SafeLessThan is unsafe

[0xDatapunk]

zk-kit.circom/packages/utils/src/safe-comparators.circom

Line 16 in 326cef9

n2b = Num2Bits_strict()(in[0] + (1<<n) - in[1]);

the inputs are not constrained to be in the range of n. A counter example is

Found SafeLessThan counter example:
n: 8
in: [1000, 512]
Expected: 0 Got: 1

A suggested change:

template SafeLessThan(n) {
    assert(n <= 252);
    signal input in[2];
    signal output out;

    // Constrain inputs to n bits
    component range_check[2];
    for (var i = 0; i < 2; i++) {
        range_check[i] = Num2Bits(n);
        range_check[i].in <== in[i];
    }

    component n2b = Num2Bits();
    n2b.in <== in[0] + (1<<n) - in[1];

    out <== 1 - n2b.out[n];
}

[0x471]

Hi, can I work on this issue? @0xDatapunk @cedoor

[cedoor]

Hi @0x471, we only assign one task at a time to contributors, you can work on other tasks as soon as #14 is closed.