ACME Working Group                                          R. Shoemaker
Internet-Draft                                                      ISRG
Intended status: Standards Track                         August 13, 2018
Expires: February 14, 2019

                   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

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on February 14, 2019.

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Table of Contents

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

1.  Introduction

   The Automatic Certificate Management Environment (ACME)
   [I-D.ietf-acme-acme] 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 alone.  In particular, this allows hosting providers,
   CDNs, and TLS-terminating load balancers to validate domain control
   without modifying the 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 incorrect.  Those incorrect assumptions weakened the
   security of those methods and are discussed in the "Design Rationale"

   This document specifies a new TLS-based challenge type, TLS-ALPN-01.
   This challenge requires negotiating a new application-layer protocol
   using the TLS Application-Layer Protocol Negotiation (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.

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2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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 to respond to specific connection
   attempts utilizing the ALPN extension with identifying information.
   The ACME server validates control of the domain name by connecting to
   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 ("=").  See [RFC4086] for additional
      information on randomness requirements.

   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 by non-ACME software.

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id-pe-acmeIdentifier OBJECT IDENTIFIER ::=  { id-pe 31 }

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.

   A client responds with an empty object ({}) to acknowledge that the
   challenge is ready to be validated by the server.  The base64url
   encoding of the protected headers and payload is described in
   [I-D.ietf-acme-acme] Section 6.1.

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

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

   On receiving a response the server constructs and stores the key
   authorization from the challenge "token" value and the current client
   account key.

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

   1.  Compute the expected SHA-256 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).

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   3.  Initiate a TLS connection with the chosen IP address, this
       connection MUST use TCP port 443.  The ClientHello that initiates
       the handshake MUST contain a ALPN extension with the 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 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 completed 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 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 to be served by User B or Host B to 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 agreeing to use the "acme-tls/1" protocol without actually
   understanding it.

   To further mitigate the risk of users claiming domain names used by
   other users on the same infrastructure hosting providers, CDNs, and

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   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

   [[RFC Editor: please replace XXXX below by the RFC number.]]

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 |
              | 31      | 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  |

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6.  Appendix: Design Rationale

   The TLS ALPN challenge exists to replace the TLS SNI challenge
   defined in the early ACME drafts.  This challenge was convenient for
   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 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 various service providers to
   illegitimately validate control of the DNS names of other users of
   the provider.  When the TLS SNI challenge was designed it was assumed
   that a user would only be able to 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 that User A wouldn't
   be able to 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 respond to SNI traffic
   for the SNI names used by the TLS SNI challenge 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
   that User A would be able to answer, proving 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 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-14 (work in progress),
              August 2018.

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   [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,

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,

   [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, <>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

Author's Address

   Roland Bracewell Shoemaker
   Internet Security Research Group


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