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(The document does seem to have the reference to RFC 2119 which the ID-Checklist requires). == Using lowercase 'not' together with uppercase 'MUST', 'SHALL', 'SHOULD', or 'RECOMMENDED' is not an accepted usage according to RFC 2119. Please use uppercase 'NOT' together with RFC 2119 keywords (if that is what you mean). Found 'MUST not' in this paragraph: The "acme-tls/1" protocol MUST only be used for validating ACME tls-alpn-01 challenges. The protocol consists of a TLS handshake in which the required validation information is transmitted. Once the handshake is complete the client MUST not exchange any further data with the server and MUST immediately close the connection. -- The document date (March 02, 2018) is 2247 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) -- Possible downref: Non-RFC (?) normative reference: ref. 'FIPS180-4' == Outdated reference: A later version (-18) exists of draft-ietf-acme-acme-09 Summary: 1 error (**), 0 flaws (~~), 4 warnings (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 ACME Working Group R. Shoemaker 3 Internet-Draft ISRG 4 Intended status: Standards Track March 02, 2018 5 Expires: September 3, 2018 7 ACME TLS ALPN Challenge Extension 8 draft-ietf-acme-tls-alpn-00 10 Abstract 12 This document specifies a new challenge for the Automated Certificate 13 Management Environment (ACME) protocol which allows for domain 14 control validation using TLS. 16 Status of This Memo 18 This Internet-Draft is submitted in full conformance with the 19 provisions of BCP 78 and BCP 79. 21 Internet-Drafts are working documents of the Internet Engineering 22 Task Force (IETF). Note that other groups may also distribute 23 working documents as Internet-Drafts. The list of current Internet- 24 Drafts is at https://datatracker.ietf.org/drafts/current/. 26 Internet-Drafts are draft documents valid for a maximum of six months 27 and may be updated, replaced, or obsoleted by other documents at any 28 time. It is inappropriate to use Internet-Drafts as reference 29 material or to cite them other than as "work in progress." 31 This Internet-Draft will expire on September 3, 2018. 33 Copyright Notice 35 Copyright (c) 2018 IETF Trust and the persons identified as the 36 document authors. All rights reserved. 38 This document is subject to BCP 78 and the IETF Trust's Legal 39 Provisions Relating to IETF Documents 40 (https://trustee.ietf.org/license-info) in effect on the date of 41 publication of this document. Please review these documents 42 carefully, as they describe your rights and restrictions with respect 43 to this document. Code Components extracted from this document must 44 include Simplified BSD License text as described in Section 4.e of 45 the Trust Legal Provisions and are provided without warranty as 46 described in the Simplified BSD License. 48 Table of Contents 50 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 51 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2 52 3. TLS with Application Level Protocol Negotiation (TLS ALPN) 53 Challenge . . . . . . . . . . . . . . . . . . . . . . . . . . 2 54 3.1. acme-tls/1 Protocol Definition . . . . . . . . . . . . . 5 55 4. Security Considerations . . . . . . . . . . . . . . . . . . . 5 56 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 57 5.1. SMI Security for PKIX Certificate Extension OID . . . . . 5 58 5.2. ACME Validation Method . . . . . . . . . . . . . . . . . 5 59 6. Appendix: Design Rationale . . . . . . . . . . . . . . . . . 6 60 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6 61 8. Normative References . . . . . . . . . . . . . . . . . . . . 6 62 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7 64 1. Introduction 66 The Automatic Certificate Management Environment (ACME) 67 [I-D.ietf-acme-acme] specification doesn't specify a TLS layer 68 validation method which limits the points at which validation can be 69 performed. This document extends the ACME specification to include a 70 TLS based validation method that uses the Application Level Protocol 71 Negotiation extension. 73 2. Terminology 75 In this document, the key words "MUST", "MUST NOT", "REQUIRED", 76 "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", 77 and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119 78 [RFC2119]. 80 3. TLS with Application Level Protocol Negotiation (TLS ALPN) Challenge 82 The TLS with Application Level Protocol Negotiation (TLS ALPN) 83 validation method proves control over a domain name by requiring the 84 client to configure a TLS server referenced by the DNS A and/or AAAA 85 Resource Records for the domain name to respond to specific 86 connection attempts utilizing the ALPN extension [RFC7301]. The 87 server validates control of the domain name by connecting to the TLS 88 server and verifying a certificate with specific content is 89 presented. 