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Turner 7 sn3rd 8 7 November 2021 10 General Purpose Extended Key Usage (EKU) for Document Signing X.509 11 Certificates 12 draft-ito-documentsigning-eku-02 14 Abstract 16 [RFC5280] specifies several extended key usages for X.509 17 certificates. This document defines a general purpose document 18 signing extended key usage for X.509 public key certificates which 19 restricts the usage of the certificates for document signing. 21 Status of This Memo 23 This Internet-Draft is submitted in full conformance with the 24 provisions of BCP 78 and BCP 79. 26 Internet-Drafts are working documents of the Internet Engineering 27 Task Force (IETF). Note that other groups may also distribute 28 working documents as Internet-Drafts. The list of current Internet- 29 Drafts is at https://datatracker.ietf.org/drafts/current/. 31 Internet-Drafts are draft documents valid for a maximum of six months 32 and may be updated, replaced, or obsoleted by other documents at any 33 time. It is inappropriate to use Internet-Drafts as reference 34 material or to cite them other than as "work in progress." 36 This Internet-Draft will expire on 5 May 2022. 38 Copyright Notice 40 Copyright (c) 2021 IETF Trust and the persons identified as the 41 document authors. All rights reserved. 43 This document is subject to BCP 78 and the IETF Trust's Legal 44 Provisions Relating to IETF Documents (https://trustee.ietf.org/ 45 license-info) in effect on the date of publication of this document. 46 Please review these documents carefully, as they describe your rights 47 and restrictions with respect to this document. Code Components 48 extracted from this document must include Revised BSD License text as 49 described in Section 4.e of the Trust Legal Provisions and are 50 provided without warranty as described in the Revised BSD License. 52 Table of Contents 54 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 55 2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3 56 3. Extended Key usage for DocumentSigning . . . . . . . . . . . 3 57 3.1. Extended Key Usage Values for Document Signing . . . . . 3 58 4. Using the Document Signing EKU in a Certificate . . . . . . . 4 59 5. Implications for a Certification Authority . . . . . . . . . 5 60 6. Security Considerations . . . . . . . . . . . . . . . . . . . 5 61 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 62 8. Normative References . . . . . . . . . . . . . . . . . . . . 6 63 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 7 64 Appendix A. ASN.1 Module . . . . . . . . . . . . . . . . . . . . 7 65 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7 67 1. Introduction 69 [RFC5280] specifies several extended key usages for X.509 70 certificates. In addition, several extended key usage had been 71 added[RFC7299] as public OID under the IANA repository. While usage 72 of any extended key usage is bad practice for publicly trusted 73 certificates, there are no public and general extended key usage 74 explicitly assigned for Document Signing certificates. The current 75 practice is to use id-kp-emailProtection, id-kp-codeSigning or vendor 76 defined Object ID for general document signing purposes. 78 In circumstances where code signing and S/MIME certificates are also 79 widely used for document signing, the technical or policy changes 80 that are made to code signing and S/MIME certificates may cause 81 unexpected behaviors or have an adverse impact such as decreased 82 cryptographic agility on the document signing ecosystem and vice 83 versa. 85 There is no issue if the vendor defined OIDs are used in a PKI (or a 86 trust program) governed by the vendor. However, if the OID is used 87 outside of the vendor governance, the usage can easily become out of 88 control (e.g. - When the end user encounters vendor defined OIDs, 89 they might want to ask that vendor about use of the certificate, 90 however, the vendor may not know about the particular use. - If the 91 issuance of the cert is not under the control of the OID owner, there 92 is no way for the OID owner to know what the impact will be if any 93 change is made to the OID in question, and it would restrict vendor's 94 choice of OID management. etc.). 96 Therefore, it is not favorable to use a vendor defined EKU for 97 signing a document that is not governed by the vendor. 99 This document defines a general Document Signing extended key usage. 