rijndael (AES): Avoid leaking sensitive data on kernel stack

[cem]

Summary

Noticed this investigating Fortuna. Remove useless duplicate stack copies
of sensitive contents when possible, or if not possible, be sure to zero
them out when we're finished.

[delphij]

Mostly LGTM.

sys/crypto/rijndael/rijndael-api-fst.c

448

Could you please change this to use some better name, e.g. something like 'cleanup' instead?

[cem]

sys/crypto/rijndael/rijndael-api-fst.c

448

I think 'out' is well understood:

$ cd sys
$ ag 'goto out;' | wc -l
    5333
$ ag 'goto cleanup;' | wc -l
     222

I can change it if you still prefer it, but I don't think "cleanup" is any better (or worse).

[cem]

Ping -- do you still want a different label or is out ok? Thanks!

[gordon]

Approved based on timeout from delphij.