Internet DRAFT - draft-suchan-acme-onion
draft-suchan-acme-onion
ACME Working Group S. Seo
Internet-Draft 10 May 2022
Intended status: Standards Track
Expires: 11 November 2022
Automated Certificate Management Environment (ACME) Onion Identifier
Validation Extension
draft-suchan-acme-onion-00
Abstract
This document specifies identifiers and challenges required to enable
the Automated Certificate Management Environment (ACME) to issue
certificates for Tor Project's onion V3 addresses.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Onion Identifier . . . . . . . . . . . . . . . . . . . . . . 2
4. Validation Challenges for Onion address . . . . . . . . . . . 3
4.1. Identify Key Possession Challenge . . . . . . . . . . . . 3
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
5.1. Identifier Types . . . . . . . . . . . . . . . . . . . . 5
5.2. Challenge Types . . . . . . . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . 5
7. Normative References . . . . . . . . . . . . . . . . . . . . 5
Appendix A. Notes and Questions to reviews . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
While onion addresses [RFC7686] are in form of DNS address, they
aren't in part of ICANN hierarchy, and onion name's self-verifying
construction warrants different verification, duce different
identifier type for them is worth consider.
2. Terminology
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 BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Onion Identifier
[RFC8555] only defines the identifier type "dns", that assumes it's
on public CA/B hierarchy, so to make clear Onion addresses are not
using normal DNS verification methods, we assign a new identifier
type for onion v3 addresses, "onion-v3".
This document only handles V3 version of onion address as defined in
[Toraddr], identified by 56 letters base domain name ends with d.
This document doesn't handle about verification of version 2 of onion
addresses, as they are retired already
An identifier for the onion address
aeceulkebl5...4zcuv5hk7fqwad.onion would be formatted like:
{"type": "onion-v3", "value": "aeceulkebl5...4zcuv5hk7fqwad.onion"}
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Keep mind in CSR this address still treated as DNS.
4. Validation Challenges for Onion address
Onion-v3 identifiers MAY be used with the existing "http-01" and
"tls-alpn-01" challenges from [RFC8555] Section 8.3 and [RFC8737]
Section 3 respectively. To use Onion identifiers with these
challenges their initial DNS resolution step MUST be skipped and the
appropriate Tor daemon that in control of CA MUST used to proxy such
request.
The existing "dns-01" challenge MUST NOT be used to validate onion
addresses.
In addition to challenges earlier RFC defined, This document create
an additional challenge called "onion-v3-csr". This challenge can be
used for wildcard subdomain of Onion address.
4.1. Identify Key Possession Challenge
With onion-csr-01 validation, the client in an ACME transaction
proves its control of an Onion address or subdomain of it by proving
the possession of Onion hidden service's identity key. The ACME
server challenges the client to sign CSR that includes the nonce it
gave as challenge.
The onion-csr-01 ACME challenge object has the following format:
type (required, string): The string "onion-v3-csr"
token (required, string): A random value that uniquely identifies
the challenge. This value MUST have at least 128 bits of entropy.
It MUST NOT contain any characters outside the base64url alphabet
as described in [RFC4648] Section 5. Trailing '=' padding
characters MUST be stripped. See [RFC4086] for additional
information on randomness requirements.
The client prepares for validation by constructing a self-signed CSR
that MUST contain an cabf caSigningNonce Attribute and a
subjectAlternativeName extension [RFC5280]. The
subjectAlternativeName extension MUST contain a single dNSName entry
where the value is the domain name being validated. The cabf
caSigningNonce Attribute MUST contain the token string as ascii
encoded for the challenge.
The cabf caSigningNonce Attribute is identified by the cabf-
caSigningNonce object identifier (OID) in the cabf arc [RFC5280].
consult [cabr] appendix B for how to construct CSR itself in detail.