91 type (required, string): The string "tls-alpn-01" 93 token (required, string): A random value that uniquely identifies 94 the challenge. This value MUST have at least 128 bits of entropy. 96 It MUST NOT contain any characters outside the base64url alphabet, 97 including padding characters ("="). 99 GET /acme/authz/1234/1 HTTP/1.1 100 Host: example.com 102 HTTP/1.1 200 OK 103 { 104 "type": "tls-alpn-01", 105 "url": "https://example.com/acme/authz/1234/1", 106 "status": "pending", 107 "token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA" 108 } 110 The client prepares for validation by constructing a self-signed 111 certificate which MUST contain a acmeValidation-v1 extension and a 112 subjectAlternativeName extension [RFC5280]. The 113 subjectAlternativeName extension MUST contain a single dNSName entry 114 where the value is the domain name being validated. The 115 acmeValidation-v1 extension MUST contain the SHA-256 digest 116 [FIPS180-4] of the key authorization [I-D.ietf-acme-acme] for the 117 challenge. The acmeValidation extension MUST be critical so that the 118 certificate isn't inadvertently used to make trust decisions. 120 id-pe-acmeIdentifier OBJECT IDENTIFIER ::= { id-pe 30 } 122 id-pe-acmeIdentifier-v1 OBJECT IDENTIFIER ::= { id-pe-acmeIdentifier 1 } 124 acmeValidation-v1 ::= OCTET STRING (SIZE (32)) 126 Once this certificate has been created it MUST be provisioned such 127 that it is returned during a TLS handshake that contains a ALPN 128 extension containing the value "acme-tls/1" and a SNI extension 129 containing the domain name being validated. 131 When ready the client acknowledges this by sending a POST message 132 containing the key authorization, as defined in [I-D.ietf-acme-acme] 133 section 8.1, to the challenge URL. 135 keyAuthorization (required, string): The key authorization for this 136 challenge. This value MUST match the token from the challenge and 137 the client's account key. 139 POST /acme/authz/1234/1 140 Host: example.com 141 Content-Type: application/jose+json 143 { 144 "protected": base64url({ 145 "alg": "ES256", 146 "kid": "https://example.com/acme/acct/1", 147 "nonce": "JHb54aT_KTXBWQOzGYkt9A", 148 "url": "https://example.com/acme/authz/1234/1" 149 }), 150 "payload": base64url({ 151 "keyAuthorization": "evaGxfADs...62jcerQ" 152 }), 153 "signature": "Q1bURgJoEslbD1c5...3pYdSMLio57mQNN4" 154 } 156 On receiving this the server MUST verify that the key authorization 157 in the request matches the "token" value in the challenge and the 158 client's account key. If they do not match then the server MUST 159 return a HTTP error in response to the POST request in which the 160 client sent the challenge. 162 The server then verifies the client's control over the domain by 163 verifying that the TLS server was configured as expected using these 164 steps: 166 1. Compute the expected SHA-256 [FIPS180-4] digest of the expected 167 key authorization. 169 2. Initiate a TLS connection with the domain name being validated, 170 this connection MUST be sent to TCP port 443. The ClientHello 171 that initiates the handshake MUST contain a ALPN extension with 172 the value "acme-tls/1" and a Server Name Indication [RFC6066] 173 extension containing the domain name being validated. 175 3. Verify that the ServerHello contains a ALPN extension containing 176 the value "acme-tls/1" and that the certificate returned contains 177 a subjectAltName extension containing the dNSName being validated 178 and no other entries and a critical acmeValidation extension 179 containing the digest computed in step 1. The comparison of 180 dNSNames MUST be case insensitive [RFC4343]. Note that as ACME 181 doesn't support Unicode identifiers all dNSNames MUST be encoded 182 using the [RFC3492] rules. 184 If all of the above steps succeed then the validation is successful, 185 otherwise it fails. Once the handshake has been completed the 186 connection should be immediately closed and no further data should be 187 exchanged. 189 3.1. acme-tls/1 Protocol Definition 191 The "acme-tls/1" protocol MUST only be used for validating ACME tls- 192 alpn-01 challenges. The protocol consists of a TLS handshake in 193 which the required validation information is transmitted. Once the 194 handshake is complete the client MUST not exchange any further data 195 with the server and MUST immediately close the connection. 197 4. Security Considerations 199 The design of this challenges relies on some assumptions centered 200 around how a server behaves during validation. 