Automated Certificate Management Environment (ACME) Extensions for ".onion" Special-Use Domain Names
draft-ietf-acme-onion-02

Automated Certificate Management Environment              Q. Misell, Ed.
Internet-Draft                                                  AS207960
Intended status: Standards Track                        27 February 2024
Expires: 30 August 2024

   Automated Certificate Management Environment (ACME) Extensions for
                   ".onion" Special-Use Domain Names
                        draft-ietf-acme-onion-02

Abstract

   The document defines extensions to the Automated Certificate
   Management Environment (ACME) to allow for the automatic issuance of
   certificates to Tor hidden services (".onion" Special-Use Domain
   Names).

Discussion

   This note is to be removed before publishing as an RFC.

   Source for this draft and an issue tracker can be found at
   https://github.com/AS207960/acme-onion.

   The project website and a reference implementation can be found at
   https://acmeforonions.org.

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
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   This Internet-Draft will expire on 30 August 2024.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Identifier  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Identifier Validation Challenges  . . . . . . . . . . . . . .   4
     3.1.  Existing challenges . . . . . . . . . . . . . . . . . . .   4
       3.1.1.  Existing "dns-01" Challenge . . . . . . . . . . . . .   4
       3.1.2.  Existing "http-01" Challenge  . . . . . . . . . . . .   4
       3.1.3.  Existing "tls-alpn-01" Challenge  . . . . . . . . . .   4
     3.2.  New "onion-csr-01" Challenge  . . . . . . . . . . . . . .   4
   4.  Client authentication to hidden services  . . . . . . . . . .   7
   5.  ACME over hidden services . . . . . . . . . . . . . . . . . .   8
   6.  Certification Authority Authorization (CAA) . . . . . . . . .   8
     6.1.  Relevant Resource Record Set  . . . . . . . . . . . . . .   9
     6.2.  When to check CAA . . . . . . . . . . . . . . . . . . . .  10
     6.3.  Preventing mis-issuance by unknown CAs  . . . . . . . . .  10
     6.4.  Alternative in-band presentation of CAA . . . . . . . . .  10
       6.4.1.  CAs requiring in-band CAA . . . . . . . . . . . . . .  11
       6.4.2.  Example in-band CAA . . . . . . . . . . . . . . . . .  12
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
     7.1.  Validation Methods  . . . . . . . . . . . . . . . . . . .  13
     7.2.  Error Types . . . . . . . . . . . . . . . . . . . . . . .  13
     7.3.  Directory Metadata Fields . . . . . . . . . . . . . . . .  14
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
     8.1.  Use of "dns" identifier type  . . . . . . . . . . . . . .  14
       8.1.1.  "http-01" Challenge . . . . . . . . . . . . . . . . .  14
       8.1.2.  "tls-alpn-01" Challenge . . . . . . . . . . . . . . .  15
       8.1.3.  "dns-01" Challenge  . . . . . . . . . . . . . . . . .  15
     8.2.  Key Authorization with "onion-csr-01" . . . . . . . . . .  15
     8.3.  Use of Tor for non ".onion" domains . . . . . . . . . . .  15
     8.4.  Security of CAA records . . . . . . . . . . . . . . . . .  15
     8.5.  In-band CAA . . . . . . . . . . . . . . . . . . . . . . .  15
     8.6.  Access of the Tor network . . . . . . . . . . . . . . . .  16
     8.7.  Anonymity of the ACME client  . . . . . . . . . . . . . .  16
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  16
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  17

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   Appendix A.  Discussion on the use of the "dns" identifier
           type  . . . . . . . . . . . . . . . . . . . . . . . . . .  18
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  18
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   The Tor network has the ability to host "Onion Services"
   [tor-rend-spec-v3] [tor-address-spec] only accessible via the Tor
   network.  These use the ".onion" Special-Use Domain Name [RFC7686] to
   identify these services.  These can be used as any other domain name
   could, but do not form part of the DNS infrastructure.

   The Automated Certificate Management Environment (ACME) [RFC8555]
   defines challenges for validating control of DNS identifiers, and
   whilst a ".onion" Special-Use Domain Name may appear as a DNS name,
   it requires special consideration to validate control of one such
   that ACME could be used on ".onion" Special-Use Domain Names.

