How to Implement a Custom X.509 Path Validation Rule

Question

I need to implement a custom path validation rule to verify an X.509 certificate chain based on a custom Certificate Policy and set of Policy Qualifiers I'm defining for a specific application.

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As a minified example:

My X.509 certificates in the chain here include a fragment like this:

extensions:
- extnID: !oid 2.5.29.32 # Certificate Policies
  critical: false
  extnValue:
  - policyIdentifier: !oid 1.3.6.1.5.5.7.13.1.1 # testPolicy
    policyQualifiers:
    - policyQualifierId: !oid 1.3.6.1.5.5.7.13.2.1 # testQualifier
      qualifier: !int 100

For testing purposes (per RFC 7229) I've assigned OIDs from the 1.3.6.1.5.5.7.13 testing arc:

• testPolicy is a Certificate Policy with OID 1.3.6.1.5.5.7.13.1.1, defined as being qualified by testQualifier.
• testQualifier is a Policy Qualifier with OID 1.3.6.1.5.5.7.13.2.1, defined as an integer.

A chain of certificates containing this extension is valid only if the value of the testPolicy's testQualifier decreases monotonically along the certificate chain. That is, in a valid chain you might have a root certificate with a value of 100, then some intermediate CA certificate might have 50, some lower CA certificate might specify 10. etc. But if any of those numbers were in a different order the chain must be rejected.

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(Note: that the actual application I'm working on will be evaluating a more complicated partial order on a more complicated structure; and it also includes additional things like custom Extended Key Usage values and will use actual OIDs allocated from a PEN arc registered with IANA. My question is not about any of those things.)

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The implementation of a rule like this doesn't feel like it should be very complicated conceptually, but there are a number of rules about how Certificate Policies are evaluated in general that also need to be respected.

I specifically want the functionality provided by other standard X.509 extensions; this is a useful feature and the primary reason for implementing this piece as a Certificate Policy rather than a separate Extension.

Just to list a few of these general behaviors:

• Require Explicit Policy: Validation needs to reject a path if it has too many certificates in a row lacking the policy, if specified by a Policy Constraint extension's requireExplicitPolicy value.
• Any Policy: Validation needs to accept an anyPolicy policy as another way for an upstream certificate to be considered a valid path.
• Inhibit AnyPolicy: Validation needs to still ignore an anyPolicy if it's inhibited by the Inhibit AnyPolicy extension.
• Policy Mapping: Validation needs to also look for OIDs considered synonyms in the appropriate Policy Mapping extensions in the chain.
• Inhibit Mapping: Validation needs to enforce the constraint on where Policy Mappings can appear, if specified by a Policy Constraint extension's inhibitMapping value.

I could theoretically parse each certificate myself with ASN.1 and run a separate validation pass with just my own logic in addition to a separate run through an X.509-specific library. But that will mean needing to re-implement other portions of this logic that are already standard, so I would like to avoid that approach both for expediency and for correctness.

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I'm not married to any particular library or any particular language for this project. I'm still prototyping the specification at this point; any tool that provides a straightforward way to implement this type of custom path validation would be useful to me.

So far, I've tried to search through the source of openSSL and pyca/cryptography, but my search skills have come up inadequate: I've not been able to find where path validation logic actually occurs, and it's unclear to me if either project necessarily has facilities to straightforwardly add this type of validation rule.

I don't expect a task nearly so esoteric to have a written how-to guide ready to go, either. What I'm looking for is specific libraries that have ways to add this, and pointers to where in the source I should be reading to figure out how.