zpaq feature request: abort if no -key for encrypted archive

[barrycarter]

I recently did "zpaq list archive.zpaq" to an encrypted archive,
forgetting to add "-key mykey", and the process took forever to report
that archive.zpaq wasn't a valid (unencrypyed) archive. Apparently,
zpaq scanned the entire file before realizing it was invalid.

Ironically, "-key wrongkey" aborts almost immediately with "password
incorrect".

Could zpaq abort rapidly if there's no key provided and the file
doesn't look like an unencrypted archive?

Quasi-related: is there an open source way to update the "magic" file that the
Unix "file" command uses? The entries below could rapidly identify
(unencrypted) zpaq archives:

0 string 7kSt ZPAQ file
0 string zPQ ZPAQ file (fragile mode)

[Matt Mahoney]

I suppose it could, but inserting random data at the beginning of an archive (like a self extracting archive) is still a valid archive.

Not sure how to update the "file" command. I'm sure this happens a lot.