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This document defines a 18 general purpose document signing KeyPurposeId for inclusion in the 19 Extended Key Usage (EKU) extension of X.509 public key certificates. 20 Document Signing applications may require that the EKU extension be 21 present and that a document signing KeyPurposeId be indicated in 22 order for the certificate to be acceptable to that Document Signing 23 application. 25 About This Document 27 This note is to be removed before publishing as an RFC. 29 Status information for this document may be found at 30 https://datatracker.ietf.org/doc/draft-ietf-lamps-documentsigning- 31 eku/. 33 Discussion of this document takes place on the Limited Additional 34 Mechanisms for PKIX and SMIME (LAMPS) Working Group mailing list 35 (mailto:spasm@ietf.org), which is archived at 36 https://mailarchive.ietf.org/arch/browse/spasm/. 38 Source for this draft and an issue tracker can be found at 39 https://github.com/lamps-wg/documentsigning-eku. 41 Status of This Memo 43 This Internet-Draft is submitted in full conformance with the 44 provisions of BCP 78 and BCP 79. 46 Internet-Drafts are working documents of the Internet Engineering 47 Task Force (IETF). Note that other groups may also distribute 48 working documents as Internet-Drafts. The list of current Internet- 49 Drafts is at https://datatracker.ietf.org/drafts/current/. 51 Internet-Drafts are draft documents valid for a maximum of six months 52 and may be updated, replaced, or obsoleted by other documents at any 53 time. It is inappropriate to use Internet-Drafts as reference 54 material or to cite them other than as "work in progress." 56 This Internet-Draft will expire on 8 September 2022. 58 Copyright Notice 60 Copyright (c) 2022 IETF Trust and the persons identified as the 61 document authors. All rights reserved. 63 This document is subject to BCP 78 and the IETF Trust's Legal 64 Provisions Relating to IETF Documents (https://trustee.ietf.org/ 65 license-info) in effect on the date of publication of this document. 66 Please review these documents carefully, as they describe your rights 67 and restrictions with respect to this document. Code Components 68 extracted from this document must include Revised BSD License text as 69 described in Section 4.e of the Trust Legal Provisions and are 70 provided without warranty as described in the Revised BSD License. 72 Table of Contents 74 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 75 2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3 76 3. Extended Key Purpose for Document Signing . . . . . . . . . . 4 77 3.1. Including the Extended Key Purpose for Document Signing in 78 Certificates . . . . . . . . . . . . . . . . . . . . . . 4 79 4. Using the Extended Key Purpose for Document Signing in a 80 Certificate . . . . . . . . . . . . . . . . . . . . . . . 5 81 5. Implications for a Certification Authority . . . . . . . . . 6 82 6. Security Considerations . . . . . . . . . . . . . . . . . . . 6 83 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 84 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 85 8.1. Normative References . . . . . . . . . . . . . . . . . . 7 86 8.2. Informative References . . . . . . . . . . . . . . . . . 7 87 Appendix A. ASN.1 Module . . . . . . . . . . . . . . . . . . . . 8 88 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 8 89 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8 91 1. Introduction 93 [RFC5280] specifies several extended key purpose identifiers 94 (KeyPurposeIds) for X.509 certificates. In addition, several 95 KeyPurposeIds have been added [RFC7299] under the IANA repository 96 "SMI Security for PKIX Extended Key Purpose". While usage of the 97 "anyExtendedKeyUsage" KeyPurposeId is bad practice for publicly 98 trusted certificates, there is no public and general KeyPurposeId 99 explicitly assigned for Document Signing. The current practice is to 100 use id-kp-emailProtection, id-kp-codeSigning or a vendor-defined 101 KeyPurposeId for general document signing purposes. 103 In circumstances where code signing and S/MIME certificates are also 104 widely used for document signing, the technical or policy changes 105 that are made to code signing and S/MIME certificates may cause 106 unexpected behaviors or have an adverse impact such as decreased 107 cryptographic agility on the document signing ecosystem and vice 108 versa. 110 There is no issue if the vendor-defined KeyPurposeIds are used in a 111 PKI (or a trust program) governed by the vendor. However, if the 112 KeyPurposeId is used outside of vendor governance, the usage can 113 easily become out of control (e.g. - When the end user encounters 114 vendor-defined KeyPurposeIds, they might want to ask that vendor 115 about use of the certificate, however, the vendor may not know about 116 the particular use. - If the issuance of the cert is not under the 117 control of the KeyPurposeId owner, there is no way for the 118 KeyPurposeId owner to know what the impact will be if any change is 119 made to the KeyPurposeId in question, and it would restrict vendor's 120 choice of OID management. etc.). 122 Therefore, it is not favorable to use a vendor-defined KeyPurposeId 123 for signing a document that is not governed by the vendor. 