101 2. Conventions and Definitions 103 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 104 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 105 "OPTIONAL" in this document are to be interpreted as described in BCP 106 14 [RFC2119] [RFC8174] when, and only when, they appear in all 107 capitals, as shown here. 109 3. Extended Key usage for DocumentSigning 111 This specification defines the KeyPurposeId id-kp-documentSigning. 112 Inclusion of this KeyPurposeId in a certificate indicates that the 113 use of any Subject names in the certificate is restricted to use by a 114 document signing. 116 Term of "Document Sign" in this document is digitally sign contents 117 that are consumed by humans. To be more precise, contents are 118 intended to be shown to human with printable or displayable form by 119 means of services or software, rather than processed by machines. 121 3.1. Extended Key Usage Values for Document Signing 123 [RFC5280] specifies the EKU X.509 certificate extension for use in 124 the Internet. The extension indicates one or more purposes for which 125 the certified public key is valid. The EKU extension can be used in 126 conjunction with the key usage extension, which indicates how the 127 public key in the certificate is used, in a more basic cryptographic 128 way. 130 The EKU extension syntax is repeated here for convenience: 132 ExtKeyUsageSyntax ::= SEQUENCE SIZE (1..MAX) OF KeyPurposeId 133 KeyPurposeId ::= OBJECT IDENTIFIER 135 This specification defines the KeyPurposeId id-kp-documentSigning. 136 Inclusion of this KeyPurposeId in a certificate indicates that the 137 use of any Subject names in the certificate is restricted to use by a 138 document signing service or a software (along with any usages allowed 139 by other EKU values). 141 id-kp OBJECT IDENTIFIER ::= 142 { iso(1) identified-organization(3) dod(6) internet(1) 143 security(5) mechanisms(5) pkix(7) 3 } 144 id-kp-documentSigning OBJECT IDENTIFIER ::= { id-kp XX } 146 4. Using the Document Signing EKU in a Certificate 148 [RFC8358] specifies the conventions for digital signatures on 149 Internet-Drafts. This is one of the intended use cases for the 150 general document signing EKU described in this document. [RFC8358] 151 uses CMS to digitally sign a wide array of files such as ASCII, PDF, 152 EPUB, HTML etc. Currently, there are no specification regarding EKU 153 for certificates signing those files except those which are defined 154 by the software vendor. 156 The signed contents of Internet-Drafts are primarily intended to be 157 consumed by human. To be more precise, contents are intended to be 158 shown to human in a printable or displayable form by means of 159 services or software, rather than processed by machines. To validate 160 the digital signature which is signed to contents intended to be 161 consumed by human, implementations MAY perform the steps below as a 162 certificate validation: 164 The implementation MAY examine the Extended Key Usage value(s): 166 1. If there are no restrictions set for the relying party and the 167 relying party software, the certificate is acceptable. 169 2. If there are restrictions set for the replying party and relying 170 party software, proceed as following. 172 Each Restriction on the EKUs can be "Excluded EKU" or "Permitted EKU" 173 and handled. 175 The procedure is intended to permit or prohibit presence of a certain 176 EKU or complete absence of EKUs. It is outside the scope of this 177 document, but the relying party can permit or prohibit conbinations 178 of EKU. A consideration on prohibiting combination of EKUs is 179 described in the security consideration section of this document. 181 2.1. Excluded EKUs procedure "Excluded EKU" is an EKU which the 182 relying party or the relying party software prohibits. Examples of 183 "Excluded EKU" are, presence of anyEKU or complete absence of EKU 184 extension on a certificate. If an EKU of the certificate meets the 185 conditions set by the "Excluded EKU" restriction, the relying party 186 or the relying party software rejects the certificate. 188 2.2. Permitted EKU procedure "Permitted EKU" is an EKU which the 189 relying party or the relying party software accepts. Examples of 190 "Permitted EKU" are, presence of this general document signing EKU 191 and/or protocol specific document signing-type EKUs. If an EKU of 192 the certificate meets the condition set by a "Permitted EKU" 193 restriction, the certificate is acceptable. Otherwise, relying party 194 or the relying party software rejects the certificate. 