   In order to allow ACME to be utilised to issue certificates to
   ".onion" Special-Use Domain Names this document specifies challenges
   suitable to validate control of these Special-Use Domain Names.
   Additionally this document defines an alternative to the DNS
   Certification Authority Authorization (CAA) Resource Record [RFC8659]
   that can be used with ".onion" Special-Use Domain Names.

1.1.  Requirements Language

   The key words MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
   SHOULD NOT, RECOMMENDED, NOT RECOMMENDED, MAY, and OPTIONAL in this
   document are to be interpreted as described in [BCP14] when, and only
   when, they appear in all capitals, as shown here.

2.  Identifier

   [RFC8555] defines the "dns" identifier type.  This identifier type
   MUST be used when requesting a certificate for a ".onion" Special-Use
   Domain Name.  The value of identifier MUST be the textual
   representation as defined in [tor-address-spec] §3.  The value MAY
   include subdomain labels.  Version 2 addresses MUST NOT be used as
   these are now considered insecure.

   Example identifiers:

   {
     "type": "dns",
     "value": "bbcweb3hytmzhn5d532owbu6oqadra5z3ar726vq5kgwwn6aucdccrad.onion"
   }

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   {
     "type": "dns",
     "value": "www.bbcweb3hytmzhn5d532owbu6oqadra5z3ar726vq5kgwwn6aucdccrad.onion"
   }

3.  Identifier Validation Challenges

   The CA/Browser Forum Baseline Requirements [cabf-br] §B.2 define
   methods accepted by the CA industry for validation of ".onion"
   Special-Use Domain Names.  This document incorporates these methods
   into ACME challenges.

3.1.  Existing challenges

3.1.1.  Existing "dns-01" Challenge

   The existing "dns-01" challenge MUST NOT be used to validate ".onion"
   Special-Use Domain Names.

3.1.2.  Existing "http-01" Challenge

   The "http-01" challenge is defined as in [RFC8555] §8.3 may be used
   to validate a ".onion" Special-Use Domain Names, with the
   modifications defined in this standard, namely Client authentication
   to hidden services and Certification Authority Authorization (CAA).

   The ACME server SHOULD follow redirects; note that these may be
   redirects to non ".onion" services, and the server SHOULD honour
   these.

3.1.3.  Existing "tls-alpn-01" Challenge

   The "tls-alpn-01" challenge is defined as in [RFC8737] may be used to
   validate a ".onion" Special-Use Domain Names, with the modifications
   defined in this standard, namely Client authentication to hidden
   services and Certification Authority Authorization (CAA).

3.2.  New "onion-csr-01" Challenge

   The two methods already defined in ACME and allowed by the CA/BF do
   not allow issuance of wildcard certificates.  This new validation
   method incorporates the specially signed CSR (as defined by [cabf-br]
   § B.2.b) into ACME to allow for the issuance of wildcard
   certificates.

   To this end a new challenge type called "onion-csr-01" is defined,
   with the following fields:

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   type (required, string)  The string "onion-csr-01"

   nonce (required, string)  A Base64 [RFC4648] encoded nonce, including
      padding characters.  It MUST contain at least 64 bits of entropy.
      It MUST NOT be valid for more than 30 days.

   authKey (optional, object)  The Ed25519 public key encoded as per
      [RFC8037].

   {
     "type": "onion-csr-01",
     "url": "https://example.com/acme/chall/bbc625c5",
     "status": "pending",
     "nonce": "bI6/MRqV4gw=",
     "authKey": { ... }
   }

   Clients prove control over the key associated with the ".onion"
   service by generating a CSR with the following additional extension
   attributes and signing it with the private key of the ".onion"
   Special-Use Domain Name:

   *  A caSigningNonce attribute containing the nonce provided in the
      challenge.  This MUST be raw bytes, and not the base64 encoded
      value provided in the challenge object.

   *  An applicantSigningNonce containing a nonce generated by the
      client.  This MUST have at least 64 bits of entropy.  This MUST be
      raw bytes.