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cabf OBJECT IDENTIFIER ::= 2.23.140
{
joint-iso-itu-t(2)
internationalorganizations(23)
a-browser-forum(140)
}
caSigningNonce ATTRIBUTE ::= {
WITH SYNTAX OCTET STRING
EQUALITY MATCHING RULE octetStringMatch
SINGLE VALUE TRUE
ID { cabf-caSigningNonce }
}
cabf-caSigningNonce OBJECT IDENTIFIER ::= { cabf 41 }
applicantSigningNonce ATTRIBUTE ::= {
WITH SYNTAX OCTET STRING
EQUALITY MATCHING RULE octetStringMatch
SINGLE VALUE TRUE
ID { cabf-applicantSigningNonce }
}
cabf-applicantSigningNonce OBJECT IDENTIFIER ::= { cabf 42 }
A client fulfills this challenge by construct the challenge CSR from
the "token" value provided in the challenge, then POST on challenge
URL with crafted CSR as payload to request validated by the server.
POST /acme/authz/1234/1
Host: example.com
Content-Type: application/jose+json
{
"protected": base64url({
"alg": "ES256",
"kid": "https://example.com/acme/acct/1",
"nonce": "XaYcb5XoTUgRWbTWw_NwkcP",
"url": "https://example.com/acme/authz/1234/1"
}),
"payload": base64url({
"csr": "MIIBBzCBugIBADAAMCo...gRYTMAhRP8nIH",
}),
"signature": "0wSzBJBgNVHREEQ...gYJKoZIhvcNAQkOM"
}
On receiving this request from client, the server verifies client's
control over the onion address by verify that CSR is crafted with
expected properties:
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1. CSR is signed with private part of identity key the requested
onion address made from.
2. A caSigningNonce attribute that contains token Value that
challenge object provided.
3. existence of applicantSigningNonce attribute. This value SHOULD
include random value from client side with at least 64bits of
entropy, and CA MUST recommend to client do so, but servers are
not expected verify such claim, as by nature it's impossible to
verify such claim of randomness.
5. IANA Considerations
5.1. Identifier Types
Adds a new type to the Identifier list in ACME Identifier Types with
the label "onion-v3" and reference I-D.THISDOC.
5.2. Challenge Types
in the ACME Validation Methods list:
Adds the raw "onion-v3-csr" to the Validation Methods.
Adds the value "onion-v3" to the Identifier Type column for the
"http-01", "onion-challenge-csr", and "tls-alpn-01" challenges. , and
reference I-D.THISDOC each of them.
6. Security Considerations
As onion addresses are able to generated in massive quantity without
financial cost, it bypasses the normal ratelimit CAs imposes. CAs
SHOULD adapt some measure to prevent DoS of CA by create hugh amount
of request for onion address. For example, imposing limit per ACME
account or require order to have at least one non-onion domain.
7. Normative References
[cabr] Forum, C., "CA/B forum baseline requirement", 26 April
2022, <https://cabforum.org/wp-content/uploads/CA-Browser-
Forum-BR-1.8.4.pdf>.
[RFC2119] 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>.
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[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/info/rfc4086>.
[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>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[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>.
[RFC8174] 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>.
[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>.
[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>.
[Toraddr] Mathewson, N., "Tor address spec", 24 August 2021,
<https://github.com/torproject/torspec/blob/main/address-
spec.txt>.
Appendix A. Notes and Questions to reviews
should this be about onion address, or all kind of alternative DNS
systems? should identifier type and challenge type include or strip
-v3 tag from its name? if we include that how about this doc name
itself? http-01 and tls-alpn-01 over tor will work as well for like
onion address V2 or V12, but csr challenge may not. but it's
reasonable to ask same identifier type should give same set of
challenges. should the as rigid as complying this will make comply
CA/B Baseline requirement? while type onion domain name just full
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onion v3 name itself with example subdomain will exceed rfc line
limit. but using ... doesn't right in context of domain name. any
alternative to express truncated FQDN? would "example.onion" work
while it wouldn't be valid onion name?
Author's Address
Seo Suchan
Email: tjtncks@gmail.com
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