avlidienbrunn/gist:652e63e4331b8251c21c

## gistfile1.html
<html>
	<head>
		<script>
		function text2bin(text){
			var pad = "00000000";
			var result = "";
			for(index in text){
				result += (pad + text.charCodeAt(index).toString(2)).slice(-8); //Pad with zeroes so that each char always becomes 8 binary chars
			}
			return result;
		}
		function bin2int(text){
			return parseInt(text, 2);
		}
		function int2bin(text){
				var pad = "00000000";
			var result = "";
			for(index in text){
				result += (pad + text.toString(2)).slice(-8); //Pad with zeroes so that each char always becomes 8 binary chars
			}
			return result;
		}
		function parseString(text){
			//WARNING: Ugly hack ahead!
			text = text.replace(/"/g, "\\x22");
			return eval('"' + text + '"')
		}

		function int2base64(char){
			var base64table = ["A","B","C","D","E","F","G","H","I","J","K","L","M","N","O","P","Q","R","S","T","U","V","W","X","Y","Za","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z","0","1","2","3","4","5","6","7","8","9","+","/"];
			return base64table[char];
		}


		function validate(startswith, desired){
			bitcount = Math.pow(2, 6 - startswith.length); //Calculate how many bits to iterate
			startswithint = bin2int((startswith + "000000").slice(6));
			valid = [];
			for(x=0;x<bitcount;x++){
				currentchar = startswithint + x;
				if(desired.indexOf(int2base64(currentchar)) != -1){
					console.log(desired);
					//The next charcter can start with the binary representation of x!
					valid.push(x);
				}
			}
			return valid;
		}
		</script>
		<script>
			var startingchars = null;
			function encodeArea(){
				var item1 = document.getElementById('textbox');
				var item2 = document.getElementById('textboxbase64');
				var parsed = parseString(item1.value);

				calulatePossible();

				item2.value = btoa(parsed);
				item1.vaue = atob(item2.value);
			}
			function calculateStartingChars(){
				var desired = document.getElementById('desired').value + "=";
				var possible = document.getElementById('possible');
				startingchars = [];
				var found = false;
				for(char1=0;char1<255;char1++){
					for(char2=0;char2<255;char2++){
						b64 = btoa(String.fromCharCode(char1) + String.fromCharCode(char2));
						if(desired.indexOf(b64[0]) != -1 && desired.indexOf(b64[1]) != -1){
							startingchars.push((String.fromCharCode(char1)));
							break;
						}
					}
					found = false;
				}
				possible.value = startingchars.join(',').replace(/(\s)/g, escape);
			}
			function calulatePossible(){
				var desired = document.getElementById('desired').value + "=";
				var possible = document.getElementById('possible');
				var parsed = parseString(document.getElementById('textbox').value);
				var possiblechars = [];
				var currentIndex = (parsed.length - 1) % 3; //Base64 works in triplets


				if(startingchars == null || parsed == ""){
					calculateStartingChars();
					possible.value = startingchars.join(',').replace(/(\s)/g, escape);
					return;
				}
				parsed = parsed.replace(/(?:.{3})*(.{1,3})/g, "$1"); //Remove every triplet before the last triplet (aaabbbcc will become cc)

				if(currentIndex == 0){
					for(char1=0;char1<256;char1++){
						b64 = btoa(parsed + String.fromCharCode(char1));
						if(desired.indexOf(b64[0]) != -1 && desired.indexOf(b64[1]) != -1){
							//Found one character that will generate DD?? (D = desired)
							for(char2=0;char2<256;char2++){
								b64inner = btoa(parsed + String.fromCharCode(char1) + String.fromCharCode(char2));
								if(desired.indexOf(b64[2]) != -1 && desired.indexOf(b64[3]) != -1){
									//Found one character that will generate DDDD (D = desired)
									possiblechars.push((String.fromCharCode(char1)));
									break;
								}
							}
						}
					}
				}
				if(currentIndex == 1){
					for(char1=0;char1<256;char1++){
						b64 = btoa(parsed + String.fromCharCode(char1));
						if(desired.indexOf(b64[2]) != -1 && desired.indexOf(b64[3]) != -1){
							//Found one character that will generate DDDD (D = desired)
							possiblechars.push((String.fromCharCode(char1)));
						}
					}
				}
				if(currentIndex == 2){
					possiblechars = startingchars;
				}
				possible.value = possiblechars.join(',').replace(/(\s)/g, escape);
			}
		</script>
		<style>
			/* CSS. Not even once. */
			textarea {width: 49%; height: 200;}
			input {width: 300px;}
			</style>
	</head>
	<body>
		Desired charset: <input type="text" id="desired" value="0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ+/"/><button onclick="calculateStartingChars()">Set charset</button><br />
		<textarea onkeyup="encodeArea()" id="textbox"></textarea>
		<textarea onkeyup="encodeArea()" id="textboxbase64"></textarea><br />
		Possible next characters: <textarea id="possible"></textarea>
	</body>
</html>
	<html>
	<head>
	<script>
	function text2bin(text){
	var pad = "00000000";
	var result = "";
	for(index in text){
	result += (pad + text.charCodeAt(index).toString(2)).slice(-8); //Pad with zeroes so that each char always becomes 8 binary chars
	}
	return result;
	}
	function bin2int(text){
	return parseInt(text, 2);
	}
	function int2bin(text){
	var pad = "00000000";
	var result = "";
	for(index in text){
	result += (pad + text.toString(2)).slice(-8); //Pad with zeroes so that each char always becomes 8 binary chars
	}
	return result;
	}
	function parseString(text){
	//WARNING: Ugly hack ahead!
	text = text.replace(/"/g, "\\x22");
	return eval('"' + text + '"')
	}

