Enforcing CA based constraints previously required the CA certificate file
to be locally installed. This is problematic from a maintencance perspective
when having many intermediate CAs, and is actually redundant if the client
sends its intermediate cert in the request.
The alternative was to use Distinguished Name matching in the subject
identity to indirectly check for the issuing CA by some RDN field, such as OU.
However, this requires trust in the intermediate CA to issue only certificates
with legitime subject identities.
This new approach checks for an intermediate CA by comparing the issuing
identity. This does not require trust in the intermediate, as long as
a path len constraint prevents that intermediate to issue further
intermediate certificates.
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