That's what I thought. And if both encodings (absent or empty
SEQUENCE) are valid, we have a DER problem. I think this is
not a problem of the particular definition of the CRL. It
is an ASN.1 DER problem where the encoding of OPTIONAL fields
in general remains unclear.
The problem is not in the DER encoding but in 2 different grammatical
codings for the same thing. For either grammatical coding, whose
semantic meaning is beyond the scope of DER, there is one DER encoding.
For example, we can imagine another use for communications with
an output device where the inclusion of the OPTIONAL
item means a title page should be outputted and ea. item in the SEQUENCE OF
should be outputted on a new page. If I want just a title page outputted,
I would include an empty SEQUENCE OF. If I want nothing outputted, I
would leave it out. I know that this is a contrived example but what I
am getting at is that there's a difference between an ambiguous grammar
and an ambiguous parser(coder)/unambiguous grammar!
i.e. this is not a shortcoming of DER but of the grammar used to specify
the CRL.
If as Paul has suggested that we change it to a DEFAULT of an empty
list, then there is only one grammatical construction and hence
one DER encoding.
-Ray