196 When a single software has capability to process various data 197 formats, the software may choose to make the excluded and permitted 198 decisions separately in accordance with the format it is handling 199 (e.g. text, pdf, etc). 201 5. Implications for a Certification Authority 203 The procedures and practices employed by a certification authority 204 MUST ensure that the correct values for the EKU extension are 205 inserted in each certificate that is issued. Unless certificates are 206 governed by a vendor specific PKI (or trust program), certificates 207 that indicate usage for document signing MAY include the id-kp- 208 documentSigning EKU extension. This does not encompass the mandatory 209 usage of the id-kp-documentSigning EKU in conjunction with the vendor 210 specific EKU. However, this does not restrict the CA from including 211 multiple EKUs related to document signing. 213 6. Security Considerations 215 The usage of id-kp-documentSigning EKU intends to prevent id-kp- 216 emailProtection from being used for none-email purposes and id-kp- 217 codeSigning used to sign objects other than binary codes. This EKU 218 does not introduce new security risks but instead reduces existing 219 security risks by providing means to separate other EKUs used for 220 communication protocols namely, TLS or S/MIME etc. in order to 221 minimize the risk of cross protocol attacks. 223 To reduce the risk of specific cross protocol attacks, the relying 224 party or relying party software may additionaly prohibit use of 225 specific combination of EKUs. 227 While a specific protocol or signing scheme may choose to come up 228 with their own EKU, some may not have significant motive or resource 229 to set up and manage thier own EKU. This general document signing 230 EKU may be used as a stop gap for those that intend to set up their 231 own EKU or those who do not intend to set up an EKU but still would 232 like to distinguish from other usage. 234 Introduction of this id-kp-documentSigning EKU value does not 235 introduce any new security or privacy concerns. 237 7. IANA Considerations 239 This document requests that IANA make two assignments. One for the 240 id-kp-documentSigning object identifier (OID), as defined in 241 Section 3.1, for the EKU from the "SMI Security for PKIX Extended Key 242 Purpose" (1.3.6.1.5.5.7.3) registry. Another for the id-mod-docsign- 243 eku, as defined in Appendix A, for the ASN.1 module [X.680] from the 244 in the "SMI Security for PKIX Module Identifier" (1.3.6.1.5.5.7.0) 245 registry. No further action is necessary by IANA. 247 8. Normative References 249 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 250 Requirement Levels", BCP 14, RFC 2119, 251 DOI 10.17487/RFC2119, March 1997, 252 . 254 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 255 Housley, R., and W. Polk, "Internet X.509 Public Key 256 Infrastructure Certificate and Certificate Revocation List 257 (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, 258 . 260 [RFC7299] Housley, R., "Object Identifier Registry for the PKIX 261 Working Group", RFC 7299, DOI 10.17487/RFC7299, July 2014, 262 . 264 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 265 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 266 May 2017, . 268 [RFC8358] Housley, R., "Update to Digital Signatures on Internet- 269 Draft Documents", RFC 8358, DOI 10.17487/RFC8358, March 270 2018, . 272 [X.680] ITU-T, "Information technology - Abstract Syntax Notation 273 One (ASN.1): Specification of basic notation", ISO/ 274 IEC 8824-1:2015, November 2015. 276 Acknowledgments 278 We would like to thank Russ Housley for verifying the ASN.1 module. 280 Appendix A. ASN.1 Module 282 The following ASN.1 module provides the complete definition of the 283 Document Signing EKU. 285 DocSignEKU { iso(1) identified-organization(3) dod(6) internet(1) 286 security(5) mechanisms(5) pkix(7) id-mod(0) id-mod-docsign-eku(TBD1) } 288 DEFINITIONS EXPLICIT TAGS ::= 290 BEGIN 292 -- EXPORTS ALL -- 294 -- IMPORTS NOTHING -- 296 -- OID Arc -- 298 id-kp OBJECT IDENTIFIER ::= { 299 iso(1) identified-organization(3) dod(6) internet(1) 300 security(5) mechanisms(5) pkix(7) kp(3) } 302 -- Document Signing Extended Key Usage -- 304 id-kp-documentSigning OBJECT IDENTIFIER ::= { id-kp TBD2 } 306 END 308 Authors' Addresses 310 Tadahiko Ito 311 SECOM CO., LTD. 313 Email: tadahiko.ito.public@gmail.com 315 Tomofumi Okubo 316 DigiCert, Inc. 318 Email: tomofumi.okubo+ietf@gmail.com 319 Sean Turner 320 sn3rd 322 Email: sean@sn3rd.com