I'd just do pointer-chasing through the file, xored with the initial hash each time. I.e. out = hash xor data[hash] xor data[hash xor data[hash]] ... Alternat

I'd just do pointer-chasing through the file, xored with the initial hash each time. I.e. out = hash xor data[hash] xor data[hash xor data[hash]] ... Alternative phrasing: out_n = data[hash xor out_(n-1)] xor out_(n-1), where out_1 = data[hash] xor hash "Random" accesses throughout the file - not the easiest to run in parallel. (That being said, I haven't checked to see if this is breakable)