125 This document defines a extended key purpose identifier for Document 126 Signing. 128 2. Conventions and Definitions 130 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 131 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 132 "OPTIONAL" in this document are to be interpreted as described in 133 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all 134 capitals, as shown here. 136 3. Extended Key Purpose for Document Signing 138 This specification defines the KeyPurposeId id-kp-documentSigning. 140 As described in [RFC5280], If the Extended Key Usage extension is 141 present, then the certificate MUST only be used for one of the 142 purposes indicated. [RFC5280] also describes that If multiple key 143 purposes are indicated the application need not recognize all 144 purposes indicated, as long as the intended purpose is present. 146 Document Signing applications MAY require that the Extended Key Usage 147 extension be present and that a id-kp-documentSigning be indicated in 148 order for the certificate to be acceptable to that Document Signing 149 application. 151 The term "Document Signing" in this document refers to digitally 152 signing contents that are consumed by people. To be more precise, 153 contents are intended to be shown to a person with printable or 154 displayable form by means of services or software, rather than 155 processed by machines. 157 3.1. Including the Extended Key Purpose for Document Signing in 158 Certificates 160 [RFC5280] specifies the EKU X.509 certificate extension for use on 161 the Internet. The extension indicates one or more purposes for which 162 the certified public key is valid. The EKU extension can be used in 163 conjunction with the key usage extension, which indicates the set of 164 basic cryptographic operations for which the certified key may be 165 used. 167 The EKU extension syntax is repeated here for convenience: 169 ExtKeyUsageSyntax ::= SEQUENCE SIZE (1..MAX) OF KeyPurposeId 170 KeyPurposeId ::= OBJECT IDENTIFIER 172 This specification defines the KeyPurposeId id-kp-documentSigning. 173 Inclusion of this KeyPurposeId in a certificate indicates that the 174 public key encoded in the certificate has been certified to be used 175 for cryptographic operations on contents that are consumed by people. 177 id-kp OBJECT IDENTIFIER ::= 178 { iso(1) identified-organization(3) dod(6) internet(1) 179 security(5) mechanisms(5) pkix(7) 3 } 180 id-kp-documentSigning OBJECT IDENTIFIER ::= { id-kp XX } 182 4. Using the Extended Key Purpose for Document Signing in a Certificate 184 [RFC8358] specifies the conventions for digital signatures on 185 Internet-Drafts. This is one of the intended use cases for the 186 general document signing key purpose described in this document. 187 [RFC8358] uses CMS to digitally sign a wide array of files such as 188 ASCII, PDF, EPUB, HTML etc. Currently, there are no specification 189 regarding key purposes for certificates signing those files except 190 those which are defined by the software vendor. 192 The signed contents of Internet-Drafts are primarily intended to be 193 consumed by people. To be more precise, contents are intended to be 194 shown to a person in a printable or displayable form by means of 195 services or software, rather than processed by machines. The digital 196 signature on the contents is to indicate to the recipient of the 197 contents that the content has not changed since it was signed by the 198 identity indicated as the subject of the certificate. To validate 199 the digital signature which is signed on contents intended to be 200 consumed by people, implementations MAY perform the steps below 201 during certificate validation: 203 The implementation MAY examine the KeyPurposeId(s) included in the 204 Extended Key Usage extension as follows: A Restriction on Extended 205 Key Usage is derived and implemented from (or configured with) the 206 policy to which the implementation conforms. 208 1. If there are no restrictions set for the relying party and the 209 relying party software, the certificate is acceptable. 211 2. If there are restrictions set for the replying party and relying 212 party software, then process the KeyPurposeId(s) as described 213 below. 215 Each restriction on "Excluded KeyPurposeId" or "Permitted 216 KeyPurposeId" is handled as described below. 218 This procedure is intended to permit or prohibit presence of a 219 certain KeyPurposeId or complete absence of KeyPurposeIds. It is 220 outside the scope of this document, but the relying party can 221 permit or prohibit combinations of KeyPurposeIds. A 222 consideration on prohibiting combinations of KeyPurposeIds is 223 described in the Security Considerations section of this 224 document. 226 Excluded KeyPurposeId procedure: "Excluded KeyPurposeId" is a 227 KeyPurposeId which the relying party or the relying party 228 software prohibits. Examples of "Excluded KeyPurposeId" are, 229 presence of the anyExtendedKeyUsage KeyPurposeId or complete 230 absence of the EKU extension in a certificate. If a 231 KeyPurposeId of the certificate meets the conditions set by 232 the "Excluded KeyPurposeId" restriction, the relying party or 233 the relying party software rejects the certificate. 