Sending encrypted file over socket

Question

I'm having some issues with sending an encrypted file over a socket (client-server communication). I think this is an issue related with the streams (more specifically the arrangement of streams seems to be rather complex, since I'm making data input/output streams from object input/output streams and then, cipher input/output streams from those)...

Client side:

private static void sendEncrypted(FileInputStream fis,DataOutputStream dos)throws Exception{

try {
        byte[] data = new byte[1024];

        CipherOutputStream cos = new CipherOutputStream(output, c);

        makeFileHeader(dos, c, key, sig);

        int i = fis.read(data);

        while(i != -1){
            cos.write(data, 0, i);
            sig.update(data);
            cos.flush();
            i = fis.read(data);
        }
        cos.close();
        fis.close();

byte[] signatureObj = sig.sign();

        output.writeObject(signatureObj);

    } catch (Exception e) {
        System.out.println(e.toString());
    }
}

dos - created from the original objectoutputstream (output, that was created from the socket) makefileheader - method that creates an object and uses output to write it over

Server side:

private static void receiveFiles(DataInputStream dis,FileOutputStream fos, int size) throws IOException{

        byte [] b = new byte[1024];
        int read = 0;
        int offset;
        while(read < size){
            offset = input.read();
            dis.read(b,0,offset);
            fos.write(b,0,offset);
            read +=offset;
        }

    }

Answer 1

Adding this answer, so the question can be closed:

I see you are writing on output directly after closing cos which is created on top of output. This is not possible.

From the docu of CipherOutputStream.close(): "This method invokes the doFinal method of the encapsulated cipher object, which causes any bytes buffered by the encapsulated cipher to be processed. The result is written out by calling the flush method of this output stream. This method resets the encapsulated cipher object to its initial state and calls the close method of the underlying output stream."

Answer 2

sig.update(data);

That should be

sig.update(data, 0, i);