import mx.utils.Base64Encoder;
import mx.utils.Base64Decoder;

// Compress a ByteArray into a Base64 String.
function compress(bytes:ByteArray):String { 
    var enc:Base64Encoder = new Base64Encoder();    
    enc.encodeBytes(bytes);
    return enc.drain().split("\n").join("");
}
 
// Uncompress a Base64 String into a ByteArray.
function uncompress(str:String):ByteArray {
    var dec:Base64Decoder = new Base64Decoder();
    dec.decode(str);
    var newByteArr:ByteArray=dec.toByteArray();        
    return newByteArr;
}

base64 Compress/Uncompress

ActionScript 3