import string


def tamper(payload, **kwargs):
    """
    Double url-encodes all characters in a given payload (not processing
    already encoded)

    Notes:
        * Useful to bypass some weak web application firewalls that do not
          double url-decode the request before processing it through their
          ruleset

    >>> tamper('SELECT FIELD FROM%20TABLE')
    '%2553%2545%254C%2545%2543%2554%2520%2546%2549%2545%254C%2544%2520%2546%2552%254F%254D%2520%2554%2541%2542%254C%2545'
    """

    retVal = payload

    if payload:
        retVal = ""
        i = 0

        while i < len(payload):
            if payload[i] == '%' and (i < len(payload) - 2) and payload[i + 1:i + 2] in string.hexdigits and payload[i + 2:i + 3] in string.hexdigits:
                retVal += '%%25%s' % payload[i + 1:i + 3]
                i += 3
            else:
                retVal += '%%25%.2X' % ord(payload[i])
                i += 1

    return retVal