Bug report: Utils.modInv assumes modulus of 26

[profbbrown]

Describe the bug
The Utils.modInv function internally uses 26 as the modulus rather than the argument y. The bug is not externally visible because it's used in the Affine Decode operation, which uses an alphabet with 26 letters.

Source code

static modInv(x, y) {
    x %= y;
    for (let i = 1; i < y; i++) {
        if ((x * i) % 26 === 1) {
            return i;
        }
    }
}

Remedy
Replace 26 with y.

Better still: replace the entire function with one that uses the Extended Euclidean Algorithm to compute the modular multiplicative inverse. It will be faster than a brute-force loop for large values of y.

[profbbrown]

Pull request #1836