Previously an incorrectly sized key was either silently padded
with NUL bytes or truncated. Especially the silent nature of this
behavior makes it extremely easy to use weak encryption. A common
mistake - which has also been extensively made in our tests - is
to use a password instead of a key.
Incorrectly sized keys will now be rejected.
Previously the code fell back on using a NUL IV if no IV was
passed and the encryption mode required it. This is dangerous and
makes no sense from a practical point of view (as you could just
as well use ECB then).
