Internet-Draft ACME for .onion June 2023
Misell Expires 24 December 2023 [Page]
Automated Certificate Management Environment
Intended Status:
Standards Track
Q. Misell, Ed.

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


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).


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

Source for this draft and an issue tracker can be found at

The project website and a reference implementation can be found at

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on 24 December 2023.

Table of Contents

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"
  "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.

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.

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": "",
  "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:

These additional attributes have the following format

  { joint-iso-itu-t(2) international-organizations(23)
    ca-browser-forum(140) }

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

caSigningNonce ATTRIBUTE ::= {
  SINGLE VALUE            TRUE
  ID                      { cabf-caSigningNonce }

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

applicantSigningNonce ATTRIBUTE ::= {
  SINGLE VALUE            TRUE
  ID                      { cabf-applicantSigningNonce }

The subject of the CSR need to 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.)
Content-Type: application/jose+json

  "protected": base64url({
    "alg": "ES256",
    "kid": "",
    "nonce": "UQI1PoRi5OuXzxuX7V7wL0",
    "url": ""
  "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.

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 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. 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 SHOULD strongly consider making 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] § 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
caa 0 issue ""
caa 0 iodef ""
caa 128 validationmethods "onion-csr-01"
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":

But these do not:

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] § 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.

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:

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

8.6. 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

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, .
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, .
Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, DOI 10.17487/RFC4648, , <>.
Appelbaum, J. and A. Muffett, "The ".onion" Special-Use Domain Name", RFC 7686, DOI 10.17487/RFC7686, , <>.
Liusvaara, I., "CFRG Elliptic Curve Diffie-Hellman (ECDH) and Signatures in JSON Object Signing and Encryption (JOSE)", RFC 8037, DOI 10.17487/RFC8037, , <>.
Barnes, R., Hoffman-Andrews, J., McCarney, D., and J. Kasten, "Automatic Certificate Management Environment (ACME)", RFC 8555, DOI 10.17487/RFC8555, , <>.
Hallam-Baker, P., Stradling, R., and J. Hoffman-Andrews, "DNS Certification Authority Authorization (CAA) Resource Record", RFC 8659, DOI 10.17487/RFC8659, , <>.
Shoemaker, R.B., "Automated Certificate Management Environment (ACME) TLS Application-Layer Protocol Negotiation (ALPN) Challenge Extension", RFC 8737, DOI 10.17487/RFC8737, , <>.
Nick Mathewson, N., "Special Hostnames in Tor", <>.
The Tor Project, "Tor Rendezvous Specification - Version 3", <>.
CA/Browser Forum, "Baseline Requirements for the Issuance and Management of Publicly-Trusted Certificates", <>.

9.2. Informative References

The Tor Project, "Set Up Your Onion Service", <>.
Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS Encrypted Client Hello", Work in Progress, Internet-Draft, draft-ietf-tls-esni-16, , <>.

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.


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:

Author's Address

Q Misell (editor)
AS207960 Cyfyngedig
13 Pen-y-lan Terrace
CF23 9EU
United Kingdom