   These additional attributes have the following format

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   cabf OBJECT IDENTIFIER ::=
     { joint-iso-itu-t(2) international-organizations(23)
       ca-browser-forum(140) }

   cabf-caSigningNonce OBJECT IDENTIFIER ::= { cabf 41 }

   caSigningNonce ATTRIBUTE ::= {
     WITH SYNTAX             OCTET STRING
     EQUALITY MATCHING RULE  octetStringMatch
     SINGLE VALUE            TRUE
     ID                      { cabf-caSigningNonce }
   }

   cabf-applicantSigningNonce OBJECT IDENTIFIER ::= { cabf 42 }

   applicantSigningNonce ATTRIBUTE ::= {
     WITH SYNTAX             OCTET STRING
     EQUALITY MATCHING RULE  octetStringMatch
     SINGLE VALUE            TRUE
     ID                      { cabf-applicantSigningNonce }
   }

   The subject of the CSR need not be meaningful and CAs SHOULD NOT
   validate its contents.  The public key presented in this CSR MUST be
   the public key corresponding to the ".onion" Special-Use Domain Name
   being validated.  It MUST NOT be the same public key presented in the
   CSR to finalize the order.

   Client respond with the following object to validate the challenge

   csr (required, string)  The CSR in the base64url-encoded version of
      the DER format.  (Note: Because this field uses base64url, and
      does not include headers, it is different from PEM.)

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   POST example.com/acme/chall/bbc625c5
   Host: example.com
   Content-Type: application/jose+json

   {
     "protected": base64url({
       "alg": "ES256",
       "kid": "https://example.com/acme/acct/evOfKhNU60wg",
       "nonce": "UQI1PoRi5OuXzxuX7V7wL0",
       "url": "https://example.com/acme/chall/bbc625c5"
     }),
     "payload": base64url({
       "csr": "MIIBPTCBxAIBADBFMQ...FS6aKdZeGsysoCo4H9P"
     }),
     "signature": "Q1bURgJoEslbD1c5...3pYdSMLio57mQNN4"
   }

   When presented with the CSR the server verifies it in the following
   manner:

   1.  The CSR is a well formatted PKCS#10 request.

   2.  The public key in the CSR corresponds to the ".onion" Special-Use
       Domain Name being validated.

   3.  The signature over the CSR validates with the ".onion" Special-
       Use Domain Name public key.

   4.  The caSigningNonce attribute is present and its contents matches
       the nonce provided to the client.

   5.  The applicantSigningNonce attribute is present and contains at
       least 64 bits of entropy.

   If all of the above are successful then validation succeeds,
   otherwise it has failed.

4.  Client authentication to hidden services

   Some hidden services do not wish to be accessible to the entire Tor
   network, and so encrypt their hidden service descriptor with the keys
   of clients authorized to connect.  Without a way for the CA to signal
   what key it will use to connect these services will not be able to
   obtain a certificate using http-01 or tls-alpn-01, nor enforce CAA
   with any validation method.

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   To this end, an additional field in the challenge object is defined
   to allow the ACME server to advertise the Ed25519 public key it will
   use (as per [tor-rend-spec-v3] INTRO-AUTH) to authenticate itself
   when retrieving the hidden service descriptor.

   authKey (optional, object)  The Ed25519 public key encoded as per
      [RFC8037].

   ACME servers MUST NOT use the same public key with multiple hidden
   services.  ACME servers MAY re-use public keys for re-validation of
   the same hidden service.

   There is no method to communicate to the CA that client
   authentication is required; instead the ACME server MUST attempt to
   calculate its CLIENT-ID as per [tor-rend-spec-v3] FIRST-LAYER-CLIENT-
   BEHAVIOR.  If no "auth-client" line in the first layer hidden service
   descriptor matches the computed client-id then the server MUST assume
   that the hidden service does not require client authentication and
   proceed accordingly.

   In the case the Ed25519 public key is novel to the client it will
   have to resign and republish its hidden service descriptor.  It
   SHOULD wait some (indeterminate) amount of time for the new
   descriptor to propagate the Tor hidden service directory servers,
   before proceeding with responding to the challenge.  This should take
   no more than a few minutes.  CAs MUST NOT expire challenges before a
   reasonable time to allow publication of the new descriptor (this
   document suggests at least 30 minutes).

5.  ACME over hidden services

   A CA offering certificates to ".onion" Special-Use Domain Names is
   RECOMMENDED to make their ACME server available as a Tor hidden
   services.  ACME clients SHOULD also support connecting to ACME
   servers over Tor, regardless of their support of "onion-csr-01", as
   their existing "http-01" and "tls-alpn-01" implementations could be
   used to obtain certificates for ".onion" Special-Use Domain Names.

