ACME Working Group                                          R. Shoemaker
Internet-Draft                                                      ISRG
Intended status: Standards Track                          March 02,                            May 30, 2018
Expires: September 3, December 1, 2018

                   ACME TLS ALPN Challenge Extension


   This document specifies a new challenge for the Automated Certificate
   Management Environment (ACME) protocol which allows for domain
   control validation using TLS.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 3, December 1, 2018.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2   3
   3.  TLS with Application Level Protocol Negotiation (TLS ALPN)
       Challenge . . . . . . . . . . . . . . . . . . . . . . . . . .   2   3
     3.1.  acme-tls/1 Protocol Definition  . . . . . . . . . . . . .   5
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5   6
     5.1.  SMI Security for PKIX Certificate Extension OID . . . . .   5   6
     5.2.  ALPN Protocol ID  . . . . . . . . . . . . . . . . . . . .   6
     5.3.  ACME Validation Method  . . . . . . . . . . . . . . . . .   5   6
   6.  Appendix: Design Rationale  . . . . . . . . . . . . . . . . .   6
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6   7
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .   6   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7   8

1.  Introduction

   The Automatic Certificate Management Environment (ACME)
   [I-D.ietf-acme-acme] specification doesn't specify a standard specifies methods for validating
   control of domain names via HTTP and DNS.  Deployment experience has
   shown it is also useful to be able to validate domain control using
   the TLS layer
   validation method which limits alone.  In particular, this allows hosting providers,
   CDNs, and TLS-terminating load balancers to validate domain control
   without modifying the points at which validation HTTP handling behavior of their backends.  This
   separation of layers can improve security and usability of ACME

   Early ACME drafts specified two TLS-based challenge types: TLS-SNI-01
   and TLS-SNI-02.  These methods were removed because they relied on
   assumptions about the deployed base of HTTPS hosting providers that
   proved to be
   performed. incorrect.  Those incorrect assumptions weakened the
   security of those methods and are discussed in the "Design Rationale"

   This document extends the ACME specification to include specifies a
   TLS based validation method that uses new TLS-based challenge type, TLS-ALPN-01.
   This challenge requires negotiating a new application-layer protocol
   using the Application Level TLS Application-Layer Protocol Negotiation extension. (ALPN) Extension
   [RFC7301].  Because no existing software implements this protocol,
   the ability to fulfill TLS-ALPN-01 challenges is effectively opt-in.
   A service provider must proactively deploy new code in order to
   implement TLS-ALPN-01, so we can specify stronger controls in that
   code, resulting in a stronger validation method.

2.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119

3.  TLS with Application Level Protocol Negotiation (TLS ALPN) Challenge

   The TLS with Application Level Protocol Negotiation (TLS ALPN)
   validation method proves control over a domain name by requiring the
   client to configure a TLS server referenced by the DNS A and/or AAAA
   Resource Records for the domain name to respond to specific connection
   attempts utilizing the ALPN extension [RFC7301]. with identifying information.
   The ACME server validates control of the domain name by connecting to the
   a TLS server at one of the addresses resolved for the domain name and
   verifying that a certificate with specific content is presented.

   type (required, string):  The string "tls-alpn-01"

   token (required, string):  A random value that uniquely identifies
      the challenge.  This value MUST have at least 128 bits of entropy.
      It MUST NOT contain any characters outside the base64url alphabet,
      including padding characters ("=").

   GET /acme/authz/1234/1 HTTP/1.1

   HTTP/1.1 200 OK
     "type": "tls-alpn-01",
     "url": "",
     "status": "pending",
     "token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA"

   The client prepares for validation by constructing a self-signed
   certificate which MUST contain a acmeValidation-v1 extension and a
   subjectAlternativeName extension [RFC5280].  The
   subjectAlternativeName extension MUST contain a single dNSName entry
   where the value is the domain name being validated.  The
   acmeValidation-v1 extension MUST contain the SHA-256 digest
   [FIPS180-4] of the key authorization [I-D.ietf-acme-acme] for the
   challenge.  The acmeValidation extension MUST be critical so that the
   certificate isn't inadvertently used to make trust decisions. by non-ACME software.

id-pe-acmeIdentifier OBJECT IDENTIFIER ::=  { id-pe 30 }

id-pe-acmeIdentifier-v1 OBJECT IDENTIFIER ::=  { id-pe-acmeIdentifier 1 }

acmeValidation-v1 ::= OCTET STRING (SIZE (32))

   Once this certificate has been created it MUST be provisioned such
   that it is returned during a TLS handshake that contains a ALPN
   extension containing the value "acme-tls/1" and a SNI extension
   containing the domain name being validated.

