The simplicity and low cost of Voice over Internet Protocol (VoIP) services has made these services increasingly popular as the Internet has grown. Unfortunately, these advantages of VoIP are attractive to both legitimate and nefarious users, and VoIP is often used by criminals to communicate and conduct illegal activities (such as fraud or blackmail) without being intercepted by Law Enforcement Agencies (LEAs). However, VoIP can also increase the efficiency of law enforcement and forensic

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... boration. Currently, VoIP researchers have only proposed a framework for this type of partnership, and have yet to provide a common protocol for forensic Internet collaboration. As a result, Internet-based collaboration between agencies is not widespread. Building from the Collaborative Forensics Mechanism (CFM) and the procedures of collaborative forensics work, this paper designs a novel application-layer Collaborative Forensics Protocol (CFP) to overcome the current framework-protocol gap. Here, CFP can exchange collaborative request and response messages between collaborative forensics region centers (CFRCs) to acquire collaborative forensics information. We present a procedure for collaborative forensics and discuss the details of protocol design. In addition, we discuss the defense of PKI working with CFM against various types of attacks and analyze the features of CFP.