In this chapter, we look at a classification of filtering called Distributed Checksum Filtering (DCF). This method of fighting spam involves a network of mail transfer agents (MTAs), which exchange mail message signatures with each other. A signature is a small unique numeric value that represents the message, similar to a checksum used in verifying data transfers.
Two approaches use the DCF method:
Exchange checksums for spam messages (Vipul’s Razor and Pyzor)
Exchange checksums and counts from all messages (Distributed Checksum Clearinghouse, or DCC)
The method of exchanging only spam message checksums theoretically reduces the amount of traffic between checksum servers because after a message is identified ...