crypto(4) and IVs

I've been trying to do useful things with crypto(4) - or more
precisely, trying to write code to pound on it in the hope of figuring
out why I'm seeing certain errors when talking to a machine with a
crypto accelerator in it.

I find that when I do a CIOCCRYPT, the IV is not modified.  How am I
supposed to get the correct IV for my next call?  Do I have to go under
the hood and "know" that for the cipher I'm using (3DES_CBC) it's the
last block of the encrypted data (output for ENCRYPT, input for
DECRYPT)?  Or is there something I'm missing?

Also, documentation is..sparse, to put it kindly.  In particular,
crypto(4) is not detailed enough to actually write code to (even after
fixing lib/30355).  Would it be worth my while to write more complete
documentation, or would that be an exercise in futility (eg, it's going
to go away totally in favour of something else)?

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