6.  Certification Authority Authorization (CAA)

   ".onion" Special-Use Domain Name are not part of the DNS, and as such
   a variation on CAA [RFC8659] is required to allow restrictions to be
   placed on certificate issuance.

   To this end a new field is added to the second layer hidden service
   descriptor [tor-rend-spec-v3] § 2.5.2.2. with the following format:

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   "caa" SP flags SP tag SP value NL
   [Any number of times]

   The contents of "flag", "tag", and "value" are as per [RFC8659] §
   4.1.1.  Multiple CAA records may be present, as is the case in the
   DNS.  CAA records in a hidden service descriptor are to be treated
   the same by CAs as if they had been in the DNS for the ".onion"
   Special-Use Domain Name.

   A hidden service's second layer descriptor using CAA may look
   something like the following:

   create2-formats 2
   single-onion-service
   caa 128 issue "test.acmeforonions.org;validationmethods=onion-csr-01"
   caa 0 iodef "mailto:security@example.com"
   introduction-point AwAGsAk5nSMpAhRqhMHbTFCTSlfhP8f5PqUhe6DatgMgk7kSL3KHCZUZ3C6tXDeRfM9SyNY0DlgbF8q+QSaGKCs=
   ...

6.1.  Relevant Resource Record Set

   In the absence of the possibility for delegation of subdomains from a
   ".onion" Special-Use Domain Name as there is in the DNS there is no
   need, nor indeed any method available to search up the DNS tree for a
   relevant CAA record set.  Similarly, it is also impossible to check
   CAA records on the "onion" Special-Use TLD, as it does not exist in
   any form except as described in [RFC7686], so implementors must not
   look here either.

   Instead all subdomains under a ".onion" Special-Use Domain Name share
   the same CAA record set.  That is all of these share a CAA record set
   with "a.onion":

   *  b.a.onion

   *  c.a.onion

   *  e.d.a.onion

   But these do not:

   *  b.c.onion

   *  c.d.onion

   *  e.c.d.onion

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6.2.  When to check CAA

   If the hidden service has client authentication enabled then it will
   be impossible for the CAA to decrypt the second layer descriptor to
   read the CAA records until the CAAs public key has been added to
   first layer descriptor.  To this end a CA SHOULD wait until the
   client responds to an authorization, and treat this as indication
   that their public key has been added and that the CA will be able to
   decrypt the second layer descriptor.

6.3.  Preventing mis-issuance by unknown CAs

   As the CAA records are in the second layer descriptor and in the case
   of a hidden service requiring client authentication it is impossible
   to read them without the hidden service trusting a CA's public key, a
   method is required to signal that there are CAA records present (but
   not reveal their contents, which may disclose unwanted information
   about the hidden service operator).

   To this end a new field is added to the first layer hidden service
   descriptor [tor-rend-spec-v3] § 2.5.1.2. with the following format:

   "caa-critical" NL
   [At most once]

   If a CA encounters this flag it MUST NOT proceed with issuance until
   it can decrypt and parse the CAA records from the second layer
   descriptor.

6.4.  Alternative in-band presentation of CAA

   A CA may not be willing to operate the infrastructure required to
   fetch, decode, and verify Tor hidden service descriptors in order to
   check CAA records.  To this end a method to signal CAA policies in-
   band of ACME is defined.

   If a hidden service does use this method to provide CAA records to a
   CA it SHOULD still publish CAA records if its CAA record set includes
   "iodef", "contactemail", or "contactphone" so that this information
   is still publicly accessible.  A hidden service operator MAY also not
   wish to publish a CAA record set in its service descriptor to avoid
   revealing information about the service operator.

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   If a CA receives a validly signed CAA record set in the finalize
   request it need not check the CAA set in the hidden service
   descriptor and can proceed with issuance on the basis of the client
   provided CAA record set only.  A CA, however, is not required to do
   anything with the client provided record set, and is free to always
   fetch the record set from the service descriptor.

   A new field is defined in the ACME finalize endpoint to contain the
   hidden service's CAA record set for each ".onion" Special-Use Domain
   Name in the order.

   onionCAA (optional, dictionary of objects)  The CAA record set for
      each ".onion" Special-Use Domain Name in the order.  The key is
      the ".onion" Special-Use Domain Name, and the value is an object
      with the following fields.

