Compressed packets have significant attack surface, both due to the
potential for denial of service (zip bombs and the like) and for code
execution via memory corruption vulnerabilities in the decompressor.
Furthermore, I am not aware of any implementation that uses them in keys
or detached signatures. Therefore, disallow their use in such contexts
When parsing detached signatures, forbid any packet that is not a
signature or marker packet. When parsing keys, return an error when
encountering a compressed packet, instead of decompressing the packet.
Furthermore, certificates, keys, and signatures are not allowed to
contain partial-length or indeterminate-length packets. Reject those in
parse_packet, rather than activating the partial-length filter code.