xforms that support processing of multiple blocks at a time (to support more
efficient modes, for example) can define the encrypt_ and decrypt_multi
interfaces. If these interfaces are not present, the generic cryptosoft
code falls back on the block-at-a-time encrypt/decrypt interfaces.
Stream ciphers may support arbitrarily sized inputs (equivalent to an input
block size of 1 byte) but may be more efficient if a larger block is passed.
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