http://h34nug3iwqjf2376fmmzdhgqivckzemcujmarlptfqeqtq7vya47yyad.onion/2007/02/18/signing-party-complete-toolkit-for-efficient-key-signing/index.html
However, caff adds additional security to that: it encrypts your signature on their key with their own key, and then mails it to them. It mails the signature for a specific user ID to the emailaddress on that user ID. 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.