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Automated Certificate Management Environment (ACME) TLS Application-Layer Protocol Negotiation (ALPN) Challenge Extension

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 8737.
Author Roland Bracewell Shoemaker
Last updated 2020-05-09 (Latest revision 2019-10-01)
Replaces draft-shoemaker-acme-tls-alpn
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Daniel McCarney
Shepherd write-up Show Last changed 2018-08-27
IESG IESG state Became RFC 8737 (Proposed Standard)
Action Holders
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Roman Danyliw
Send notices to Daniel McCarney <>
IANA IANA review state IANA - Not OK
IANA action state RFC-Ed-Ack
IANA expert review state Reviews assigned
IANA expert review comments Waiting for ACME review and at least one more TLS approval.
ACME Working Group                                          R. Shoemaker
Internet-Draft                                                      ISRG
Intended status: Standards Track                        October 01, 2019
Expires: April 3, 2020

                   ACME TLS ALPN Challenge Extension


   This document specifies a new challenge for the Automated Certificate
   Management Environment (ACME) protocol that 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 April 3, 2020.

Copyright Notice

   Copyright (c) 2019 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.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  TLS with Application Layer Protocol Negotiation (TLS ALPN)
       Challenge . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  acme-tls/1 Protocol Definition  . . . . . . . . . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  SMI Security for PKIX Certificate Extension OID . . . . .   6
     6.2.  ALPN Protocol ID  . . . . . . . . . . . . . . . . . . . .   6
     6.3.  ACME Validation Method  . . . . . . . . . . . . . . . . .   7
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .   7
   Appendix A.  Design Rationale . . . . . . . . . . . . . . . . . .   9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   The Automatic Certificate Management Environment (ACME) [RFC8555]
   specification describes 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 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 this protocol does not build on a preexisting
   deployment base, 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

   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.

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

   The TLS with Application Layer Protocol Negotiation (TLS ALPN)
   validation method proves control over a domain name by requiring the
   ACME client to configure a TLS server to respond to specific
   connection attempts using 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

   The tls-alpn-01 ACME challenge object has the following format:

   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
      as described in [RFC4648] Section 5.  Trailing '=' padding
      characters MUST be stripped.  See [RFC4086] for additional
      information on randomness requirements.

   The client prepares for validation by constructing a self-signed
   certificate that MUST contain an acmeIdentifier 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
   acmeIdentifier extension MUST contain the SHA-256 digest [FIPS180-4]
   of the key authorization [RFC8555] for the challenge.  The
   acmeIdentifier extension MUST be critical so that the certificate
   isn't inadvertently used by non-ACME software.

   The acmeIdentifier extension is identified by the id-pe-
   acmeIdentifier object identifier (OID) in the id-pe arc [RFC5280]:

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

   The extension has the following ASN.1 [X.680] format :

   Authorization ::= OCTET STRING (SIZE (32))

   The extnValue of the id-pe-acmeIdentifier extension is the ASN.1 DER
   encoding [X.690] of the Authorization structure, which contains the
   SHA-256 digest of the key authorization for the challenge.

   Once this certificate has been created it MUST be provisioned such
   that it is returned during a TLS handshake where the "acme-tls/1"
   application-layer protocol has been negotiated and a Server Name

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   Indication (SNI) extension [RFC6066] has been provided containing the
   domain name being validated.

   A client responds by POSTing an empty JSON object ({}) to the
   challenge URL 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 [RFC8555] 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 this request from a client 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.  The ACME server computes the expected SHA-256 digest of the key

   2.  The ACME server resolves the domain name being validated and
       chooses one of the IP addresses returned for validation (the
       server MAY validate against multiple addresses if more than one
       is returned).

   3.  The ACME server initiates a TLS connection to the chosen IP
       address.  This connection MUST use TCP port 443.  The ACME server
       MUST provide an ALPN extension with the single protocol name
       "acme-tls/1" and an SNI extension containing only the domain name
       being validated during the TLS handshake.

