A proxy can help you listen to what is being sent back and forth. The java probably does something with the return value that you may not have included in the code such as an xml file. Again a proxy can help you get more information especially if there is a filter feature.

sendDataToServer ("post_login.cgi?data=" + convertToBase64(a),
loginReturnValue)
alert (loginReturnValue);
alert (convertToBase64(a));
}
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