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If not, you may need to add the pre-RFC5378 disclaimer. (See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (December 2, 2008) is 5623 days in the past. Is this intentional? Checking references for intended status: Informational ---------------------------------------------------------------------------- == Missing Reference: 'RFC2459' is mentioned on line 85, but not defined ** Obsolete undefined reference: RFC 2459 (Obsoleted by RFC 3280) ** Obsolete normative reference: RFC 2560 (Obsoleted by RFC 6960) Summary: 4 errors (**), 0 flaws (~~), 2 warnings (==), 7 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Internet Engineering Task Force P. Hallam-Baker 3 Internet-Draft VeriSign Inc 4 Intended status: Informational December 2, 2008 5 Expires: June 5, 2009 7 OCSP Algorithm Agility 8 draft-hallambaker-ocspagility-02 10 Status of this Memo 12 By submitting this Internet-Draft, each author represents that any 13 applicable patent or other IPR claims of which he or she is aware 14 have been or will be disclosed, and any of which he or she becomes 15 aware will be disclosed, in accordance with Section 6 of BCP 79. 17 Internet-Drafts are working documents of the Internet Engineering 18 Task Force (IETF), its areas, and its working groups. Note that 19 other groups may also distribute working documents as Internet- 20 Drafts. 22 Internet-Drafts are draft documents valid for a maximum of six months 23 and may be updated, replaced, or obsoleted by other documents at any 24 time. It is inappropriate to use Internet-Drafts as reference 25 material or to cite them other than as "work in progress." 27 The list of current Internet-Drafts can be accessed at 28 http://www.ietf.org/ietf/1id-abstracts.txt. 30 The list of Internet-Draft Shadow Directories can be accessed at 31 http://www.ietf.org/shadow.html. 33 This Internet-Draft will expire on June 5, 2009. 35 Abstract 37 The OSCP specification defined in RFC 2560 requires server responses 38 to be signed but does not specify a mechanism for selecting the 39 signature algorithm to be used leading to possible interoperability 40 failures in contexts where multiple signature algorithms are in use. 41 This document specifies an algorithm for server signature algorithm 42 selection and an extension that allows a client to advise a server 43 that specific signature algorithms are supported. 45 Table of Contents 47 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 48 1.1. Requirements Language . . . . . . . . . . . . . . . . . . . 3 49 2. OCSP Algorithm Agility Requirements . . . . . . . . . . . . . . 3 50 3. Client Indication of Preferred Signature Algorithms . . . . . . 4 51 4. Responder Signature Algorithm Selection . . . . . . . . . . . . 5 52 4.1. Dynamic Response . . . . . . . . . . . . . . . . . . . . . 5 53 4.2. Static Response . . . . . . . . . . . . . . . . . . . . . . 5 54 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6 55 6. Security Considerations . . . . . . . . . . . . . . . . . . . . 6 56 6.1. Use of insecure algorithms . . . . . . . . . . . . . . . . 6 57 6.2. Man in the Middle Downgrade Attack . . . . . . . . . . . . 6 58 7. Normative References . . . . . . . . . . . . . . . . . . . . . 7 59 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 7 60 Intellectual Property and Copyright Statements . . . . . . . . . . 8 62 1. Introduction 64 1.1. Requirements Language 66 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 67 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 68 document are to be interpreted as described in RFC 2119 [RFC2119]. 70 2. OCSP Algorithm Agility Requirements 72 OCSP RFC 2560 [RFC2560] defines a protocol for obtaining certificate 73 status information from an online service. A particular OCSP server 74 may or may not be provided by the CA that issued the certificate 75 whose status is being queried and may or may provide a realtime 76 indication of the certificate status or a time delayed status 77 indication. 79 RFC 2560 [RFC2560] specifies a means for an OCSP responder to 80 indicate the signature and digest algorithms used in a response but 81 not how those algorithms are specified. The only algorithm mandated 82 by the protocol specification is that the OCSP client SHALL support 83 the DSA sig-alg-oid specified in section 7.2.2 of [RFC2459] and 84 SHOULD be capable of processing RSA signatures as specified in 85 section 7.2.1 of [RFC2459]. The only requirement placed on 86 responders is that they SHALL support the SHA1 hashing algorithm. 88 This requirement is clearly insufficient to ensure interoperabilty. 90 While the responder may apply heuristics such as using the signature 91 algorithm employed by the certificate issuer, such heuristics fail in 92 many common real-world situations where multiple signature algorithms 93 are employed: 95 o The algorithm used to sign the certificate may differ from the 96 subject key algorithm 98 o The properties of the OCSP responder certificate chain are 99 frequently only known to the responder after the fact. 101 o In an extended PKI deployment, the task of verifying certificate 102 status may be separated from the task(s) that make use of the 103 certificate. 105 o A responder cannot infer anything if a request for an unknown 106 certificate is issued. 108 The last criterion is significant as it occurs frequently in real 109 world PKI deployments and cannot be resolved through the information 110 available from in-band signalling using the RFC 2560 [RFC2560] 111 protocol without modification. 113 In addition, a system that employs a signature algorithm other than 114 the de-facto default is frequently doing so to achieve very specific 115 security properties that may not be captured by a heuristic 116 assumptuion designed to facilitate interoperability rather than 117 performance. In particular: 119 o An implementation may intentionally employ an algorithm for 120 certificate status response that is less computationally demanding 121 than for signing the certificate itself, thus allowing for more 122 frequent certificate status validation. 124 o An implementation may intentionally wish to guard against the 125 possibility of a compromise resulting from a signature algorithm 126 compromise by employing two separate encryption algorithms. 128 This document describes: 130 o A mechanism that allows a client to indicate the set of preferred 131 signature algorithms. 133 o An algorithm for signature algorithm selection that maximizes the 134 probability of successful operation in the case that no supported 135 prefered algorithm(s) are specified. 