A method of generating a signature for a group of electronic messages that each include a plurality of characters comprises extracting a plurality of blocks of characters from each of the electronic messages, mathematically processing each of the blocks of characters from each electronic message, and generating a signature for the group of electronic messages based at least in part on the mathematically processed blocks of characters. In some embodiments a counting Bloom filter may be used to generate the signature. The signatures generated by these methods may be used to identify spam.