Base64 encodes a string using the URL-safe characters specified by
        Amazon.

def _url_base64_encode(msg):
        """
        Base64 encodes a string using the URL-safe characters specified by
        Amazon.
        """
        msg_base64 = base64.b64encode(msg)
        msg_base64 = msg_base64.replace('+', '-')
        msg_base64 = msg_base64.replace('=', '_')
        msg_base64 = msg_base64.replace('/', '~')
        return msg_base64