how to use SecFld.array() correctly

[devifaustine]

I've been having trouble creating a secure field array these past few days. I want to implement operations done in bytes and the only way I see in the demos is to use an array in a secure field. But the program never terminates. Consider the following program, which receives two strings as inputs, converts them into an array in a secure field, and outputs the secure objects created. This program is stuck in the loop. I thought it had something to do with numpy instances used, and I tried using different numpy instances, but this still does not work. Am I missing something? Originally I wanted to concatenate these two bytes, but even outputting those bytes is impossible. I appreciate any help you can provide.

import numpy as np
async def main():
    secfld = mpc.SecFld(2)
    await mpc.start()

    in_ = input("Give your input: ")
    # encode() converts string to bytes

    in_bit = ''.join(format(ord(i), '08b') for i in in_)
    print(in_bit)

    in_bit_list = [int(i) for i in in_bit]
    print(in_bit_list)

    inputs = mpc.input(secfld.array(np.array(in_bit_list)))

    print("input: ", await mpc.output(inputs))

    await mpc.shutdown()

[lschoe]

If I run the program with one party, there is no problem. So, I think you mean you experience this problem if you run the program with more than one party? For instance, with -M 2?

In that case, you may add --output-windows on the command line, to see the other party. For that party also an input needs to be entered, otherwise there will be no progress.

[devifaustine]

Thanks for the fast reply.
Yes, the problem occurs with -M2. I have already given the inputs in the command line for both parties using two different terminals.

And the program just stops there without terminating.

When I tried to debug it, it outputs runtime error and behaves differently than when I'm just running it normally.

Traceback (most recent call last):
  File "/usr/lib/python3.10/asyncio/events.py", line 80, in _run
    self._context.run(self._callback, *self._args)
python-BaseException  File "/home/devifaustine/.local/lib/python3.10/site-packages/mpyc/asyncoro.py", line 431, in <lambda>

    task.add_done_callback(lambda t: _reconcile(decl, t))
  File "/home/devifaustine/.local/lib/python3.10/site-packages/mpyc/asyncoro.py", line 355, in _reconcile
    givn = task.result()
  File "/home/devifaustine/.local/lib/python3.10/site-packages/mpyc/asyncoro.py", line 283, in _wrap_in_coro
    return await awaitable
  File "/home/devifaustine/.local/lib/python3.10/site-packages/mpyc/asyncoro.py", line 271, in __await__
    val = self.coro.send(None)
  File "/home/devifaustine/.local/lib/python3.10/site-packages/mpyc/runtime.py", line 452, in _distribute
    y = [[field.array(unmarshal(r), check=False).reshape(shape) for r in shares]]
  File "/home/devifaustine/.local/lib/python3.10/site-packages/mpyc/runtime.py", line 452, in <listcomp>
    y = [[field.array(unmarshal(r), check=False).reshape(shape) for r in shares]]
  File "/home/devifaustine/.local/lib/python3.10/site-packages/mpyc/finfields.py", line 1289, in reshape
    return type(self)(self.value.reshape(*args, **kwargs), check=False)
ValueError: cannot reshape array of size 8 into shape (16,)
Traceback (enclosing MPyC coroutine call):
  File "/home/devifaustine/Documents/Post-Quantum-Blind-Signature-with-MPC/test_secobj.py", line 237, in main11
    print("input: ", await mpc.output(inputs))
  File "/home/devifaustine/.local/lib/python3.10/site-packages/mpyc/runtime.py", line 396, in input
    return y
Traceback (most recent call last):
  File "/snap/pycharm-community/364/plugins/python-ce/helpers/pydev/pydevd.py", line 1534, in _exec
    pydev_imports.execfile(file, globals, locals)  # execute the script
  File "/snap/pycharm-community/364/plugins/python-ce/helpers/pydev/_pydev_imps/_pydev_execfile.py", line 18, in execfile
    exec(compile(contents+"\n", file, 'exec'), glob, loc)
  File "/home/devifaustine/Documents/Post-Quantum-Blind-Signature-with-MPC/test_secobj.py", line 463, in <module>
    mpc.run(main11())
  File "/home/devifaustine/.local/lib/python3.10/site-packages/mpyc/runtime.py", line 181, in run
    return self._loop.run_until_complete(f)
  File "/usr/lib/python3.10/asyncio/base_events.py", line 647, in run_until_complete
    raise RuntimeError('Event loop stopped before Future completed.')
RuntimeError: Event loop stopped before Future completed.

Process finished with exit code 1

[lschoe]

Right, I see. Both parties input a string of possibly different lengths, but they don't know the input length used by the other party. To fix this you can set a maximum length for both parties, and let each party pad its input string in_ to this maximum length by concatenating " "s for instance. Then all parties will end up using a secure array of the same length (hence of the same shape), which is a prerequisite for mpc.input() to work with arrays.

Another thing is that mpc.output() currently does not support lists of secure arrays. So, to see everything in inputs, you can put this print("input: ", i, await mpc.output(inputs[i])) in a for i in range(len(inputs)) loop.

[devifaustine]

I tried padding it with blankspace and use a for loop for the mpc.output(), but for some reasons the input from the second party cannot be accessed and it just stops after printing out input[0].

Even if I change the for loop to just print("input 1: ", await mpc.output(input[1])) the program does not execute this.

async def main():
    secfld = mpc.SecFld(2)
    await mpc.start()

    in_ = input("Give your input: ")
    
    # pad the input to max length 
    max = 10
    if len(in_) < max:
        in_ = pad(in_, max)

    in_bit = ''.join(format(ord(i), '08b') for i in in_)
    print(in_bit)

    in_bit_list = [int(i) for i in in_bit]
    print(in_bit_list)

    inputs = mpc.input(secfld.array(np.array(in_bit_list)))

    # mpc.output() does not support outputting list
    for i in range(len(inputs)):
        print("input ", i, ": ", await mpc.output(inputs[i]))

    await mpc.shutdown()

def pad(string, length):
    while len(string) < length:
        string += " "
    return string

Thanks again and sorry to bother you :)

[lschoe]

Your program works for me, but when I run it with MPyC v0.9 the same issue arises.

So, if you update to the latest version, e.g., using pip install git+https://github.com/lschoe/mpyc the problem will probably be gone.

[devifaustine]

Yeah, that seemed to be the problem :D, thank you so much for your help.