[Ulya]

Вот, может кому тоже поможет. Короткий вариант.

var base64chars:String = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";
function encode(src:String):String {
	var i:Number = 0;
	var output:String = new String("");
	var chr1:Number, chr2:Number, chr3:Number;
	var enc1:Number, enc2:Number, enc3:Number, enc4:Number;
	while (i<src.length) {
		chr1 = src.charCodeAt(i++);
		chr2 = src.charCodeAt(i++);
		chr3 = src.charCodeAt(i++);
		enc1 = chr1 >> 2;
		enc2 = ((chr1 & 3) << 4) | (chr2 >> 4);
		enc3 = ((chr2 & 15) << 2) | (chr3 >> 6);
		enc4 = chr3 & 63;
		if (isNaN(chr2)) {
			enc3 = enc4=64;
		} else if (isNaN(chr3)) {
			enc4 = 64;
		}
		output += base64chars.charAt(enc1)+base64chars.charAt(enc2);
		output += base64chars.charAt(enc3)+base64chars.charAt(enc4);
	}
	//trace(output);
	return output;
}
var unEncoded:String = "ABC:123";
var A:String = encode(unEncoded);
trace("A="+A)