WeChat:
据说
A (encryption) => WeCha Server => decryption => Censorship => encryption => B (decryption)
如果A and B都不在墙内,那么 Censorship 这一步没有
Signal:
A (encryption) => Signal Server => B (decryption)
Skype:
墙外下载的软件,有 end-to-end encryption 选择,但是不支持 audio/video encryption
墙内下载的软件,据说没有end-to-end encryption 选择,所以与 WeChat 私密性差不多