Automated Certificate Management Environment (ACME) Extensions for ".onion" Domain Names
draft-misell-acme-onion-00
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Replaced".
|
|
|---|---|---|---|
| Author | Q Misell | ||
| Last updated | 2023-02-24 | ||
| Replaced by | draft-ietf-acme-onion | ||
| RFC stream | (None) | ||
| Formats | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | I-D Exists | |
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-misell-acme-onion-00
Internet Engineering Task Force Q. Misell, Ed.
Internet-Draft AS207960
Updates: RFC8555 (if approved) 24 February 2023
Intended status: Standards Track
Expires: 28 August 2023
Automated Certificate Management Environment (ACME) Extensions for
".onion" Domain Names
draft-misell-acme-onion-00
Abstract
The documents defines extensions to the Automated Certificate
Management Environment (ACME) to allow for the automatic issuance of
certificates to Tor hidden services (".onion" domains).
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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 28 August 2023.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
Misell Expires 28 August 2023 [Page 1]
Internet-Draft ACME for .onion Domains February 2023
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
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
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 . . . . . . . . . . . . . . 5
4. Certification Authority Authorization (CAA) . . . . . . . . . 7
4.1. Relevant Resource Record Set . . . . . . . . . . . . . . 7
4.2. When to check CAA . . . . . . . . . . . . . . . . . . . . 8
4.3. Preventing mis-issuance by unknown CAs . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
5.1. Validation Methods . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6.1. Use of "dns" identifier type . . . . . . . . . . . . . . 9
6.1.1. "http-01" Challenge . . . . . . . . . . . . . . . . . 9
6.1.2. "tls-alpn-01" Challenge . . . . . . . . . . . . . . . 9
6.1.3. "dns-01" Challenge . . . . . . . . . . . . . . . . . 9
6.2. Key Authorization with "onion-csr-01" . . . . . . . . . . 9
6.3. Use of Tor for non ".onion" domains . . . . . . . . . . . 10
6.4. Security of CAA records . . . . . . . . . . . . . . . . . 10
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Normative References . . . . . . . . . . . . . . . . . . 10
7.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. Discussion on the use of the "dns" identifier
type . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 12
Misell Expires 28 August 2023 [Page 2]
Internet-Draft ACME for .onion Domains February 2023
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 special use ".onion" top-level domain
[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" domain may appear as a DNS name, it requires
special consideration to validate control of one such that ACME could
be used on ".onion" domains.
In order to allow ACME to be utilised to issue certificates to
".onion" domains this document specifies challenges suitable to
validate control of these domains. Additionally this document
defines an alternative to the DNS Certification Authority
Authorization (CAA) Resource Record [RFC8659] that can be used with
".onion" domains.
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" domain.
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"
}
Misell Expires 28 August 2023 [Page 3]
Internet-Draft ACME for .onion Domains February 2023
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"
domains. 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"
domains.
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" domain, with the following modifications.
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.
An additional field 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 during the introduction. This allows
the hidden service to obtain a certificate,
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.
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" domain, with the following modifications.
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.
Misell Expires 28 August 2023 [Page 4]
Internet-Draft ACME for .onion Domains February 2023
An additional field 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 during the introduction. This allows
the hidden service to obtain a certificate,
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.
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 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": { ... }
}
An "authKey" field is defined to allow the ACME server to advertise
the ed25519 public key it will use to decrypt the second layer
descriptor to check CAA records.
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.
Misell Expires 28 August 2023 [Page 5]
Internet-Draft ACME for .onion Domains February 2023
Clients may prove control over the key associated with the ".onion"
service by generating their CSR with the following additional
attributes and signing it with the private key of the ".onion"
domain:
* A caSigningNonce attribute containing the nonce provided in the
challenge. This MUST NOT be base64 encoded in the CSR.
* An applicantSigningNonce containing a nonce generated by the
client. This MUST have at least 64 bits of entropy.
These additional attributes have the following format
id-pkix OBJECT IDENTIFIER ::=
{ iso(1) identified-organization(3) dod(6) internet(1) security(5)
mechanisms(5) pkix(7) }
id-pe OBJECT IDENTIFIER ::= { id-pkix 1 }
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 }
}
In a variation to the usual state machine of ACME, a client need not
respond to the challenge. The act of POSTing a CSR to the
finalization endpoint is in itself a response to the challenge. The
challenge and order progress directly to either the "valid" or
"invalid" state without passing through "processing" or "ready"
(respectively).
