHA-256 and SHA-512, this document will use the name 98 SHA-2. This is done to improve readability. When a part of text is 99 specific for either SHA-256 or SHA-512, their specific names are 100 used. The same goes for RSA/SHA-256 and RSA/SHA-512, which will be 101 grouped using the name RSA/SHA-2. 103 The term "SHA-2" is not officially defined, but is usually used to 104 refer to the collection of the algorithms SHA-224, SHA-256, SHA-384 105 and SHA-512. Since SHA-224 and SHA-384 are not used in DNSSEC, SHA-2 106 will only refer to SHA-256 and SHA-512 in this document. 108 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 109 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 110 document are to be interpreted as described in [RFC2119]. 112 2. DNSKEY Resource Records 114 The format of the DNSKEY RR can be found in RFC 4034 [RFC4034]. RFC 115 3110 [RFC3110] describes the use of RSA/SHA-1 for DNSSEC signatures. 117 2.1. RSA/SHA-256 DNSKEY Resource Records 119 RSA public keys for use with RSA/SHA-256 are stored in DNSKEY 120 resource records (RRs) with the algorithm number {TBA1}. 122 For interoperability, as in RFC 3110 [RFC3110], the key size of RSA/ 123 SHA-256 keys MUST NOT be less than 512 bits, and MUST NOT be more 124 than 4096 bits. 126 2.2. RSA/SHA-512 DNSKEY Resource Records 128 RSA public keys for use with RSA/SHA-512 are stored in DNSKEY 129 resource records (RRs) with the algorithm number {TBA2}. 131 The key size of RSA/SHA-512 keys MUST NOT be less than 1024 bits, and 132 MUST NOT be more than 4096 bits. 134 3. RRSIG Resource Records 136 The value of the signature field in the RRSIG RR follows the RSASSA- 137 PKCS1-v1_5 signature scheme, and is calculated as follows. The 138 values for the RDATA fields that precede the signature data are 139 specified in RFC 4034 [RFC4034]. 141 hash = SHA-XXX(data) 143 Here XXX is either 256 or 512, depending on the algorithm used, as 144 specified in FIPS PUB 180-3 [FIPS.180-3.2008], and "data" is the wire 145 format data of the resource record set that is signed, as specified 146 in RFC 4034 [RFC4034]. 148 signature = ( 00 | 01 | FF* | 00 | prefix | hash ) ** e (mod n) 150 Here "|" is concatenation, "00", "01", "FF" and "00" are fixed octets 151 of corresponding hexadecimal value, "e" is the private exponent of 152 the signing RSA key, and "n" is the public modulus of the signing 153 key. The FF octet MUST be repeated the exact number of times so that 154 the total length of the concatenated term in parentheses equals the 155 length of the modulus of the signer's public key ("n"). 157 The "prefix" is intended to make the use of standard cryptographic 158 libraries easier. These specifications are taken directly from the 159 specifications of RSASSA-PKCS1-v1_5 in PKCS #1 v2.1 section 8.2 160 [RFC3447], and EMSA-PKCS1-v1_5 encoding in PKCS #1 v2.1 section 9.2 161 [RFC3447]. The prefixes for the different algorithms are specified 162 below. 164 3.1. RSA/SHA-256 RRSIG Resource Records 166 RSA/SHA-256 signatures are stored in the DNS using RRSIG resource 167 records (RRs) with algorithm number {TBA1}. 169 The prefix is the ASN.1 DER SHA-256 algorithm designator prefix as 170 specified in PKCS #1 v2.1 [RFC3447]: 172 hex 30 31 30 0d 06 09 60 86 48 01 65 03 04 02 01 05 00 04 20 174 3.2. RSA/SHA-512 RRSIG Resource Records 176 RSA/SHA-512 signatures are stored in the DNS using RRSIG resource 177 records (RRs) with algorithm number {TBA2}. 179 The prefix is the ASN.1 DER SHA-512 algorithm designator prefix as 180 specified in PKCS #1 v2.1 [RFC3447]: 182 hex 30 51 30 0d 06 09 60 86 48 01 65 03 04 02 03 05 00 04 40 184 4. Deployment Considerations 186 4.1. Key Sizes 188 Apart from the restrictions in section 2, this document will not 189 specify what size of keys to use. That is an operational issue and 190 depends largely on the environment and intended use. A good starting 191 point for more information would be NIST SP 800-57 [NIST800-57]. 193 4.2. Signature Sizes 195 In this family of signing algorithms, the size of signatures is 196 related to the size of the key, and not the hashing algorithm used in 197 the signing process. Therefore, RRSIG resource records produced with 198 RSA/SHA-256 or RSA/SHA-512 will have the same size as those produced 199 with RSA/SHA-1, if the keys have the same length. 201 5. Implementation Considerations 203 5.1. Support for SHA-2 signatures 205 DNSSEC aware implementations SHOULD be able to support RRSIG and 206 DNSKEY resource records created with the RSA/SHA-2 algorithms as 207 defined in this document. 209 5.2. Support for NSEC3 Denial of Existence 211 RFC 5155 [RFC5155] defines new algorithm identifiers for existing 212 signing algorithms, to indicate that zones signed with these 213 algorithm identifiers can use NSEC3 as well as NSEC records to 214 provide denial of existence. That mechanism was chosen to protect 215 implementations predating RFC5155 from encountering resource records 216 they could not know about. This document does not define such 217 algorithm aliases, and support for NSEC3 denial of existence is 218 implicitly signaled with support for one of the algorithms defined in 219 this document. 