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ACME TLS ALPN Challenge Extension

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This is an older version of an Internet-Draft that was ultimately published as RFC 8737.
Author Roland Bracewell Shoemaker
Last updated 2018-03-02
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ACME Working Group                                          R. Shoemaker
Internet-Draft                                                      ISRG
Intended status: Standards Track                          March 02, 2018
Expires: September 3, 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, 2018.

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   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
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   described in the Simplified BSD License.

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Table of Contents

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

1.  Introduction

   The Automatic Certificate Management Environment (ACME)
   [I-D.ietf-acme-acme] specification doesn't specify a TLS layer
   validation method which limits the points at which validation can be
   performed.  This document extends the ACME specification to include a
   TLS based validation method that uses the Application Level Protocol
   Negotiation extension.

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].  The
   server validates control of the domain name by connecting to the TLS
   server and verifying a certificate with specific content is

   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.

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      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.

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 client acknowledges this by sending a POST message
   containing the key authorization, as defined in [I-D.ietf-acme-acme]
   section 8.1, to the challenge URL.

   keyAuthorization (required, string):  The key authorization for this
      challenge.  This value MUST match the token from the challenge and
      the client's account key.

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

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

   On receiving this the server MUST verify that the key authorization
   in the request matches the "token" value in the challenge and the
   client's 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

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

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

   3.  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 handshake has been completed the

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   connection should 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 the client MUST 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 be able to serve that
   request.  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 blindly agree to use
   the acme-tls/1 protocol without actually knowing about the protocol
   itself, which is a violation of [RFC7301].

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
   include the following entry:

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

5.2.  ACME Validation Method

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

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               | 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 ACME document.  This challenge allowed
   validation of domain control purely within the TLS layer which
   provided convenience for server operators 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.

   A security issue was discovered in the TLS SNI challenge which
   allowed users of certain service providers to illegitimately validate
   control of the DNS names of other users, as long as those users were
   also using those service providers.  When the TLS SNI challenge was
   designed it was assumed that a user would only be able to claim TLS
   traffic via SNI for domain names they controlled (i.e. if User A
   registered Host A with a service provider they wouldn't be able to
   claim 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 SNI traffic for the non-valid SNI names
   the TLS SNI challenge used to signal what was being validated to the
   server.  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.

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,
   Marcin Walas, and Martin Thomson.

8.  Normative References

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

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

   [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


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