Python version of http://stackoverflow.com/a/7530325/345480

IntCrypt.py

"""
/**
 * IntegerPerm is a reversible keyed permutation of the integers.
 * This class is not cryptographically secure as the F function
 * is too simple and there are not enough rounds.
 *
 * @author Martin Ross
 */
"""
def rshift(val, n): return val>>n if val >= 0 else (val+0x100000000)>>n

class IntCrypt():
    DEFAULT_KEY = 0x6CFB18E2
    LOW_16_MASK = 0xFFFF
    HALF_SHIFT = 16
    NUM_ROUNDS = 4

    def __init__(self, key=None):
        self.mRoundKeys = [0,] * self.NUM_ROUNDS
        self.setkey(key if not key==None else self.DEFAULT_KEY)
    
    def setkey(self, key):
        self.mKey = key;
        self.mRoundKeys[0] = self.mKey & self.LOW_16_MASK
        self.mRoundKeys[1] = ~(self.mKey & self.LOW_16_MASK)
        self.mRoundKeys[2] = rshift(self.mKey, self.HALF_SHIFT)
        self.mRoundKeys[3] = ~(rshift(self.mKey,self.HALF_SHIFT))

    def encrypt(self, plain):
        rhs = plain & self.LOW_16_MASK
        lhs = rshift(plain, self.HALF_SHIFT)
        
        for i in range(self.NUM_ROUNDS):
            if i>0:
                temp = lhs
                lhs = rhs
                rhs = temp
            f = self.F(lhs, i)
            rhs ^= self.F(lhs, i)
            rhs = rhs & self.LOW_16_MASK
            #print "lhs %s rhs %s" % (lhs, rhs)
        r = (lhs  << self.HALF_SHIFT) + (rhs & self.LOW_16_MASK)
        return r

    def decrypt(self, cypher):
        rhs = cypher & self.LOW_16_MASK
        lhs = rshift(cypher, self.HALF_SHIFT)
        for i in range(self.NUM_ROUNDS):
            if i>0:
                temp = lhs
                lhs = rhs
                rhs = temp
            f = self.F(lhs, self.NUM_ROUNDS - 1 - i)
            rhs ^= f
            rhs = rhs & self.LOW_16_MASK
            #print "lhs %s rhs %s" % (lhs, rhs)
        return (lhs << self.HALF_SHIFT) + (rhs & self.LOW_16_MASK)

    def F(self, num, round):
        num ^= self.mRoundKeys[round]
        num *= num;
        return rshift(num, self.HALF_SHIFT) ^ (num & self.LOW_16_MASK)