   4.  The ACME server verifies that during the TLS handshake the
       application-layer protocol "acme-tls/1" was successfully

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       negotiated (and that the ALPN extension contained only the value
       "acme-tls/1") and that the certificate returned contains:

       *  a subjectAltName extension containing the dNSName being
          validated and no other entries

       *  a critical acmeIdentifier extension containing the expected
          SHA-256 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 [RFC3492] rules.

   If all of the above steps succeed then the validation is successful,
   otherwise it fails.

4.  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.  The "acme-
   tls/1" protocol does not carry application data, once the handshake
   is completed the client MUST NOT exchange any further data with the
   server and MUST immediately close the connection.  While this
   protocol uses X.509 certificates, it does not use the authentication
   method described in [RFC5280] and as such does not require a valid
   signature on the provided certificate nor require the TLS handshake
   to complete successfully.  An ACME server may wish to use an off the
   shelf TLS stack where it is not simple to allow these divergences in
   the protocol as defined.  Because of this, an ACME server MAY choose
   to withhold authorization if either the certificate signature is
   invalid or the handshake doesn't fully complete.

   ACME servers that implement "acme-tls/1" MUST only negotiate TLS 1.2
   [RFC5246] or higher when connecting to clients for validation.

5.  Security Considerations

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

   The first assumption is that when a HTTPS server is being used to
   serve content for multiple DNS names from a single IP address 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 HTTPS server should not allow a TLS request
   using an SNI value for Host A to be served by User B or a TLS
   connection with a server_name extension identifying Host B to be

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   answered by User A.  If the HTTPS 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
   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.

   The extensions to the ACME protocol described in this document build
   upon the Security Considerations and threat model defined in
   [RFC8555] Section 10.1.

6.  IANA Considerations

   [[RFC Editor: please replace I-D.ietf-acme-tls-alpn below by the RFC

6.1.  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 | I-D.ietf-acme-tls-alpn |

6.2.  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:

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   | Protocol   | Identification Sequence     | Reference              |
   | acme-tls/1 | 0x61 0x63 0x6d 0x65 0x2d    | I-D.ietf-acme-tls-alpn |
   |            | 0x74 0x6c 0x73 0x2f 0x31    |                        |
   |            | ("acme-tls/1")              |                        |

6.3.  ACME Validation Method

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

     | Label       | Identifier Type | ACME | Reference              |
     | tls-alpn-01 | dns             | Y    | I-D.ietf-acme-tls-alpn |

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, Martin Thomson and especially Frans
   Rosen, who discovered the vulnerability in the TLS SNI method that
   necessitated the writing of this specification.

8.  Normative References

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

   [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,

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   [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,

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,

   [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, <>.

   [RFC8555]  Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
              Kasten, "Automatic Certificate Management Environment
              (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,

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   [X.680]    International Telecommunication Union, ., "Information
              technology -- Abstract Syntax Notation One (ASN.1):
              Specification of basic notation", 2015,

   [X.690]    International Telecommunication Union, ., "Information
              Technology -- ASN.1 encoding rules: Specification of Basic
              Encoding Rules (BER), Canonical Encoding Rules (CER) and
              Distinguished Encoding Rules (DER)", 2015,

Appendix A.  Design Rationale

   The TLS ALPN challenge exists to iterate on the TLS SNI challenge
   defined in the early ACME drafts.  The TLS SNI 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.  The value provided by the TLS SNI challenge was considered
   large enough that this document was written in order to provide a new
   challenge type that addressed the existing security concerns.

   A security issue in the TLS SNI challenge was discovered 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 had registered with a service provider (i.e., if a
   user registered 'a.example' they would only be able to respond to SNI
   requests for 'a.example' and not for SNI requests for 'b.example').
   It turns out that a number of large service providers do not honor
   this property.  Because of this, users were able to respond to SNI
   requests for the 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
   constructed SNI challenge name for Host B and when the validation
   connection was made that User A would be able to answer, proving
   'control' of Host B.  As the SNI name used was a subdomain of the
   domain name being validated, rather than the domain name itself, it
   was likely to not already be registered with the service provider for
   traffic routing, making it much easier for a hijack to occur.

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Author's Address

   Roland Bracewell Shoemaker
   Internet Security Research Group


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