"Discovery keys are used for finding other peers who are interested in the same Dat as you.
If you know a Dat’s public key then you can calculate the discovery key easily, however if you only know a discovery key you cannot work backwards to find the corresponding public key. This prevents eavesdroppers learning of Dat URLs (and therefore being able to read their contents) by observing network traffic.
However eavesdroppers can confirm that peers are talking about a specific Dat and read all communications between those peers if they know its public key already. Eavesdroppers who do not know the public key can still get an idea of how many Dats are popular on the network, their approximate sizes, which IP addresses are interested in them and potentially the IP address of the creator by observing handshakes, traffic timing and volumes. Dat makes no attempt to hide IP addresses."