Question

I’m looking for a PRNG (pseudo randomness) that you initially seed with an arbitrary array of bytes.

Heard of any?

Answer 1

Hashing your arbitrary length seed (instead of using XOR as paxdiablo suggested) will ensure that collisions are extremely unlikely, i.e. equal to the probability of a hash collision, with something such as SHA1/2 this is a practical impossibility.

You can then use your hashed seed as the input to a decent PRNG such as my favourite, the Mersenne Twister.

UPDATE

The Mersenne Twister implementation available here already seems to accept an arbitrary length key: http://code.msdn.microsoft.com/MersenneTwister/Release/ProjectReleases.aspx?ReleaseId=529

UPDATE 2

For an analysis of just how unlikely a SHA2 collision is see how hard someone would have to work to find one, quoting http://en.wikipedia.org/wiki/SHA_hash_functions#SHA-2 :

There are two meet-in-the-middle preimage attacks against SHA-2 with a reduced number of rounds. The first one attacks 41-round SHA-256 out of 64 rounds with time complexity of 2^253.5 and space complexity of 2^16, and 46-round SHA-512 out of 80 rounds with time 2^511.5 and space 2^3. The second one attacks 42-round SHA-256 with time complexity of 2^251.7 and space complexity of 2^12, and 42-round SHA-512 with time 2^502 and space 2^22.