Abstract: In a secure electronic payment system, authentication data based on a payment account (e.g., a credit card account) is sent from an authentication server, through a user's Web browser, to a merchant's computer. The merchant's computer sends the authentication data to a computer operated by the issuer of the payment account, either through a payment organization computer or through an acquirer computer operated by the merchant's acquirer. The issuer's computer verifies the authorization request message, thereby generating an authorization response message. The authorization response message is forwarded to the merchant's computer, either through the payment organization computer or through the acquirer computer. If the authorization response message indicates that the verification was successful, the transaction is completed.