crypto1

var refText, encodedMessage, decodedMessage;

function decodeMessage(ref, key) {
    var keyArr = key.split('');
    var keyArrCopy = [].concat(keyArr);
    var dict = [];
    var result = '';
    
    while(keyArr.length > 0) {
        var k = keyArr.splice(0, keyArr.shift()).join('');
        var c = ref.charAt( Number(k) );
        
        result += c;
    }
    
    return result;
}

function encodeMessage(ref, message) {
    var c, r, t, refMap = {}, key = [];
    
    ref.toLowerCase().split('').map( function(char, i) {
         refMap[char] && refMap[char].push(i) || (refMap[char] = [i]); 
    });
    
    message.toLowerCase().split('').map( function(char, i) {
        c = refMap[char] || [];
        r = getRandomInt(0,c.length-1);
        t = (c[ r ].toString().length).toString() + c[ r ].toString();
        
        key.push( t );
    });
    
    return key.join('');
}

function getRandomInt(min, max) {
    return Math.floor(Math.random() * (max - min + 1)) + min;
}

refText = 'A quick movement of the enemy will jeopardize six gunboats.';

encodedMessage = encodeMessage(refText, 'encoded message here');
decodedMessage = decodeMessage(refText, encodedMessage);
console.log('Encoded Key: ', encodedMessage);
console.log('Decoded Message: ', decodedMessage);