The protocol itself is highly obfuscated, but from my understanding of what has been published it works something like this: (lots of disclaimers here that nobo

The protocol itself is highly obfuscated, but from my understanding of what has been published it works something like this: (lots of disclaimers here that nobody outside of Microsoft/Skype really knows for sure) When logging in an RSA public/private key pair is generated and the public key is sent up to the server. The username to public key mapping is seeded to supernodes and inserted into the global address book. A calling party looks up the username on a supernode and receives the public key of the answerer as well as some magic to help them establish a direct connection even if both are behind NAT. The caller generates a single use AES256 key for the session, encrypts it N times where N is the number of other parties on the call plus a number of built-in "observer" certificates. These encrypted keys are all sent over the wire to the other parties, whom are each able to decrypt 1 of the N encrypted payloads. Each party encrypts traffic to the others using the session specific AES key. If you are a government agency with a private key that matches one of the observer public keys (Russia, China, and India have openly claimed to have these), and you are able to record the setup for the call, you are effectively another party in the group chat and have access to the session key.