Abstract: Commercial transactions conducted over a telecommunications link are verified using a transaction key available at both ends of the link. The transaction key is produced by combining (1) data supplied to the retailer by a customer and stored by the bank with (2) data stored by the retailer. The telecommunications link includes a node which passes the retailer's data to the bank in the form of a label obtained by encrypting the retailer's code with a client code. The bank retrieves the bank code and decrypts the label to obtain the retailer's code. The bank also retrieves the customer's data and combines the two elements to obtain the same transaction key that was created at the retailer's terminal.