Abstract: A personal computer transmits encrypted information identifying a transaction card, together with a serial number of an encryption unit in which the information way encrypted, over a public network to a payment server. The payment server generates a single-use credit card number, which is returned to the personal computer over the public network, and stores the single-use credit card number together with the serial number and the encrypted information. The personal computer then transmits the single-use credit card number to a merchant's server through which a transaction is to be made. The single-use credit card number is transmitted to the payment server, which is identified according to a portion of the single-use credit card number. The payment server then decodes a portion of the encoded information according to a cryptogram located within a data base according to the serial number stored with the single-use credit card number.