221 5.2.1. NSEC3 in Authoritative servers 223 An authoritative server that does not implement NSEC3 MAY still serve 224 zones that use RSA/SHA-2 with NSEC denial of existence. 226 5.2.2. NSEC3 in Validators 228 A DNSSEC validator that implements RSA/SHA-2 MUST be able to handle 229 both NSEC and NSEC3 [RFC5155] negative answers. If this is not the 230 case, the validator MUST treat a zone signed with RSA/SHA-256 or RSA/ 231 SHA-512 as signed with an unknown algorithm, and thus as insecure. 233 6. IANA Considerations 235 This document updates the IANA registry "DNS SECURITY ALGORITHM 236 NUMBERS -- per [RFC4035] " 237 (http://www.iana.org/assignments/dns-sec-alg-numbers). The following 238 entries are added to the registry: 240 Zone Trans. 241 Value Description Mnemonic Signing Sec. References 242 {TBA1} RSA/SHA-256 RSASHA256 y * {this memo} 243 {TBA2} RSA/SHA-512 RSASHA512 y * {this memo} 245 * There has been no determination of standardization of the use of this 246 algorithm with Transaction Security. 248 7. Security Considerations 250 7.1. SHA-1 versus SHA-2 Considerations for RRSIG Resource Records 252 Users of DNSSEC are encouraged to deploy SHA-2 as soon as software 253 implementations allow for it. SHA-2 is widely believed to be more 254 resilient to attack than SHA-1, and confidence in SHA-1's strength is 255 being eroded by recently-announced attacks. Regardless of whether or 256 not the attacks on SHA-1 will affect DNSSEC, it is believed (at the 257 time of this writing) that SHA-2 is the better choice for use in 258 DNSSEC records. 260 SHA-2 is considered sufficiently strong for the immediate future, but 261 predictions about future development in cryptography and 262 cryptanalysis are beyond the scope of this document. 264 The signature scheme RSASSA-PKCS1-v1_5 is chosen to match the one 265 used for RSA/SHA-1 signatures. This should ease implementation of 266 the new hashing algorithms in DNSSEC software. 268 7.2. Signature Type Downgrade Attacks 270 Since each RRSet MUST be signed with each algorithm present in the 271 DNSKEY RRSet at the zone apex (see [RFC4035] Section 2.2), a 272 malicious party cannot filter out the RSA/SHA-2 RRSIG, and force the 273 validator to use the RSA/SHA-1 signature if both are present in the 274 zone. This should provide resilience against algorithm downgrade 275 attacks, if the validator supports RSA/SHA-2. 277 8. Acknowledgments 279 This document is a minor extension to RFC 4034 [RFC4034]. Also, we 280 try to follow the documents RFC 3110 [RFC3110] and RFC 4509 [RFC4509] 281 for consistency. The authors of and contributors to these documents 282 are gratefully acknowledged for their hard work. 284 The following people provided additional feedback and text: Jaap 285 Akkerhuis, Mark Andrews, Roy Arends, Rob Austein, Francis Dupont, 286 Miek Gieben, Alfred Hoenes, Paul Hoffman, Peter Koch, Michael St. 287 Johns, Scott Rose and Wouter Wijngaards. 289 9. References 291 9.1. Normative References 293 [FIPS.180-3.2008] 294 National Institute of Standards and Technology, "Secure 295 Hash Standard", FIPS PUB 180-3, October 2008. 297 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 298 Requirement Levels", RFC 2119, March 1997. 300 [RFC3110] Eastlake, D., "RSA/SHA-1 SIGs and RSA KEYs in the Domain 301 Name System (DNS)", RFC 3110, May 2001. 303 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 304 Rose, "DNS Security Introduction and Requirements", 305 RFC 4033, March 2005. 307 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. 308 Rose, "Resource Records for the DNS Security Extensions", 309 RFC 4034, March 2005. 311 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. 312 Rose, "Protocol Modifications for the DNS Security 313 Extensions", RFC 4035, March 2005. 315 9.2. Informative References 317 [NIST800-57] 318 Barker, E., Barker, W., Burr, W., Polk, W., and M. Smid, 319 "Recommendations for Key Management", NIST SP 800-57, 320 March 2007. 322 [RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography 323 Standards (PKCS) #1: RSA Cryptography Specifications 324 Version 2.1", RFC 3447, February 2003. 326 [RFC4509] Hardaker, W., "Use of SHA-256 in DNSSEC Delegation Signer 327 (DS) Resource Records (RRs)", RFC 4509, May 2006. 329 [RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS 330 Security (DNSSEC) Hashed Authenticated Denial of 331 Existence", RFC 5155, March 2008. 333 Author's Address 335 Jelte Jansen 336 NLnet Labs 337 Kruislaan 419 338 Amsterdam 1098VA 339 NL 341 Email: jelte@NLnetLabs.nl 342 URI: http://www.nlnetlabs.nl/