In an Internet commercial transaction, the secure Web server and the buyer’s computer authenticate each other and encipher the data transmitted using transport layer security (TLS) or secure socket layer (SSL) protocols. When a purchase is made online using a credit card, does the customer’s bank need to know what was purchased? Not really. Does the seller need to know the customer’s credit card number? Actually, the answer is no. The responses to these questions were the main premises of the secure electronic transaction (SET). In the late 1990’s, SET was approved as the credit card standard, but it failed to be accepted because of its cost and the problems regarding distribution of end-user certificates. However, SET is explained in this chapter as an ideal protocol, from the point of view of certificates, digital signatures, and cryptography for securing credit card transactions over the Internet.