MPC: wrong output when using if

[sebw42]

Hi, I wanted to let you know that there seems to be an issue with the MPC compiler (on the mpc_aws branch) when using if-then-else.
Here is an example (with a set to 1 in the input file):

int main(__attribute__((private(0))) int a) {
    if ((a * 2) >= a) {
        return 0;
    }
    return a;
}

Build command:

export CARGO_MANIFEST_DIR="$PWD"; export RUST_BACKTRACE=1; ./target/release/examples/circ --parties 2 examples/C/mpc/program.c mpc  --cost-model "empirical"

Execution command:

/path/to/ABY/build/bin/aby_interpreter -m mpc -f ./scripts/aby_tests/tests/program_c -t ./scripts/aby_tests/test_inputs/program.txt -r 0 & /path/to/ABY/build/bin/aby_interpreter -m mpc -f ./scripts/aby_tests/tests/program_c -t ./scripts/aby_tests/test_inputs/program.txt -r 1

The output seems to be a random number (a different one with every execution).

I tried some things and found that the error disappears in the following cases:

• the guard evaluates to false (e.g. (a * 2) >= (a * 3))
• the left comparator in the guard does not contain secrets (e.g. 100 >= a)
• we return a constant instead of a at the end

The error also vanishes if we use a second input for the comparison (a set to 1, b set to 3 in this example):

int main(__attribute__((private(0))) int a, __attribute__((private(1))) int b) {
    if ((4 * a) >= b) {
        return 0;
    }
    return a;
}

Thank you for looking into this!