The American Jeffrey Knockel started the research in 2011 joining a project at the University of New Mexico. After years of study, Jeffrey Knockel, discovered that Skype tracks politically sensitive text messages on its Chinese videophone and texting service, called TOM-Skype.
Knockel has been able to crack the sophisticated surveillance system included in Skype’s Chinese service. The great discovery is the identification of thousands of terms – such as “Tiananmen”, “Amnesty International” and “BBC News” – that provoke control of the conversation. Specifically, the TOM-Skype scans specific words and sentences in any message and as soon as it finds a match between what people are speaking about and the “black-words-list”, it sends a copy of the message with all details, such as username, time and date of transmission, and whether the message was sent or received by the user. Currently it is still not clear if the information is then shared with the Chinese government.
Here you can find the list of forbidden words (in Chinese!).