The cx509 extension essentially wraps a machine-generated pure C X.509
certificate parser. We use asn1c to generate the parser code; the
setup.py file will retrieve a suitable copy if you don't have one already.

The relevant files for X.509 prasing reside under

  asn1c/examples/sample.source.PKIX1

and

  asn1c/examples/sample.source.PKCS1

The .c and .h files in these directories have been generated from the
ASN.1 syntax definitions in RFC 5280 (unmodified) and pkcs-1v2-1.asn
(with several modifications). You can get the unmodified ASN.1 module
text from here:

  ftp://ftp.rsasecurity.com/pub/pkcs/pkcs-1/pkcs-1v2-1.asn

Note that you should not need to build and run the asn1c compiler
unless you want to update the code to use later ASN.1 definitions; the
asn1c directory linked here already has pre-built code that should be
cross-platform compatible.