Re: [pkix] RFC 5280 interpretation of trust anchor certificates

[Corey Bonnell <[e-mail]>]

Hi Niklas,

Section 6.1.1 (d) says:
" The trust anchor information may be provided to the path
      processing procedure in the form of a self-signed certificate.
      When the trust anchor information is provided in the form of a
      certificate, the name in the subject field is used as the trusted
      issuer name and the contents of the subjectPublicKeyInfo field is
      used as the source of the trusted public key algorithm and the
      trusted public key."

The first sentence provides an illustrative example how trust anchor
information can be distributed. The second sentence describes how the name
and public key can be extracted from a certificate. Note that the second
sentence does not say "self-signed certificate", but rather merely says
"certificate". Given this, I believe this sentence is also applicable to
non- self-signed (or even non- self-issued) certificates that are used as
trust anchors.

Thanks,
Corey

-----Original Message-----
From: pkix <[e-mail]> On Behalf Of Niklas Matthies
Sent: Tuesday, October 11, 2022 4:26 PM
To: [e-mail]
Subject: Re: [pkix] RFC 5280 interpretation of trust anchor certificates

Russ,

The context of this thread concerns the European Trusted Lists, which are
intended to provide a set of trust anchors for various services. 
The service entries in a Trusted List contain trusted certificates, however
it remains unspecified how to map those certificates (plus, potentially,
associated metadata in the service entries) to the specific form of trust
anchor information required as input for the RFC 5280 path validation
algorithm (item (d) on page 76). Most of those certificates are not
self-signed.

The argument has been made on the ESI mailing list that RCF 5280 actually
does specify how to do that mapping, but in my reading it only does that for
self-signed certificates, if at all. Hence I started this thread to get a
clarification on that point.

My current conclusion is still that RFC 5280 at best only specifies a
mapping for self-signed certificates. Even for those, it's not clear whether
the use of that specific mapping is mandatory, due to the lack of "shall" or
of other keywords indicating the requirement level. This to me indicates a
technical gap in the ETSI specifications regarding the use of the Trusted
Lists.

Niklas


On Mon 2022-10-10 at 12:13h, Russ Housley wrote on pkix:
>Niklas:
>
>The relying party gets to decide the set of trust anchors that it will
accept.
>
>A very common way to distribute trust anchors is self-signed 
>certificates.  This provides the public key, the algorithm identifier 
>for the public key, and the distinguished name for the trust anchor.
>As noted by others, RFC 5914 defines an alternative format to supply 
>the trust anchor public key, the algorithm identifier for the public 
>key, and the distinguished name.  This information are needed to 
>validate certification paths that terminate at the trust anchor.
>
>The relying party can use a self-signed certificate or any other 
>certificate or any other source to update their trust anchor store.
>In practice, a self-signed certificate is normally used; the creation 
>of a self-signed certificate illustrates an expectation for it to be 
>used as a trust anchor.
>
>Russ
>
>
>> On Oct 9, 2022, at 1:52 PM, Niklas Matthies <[e-mail]> wrote:
>>
>> Dear all,
>>
>> On the ESI (ETSI) mailing list, the question came up whether RFC 5280
says anything about trust anchors provided in the form of certificates that
are _not_ self-signed.
>>
>> In section 6.1.1, there is the following wording on page 76:
>>
>>      The trust anchor information may be provided to the path
>>      processing procedure in the form of a self-signed certificate.
>>      When the trust anchor information is provided in the form of a
>>      certificate, the name in the subject field is used as the trusted
>>      issuer name and the contents of the subjectPublicKeyInfo field is
>>      used as the source of the trusted public key algorithm and the
>>      trusted public key.
>>
>> The first sentence seems to indicate that the case of providing a trust
anchor in the form of a non-self-signed certicate is not considered here.
But the second sentence doesn't repeat the "self-signed" bit, which can be
interpreted as that sentence also applying to non-self-signed certificates.
However, if that is the case, why does the first sentence restrict itself to
specifically self-signed certificates?
>>
>> On page 74 there is also the following wording:
>>      When the trust anchor is provided in the form of a self-signed
>>      certificate, this self-signed certificate is not included as
part of the prospective certification path.
>>
>> If RFC 5280 also allows the possibility of trust anchors being 
>> provided in the form of non-self-signed certificates, then it would seem
that the above restriction would not apply to those, i.e., that they may be
included as part of the prospective certifcation path. However, I don't see
how that would make any sense.
>>
>> All the wording taken together, my conclusion up to now was that RFC 5280
simply does not consider the possibility that the trust anchor could be
provided in the form of a non-self-signed certificate, and that therefore,
specifications which *do* allow for that possibility (such as in the context
of ETSI trusted lists) have to clarify how that case maps onto what RFC 5280
expects.
>>
>> If that interpretation is incorrect, that is, if RFC 5280 doesn't
actually care about whether a trust-anchor-representing certificate provided
as input to the path validation algorithm is self-signed or not, then maybe
an erratum is in order?
>>
>> Kind regards,
>> Niklas
>>
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>

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