   When ready the

   A client acknowledges this by sending a POST message
   containing the key authorization, as defined in [I-D.ietf-acme-acme]
   section 8.1, responds with an empty object ({}) to acknowledge that the
   challenge URL.

   keyAuthorization (required, string):  The key authorization for this
      challenge.  This value MUST match the token from the challenge and is ready to be validated by the client's account key. server.

   POST /acme/authz/1234/1
   Content-Type: application/jose+json

     "protected": base64url({
       "alg": "ES256",
       "kid": "",
       "nonce": "JHb54aT_KTXBWQOzGYkt9A",
       "url": ""
     "payload": base64url({
       "keyAuthorization": "evaGxfADs...62jcerQ"
     }), base64url({}),
     "signature": "Q1bURgJoEslbD1c5...3pYdSMLio57mQNN4"

   On receiving this a response the server MUST verify that constructs and stores the key
   in the request matches from the challenge "token" value in the challenge and the
   client's current client
   account key.  If they do not match then the server MUST
   return a HTTP error in response to the POST request in which the
   client sent the challenge.

   The server then verifies the client's control over the domain by
   verifying that the TLS server was configured as expected using these the
   following steps:

   1.  Compute the expected SHA-256 [FIPS180-4] digest of the expected key

   2.  Resolve the domain name being validated and choose one of the IP
       addresses returned for validation (the server MAY validate
       against multiple addresses if more than one is returned, but this
       is not required).

   3.  Initiate a TLS connection with the domain name being validated, chosen IP address, this
       connection MUST be sent to use TCP port 443.  The ClientHello that initiates
       the handshake MUST contain a ALPN extension with the value single
       protocol name "acme-tls/1" and a Server Name Indication [RFC6066]
       extension containing the domain name being validated.


   4.  Verify that the ServerHello contains a ALPN extension containing
       the value "acme-tls/1" and that the certificate returned contains
       a subjectAltName extension containing the dNSName being validated
       and no other entries and a critical acmeValidation extension
       containing the digest computed in step 1.  The comparison of
       dNSNames MUST be case insensitive [RFC4343].  Note that as ACME
       doesn't support Unicode identifiers all dNSNames MUST be encoded
       using the [RFC3492] rules.

   If all of the above steps succeed then the validation is successful,
   otherwise it fails.  Once the TLS handshake has been completed the
   connection should MUST be immediately closed and no further data should be

3.1.  acme-tls/1 Protocol Definition

   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 completed the client MUST not NOT exchange any further data
   with the server and MUST immediately close the connection.

4.  Security Considerations

   The design of this challenges relies on some assumptions centered
   around how a server behaves during validation.

   The first assumption is that when a server is being used to serve
   content for multiple DNS names from a single IP address that it
   properly segregates control of those names to the users on the server that own
   them.  This means that if User A registers Host A and User B
   registers Host B the server should not allow a TLS request using a
   SNI value for Host A that only User A should to be able served by User B or Host B to serve that
   request. be served by
   User A.  If the server allows User B to serve this request it allows
   them to illegitimately validate control of Host A to the ACME server.

   The second assumption is that a server will not violate [RFC7301] by
   blindly agree agreeing to use the acme-tls/1 "acme-tls/1" protocol without actually knowing about
   understanding it.

   To further mitigate the protocol
   itself, which is a violation risk of [RFC7301]. users claiming domain names used by
   other users on the same infrastructure hosting providers, CDNs, and
   other service providers should not allow users to provide their own
   certificates for the TLS ALPN validation process.  If providers wish
   to implement TLS ALPN validation they SHOULD only generate
   certificates used for validation themselves and not expose this
   functionality to users.

