`SecInt` bit widths seemingly not enforced

Suppose I run the following program on three parties, with input hardcoded purely to demonstrate this issue:

from mpyc.runtime import mpc
async def main():
    await mpc.start()
    a0 = mpc.input(mpc.SecInt(8)(300))
    a1 = a0[0] + a0[1] + a0[2]
    print(await mpc.output(a1))
    await mpc.shutdown()
mpc.run(main())

(I’m executing this program on localhost in three separate shells via python prog.py -M 3 -I {0,1,2} -B 8000, Python 3.10.5.)

At first glance, I would expect this program to either:

• Produce an error at the attempt to initialize SecInt(8) with a value that overflows the type’s nominal range,
• Produce an error at the attempt to output the result of the overflowing addition, or
• Silently overflow and produce the result -124 (i.e. (300 + 300 + 300) & 0xff interpreted as a two’s-complement signed value)

Instead, the program produces the result 900. Should I expect this behavior, or is there some way I can leverage the library’s other types to achieve one of the potential behaviors I listed above?

[lschoe]

MPyC v0.8.8 with support for secure (secret-shared) NumPy arrays is now available on GitHub, check it out!

Threshold SHA3 is one of the new demos, see sha3.py.

[samcowger]

Cool! Thank you for the advice, and for the SHA3 pointer. I think my original question has been sufficiently addressed, so I’ll go ahead and close this issue.