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Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document date (June 2, 2017) is 1807 days in the past. Is this intentional? Checking references for intended status: Informational ---------------------------------------------------------------------------- == Missing Reference: 'FIPS180-4' is mentioned on line 264, but not defined ** Obsolete normative reference: RFC 3501 (Obsoleted by RFC 9051) ** Obsolete normative reference: RFC 5751 (Obsoleted by RFC 8551) == Outdated reference: draft-ietf-acme-acme has been published as RFC 8555 Summary: 3 errors (**), 0 flaws (~~), 4 warnings (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group A. Melnikov 3 Internet-Draft Isode Ltd 4 Intended status: Informational June 2, 2017 5 Expires: December 4, 2017 7 Extensions to Automatic Certificate Management Environment for email TLS 8 and S/MIME 9 draft-melnikov-acme-email-tls-smime-00 11 Abstract 13 This document specifies identifiers and challenges required to enable 14 the Automated Certificate Management Environment (ACME) to issue 15 certificates for use by TLS email services and for email recipients 16 that want to use S/MIME. 18 Status of This Memo 20 This Internet-Draft is submitted in full conformance with the 21 provisions of BCP 78 and BCP 79. 23 Internet-Drafts are working documents of the Internet Engineering 24 Task Force (IETF). Note that other groups may also distribute 25 working documents as Internet-Drafts. The list of current Internet- 26 Drafts is at http://datatracker.ietf.org/drafts/current/. 28 Internet-Drafts are draft documents valid for a maximum of six months 29 and may be updated, replaced, or obsoleted by other documents at any 30 time. It is inappropriate to use Internet-Drafts as reference 31 material or to cite them other than as "work in progress." 33 This Internet-Draft will expire on December 4, 2017. 35 Copyright Notice 37 Copyright (c) 2017 IETF Trust and the persons identified as the 38 document authors. All rights reserved. 40 This document is subject to BCP 78 and the IETF Trust's Legal 41 Provisions Relating to IETF Documents 42 (http://trustee.ietf.org/license-info) in effect on the date of 43 publication of this document. Please review these documents 44 carefully, as they describe your rights and restrictions with respect 45 to this document. Code Components extracted from this document must 46 include Simplified BSD License text as described in Section 4.e of 47 the Trust Legal Provisions and are provided without warranty as 48 described in the Simplified BSD License. 50 Table of Contents 52 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 53 2. Conventions Used in This Document . . . . . . . . . . . . . . 2 54 3. Use of ACME for use by TLS-protected SMTP and IMAP services . 2 55 3.1. "service" JWS header parameter . . . . . . . . . . . . . 3 56 3.2. "port" JWS header parameter . . . . . . . . . . . . . . . 4 57 3.3. TLS with Server Name Indication (SNI) challenge for email 58 services . . . . . . . . . . . . . . . . . . . . . . . . 4 59 3.4. DNS challenge for email services . . . . . . . . . . . . 4 60 3.5. CAPABILITY challenge for email services . . . . . . . . . 5 61 4. Use of ACME for issuing end user S/MIME certificates . . . . 6 62 5. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 6 63 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 64 7. Security Considerations . . . . . . . . . . . . . . . . . . . 7 65 8. Normative References . . . . . . . . . . . . . . . . . . . . 7 66 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8 68 1. Introduction 70 [I-D.ietf-acme-acme] is a mechanism for automating certificate 71 management on the Internet. It enables administrative entities to 72 prove effective control over resources like domain names, and 73 automates the process of generating and issuing certificates. 75 This document describes extensions to ACME for use by email services. 76 Section 3 defines extensions for how email services (such as SMTP, 77 IMAP) can get certificates for use with TLS. Section 4 defines 78 extensions for issuing end user S/MIME [RFC5751] certificates. 80 2. Conventions Used in This Document 82 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 83 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 84 document are to be interpreted as described in [RFC2119]. 