`mpc.xor` producing incorrect value

[SynthLuvr]

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import sys

from mpyc import mpctools
from mpyc.runtime import mpc


async def main():
    my_input = int(sys.argv[1], 16)
    secint = mpc.SecInt(256)

    await mpc.start()
    all_inputs = mpc.input(secint(my_input))
    combined_inputs = mpctools.reduce(mpc.xor, all_inputs)
    entropy = await mpc.output(combined_inputs)
    await mpc.shutdown()

    key = hex(entropy)[2:]
    print(key)


mpc.run(main())

Run with:

➜ python src/compute.py -M2 -I0 --no-log 6bf558aeeb81970a5d82e63f8c785f6a03b0f5355f276ef986b8b8b1cff1c6cb
➜ python src/compute.py -M2 -I1 --no-log 96dc2474ff71bab6001c1b81c0d1ff13a8e64264e68c031666cc521a662dbe14
102d17d23eaf351c05d9f01c14d4a5e7dac97379a45b3720fed850acc361f84df

The first issue is there's an overflow, 257 bit instead of 256. Second issue is that this value is incorrect. Correct result:

hex1 = "6bf558aeeb81970a5d82e63f8c785f6a03b0f5355f276ef986b8b8b1cff1c6cb"
hex2 = "96dc2474ff71bab6001c1b81c0d1ff13a8e64264e68c031666cc521a662dbe14"

num1 = int(hex1, 16)
num2 = int(hex2, 16)

final = num1 ^ num2

key = hex(final)[2:]
print(key)

Running this produces fd297cda14f02dbc5d9efdbe4ca9a079ab56b751b9ab6defe074eaaba9dc78df, whereas running via mpc.xor produces 102d17d23eaf351c05d9f01c14d4a5e7dac97379a45b3720fed850acc361f84df

[SynthLuvr]

I think I found the fix. The xor function is implemented incorrectly. By doing a loop and the xor manually it produces the correct result:

import sys

from mpyc import mpctools
from mpyc.runtime import mpc


async def main():
    my_input = int(sys.argv[1], 16)
    secint = mpc.SecInt(256)

    await mpc.start()
    all_inputs = mpc.input(secint(my_input))
    combined_inputs = all_inputs[0]

    for input_value in all_inputs[1:]:
        a_bits = mpc.to_bits(combined_inputs)
        b_bits = mpc.to_bits(input_value)
        and_bits = mpc.schur_prod(a_bits, b_bits)
        xor_bits = mpc.vector_sub(
            mpc.vector_add(a_bits, b_bits), mpc.vector_add(and_bits, and_bits)
        )
        combined_inputs = mpc.from_bits(xor_bits)

    entropy = await mpc.output(combined_inputs)
    await mpc.shutdown()

    key = hex(entropy)[2:]
    print(key)


mpc.run(main())

[SynthLuvr]

Instead of:

def xor(self, a, b):
    """Secure bitwise xor of a and b."""
    return a + b

I think the implementation should be something like:

def xor(self, a, b):
    """Secure bitwise xor of a and b."""
    a_bits = self.to_bits(a)
    b_bits = self.to_bits(b)
    and_bits = self.schur_prod(a_bits, b_bits)
    xor_bits = self.vector_sub(
        self.vector_add(a_bits, b_bits),
        self.vector_add(and_bits, and_bits),
    )
    return self.from_bits(xor_bits)

[lschoe]

Please see issue #36. The implementation of mpc.xor() is not meant to cover the general case because that's very costly.

[SynthLuvr]

It'd be great if this was documented, and also a code sample added to show how to do it