[Cryptography] "Perpetual Encryption"

[Allen]

> if you have (Mi XOR Ri) and (Pi XOR Ri) the logical next step is
> (Mi XOR Ri) XOR (Pi XOR Ri) leaving you with Mi^Pi

I simplified some--the scheme may use something more complicated than
an XOR, something like reversibly mixing bits in blocks (as in a block
cipher)--it's not at all clear though from the whitepaper.
Fundamentally, this scheme may be equivalent to a block or stream
cipher with a random IV that is the same length as the message.