86 3. Use of ACME for use by TLS-protected SMTP and IMAP services 88 SMTP [RFC5321] (including SMTP submission) and IMAP [RFC3501] servers 89 use TLS to provide server identity authentication, data 90 confidentiality and integrity services. Such TLS protected email 91 services either use STARTTLS command or run on a separate TLS- 92 protected port. 94 [I-D.ietf-acme-acme] defines several challenge types that can be 95 extended for use by email services. This document also defines some 96 new challenge types specific to SMTP and IMAP. 98 In order to use these challenges JWS [RFC7515] object used by 99 [I-D.ietf-acme-acme] is extended. The following extra requirements 100 are in addition to requirements on JWS objects sent in ACME defined 101 in Section 6.2 of [I-D.ietf-acme-acme]: 103 1. "service" JWS header parameter MUST be included. See Section 3.1 104 for more details. 106 2. "port" JWS header parameter MUST (SHOULD?) be included. See 107 Section 3.2 for more details. 109 For example, if the client were to respond to the "tls-sni-email-00" 110 challenge, it would send the following request: 112 POST /acme/authz/asdf/0 HTTP/1.1 113 Host: example.com 114 Content-Type: application/jose+json 116 { 117 "protected": base64url({ 118 "alg": "ES256", 119 "kid": "https://example.com/acme/acct/1", 120 "nonce": "Q_s3MWoqT05TrdkM2MTDcw", 121 "url": "https://example.com/acme/authz/asdf/0", 122 "service": "smtp", 123 "port": 25 124 }), 125 "payload": base64url({ 126 "type": "tls-sni-email-00", 127 "keyAuthorization": "IlirfxKKXA...vb29HhjjLPSggQiE" 128 }), 129 "signature": "7cbg5JO1Gf5YLjjF...SpkUfcdPai9uVYYU" 130 } 132 Figure 1 134 3.1. "service" JWS header parameter 136 The "service" JWS header parameter specifies the service for which 137 TLS server certificate should be issued. Valid values come from 138 "Service Names and Transport Protocol Port Numbers" IANA registry 139 . ACME server MAY include SRV-ID [RFC6125] 141 subjectAltNames in issued certificates. 143 3.2. "port" JWS header parameter 145 The "port" JWS header parameter specifies the TCP port number where 146 the corresponding service is running. 148 [[This parameter might have applicability beyond email services.]] 150 3.3. TLS with Server Name Indication (SNI) challenge for email services 152 "tls-sni-email-00" is very similar to "tls-sni-01" defined in 153 Section 8.3 of [I-D.ietf-acme-acme]. 155 The difference between processing of "tls-sni-email-00" and "tls-sni- 156 01" are listed below: 158 1. SAN A MUST be constructed as follows: compute the SHA-256 digest 159 [FIPS180-4] of the challenge token and encode it in lowercase 160 hexadecimal form. The dNSName is "...acme.invalid", 161 where is the first half of the hexadecimal representation and 162 is the second half, and was generated by the ACME 163 server. SAN B MUST be constructed as follows: compute the 164 SHA-256 digest of the key authorization and encode it in 165 lowercase hexadecimal form. The dNSName is 166 "...acme.invalid" where is the first half of the 167 hexadecimal representation and is the second half, and 168 is the key authorization. [[OPEN ISSUE: Should service name and 169 port number be incorporated into SAN A and B?]] 171 2. When verifying the client's control of the domain/service, ACME 172 server connects to port as specified in "port" JWS header 173 parameter (Section 3.2), instead of port 443. When connecting to 174 ports 25, 143 and 587, ACME server needs to use STARTTLS command. 175 When connecting to ports 465 or 993, ACME server initiate TLS 176 negotiation immediately upon connection to the corresponding 177 ports. In all cases ACME server presents SAN A in the SNI field, 178 constructed as specified above. 180 3.4. DNS challenge for email services 182 "dns-email-00" is very similar to "dns-01" defined in Section 8.4 of 183 [I-D.ietf-acme-acme]. 185 The difference between processing of "dns-email-00" and "dns-01" are 186 listed below: 188 1. The TXT record used to validate this challenge is 189 _.__acme-challenge.. For example, for 190 domain "example.com" and IMAP service running on port 993, the 191 TXT record name is _993._imaps._acme-challenge.example.com. For 192 domain "example.net" and IMAP service running on port 143, the 193 TXT record name is _143._imap._acme-challenge.example.next. 195 2. [[OPEN ISSUE: Should service name and port number be incorporated 196 into the hash?]] 198 3.5. CAPABILITY challenge for email services 200 For "capability-smtp-00" challenge, ACME client (== SMTP server) 201 constructs a key authorization from the "token" value provided in the 202 challenge and the client's account key. The client then computes the 203 SHA-256 digest [FIPS180-4] of the key authorization. SMTP server 204 than returns the base64url encoding of this digest as a value of the 205 "ACME" EHLO capability: 207 250-smtp.example.com 208 250-SIZE 209 250-8BITMIME 210 250-BINARYMIME 211 250-PIPELINING 212 250-HELP 213 250-DSN 214 250-CHUNKING 215 250-AUTH SCRAM-SHA-1 216 250-AUTH=SCRAM-SHA-1 217 250-STARTTLS 218 250-ACME gfj9Xq...Rg85nM 