[Cryptography] Simple provably secure stream cipher

[Bill Cox]

For any prime p suitable for Diffie-Hellman key agreement with group
generator g = 2, simply generate the binary digits of fraction(2^n/p),
where n is a shared secret.  XOR these digits over the message stream for
both encryption and decryption.

I'm ignoring issues such as the need for a unique nonce, and maliability
defense.  The standard fixes apply.  The ability to determine n is
trivially equivalent to solving the discrete log problem.

Is this well known?  I'm pretty much finding that everything seems to be
already known in crypto...

Bill
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