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ACME TLS ALPN Challenge Extension

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 8737.
Author Roland Bracewell Shoemaker
Last updated 2019-09-28 (Latest revision 2019-09-05)
Replaces draft-shoemaker-acme-tls-alpn
RFC stream Internet Engineering Task Force (IETF)
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 IESG Evaluation
Consensus boilerplate Yes
Telechat date (None)
Needs 7 more YES or NO OBJECTION positions to pass.
Responsible AD Roman Danyliw
Send notices to Daniel McCarney <>
IANA IANA review state IANA - Not OK
IANA expert review state Reviews assigned
ACME Working Group                                          R. Shoemaker
Internet-Draft                                                      ISRG
Intended status: Standards Track                      September 05, 2019
Expires: March 8, 2020

                   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 March 8, 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
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   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 . . . . . . . . . . . . . . . . . . . . . . . . .   3
   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  . . . . . . . . . . . . . . . . .   6
   7.  Appendix: Design Rationale  . . . . . . . . . . . . . . . . .   7
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   9.  Normative References  . . . . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   The Automatic Certificate Management Environment (ACME) [RFC8555]
   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 validation.

   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 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
   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.

   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 which MUST contain a 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))

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   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
   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 AMCE server initiates a TLS connection to the chosen IP
       address, this connection MUST use TCP port 443.  The ACME server

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       MUST provide a ALPN extension with the single protocol name
       "acme-tls/1" and a 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
       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.  Once the
   handshake is completed the client MUST NOT exchange any further data
   with the server and MUST immediately close the connection.

5.  Security Considerations

   The design of this challenge 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.

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   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 XXXX below by the RFC number.]]

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 | RFC XXXX   |

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:

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

6.3.  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  |

7.  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 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.

8.  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 which
   necessitated the writing of this specification.

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9.  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,

   [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,

   [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, <>.

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   [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,

   [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,

Author's Address

   Roland Bracewell Shoemaker
   Internet Security Research Group


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