In the good old days when Skype was P2P, Skype users with an appropriate connection type would be promoted to ‘supernodes’, relaying connections for other Skype users locked behind NAT. Back in 2012 Microsoft had bought Skype from eBay and promptly changed from a P2P architecture to a centralised one. The move was advertised as one of security and reliability. We now know otherwise, and the NSA and Microsoft planned this architectural change to make spying on Skype users much easier as part of Project Chess. Here is what was said about the changes back in 2012:
Kortchinskys analysis, which has not yet been confirmed by Microsoft, shows that Skype is now being powered by a little more than 10,000 supernodes that are all hosted by the company. Its currently not possible for regular users to be promoted to supernode status. Whats more, the boxes are running a version of Linux using grsecurity, a collection of patches and configurations designed to make servers more resistant to attacks. In addition to hardening them to hacks, the Microsoft-hosted boxes are able to accommodate significantly more users. Supernodes under the old system typically handled about 800 end users, Kortchinsky said, whereas the newer ones host about 4,100 users and have a theoretical limit of as many as 100,000 users.
“Its pretty good for security reasons because then you don’t rely on random people running random stuff on their machine,” Kortchinsky told Ars. “You just have something that’s centralized and secure.”