   The contents of the "onionCAA" object are:

   caa (required, string or null)  The CAA record set as a string,
      encoded in the same way as if was included in the hidden service
      descriptor.  If the hidden service does not have a CAA record set
      then this MUST be null.

   expiry (required, integer)  The Unix timestamp at which this CAA
      record set will expire.  This SHOULD NOT be more than 8 hours in
      the future.  CAs MUST process this as at least a 64-bit integer to
      ensure functionality beyond 2038.

   signature (required, string)  The Ed25519 signature of the CAA record
      set using the private key corresponding to the ".onion" Special-
      Use Domain Name, encoded using base64url.  The signature is
      defined below.

   The data that the signature is calculated over is the concatenation
   of the following, encoded in UTF-8 [RFC3629]:

   "onion-caa|" || expiry || "|" || caa

   Where "|" is the ASCII character 0x7C, and expiry is the expiry field
   as a decimal string with no leading zeros.

6.4.1.  CAs requiring in-band CAA

   If a CA does not support fetching a service's CAA record set from its
   service descriptor it, and the ACME client does not provide an
   "onionCAA" object in its finalize request the CA MUST respond with an
   "onionCAARequired" error to indicate this.

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   Additionally, a new field is defined in the directory "meta" object
   to signal this.

   inBandOnionCAARequired (optional, boolean)  If true, the CA requires
      the client to provide the CAA record set in the finalize request.
      If false or absent the CA does not require the client to provide
      the CAA record set is this manner.

   A directory of such a CA may look like

   HTTP/1.1 200 OK
   Content-Type: application/json

   {
     "newNonce": "https://example.com/acme/new-nonce",
     "newAccount": "https://example.com/acme/new-account",
     "newOrder": "https://example.com/acme/new-order",
     "revokeCert": "https://example.com/acme/revoke-cert",
     "keyChange": "https://example.com/acme/key-change",
     "meta": {
       "termsOfService": "https://example.com/acme/terms/2023-10-13",
       "website": "https://acmeforonions.org/",
       "caaIdentities": ["test.acmeforonions.org"],
       "inBandOnionCAARequired": true
     }
   }

6.4.2.  Example in-band CAA

   Given the following example CAA record set for
   5anebu2glyc235wbbop3m2ukzlaptpkq333vdtdvcjpigyb7x2i2m2qd.onion:

   caa 128 issue "test.acmeforonions.org; validationmethods=onion-csr-01"
   caa 0 iodef "mailto:example@example.com"

   The following would be submitted to the CA's finalize endpoint

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   POST /acme/order/TOlocE8rfgo/finalize
   Host: example.com
   Content-Type: application/jose+json

   {
     "protected": base64url({
       "alg": "ES256",
       "kid": "https://example.com/acme/acct/evOfKhNU60wg",
       "nonce": "MSF2j2nawWHPxxkE3ZJtKQ",
       "url": "https://example.com/acme/order/TOlocE8rfgo/finalize"
     }),
     "payload": base64url({
       "csr": "MIIBPTCBxAIBADBFMQ...FS6aKdZeGsysoCo4H9P",
       "onionCAA": {
         "5anebu2glyc235wbbop3m2ukzlaptpkq333vdtdvcjpigyb7x2i2m2qd.onion": {
           "caa": "caa 128 issue \"test.acmeforonions.org; validationmethods=onion-csr-01\"\ncaa 0 iodef \"mailto:example@example.com\"",
           "expiry": 1697210719,
           "signature": "u_iP6JZ4JZBrzQUKH6lSrWejjRfeQmkTuehc0_FaaTNPAV0RVxpUz9r44DRdy6kgy0ofnx18KIhMrP7N1wpxAA=="
         }
       }
     }),
     "signature": "uOrUfIIk5RyQ...nw62Ay1cl6AB"
   }

7.  IANA Considerations

7.1.  Validation Methods

   Per this document, one new entry has been added to the "ACME
   Validation Methods" registry defined in [RFC8555] §9.7.8.  This entry
   is defined below:

         +==============+=================+======+===============+
         | Label        | Identifier Type | ACME | Reference     |
         +==============+=================+======+===============+
         | onion-csr-01 | dns             | Y    | This document |
         +--------------+-----------------+------+---------------+

                            Table 1: New entries

7.2.  Error Types

   Per this document, one new entry has been added to the "ACME Error
   Types" registry defined in [RFC8555] §9.7.8.  This entry is defined
   below:

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     +==================+===============================+===========+
     | Type             | Description                   | Reference |
     +==================+===============================+===========+
     | onionCAARequired | The CA only supports checking | This      |
     |                  | CAA for hidden services in-   | document  |
     |                  | band, but the client has not  |           |
     |                  | provided an in-band CAA       |           |
     +------------------+-------------------------------+-----------+

                           Table 2: New entries

7.3.  Directory Metadata Fields

   Per this document, one new entry has been added to the "ACME
   Directory Metadata Fields" registry defined in [RFC8555] §9.7.8.
   This entry is defined below:

             +==================+============+===============+
             | Field name       | Field type | Reference     |
             +==================+============+===============+
             | onionCAARequired | boolean    | This document |
             +------------------+------------+---------------+

                            Table 3: New entries

8.  Security Considerations

8.1.  Use of "dns" identifier type

   The re-use of the "dns" identifier type for a Special-Use Domain Name
   not actually in the DNS infrastructure raises questions regarding its
   suitability.  The reasons the author wishes to pursue this path in
   the first place are detailed in Appendix A.  It is felt that there is
   little security concern in reuse of the "dns" identifier type with
   regards the mis-issuance by CAs that are not aware of ".onion"
   Special-Use Domain Names, as CAs would not be able to resolve the
   identifier in the DNS.

8.1.1.  "http-01" Challenge

   The CA would follow the procedure set out in [RFC8555] §8.3 which
   specifies that the CA should "Dereference the URL using an HTTP GET
   request".  Given that ".onion" Special-Use Domain Names require
   special handling to dereference, this de-referencing will fail,
   disallowing issuance.

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8.1.2.  "tls-alpn-01" Challenge

   The CA would follow the procedure set out in [RFC8737] §3 which
   specifies that the CA "resolves the domain name being validated and
   chooses one of the IP addresses returned for validation".  Given that
   ".onion" Special-Use Domain Names are not resolvable to IP addresses,
   this de-referencing will fail, disallowing issuance.

8.1.3.  "dns-01" Challenge

   The CA would follow the procedure set out in [RFC8555] §8.4 which
   specifies that the CA should "query for TXT records for the
   validation domain name".  Given that ".onion" Special-Use Domain
   Names are not present in the DNS infrastructure, this query will
   fail, disallowing issuance.

8.2.  Key Authorization with "onion-csr-01"

   The "onion-csr-01" challenge does not make use of the key
   authorization string defined in [RFC8555] §8.1.  This does not weaken
   the integrity of authorizations.

   The key authorization exists to ensure that whilst an attacker
   observing the validation channel may observe the correct validation
   response, they cannot compromise the integrity of authorizations as
   the response can only be used with the account key for which it was
   generated.  As the validation channel for this challenge is ACME
   itself, and ACME already requires that the request be signed by the
   account, the key authorization is not required.

8.3.  Use of Tor for non ".onion" domains

   An ACME server MUST NOT utilise Tor for the validation of non
   ".onion" domains, due to the risk of possible exit hijacking.

8.4.  Security of CAA records

   The second layer descriptor is signed, encrypted and MACed in a way
   that only a party with access to the secret key of the hidden service
   could manipulate what is published there.  For more information about
   this process see [tor-rend-spec-v3] § 2.5.3.

8.5.  In-band CAA

   Tor directory servers are inherently untrusted entities, and as such
   there is no difference in the security model for accepting CAA
   records directly from the ACME client or fetching them over Tor. CAA
   records are still verified against the same hidden service key.

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8.6.  Access of the Tor network

   The ACME server MUST make its own connection to the hidden service
   via the Tor network, and MUST NOT outsource this, such as by using
   Tor2Web.

8.7.  Anonymity of the ACME client

   ACME clients requesting certificates for ".onion" Special-Use Domain
   Names may expose the existence of a hidden service on the host to
   unintended parties - even when features such as ECH
   [I-D.ietf-tls-esni] are utilised, as the IP addresses of ACME servers
   are generally well-known, static, and not used for any other purpose.

   ACME clients SHOULD connect to ACME servers over the Tor network to
   alleviate this, preferring a hidden service endpoint if the CA
   provides such a service.