235 Permitted KeyPurposeId procedure: 237 : "Permitted KeyPurposeId" is a KeyPurposeId which the relying 238 party or the relying party software accepts. Examples of 239 "Permitted KeyPurposeId" are, presence of this general document 240 signing KeyPurposeId and/or protocol specific document signing- 241 type KeyPurposeIds. If a KeyPurposeId of the certificate meets 242 the condition set by a "Permitted KeyPurposeId" restriction, the 243 certificate is acceptable. Otherwise, relying party or the 244 relying party software rejects the certificate. 246 When a single application has the capability to process various data 247 formats, the software may choose to make the excluded and permitted 248 decisions separately in accordance with the format it is handling 249 (e.g. text, pdf, etc). 251 5. Implications for a Certification Authority 253 The procedures and practices employed by a certification authority 254 MUST ensure that the correct values for the EKU extension are 255 inserted in each certificate that is issued. Unless certificates are 256 governed by a vendor specific PKI (or trust program), certificates 257 that indicate usage for document signing MAY include the id-kp- 258 documentSigning KeyPurposeId. The inclusion of the id-kp- 259 documentSigning KeyPurposeId does not preclude the inclusion of other 260 KeyPurposeIds. 262 6. Security Considerations 264 The usage of id-kp-documentSigning KeyPurposeId is to provide an 265 alternative to id-kp-emailProtection being used for non-email 266 purposes and id-kp-codeSigning being used to sign objects other than 267 binary code. This extended key purpose does not introduce new 268 security risks but instead reduces existing security risks by 269 providing means to separate other extended key purposes used for 270 communication protocols namely, TLS or S/MIME etc. in order to 271 minimize the risk of cross-protocol attacks. 273 To reduce the risk of specific cross-protocol attacks, the relying 274 party or relying party software may additionally prohibit use of 275 specific combinations of KeyPurposeIds. 277 While a specific protocol or signing scheme may choose to come up 278 with their own KeyPurposeIds, some may not have significant motive or 279 resources to set up and manage their own KeyPurposeIds. This general 280 document signing KeyPurposeId may be used as a stop-gap for those 281 that intend to define their own KeyPurposeId or those who do not 282 intend to set up an KeyPurposeId but still would like to distinguish 283 document signing from other usages. 285 Introduction of this id-kp-documentSigning KeyPurposeId does not 286 introduce any new security or privacy concerns. 288 7. IANA Considerations 290 This document requests that IANA make two assignments. One 291 assignment is for the addition of the id-kp-documentSigning object 292 identifier (OID), as defined in Section 3.1, to the "SMI Security for 293 PKIX Extended Key Purpose" (1.3.6.1.5.5.7.3) registry. The other 294 assignment is for the addition of the id-mod-docsign-eku ASN.1 module 295 [X.680] object identifier (OID), as defined in Appendix A, to the 296 "SMI Security for PKIX Module Identifier" (1.3.6.1.5.5.7.0) registry. 297 No further action is necessary by IANA. 299 8. References 301 8.1. Normative References 303 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 304 Requirement Levels", BCP 14, RFC 2119, 305 DOI 10.17487/RFC2119, March 1997, 306 . 308 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 309 Housley, R., and W. Polk, "Internet X.509 Public Key 310 Infrastructure Certificate and Certificate Revocation List 311 (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, 312 . 314 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 315 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 316 May 2017, . 318 [X.680] ITU-T, "Information technology - Abstract Syntax Notation 319 One (ASN.1): Specification of basic notation", ISO/ 320 IEC 8824-1:2015, November 2015. 322 8.2. Informative References 324 [RFC7299] Housley, R., "Object Identifier Registry for the PKIX 325 Working Group", RFC 7299, DOI 10.17487/RFC7299, July 2014, 326 . 328 [RFC8358] Housley, R., "Update to Digital Signatures on Internet- 329 Draft Documents", RFC 8358, DOI 10.17487/RFC8358, March 330 2018, . 332 Appendix A. ASN.1 Module 334 The following ASN.1 module provides the complete definition of the 335 Document Signing KeyPurposeId. 337 DocSignEKU { iso(1) identified-organization(3) dod(6) internet(1) 338 security(5) mechanisms(5) pkix(7) id-mod(0) 339 id-mod-docsign-eku(TBD1) } 341 DEFINITIONS EXPLICIT TAGS ::= 343 BEGIN 345 -- EXPORTS ALL -- 347 -- IMPORTS NOTHING -- 349 -- OID Arc -- 351 id-kp OBJECT IDENTIFIER ::= { 352 iso(1) identified-organization(3) dod(6) internet(1) 353 security(5) mechanisms(5) pkix(7) kp(3) } 355 -- Document Signing Extended Key Usage -- 357 id-kp-documentSigning OBJECT IDENTIFIER ::= { id-kp TBD2 } 359 END 361 Acknowledgments 363 We would like to thank Russ Housley for verifying the ASN.1 module. 365 Authors' Addresses 367 Tadahiko Ito 368 SECOM CO., LTD. 369 Email: tadahiko.ito.public@gmail.com 370 Tomofumi Okubo 371 DigiCert, Inc. 372 Email: tomofumi.okubo+ietf@gmail.com 374 Sean Turner 375 sn3rd 376 Email: sean@sn3rd.com