On Privacy of Skype VoIP Calls

Skype is one of the most popular voice-over-IP (VoIP) service providers. One of the main reasons for the popularity of Skype VoIP services is its unique set of features to protect privacy of VoIP calls such as strong encryption, proprietary protocol, unknown codec, dynamic path selection, and constant packet rate. In this paper, we propose a class of passive traffic analysis attacks to compromise privacy of Skype VoIP calls. The proposed attacks are based on application-level features extracted from VoIP call traces. The proposed attacks are evaluated by extensive experiments over different types of networks including commercialized anonymity networks and our campus network. The experiments show that the proposed traffic analysis attacks can detect speaker and speech of Skype calls with 0.33 and 0.44 detection rate, about 30-fold and 15-fold improvement over random guess respectively.