class ctr :?iv:string -> ?inc:int -> block_cipher ->
block_cipher
Add Counter mode to the given block cipher. Viewing the IV
as a blocksize
-byte integer in big-endian representation,
the blocks IV
, IV+1
, IV+2
, ... are encrypted using
the given block cipher, and the result is xor-ed with the
input blocks to produce the output blocks. The additions
IV+n
are performed modulo 2 to the 8 * inc
power.
In other words, only the low inc
bytes of the IV
are
subject to incrementation; the high blocksize - inc
bytes
are unaffected. inc
defaults to blocksize
.
The returned block cipher has the same block size as
the underlying block cipher, and is usable both for
encryption and decryption.