This paper reflects work done during 2004 and 2005. It is a snapshot of the Skype cryptosystem at that time. Many readers observe that the work is ancient, and ask why I have not written a new report. The answer is the project was over. I have not had further access to Skype code or engineers. Skype, the company, has been sold three times since I did this work. The creators of the system are long gone, taking with them their skills and their values.
This paper contains the first authorized description of the Skype cryptosystem. Skype P2P sessions are encrypted end-to-end at the session layer. Session keys are created using a key-agreement protocol which provides each peer with proofs of freshness and authenticity, and which allows each peer to contribute bits toward the session key. Authenticity and identity are rooted in the Skype Certificate Authority. We analyze the cryptosystem as of Skype Version 1.3. We conclude that is is generally well-designed and correctly implemented.
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