Steven M. Bellovin wrote:
It's moderately complex if you're trying to conserve bandwidth (which
translates to power) and preserve a datagram model.  The latter
constraint generally rules out stream ciphers; the former rules out
things like encrypting the keystroke plus seven random bytes with a
64-bit block cipher.  Power is also an issue if your cipher uses very
much CPU time or custom hardware.

I"m sure most readers of this list can propose *some* solution.  It's
instructive, though, to consider everything that needs to go into a
full system solution, including the ability to resynchronize cipher
states and the need to avoid confusing naive users if the cat happened
to fall asleep on the space bar while the CPU was turned off.

Use CFB mode. That takes care of all the above problems. You can transmit any small bunch of bits, don't need to transmit a complete block, and if the keyboard and the receiver get out sync, the keyboard's signal will be decrypted as garbage for the first 128 bits. If one has the keyboard regularly transmit "no key's pressed" from time to time, and if valid key press representations have a couple of check bits redundancy, with several keypresses being ignored after any invalid key signal, keyboard and receiver will synchronize with no fuss.


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