137 3. Client Indication of Preferred Signature Algorithms 139 A client MAY declare a preferred set of algorithms algorithms in a 140 request using the preferred signature algorithm extension. 142 id-pkix-ocsp-preferred-signature-algorithms OBJECT IDENTIFIER ::= { 143 id-pkix-ocsp x } 145 PreferredSignatureAlgorithms ::= SEQUENCE { 146 Algorithms SEQUENCE OF AlgorithmIdentifier 147 } 149 If a set of preferred signature algorithms is declared the client 150 MUST support each of the specified algorithms. 152 If a set of preferred algorithms is declared the OCSP responder 153 SHOULD use one of the specified signing algorithms. 155 4. Responder Signature Algorithm Selection 157 RFC 2560 [RFC2560] does not specify a mechanism for deciding the 158 signature algorithm to be used in an OCSP response. As previously 159 noted this does not provide a sufficient degree of certainty as to 160 the algorithm selected to guarantee interoperation. 162 4.1. Dynamic Response 164 A responder MAY maximize the potential for ensuring interoperability 165 by selecting a supported signature algorithm using the following 166 order of precedence where the first method has the highest 167 precedence: 169 1. Using an algorithm specified as a preferred signing algorithm in 170 the client request. 172 2. Using the signing algorithm used to sign the CertID specified in 173 the query. 175 3. Using the signing algorithm used to sign a CRL issued by the 176 certificate issuer providing status information for the 177 certificate specified by CertID. 179 4. Using a signature algorithm that has been advertised as being the 180 default signature algorithm for the signing service using an out 181 of band mechanism 183 5. Using a mandatory signing algorithm specified for the version of 184 the OCSP protocol in use. 186 A responder SHOULD always apply the lowest numbered selection 187 mechanism that is known, supported and meets the responder's criteria 188 for cryptographic algorithm strength. 190 4.2. Static Response 192 For purposes of efficiency, an OCSP responder is permitted to 193 generate static responses in advance of a request. Although this 194 case does not permit the responder to make use of the client data 195 directly, the responder may anticipate the client request and 196 generate a set of signed responses so as to maximize the probability 197 that it is possible to generate a response that is assigned the 198 highest preference weighting. 200 5. Acknowledgements 202 The author acknowleges the helpful comments made on earlier drafts of 203 this work by Santosh Chokhani and Stefan Santesson 205 6. Security Considerations 207 The mechanism used to choose the response signing algorithm MUST be 208 considered to be sufficiently secure against cryptanalytic attack for 209 the intended application. 211 In most applications it is sufficient for the signing algorithm to be 212 at least as secure as the signing algorithm used to sign the original 213 certificate whose status is being queried. This criteria may not 214 hold in long term archival applications however in which the status 215 of a certificate is being queried for a date in the distant past, 216 long after the signing algorithm has ceased being considered 217 trustworthy. 219 6.1. Use of insecure algorithms 221 It is not always possible for a responder to generate a response that 222 the client is expected to understand and meets contemporary standards 223 for cryptographic security. In such cases an application MUST 224 balance the risk of employing a compromised security solution and the 225 cost of mandating an upgrade, including the risk that the alternative 226 chosen by end users will offer even less security or no security. 228 In archival applications it is quite possible that an OCSP responder 229 might be asked to report the validity of a certificate on a date in 230 the distant past. Such a certificate might employ a signing method 231 that is no longer considered acceptably secure. In such 232 circumstances the responder MUST NOT generate a signature for a 233 signing mechanism that is considered unacceptably insecure. 235 A client MUST accept any signing algorithm in a response that it 236 specified as a preferred signing algorithm in the request. It 237 follows therefore that a client MUST NOT specify as a preferred 238 signing algorithm any signing algorithm that is either not supported 239 or not considered acceptably secure. 241 6.2. Man in the Middle Downgrade Attack 243 The mechanism to support client indication of preferred signature 244 algorithms is not protected against a man in the middle downgrade 245 attack. This constraint is not considered to be a significant 246 security concern as the client MUST NOT accept any signing algorithm 247 that does not meet its own criteria for acceptable cryptographic 248 security no matter what mechanism is used to determine the signing 249 algorithm of the response. 251 7. Normative References 253 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 254 Requirement Levels", BCP 14, RFC 2119, March 1997. 256 [RFC2560] Myers, M., Ankney, R., Malpani, A., Galperin, S., and C. 257 Adams, "X.509 Internet Public Key Infrastructure Online 258 Certificate Status Protocol - OCSP", RFC 2560, June 1999. 260 Author's Address 262 Phillip Hallam-Baker 263 VeriSign Inc 265 Email: pbaker@verisign.com 267 Full Copyright Statement 269 Copyright (C) The IETF Trust (2008). 271 This document is subject to the rights, licenses and restrictions 272 contained in BCP 78, and except as set forth therein, the authors 273 retain all their rights. 275 This document and the information contained herein are provided on an 276 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 277 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND 278 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS 279 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF 280 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 281 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 283 Intellectual Property 285 The IETF takes no position regarding the validity or scope of any 286 Intellectual Property Rights or other rights that might be claimed to 287 pertain to the implementation or use of the technology described in 288 this document or the extent to which any license under such rights 289 might or might not be available; nor does it represent that it has 290 made any independent effort to identify any such rights. 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