We construct an on-line transaction scheme based on the analysis in the previous section. We define three procedures for access control and electronic transactions: a registration procedure, a transaction procedure, and a secret-revocation procedure. The principals in these transactions are a customer C, a merchant M, and a bank B. Merchants include Internet shopping malls, book sellers, on-line travel agents, news providers, etc.
We describe four procedures with brief protocol flows.
In the protocol description, the notation 
stands for
protocols between A and B,
a message from A to B. The symbol := denotes an assignment.
The notation (x,y) is the concatenation of x and y.
Figure 1 shows the interfaces between the principals.
Figure 1: Principals and their interfaces