ASN.1 defines MANDATORY and OPTIONAL fields. All the MANDATORY fields must be present in the encoding of an ASN.1 type, whereas OPTIONAL fields may be missing. An ASN.1 parser should be able to handle missing OPTIONAL fields, and not expect them to be present in the encoding.

Currently, the ASN.1 parser for PKCS#7 formatted messages in the Sun package expects all fields (even the ones defined as OPTIONAL) to be present in the encoding, and will break if an OPTIONAL field is missing in the encoding.