Internet DRAFT - draft-ietf-acme-onion
draft-ietf-acme-onion
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
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 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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