OTP

otp.js

// JavasScript solution to the NSA's OTP puzzle
// https://www.facebook.com/NSACareers/photos/a.10150165394744358.374663.38534064357/10155202632259358/?type=3&theater
var cipherText = "6097703920902805098792458100127006308920278750110017283152904512008635073921961285410397244195102032905201942802717080593227";
var key = "6981642705701301086201207791115091207421138236919216132358913111926129022415841781360483274671901231854407951401635567442416";
var message = "";

// offset this so that letters.charAt(1) returns A and letters.charAt(26) returns Z
var letters = " abcdefghijklmnopqrstuvwxyz";

// Doing the second step here: https://en.wikipedia.org/wiki/One-time_pad#Example
// Stepping through the cipherText and the key one character at a time
// If the cipherText character (c) is less than the key character (k), add 10
for (var i = 0; i<cipherText.length; i++) {
    var c = Number(cipherText.charAt(i));
    var k = Number(key.charAt(i));
    if (c < k) {
        c += 10;
    }
    var p = c - k;
    message += p.toString();
}

// message now contains a string of numbers. To convert these to letters, first split them into
// two digit chunks.
var plainTextArray = message.match(/.{1,2}/g);

var answer = "";

// Now loop through each chunk, convert it to a number, and then add the coresponding letter value 
// to the answer
plainTextArray.forEach(function(element, index, array) {
    // Each two digit number is a letter. So "01" is a, "26" is z. For values > 26
    // go around to the start of the alphabet
    var letterIndex = Number(element) % 26;    
    answer += letters[letterIndex];
})

// output the answer
console.log(answer);