5.  IANA Considerations

5.1.  SMI Security for PKIX Certificate Extension OID

   Within the SMI-numbers registry, the "SMI Security for PKIX
   Certificate Extension (" table is to be updated to
   add the following entry:

              | Decimal | Description          | References |
              | 30      | id-pe-acmeIdentifier | RFC XXXX   |

5.2.  ALPN Protocol ID

   Within the Transport Layer Security (TLS) Extensions registry, the
   "Application-Layer Protocol Negotiation (ALPN) Protocol IDs" table is
   to be updated to add the following entry:

   | Protocol   | Identification Sequence                  | Reference |
   | ACME-TLS/1 | 0x61 0x63 0x6d 0x65 0x2d 0x74 0x6c 0x73  | RFC XXXX  |
   |            | 0x2f 0x31 ("acme-tls/1")                 |           |

5.3.  ACME Validation Method

   The "ACME Validation Methods" registry is to be updated to include
   the following entry:

               | Label       | Identifier Type | Reference |
               | tls-alpn-01 | dns             | RFC XXXX  |

6.  Appendix: Design Rationale

   The TLS ALPN challenge exists to replace the TLS SNI challenge
   defined in the original early ACME document. drafts.  This challenge allowed
   validation of domain control purely within the TLS layer which
   provided convenience was convenient for server operators
   service providers who were either operating large TLS layer load
   balancing systems at which they wanted to perform validation or
   running servers fronting large numbers of DNS names from a single host.
   host as it allowed validation purely within the TLS layer.

   A security issue was discovered in the TLS SNI challenge by Frans
   Rosen which allowed users of certain various service providers to
   illegitimately validate control of the DNS names of other users, as long as those users were
   also using those service providers. of
   the provider.  When the TLS SNI challenge was designed it was assumed
   that a user would only be able to claim respond to TLS traffic via SNI for
   domain names they controlled (i.e. if User A registered Host A and
   User B registered Host B with a service provider they that User A wouldn't
   be able to
   claim respond to SNI traffic for Host B).  This turns out not to
   be a security property provided by a number of large service
   providers.  Because of this users were able to claim respond to SNI traffic
   for the non-valid SNI names used by the TLS SNI challenge used to signal what was being validated to the
   server. validation process.
   This meant that if User A and User B had registered Host A and Host B
   respectively User A would be able to claim the SNI name for a
   validation for Host B and when the validation connection was made to the shared IP address
   that User A would be able to answer, proving control. control of Host B.

7.  Acknowledgements

   The author would like to thank all those whom have provided design
   insights and editorial review of this document, including Richard
   Barnes, Ryan Hurst, Adam Langley, Ryan Sleevi, Jacob Hoffman-Andrews,
   Daniel McCarney, Marcin Walas, and Martin Thomson. Thomson and especially
   Frans Rosen who discovered the vulnerability in the TLS SNI method
   which necessitated the writing of this specication.

8.  Normative References

              Department of Commerce, National., "NIST FIPS 180-4,
              Secure Hash Standard", March 2012,

              Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
              Kasten, "Automatic Certificate Management Environment
              (ACME)", draft-ietf-acme-acme-09 draft-ietf-acme-acme-12 (work in progress),
              December 2017. April

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC3492]  Costello, A., "Punycode: A Bootstring encoding of Unicode
              for Internationalized Domain Names in Applications
              (IDNA)", RFC 3492, DOI 10.17487/RFC3492, March 2003,

   [RFC4343]  Eastlake 3rd, D., "Domain Name System (DNS) Case
              Insensitivity Clarification", RFC 4343,
              DOI 10.17487/RFC4343, January 2006,

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,

   [RFC7301]  Friedl, S., Popov, A., Langley, A., and E. Stephan,
              "Transport Layer Security (TLS) Application-Layer Protocol
              Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
              July 2014, <>.

Author's Address

   Roland Bracewell Shoemaker
   Internet Security Research Group