A stream that named its own crypt filter (/Filter /Crypt with
/DecodeParms /Name) kept that name when the document was given new
password protection. The new /Encrypt defines only /StdCF, so the
inherited name resolved to /Identity and the stream was written in
clear into a file the user asked to protect. When saving with protection
applied by set_encryption, a selector the new handler does not define is
removed with its parameters and the stream follows the new policy.
Saving with the original handler keeps the source's selectors.