The previous solution (ad-hoc stream-parsing the CRLs to obtain their
issuers and nextUpdate) was too cute. It began breaking on CRLs that had
an addition hex 0x30 byte somewhere in their header. I thought that 0x30
was a reserved character only to be used for tags in ASN1 encoded with
DER; turns out that's not true. Rather than write a full-fledged ASN1
stream-parser, the simplest solution is to just maintain the list of
issuers as a constant in the codebase. This is fine because the issuer
for a specific CRL URI should not change. If it does, we've probably got
bigger problems.
This also removes the Flask app's functionality for updating the local
CRL cache. This is being handled out-of-band by a Kubernetes CronJob
and is not a concern of the app's. This means that instances of the
CRLCache do not have to explicitly track expirations for CRLs.
Previously, the in-memory dictionary or CRL issuers and locations
included expirations; now it is flattened to not include that
information.
The CRLCache class has been updated to accept a crl_list kwargs so that
unit tests can provide their own alternative CRL lists, since we now
hard-code the expected CRLs and issuers. The nightly CRL check job has
been updated to check that the hard-coded list of issuers matches what
we get when we actually sync the CRLs.
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