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

Internet-Draft	ACME for .onion	February 2024
Misell	Expires 30 August 2024	[Page]

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).¶

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:¶

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¶

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.)¶

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:¶

The CSR is a well formatted PKCS#10 request.¶
The public key in the CSR corresponds to the ".onion" Special-Use Domain Name being validated.¶
The signature over the CSR validates with the ".onion" Special-Use Domain Name public key.¶
The caSigningNonce attribute is present and its contents matches the nonce provided to the client.¶
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.¶

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).¶

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:¶

"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¶

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.¶

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.¶

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¶

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"
}

Table 1: New entries
Label	Identifier Type	ACME	Reference
onion-csr-01	dns	Y	This document

Table 2: New entries
Type	Description	Reference
onionCAARequired	The CA only supports checking CAA for hidden services in-band, but the client has not provided an in-band CAA	This document

Table 3: New entries
Field name	Field type	Reference
onionCAARequired	boolean	This document

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.¶

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.¶

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>.
[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>.