9.  References

9.1.  Normative References

   [BCP14]    Best Current Practice 14,
              <https://www.rfc-editor.org/info/bcp14>.
              At the time of writing, this BCP comprises the following:

              Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

              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>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/info/rfc4648>.

   [RFC7686]  Appelbaum, J. and A. Muffett, "The ".onion" Special-Use
              Domain Name", RFC 7686, DOI 10.17487/RFC7686, October
              2015, <https://www.rfc-editor.org/info/rfc7686>.

   [RFC8037]  Liusvaara, I., "CFRG Elliptic Curve Diffie-Hellman (ECDH)
              and Signatures in JSON Object Signing and Encryption
              (JOSE)", RFC 8037, DOI 10.17487/RFC8037, January 2017,
              <https://www.rfc-editor.org/info/rfc8037>.

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   [RFC8555]  Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
              Kasten, "Automatic Certificate Management Environment
              (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,
              <https://www.rfc-editor.org/info/rfc8555>.

   [RFC8659]  Hallam-Baker, P., Stradling, R., and J. Hoffman-Andrews,
              "DNS Certification Authority Authorization (CAA) Resource
              Record", RFC 8659, DOI 10.17487/RFC8659, November 2019,
              <https://www.rfc-editor.org/info/rfc8659>.

   [RFC8737]  Shoemaker, R.B., "Automated Certificate Management
              Environment (ACME) TLS Application-Layer Protocol
              Negotiation (ALPN) Challenge Extension", RFC 8737,
              DOI 10.17487/RFC8737, February 2020,
              <https://www.rfc-editor.org/info/rfc8737>.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <https://www.rfc-editor.org/info/rfc3629>.

   [tor-address-spec]
              Nick Mathewson, N., "Special Hostnames in Tor",
              <https://spec.torproject.org/address-spec>.

   [tor-rend-spec-v3]
              The Tor Project, "Tor Rendezvous Specification - Version
              3", <https://spec.torproject.org/rend-spec-v3>.

   [cabf-br]  CA/Browser Forum, "Baseline Requirements for the Issuance
              and Management of Publicly-Trusted Certificates",
              <https://cabforum.org/wp-content/uploads/CA-Browser-Forum-
              BR-1.8.6.pdf>.

9.2.  Informative References

   [onion-services-setup]
              The Tor Project, "Set Up Your Onion Service",
              <https://community.torproject.org/onion-services/setup/>.

   [I-D.ietf-tls-esni]
              Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS
              Encrypted Client Hello", Work in Progress, Internet-Draft,
              draft-ietf-tls-esni-17, 9 October 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-tls-
              esni-17>.

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Appendix A.  Discussion on the use of the "dns" identifier type

   The reasons for utilising the "dns" identifier type in ACME and not
   defining a new identifier type for ".onion" s may not seem obvious at
   first glance.  After all, ".onion" Special-Use Domain Names are not
   part of the DNS infrastructure and as such why should they use the
   "dns" identifier type?

   The CA/Browser Forum Baseline Requirements [cabf-br] §B.2.a.ii
   define, and this standard allows, using the "http-01" or "tls-alpn-
   01" validation methods already present in ACME (with some
   considerations).  Given the situation of a web server placed behind a
   Tor terminating proxy (as per the setup suggested by the Tor project
   [onion-services-setup]), existing ACME tooling can be blind to the
   fact that a ".onion" Special-Use Domain Name is being utilised, as
   they simply receive an incoming TCP connection as they would
   regardless (albeit from the Tor terminating proxy).

   An example of this would be Certbot placing the ACME challenge
   response file in the webroot of an NGINX web server.  Neither Certbot
   nor NGINX would require any modification to be aware of any special
   handling for ".onion" Special-Use Domain Names.

   This does raise some questions regarding security within existing
   implementations, however the authors believe this is of little
   concern, as per Section 8.1.

Acknowledgements

   With thanks to the Open Technology Fund for funding the work that
   went into this document.

   The authors also wish to thank the following for their input on this
   document:

   *  Iain R.  Learmonth

   *  Jan-Frederik Rieckers

Author's Address

   Q Misell (editor)
   AS207960 Cyfyngedig
   13 Pen-y-lan Terrace
   Caerdydd
   CF23 9EU
   United Kingdom
   Email: q@as207970.net, q@magicalcodewit.ch

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   URI:   https://magicalcodewit.ch

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