Abstract: Embodiments of the present invention include a method and apparatus for encoding the signature string X into a first part B and a second part R with reference to a dictionary comprising a plurality of codes. The first part B identifies which, if any, characters of the signature string X are wildcard characters. The second part R is formed by, for each character in the signature string X that is not a wildcard character, retrieving a code from the dictionary based on the character and its position within the signature string X, the dictionary holding a different code for each such character-position pairing, and combining the retrieved codes according to a predetermined logical operation (e.g. XOR) to form the second part R.