202 The first assumption is that when a server is being used to serve 203 content for multiple DNS names from a single IP address that it 204 properly segregates control of those names to the users on the server 205 that own them. This means that if User A registers Host A and User B 206 registers Host B the server should not allow a TLS request using a 207 SNI value for Host A that only User A should be able to serve that 208 request. If the server allows User B to serve this request it allows 209 them to illegitimately validate control of Host A to the ACME server. 211 The second assumption is that a server will not blindly agree to use 212 the acme-tls/1 protocol without actually knowing about the protocol 213 itself, which is a violation of [RFC7301]. 215 5. IANA Considerations 217 5.1. SMI Security for PKIX Certificate Extension OID 219 Within the SMI-numbers registry, the "SMI Security for PKIX 220 Certificate Extension (1.3.6.1.5.5.7.1)" table is to be updated to 221 include the following entry: 223 +---------+----------------------+------------+ 224 | Decimal | Description | References | 225 +---------+----------------------+------------+ 226 | 30 | id-pe-acmeIdentifier | RFC XXXX | 227 +---------+----------------------+------------+ 229 5.2. ACME Validation Method 231 The "ACME Validation Methods" registry is to be updated to include 232 the following entry: 234 +-------------+-----------------+-----------+ 235 | Label | Identifier Type | Reference | 236 +-------------+-----------------+-----------+ 237 | tls-alpn-01 | dns | RFC XXXX | 238 +-------------+-----------------+-----------+ 240 6. Appendix: Design Rationale 242 The TLS ALPN challenge exists to replace the TLS SNI challenge 243 defined in the original ACME document. This challenge allowed 244 validation of domain control purely within the TLS layer which 245 provided convenience for server operators who were either operating 246 large TLS layer load balancing systems at which they wanted to 247 perform validation or running servers fronting large numbers of DNS 248 names from a single host. 250 A security issue was discovered in the TLS SNI challenge which 251 allowed users of certain service providers to illegitimately validate 252 control of the DNS names of other users, as long as those users were 253 also using those service providers. When the TLS SNI challenge was 254 designed it was assumed that a user would only be able to claim TLS 255 traffic via SNI for domain names they controlled (i.e. if User A 256 registered Host A with a service provider they wouldn't be able to 257 claim SNI traffic for Host B). This turns out not to be a security 258 property provided by a number of large service providers. Because of 259 this users were able to claim SNI traffic for the non-valid SNI names 260 the TLS SNI challenge used to signal what was being validated to the 261 server. This meant that if User A and User B had registered Host A 262 and Host B respectively User A would be able to claim the SNI name 263 for a validation for Host B and when the validation connection was 264 made to the shared IP address that User A would be able to answer, 265 proving control. 267 7. Acknowledgements 269 The author would like to thank all those whom have provided design 270 insights and editorial review of this document, including Richard 271 Barnes, Ryan Hurst, Adam Langley, Ryan Sleevi, Jacob Hoffman-Andrews, 272 Marcin Walas, and Martin Thomson. 274 8. Normative References 276 [FIPS180-4] 277 Department of Commerce, National., "NIST FIPS 180-4, 278 Secure Hash Standard", March 2012, 279 . 282 [I-D.ietf-acme-acme] 283 Barnes, R., Hoffman-Andrews, J., McCarney, D., and J. 284 Kasten, "Automatic Certificate Management Environment 285 (ACME)", draft-ietf-acme-acme-09 (work in progress), 286 December 2017. 288 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 289 Requirement Levels", BCP 14, RFC 2119, 290 DOI 10.17487/RFC2119, March 1997, 291 . 293 [RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode 294 for Internationalized Domain Names in Applications 295 (IDNA)", RFC 3492, DOI 10.17487/RFC3492, March 2003, 296 . 298 [RFC4343] Eastlake 3rd, D., "Domain Name System (DNS) Case 299 Insensitivity Clarification", RFC 4343, 300 DOI 10.17487/RFC4343, January 2006, 301 . 303 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 304 Housley, R., and W. Polk, "Internet X.509 Public Key 305 Infrastructure Certificate and Certificate Revocation List 306 (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, 307 . 309 [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) 310 Extensions: Extension Definitions", RFC 6066, 311 DOI 10.17487/RFC6066, January 2011, 312 . 314 [RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan, 315 "Transport Layer Security (TLS) Application-Layer Protocol 316 Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301, 317 July 2014, . 319 Author's Address 321 Roland Bracewell Shoemaker 322 Internet Security Research Group 324 Email: roland@letsencrypt.org