[Cryptography] Simple provably secure stream cipher

[Ben Laurie]

On 6 June 2015 at 08:04, Bill Cox <waywardgeek at gmail.com> wrote:
> 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...

One thing that is already known is that if you rely on the DLP, you're
not provably secure.