RFC8737: Automated Certificate Management Environment (ACME) TLS Application-Layer Protocol Negotiation (ALPN) Challenge Extension

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Internet Engineering Task Force (IETF)                    R.B. Shoemaker
Request for Comments: 8737                                          ISRG
Category: Standards Track                                  February 2020
ISSN: 2070-1721

        Automated Certificate Management Environment (ACME) TLS
   Application-Layer Protocol Negotiation (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 is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

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

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, Content
   Distribution Networks (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 pre-existing
   deployment base, the ability to complete tls-alpn-01 challenges
   requires changes by service providers, making it explicitly an opt-in
   process.  Because service providers must proactively deploy new code
   in order to implement tls-alpn-01, 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.

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 Section 5 of [RFC4648].  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
   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 Section 6.1 of [RFC8555].

   POST /acme/authz/1234/1
   Host: example.com
   Content-Type: application/jose+json

     "protected": base64url({
       "alg": "ES256",
       "kid": "https://example.com/acme/acct/1",
       "nonce": "JHb54aT_KTXBWQOzGYkt9A",
       "url": "https://example.com/acme/authz/1234/1"
     "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
       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 the rules of [RFC3492].

   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 an HTTPS server behaves during validation.

   The first assumption is that when an 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
   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
   Section 10.1 of [RFC8555].

6.  IANA Considerations

6.1.  SMI Security for PKIX Certificate Extension OID

   Within the "Structure of Management Information (SMI) Numbers (MIB
   Module Registrations)" registry, the following entry has been added
   to the "SMI Security for PKIX Certificate Extension"
   ( table.

              | Decimal | Description          | References |
              | 31      | id-pe-acmeIdentifier | RFC 8737   |

                                  Table 1

6.2.  ALPN Protocol ID

   Within the "Transport Layer Security (TLS) Extensions" registry, the
   following entry has been added to the "TLS Application-Layer Protocol
   Negotiation (ALPN) Protocol IDs" table.

      | Protocol   | Identification Sequence            | Reference |
      | acme-tls/1 | 0x61 0x63 0x6d 0x65 0x2d 0x74 0x6c | RFC 8737  |
      |            | 0x73 0x2f 0x31 ("acme-tls/1")      |           |

                                  Table 2

6.3.  ACME Validation Method

   Within the "Automated Certificate Management Environment (ACME)
   Protocol" registry, the following entry has been added to the "ACME
   Validation Methods" registry.

           | Label       | Identifier Type | ACME | Reference |
           | tls-alpn-01 | dns             | Y    | RFC 8737  |

                                 Table 3

7.  Normative References

              National Institute of Standards and Technology (NIST),
              "Secure Hash Standard (SHS)", FIPS PUB 180-4,
              DOI 10.6028/NIST.FIPS.180-4, August 2015,

   [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,

   [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, <https://www.rfc-editor.org/info/rfc7301>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [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]    ITU-T, "Information technology -- Abstract Syntax Notation
              One (ASN.1): Specification of basic notation", ITU-T
              Recommendation X.680, ISO/IEC 8824-1:2015, August 2015,

   [X.690]    ITU-T, "Information Technology -- ASN.1 encoding rules:
              Specification of Basic Encoding Rules (BER), Canonical
              Encoding Rules (CER) and Distinguished Encoding Rules
              (DER)", ITU-T Recommendation X.690, ISO/IEC 8825-1:2015,
              August 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 (1) 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 (2) when the
   validation connection was made, User A would be able to answer,
   thereby 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.


   The author would like to thank all those that 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.

Author's Address

   Roland Bracewell Shoemaker
   Internet Security Research Group

   Email: roland@letsencrypt.org