rsa_example.js

/* 

Not putting any comments, just follow this excellent tutorial
https://www.pagedon.com/rsa-explained-simply/programming

Some more credits:
https://gist.github.com/krzkaczor/0bdba0ee9555659ae5fe
https://www.pagedon.com/modular-exponentiation/programming

*/

p = 29
q = 31
n = p * q
t = (p - 1) * (q - 1)


function isPrime(value) {
    for(var i = 2; i < value; i++) {
        if(value % i === 0) {
            return false;
        }
    }
    return value > 1;
}

e = 1

while (true) {
    if (!(t % e === 0)) {
        if (isPrime(e)) {
            break
        }
    }
    e++
}

d = 1

while (true) {
    if ((d * e) % t !== 1) {
        d++
    } else {
        break;
    }
}


var modexp = function(a, b, n) {
  a = a % n;
  var result = 1;
  var x = a;

  while(b > 0){
    var leastSignificantBit = b % 2;
    b = Math.floor(b / 2);

    if (leastSignificantBit == 1) {
      result = result * x;
      result = result % n;
    }

    x = x * x;
    x = x % n;
  }
  return result;
};

function enc(s) {
    return s.split('').map(function(c) {
        char_id = c.charCodeAt(0)
        char_id_enc = modexp(char_id,e,n)
        //Math.pow(char_id,e) % n
        return char_id_enc
        })
}

function dec(a) {
    return a.map(function(char_id_enc) {
        char_id = modexp(char_id_enc,d,n)
        return char_id
        }).map(function(c) {
            return String.fromCharCode(c)
    }).join("")
}

encrypted = enc("This is top - secret")
dec(encrypted)

Thanks for this code. Strangely, it seems to break when p = 5 and q = 11