Misell Expires 28 August 2023 [Page 6]
Internet-Draft ACME for .onion Domains February 2023
In the case of a CSR posted to the finalization endpoint that does
not include the above extensions the order SHOULD remain in the
"pending" state and SHOULD NOT transition to the "invalid" state.
Only in the case that an "onion-csr-01" CSR is POSTed to the
finalization endpoint that subsequently fails validation MUST the
order transition to the "invalid" state.
4. Certification Authority Authorization (CAA)
".onion" domains 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 at the DNS for the ".onion"
domain.
A hidden service's second layer descriptor using CAA may look
something like the following:
create2-formats 2
single-onion-service
caa 0 issue "example.com"
caa 0 iodef "mailto:security@example.com"
caa 128 validationmethods "onion-csr-01"
introduction-point AwAGsAk5nSMpAhRqhMHbTFCTSlfhP8f5PqUhe6DatgMgk7kSL3KHCZUZ3C6tXDeRfM9SyNY0DlgbF8q+QSaGKCs=
...
4.1. Relevant Resource Record Set
In the absence of the possibility of delegation from a ".onion"
domain as there is in the DNS there is no need, nor indeed any
possibility to search up a the DNS tree for a relevant CAA record
set. Instead all subdomains under a ".onion" domain 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
Misell Expires 28 August 2023 [Page 7]
Internet-Draft ACME for .onion Domains February 2023
* e.d.a.onion
But these do not:
* b.c.onion
* c.d.onion
* e.c.d.onion
4.2. 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.
4.3. 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 CAs 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.
5. IANA Considerations
5.1. 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:
Misell Expires 28 August 2023 [Page 8]
Internet-Draft ACME for .onion Domains February 2023
+==============+============+====+================================+
| Label | Identifier |ACME| Reference |
| | Type | | |
+==============+============+====+================================+
| onion-csr-01 | dns |Y | draft-misell-acme-onion-latest |
+--------------+------------+----+--------------------------------+
Table 1: New entries
6. Security Considerations
6.1. Use of "dns" identifier type
The re-use of the "dns" identifier type for a domain not actually in
the DNS infrastructure raises questions regarding its suitability.
The reasons the author wish 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" domains.
6.1.1. "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" require special handling to
dereference, this de-referencing will fail, disallowing issuance.
6.1.2. "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" are not resolvable to IP addresses, this de-referencing will
fail, disallowing issuance.
6.1.3. "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" are not present in the
DNS infrastructure, this query will fail, disallowing issuance.
6.2. 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.
Misell Expires 28 August 2023 [Page 9]
Internet-Draft ACME for .onion Domains February 2023
The key authorization exists to ensure that an attacker observing the
validation channel can observe the correct validation response, but
cannot compromise the integrity of authorizations as the response
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.
6.3. 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.
6.4. Security of CAA records
The second layer descriptor is 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.
7. References
7.1. Normative References
[BCP14] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, May 2017.
<https://www.rfc-editor.org/info/bcp14>
[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>.
Misell Expires 28 August 2023 [Page 10]
Internet-Draft ACME for .onion Domains February 2023
[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>.
[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>.
7.2. Informative References
[onion-services-setup]
The Tor Project, "Set Up Your Onion Service",
<https://community.torproject.org/onion-services/setup/>.
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" domains may not seem
obvious at first glance. After all, ".onion" domains 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
Misell Expires 28 August 2023 [Page 11]
Internet-Draft ACME for .onion Domains February 2023
[onion-services-setup]), existing ACME tooling can be blind to the
fact that a ".onion" domain 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" domains.
This does raise some questions regarding security within existing
implementations, however the authors believe this is of little
concern, as per Section 6.1.
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
Author's Address
Q Misell (editor)
AS207960 Cyfyngedig
13 Pen-y-lan Terrace
Caerdydd
CF23 9EU
United Kingdom
Email: q@magicalcodewit.ch, q@as207970.net
URI: https://magicalcodewit.ch
Misell Expires 28 August 2023 [Page 12]