http://h34nug3iwqjf2376fmmzdhgqivckzemcujmarlptfqeqtq7vya47yyad.onion/2007/02/18/signing-party-complete-toolkit-for-efficient-key-signing/index.html
This brings additional security: before the recipient can add your signature to their key, they must decrypt it with their private key, proving that they indeed have access to the key they claimed to be theirs. By mailing to the email addresses on the key user ID’s, it is also verified that the key owner can indeed read that address.