PDF has a feature where streams can be encrypted with a crypt filter that is specified via DecodeParms that refers to the /CF dictionary of the /Encrypt dictionary. If DecodeParms is missing the Identity filter is used (raw data unchanged)
From 7.6.5 Crypt Filters (PDF32000_2008.PDF):
A stream filter type, the Crypt filter (see 7.4.10, "Crypt Filter") can be specified for any stream in the
document to override the default filter for streams.
For example here is a case where /Crypt is specified and DecodeParms is missing (Identity filter) so the data is left in tact. Seems to be used for metadata sometimes.
Current state
PDF has a feature where streams can be encrypted with a crypt filter that is specified via DecodeParms that refers to the /CF dictionary of the /Encrypt dictionary. If DecodeParms is missing the Identity filter is used (raw data unchanged)
From 7.6.5 Crypt Filters (PDF32000_2008.PDF):
For example here is a case where /Crypt is specified and DecodeParms is missing (Identity filter) so the data is left in tact. Seems to be used for metadata sometimes.
Another example from PDF32000_2008.PDF (p. 77)
Proposed change
Add support for supporting the Crypt filter in a similar fashion as other stream filters in core/stream.go