http://bitmsgdyvsmwgsimkxplisxbzpewvkhfm3fmomzd63apaymaxiznz6ad.onion/index.php?title=Protocol_specification&diff=prev
This prevents people from gathering pubkeys sent around the network and using the data from them to create messages to be used in spam or in flooding attacks. In order to encrypt the pubkey data, a double SHA-512 hash is calculated from the address version number, stream number, and ripe hash of the Bitmessage address that the pubkey corresponds to.