Characterising network traffic for Skype forensics

Voice over IP (VoIP) is increasingly replacing fixed line telephone systems globally due to lower cost, call quality improvements over digital lines and ease of availability. At the same time, criminals have also transitioned to using this environment, creating challenges for law enforcement, since interception of VoIP traffic is more difficult than a traditional telephony environment. One key problem for proprietary VoIP algorithms like Skype is being able to reliably identify and characterize network traffic. In this paper, the latest Skype version and its components are analyzed, in terms of network traffic behavior for logins, calls establishment, call answering and the change status phases. Network conditions tested included blocking different port numbers, inbound connections and outbound connections. The results provide a clearer view of the difficulties in characterizing Skype traffic in forensic contexts. We also found different changes from previous investigations into older versions of Skype.