http://bkg5ofjslnbyqvu55sjonpdyp3wmfsgq23u7vpk3wxn7ewgdeocpgwid.onion?page=article&article=Why+You+Should+Use+a+Tor-Based+PGP+Key+Server.html
Mitigating Metadata Correlation Attacks Even when encryption is secure, metadata—such as who is communicating with whom, when, and from where—can still be exploited by adversaries. If you interact with a clearnet key server, your IP address, timestamps, and request logs may be captured, which could later be correlated with your other online activities, even if encrypted.