Trying to send a compressed base64 encoded string from JS to AS3 via Websocket. Problem with uncompr-CodePudding

Hello and to a very happy and more healthy 2021!

I am setting up a small communication interface between a JS client and a AS3 Server based on the Websockets protocol. For various reasons I need to compress and base64 encode the payloads. From AS3 to JS everything works like this (using https://github.com/blooddy/blooddy_crypto to handle the Base64 en/decryption ) :

function encodeMessage(message:String):String{
    var rawData:ByteArray=new ByteArray()
    rawData.writeUTFBytes( encodeURIComponent(message) );
    rawData.compress();
    var b64 = Base64.encode(rawData);
    return b64;
};

decoding in JS with pako for inflation (https://github.com/nodeca/pako):

decodePayload(payload){
    let rawfile = (atob(payload));
    var bytes = [];
    for (var fileidx = 0; fileidx < rawfile.length; fileidx  ) {
        var abyte = rawfile.charCodeAt(fileidx) & 0xff;
        bytes.push(abyte);
    }
    var plain = pako.inflate(bytes);
    var enc = "";
    for (var i = 0; i < plain.length; i  ) {         
         enc  = String.fromCharCode(plain[i]);
    }
    return decodeURIComponent(enc);
}

now the other direction creates some problems: in JS I use:

encodeMessage(message){
    let enc = encodeURIComponent(message)
    let zlib = pako.deflate(enc)
    let b64 = btoa(zlib);
    return b64;
}

but then I am running into issues on the AS3 side:

function decodePayload(payload:String){
    var ba:ByteArray = Base64.decode(payload);
    //this is where the error happens
    ba.uncompress();
   
}

the error is a "Error: Error #2058: There was an error decompressing the data." I suspect that the bytearry i receive from pako.deflate is different from what AS3 is using?

Any pointers welcome!

CodePudding user response：

here is the solution - a stupid little oversight of course ;)

in the encode function in JS the Uint8Array needs to be converted into a BinaryString before it gets Base64 encoded:

function encodeMessage(message){
    let enc = encodeURIComponent(message);
    let zlib = pako.deflate(enc);
    let binstring = convertUint8ArrayToBinaryString(zlib); 
    let b64 = btoa(binstring);
    return b64;
};
function convertUint8ArrayToBinaryString(u8Array) {
    
    var i, len = u8Array.length, b_str = "";
    for (i=0; i<len; i  ) {
        b_str  = String.fromCharCode(u8Array[i]);
    }
    return b_str;
}

Then all is well in Roswell.

CodePudding user response：

It's not obvious if your AS3 code is used in just an SWF file or you have an AIR project.

If making an AIR project:

Try using uncompress.apply... (code not yet tested).

//# this is where the error happens
//ba.uncompress();

//# try this..
ba.uncompress.apply(ba, ["deflate"]);

If making an SWF project:

When I needed Deflate algorithm inside an SWF running on browser I used a port of Zlib.

Get as3zlib at: https://github.com/BenV/as3zlib (is a port of Zlib for Java).

note: While in JS it's better to make local vars rather than global, this a quirk of JavaScript and actually in most C-like languages (Java, C#, AS3, etc) this is opposite, a global variable is less expensive than constantly making local vars every time the function is called.

Here is a usage example:

//# import Zlib Class
import Zlib;

//# setup global vars
var zlibdecomp :Zlib;
var ba :ByteArray = new ByteArray;
var decoded_BA :ByteArray = new ByteArray;

//# try Deflate algo...
function decodePayload(payload:String)
{
    zlibdecomp = new Zlib;
    ba.clear(); //in case it already has data from a prev decode
    
    //# all code below can be done in this one line
    //ba = zlibdecomp.uncompress( Base64.decode(payload) );
    
    ba = Base64.decode(payload);
    
    //# this is where the error happens...
    //ba.uncompress();
    
    //# try this...
    decoded_BA = zlibdecomp.uncompress( ba ); //inflate to original big from small/compressed
    //ba = decoded_BA; //if this way of updating is needed
}