The wheel has already been invented once so the client could just call GnuPG to encrypt files add/get keys etc over the users (normal) network via a simple call to gpg --keyserver certserver.pgp.com --recv-key 0xBB7576AC.

Encrypting a file does not increase the file size (+- a few bytes) so the (Gnutella) network speed will not really be affected. User download time may notice a few seconds delay for the transfer to start while the acting server encrypts the file.

I suggested encrypting not out a fear of what is being shared but rather a mechanism to stop agencies claims that then network is being used for (insert what you share) as they will not be able to show in court that xxxGb of copyrighted material was transferred over the Gnutella network as without all keys to prove this you would be slandering Gnutella users.