I believe that PEM implementations do have to understand the matching rules,
for the following reason:
Suppose my certificate has:
Issuer c=GB;o=Cambridge University
Subject: c=GB;o=Cambridge University;ou=Computer Lab;cn=Michael Roe
and my CA's certificate has:
Issuer: c=GB;o=JNT Policy CA
Subject: c=GB;o=CAMBRIDGE UNIVERSITY
I can use these two certificates to form a valid certification path up
to the policy CA because the matching rules say that o (organisation)
is caseIgnoreString and hence "Cambridge University"="CAMBRIDGE UNIVERSITY".
However, the certification path checking routine much understand the
matching rules for the naming attributes in order to realise that this
is a valid path,.
Mike