219 250-MT-PRIORITY 220 250 ENHANCEDSTATUSCODES 222 Figure 2 224 Similarly, "capability-imap-00" challenge, ACME client (== IMAP 225 server) constructs a key authorization from the "token" value 226 provided in the challenge and the client's account key. The client 227 then computes the SHA-256 digest [FIPS180-4] of the key 228 authorization. SMTP server than returns the base64url encoding of 229 this digest as a value of the "ACME" capability: 231 * OK [CAPABILITY IMAP4rev1 LOGINDISABLED LITERAL+ ENABLE STARTTLS ACME=gfj9Xq...Rg85nM] Example IMAP4rev1 server ready 233 or 235 * CAPABILITY IMAP4rev1 LOGINDISABLED LITERAL+ ENABLE STARTTLS ACME=gfj9Xq...Rg85nM 237 Figure 3 239 4. Use of ACME for issuing end user S/MIME certificates 241 [I-D.ietf-acme-acme] defines "dns" Identifier Type that is used to 242 verify that a particular entity has control over a domain or specific 243 service associated with the domain. In order to be able to issue 244 end-user S/MIME certificates, ACME needs a new Identifier Type that 245 proves ownership of an email address. 247 This document defines a new Identifier Type "email" which corresponds 248 to an (all ASCII) email address [RFC5321]. This can be used with 249 S/MIME or other similar service that requires posession of a 250 certificate tied to an email address. 252 A new challenge type "email-reply-00" is used with "email" Identifier 253 Type, which provides proof that an ACME client has control over an 254 email address: [[Very rough outline follows]] 256 1. ACME server generates an email message with the subject 257 containing "ACME ", where is the 258 base64url encoded first part of the token, which contains at 259 least 64 bit of entropy. The second part of the token (token- 260 part2, which also contains at least 64 bit of entropy) is 261 returned over HTTPS to the ACME client. ACME client concatenates 262 "token-part1" and "token-part2" to create "token", calculates 263 key-authz (as per Section 8.1 of [I-D.ietf-acme-acme]), then 264 included the base64url encoded SHA-256 digest [FIPS180-4] of the 265 key authorization in a response email message. The response 266 email message has a single text/plain MIME body part. [[Do we 267 need to handle text/html or multipart/alternative? Simplicity 268 suggests "no".]] 270 [[Do we need a proof that ACME client can submit email on behalf of 271 the user, not just read the challenge using IMAP?]] 273 5. Open Issues 275 [[This section should be empty before publication]] 277 1. One possible alternative for issuing TLS certificates for email 278 services is to define a new Identifier Type that specifies 279 service@domain. The current version of the document just reuses 280 "dns". 282 6. IANA Considerations 284 IANA is requested to register the following ACME challenge types that 285 are used with Identifier Type "dns": "tls-sni-email", "dns-email", 286 "capability-smtp" and "capability-imap". The reference for all of 287 them is this document. 289 IANA is requested to register a new Identifier Type "email" which 290 corresponds to an (all ASCII) email address [RFC5321]. 292 And finally, IANA is requested to register the following ACME 293 challenge types that are used with Identifier Type "email": "email- 294 reply". The reference for it is this document. 296 7. Security Considerations 298 TBD. 300 8. Normative References 302 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 303 Requirement Levels", BCP 14, RFC 2119, 304 DOI 10.17487/RFC2119, March 1997, 305 . 307 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 308 DOI 10.17487/RFC5321, October 2008, 309 . 311 [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 312 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, 313 . 315 [RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet 316 Mail Extensions (S/MIME) Version 3.2 Message 317 Specification", RFC 5751, DOI 10.17487/RFC5751, January 318 2010, . 320 [RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web 321 Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May 322 2015, . 324 [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and 325 Verification of Domain-Based Application Service Identity 326 within Internet Public Key Infrastructure Using X.509 327 (PKIX) Certificates in the Context of Transport Layer 328 Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March 329 2011, . 331 [I-D.ietf-acme-acme] 332 Barnes, R., Hoffman-Andrews, J., and J. Kasten, "Automatic 333 Certificate Management Environment (ACME)", draft-ietf- 334 acme-acme-06 (work in progress), March 2017. 336 Author's Address 338 Alexey Melnikov 339 Isode Ltd 340 14 Castle Mews 341 Hampton, Middlesex TW12 2NP 342 UK 344 EMail: Alexey.Melnikov@isode.com