[ruslan_mart]

Можно как-то так:

var Shuffler = (function() {

	var joinReplaceCallback = function() {
		return Math.random() > 0.5 ? 'e' : 'f';
	};

	return {
		decode: function(array, key) {
			var tempArray = array.slice(); //copy source array

			array.splice(0, array.length); //clear source array
			key = key.split(/[ef]/);

			for(var i = 0, value; i !== key.length; i++) {
				value = parseInt(key[i], 14);
				array[value] = tempArray[i];
			}
		},
		encode: function(array) {
			var indexes = [],
				key = [],
				tempArray = array.slice(), //copy source array
				value;

			array.splice(0, array.length); //clear source array

			while(array.length !== tempArray.length) {
				value = Math.floor(Math.random() * tempArray.length);

				if(indexes.indexOf(value) === -1) {
					array.push(tempArray[value]);
					indexes.push(value);
					key.push(value.toString(14));
				}
			}

			return key.join('$').replace(/\$/g, joinReplaceCallback);
		}
	};

})();

var array = ['a', 'b', 'c', 'd', 'e', 'f', 'g'];

var secretKey = Shuffler.encode(array);

console.log(array, secretKey);

Shuffler.decode(array, secretKey);

console.log(array);