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import hashlib | |
import math | |
def prf_under_n(N, key, tweak): | |
b = math.ceil(math.log(N, 2)) | |
c = 0 | |
while True: | |
h = "" | |
while len(h) * 4 < b: | |
h += hashlib.sha256("prf_under_n,{},{},{},{}".format(c, N, key, tweak).encode("ascii")).hexdigest() | |
c += 1 | |
rand = int(h, 16) & (2**b - 1) | |
if rand < N: | |
return rand | |
def prf_coinflip(i, key, tweak): | |
return hashlib.sha256("prf_coinflip,{},{},{}".format(i, key, tweak).encode("ascii")).digest()[0] & 1 | |
class SometimeShuffle: | |
def __init__(self, N, key): | |
self.N = N | |
self.key = key | |
self.rounds = 7 * math.ceil(math.log(N, 2)) | |
self.round_keys = [prf_under_n(N, key, r) for r in range(self.rounds)] | |
if N > 1: | |
self.recurse = SometimeShuffle(N//2, key) | |
def __call__(self, x): | |
if self.N == 1: | |
return x | |
for r, K in enumerate(self.round_keys): | |
x2 = (K - x) % self.N | |
if prf_coinflip(max(x, x2), self.key, r): | |
x = x2 | |
if x < self.N // 2: | |
return self.recurse(x) | |
return x | |
p = SometimeShuffle(10, 2) | |
print([p(i) for i in range(10)]) | |
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