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

@staticmethod
def _url_base64_encode(msg):

    msg_base64 = base64.b64encode(msg)
    msg_base64 = msg_base64.replace('+', '-')
    msg_base64 = msg_base64.replace('=', '_')
    msg_base64 = msg_base64.replace('/', '~')
    return msg_base64
