Modes ECB and CBC can only handle full 16-byte frames. This means
the length of every strict bytestring passed in must be a multiple
of 16; when using lazy bytestrings, its component strict
bytestrings must all satisfy this.
Other modes can handle bytestrings of any length. However,
encrypting a bytestring of length 5 and then one of length 4 is not
the same operation as encrypting a single bytestring of length 9;
they are internally padded to a multiple of 16 bytes.
For OFB and CTR, Encrypt and Decrypt are the same operation. For
CTR, the IV is the initial value of the counter.
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