	function int2base64(char){
	var base64table = ["A","B","C","D","E","F","G","H","I","J","K","L","M","N","O","P","Q","R","S","T","U","V","W","X","Y","Za","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z","0","1","2","3","4","5","6","7","8","9","+","/"];
	return base64table[char];
	}


	function validate(startswith, desired){
	bitcount = Math.pow(2, 6 - startswith.length); //Calculate how many bits to iterate
	startswithint = bin2int((startswith + "000000").slice(6));
	valid = [];
	for(x=0;x<bitcount;x++){
	currentchar = startswithint + x;
	if(desired.indexOf(int2base64(currentchar)) != -1){
	console.log(desired);
	//The next charcter can start with the binary representation of x!
	valid.push(x);
	}
	}
	return valid;
	}
	</script>
	<script>
	var startingchars = null;
	function encodeArea(){
	var item1 = document.getElementById('textbox');
	var item2 = document.getElementById('textboxbase64');
	var parsed = parseString(item1.value);

	calulatePossible();

	item2.value = btoa(parsed);
	item1.vaue = atob(item2.value);
	}
	function calculateStartingChars(){
	var desired = document.getElementById('desired').value + "=";
	var possible = document.getElementById('possible');
	startingchars = [];
	var found = false;
	for(char1=0;char1<255;char1++){
	for(char2=0;char2<255;char2++){
	b64 = btoa(String.fromCharCode(char1) + String.fromCharCode(char2));
	if(desired.indexOf(b64[0]) != -1 && desired.indexOf(b64[1]) != -1){
	startingchars.push((String.fromCharCode(char1)));
	break;
	}
	}
	found = false;
	}
	possible.value = startingchars.join(',').replace(/(\s)/g, escape);
	}
	function calulatePossible(){
	var desired = document.getElementById('desired').value + "=";
	var possible = document.getElementById('possible');
	var parsed = parseString(document.getElementById('textbox').value);
	var possiblechars = [];
	var currentIndex = (parsed.length - 1) % 3; //Base64 works in triplets


	if(startingchars == null \|\| parsed == ""){
	calculateStartingChars();
	possible.value = startingchars.join(',').replace(/(\s)/g, escape);
	return;
	}
	parsed = parsed.replace(/(?:.{3})*(.{1,3})/g, "$1"); //Remove every triplet before the last triplet (aaabbbcc will become cc)

	if(currentIndex == 0){
	for(char1=0;char1<256;char1++){
	b64 = btoa(parsed + String.fromCharCode(char1));
	if(desired.indexOf(b64[0]) != -1 && desired.indexOf(b64[1]) != -1){
	//Found one character that will generate DD?? (D = desired)
	for(char2=0;char2<256;char2++){
	b64inner = btoa(parsed + String.fromCharCode(char1) + String.fromCharCode(char2));
	if(desired.indexOf(b64[2]) != -1 && desired.indexOf(b64[3]) != -1){
	//Found one character that will generate DDDD (D = desired)
	possiblechars.push((String.fromCharCode(char1)));
	break;
	}
	}
	}
	}
	}
	if(currentIndex == 1){
	for(char1=0;char1<256;char1++){
	b64 = btoa(parsed + String.fromCharCode(char1));
	if(desired.indexOf(b64[2]) != -1 && desired.indexOf(b64[3]) != -1){
	//Found one character that will generate DDDD (D = desired)
	possiblechars.push((String.fromCharCode(char1)));
	}
	}
	}
	if(currentIndex == 2){
	possiblechars = startingchars;
	}
	possible.value = possiblechars.join(',').replace(/(\s)/g, escape);
	}
	</script>
	<style>
	/* CSS. Not even once. */
	textarea {width: 49%; height: 200;}
	input {width: 300px;}
	</style>
	</head>
	<body>
	Desired charset: <input type="text" id="desired" value="0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ+/"/><button onclick="calculateStartingChars()">Set charset</button><br />
	<textarea onkeyup="encodeArea()" id="textbox"></textarea>
	<textarea onkeyup="encodeArea()" id="textboxbase64"></textarea><br />
	Possible next characters: <textarea id="possible"></textarea>
	</body>
	</html>