TTP (trusted third party) plays an important role in electronic commerce protocols. Different roles of TTP in inline TTP protocols, online TTP protocols and offline TTP protocols are pointed out through three protocols of different types, i.e., Coffey-Saidha protocol, CMP1 protocol and Asoken-Shoup-Waidner protocol. The above protocols are analyzed thoroughly, and their characteristics, defects and approaches to further improvement are discussed respectively in this paper.