mpc.convert() from mpc.SecFld() has issues with negative numbers

It appears that the conversion from (signed) SecFld objects to other objects yields incorrect results when the shared secret has a negative value.

from mpyc.runtime import mpc

async def main():
    await mpc.start()
    secint = mpc.SecInt()
    secfld = mpc.SecFld(min_order=2**10, signed=True)

    a = secint(3)
    a = mpc.input(a, 0)
    print(await mpc.output(a))
    b = mpc.convert(a, secfld)
    print(await mpc.output(b))
    c = mpc.convert(b, secint)
    print(await mpc.output(c))

    await mpc.shutdown()

if __name__ == "__main__":
    mpc.run(main())

Result:

2020-03-03 16:55:50,690 Install package gmpy2 for better performance.
2020-03-03 16:55:50,691 Start MPyC runtime v0.6
-3
-3
1028
2020-03-03 16:55:50,700 Stop MPyC runtime -- elapsed time: 0:00:00.009001

Of course, we’d expect the result to be -3. Help would be appreciated!

[lschoe]

OK, I’ve now merged your fix. Thanks again! Implementation of function mpc.convert() is still quite limited so will need some further improvements later on. I’ll also reconsider the use of slots for the Runtime, probably better to drop this indeed.

[lschoe]

Yeah, also a good point. This is not so easy to fix, but certainly possible.

Near the end of _convert() in runtime.py there is this call “self._mod(ttype(x[i]), stype.field.modulus)” (line 490). This results in an unsigned value between 0 and the modulus.

To fix this issue, function _mod() needs to be extended to output a signed modulo value in the range -modulus/2 to modulus/2. That can be done at the expense of an extra comparison.

As a workaround you can add this comparison with modulus/2 in your application code. Do you want to try that?