Abstract: A device to secure payments or transactions made with a smart card and validate the identity of the smart card holder using a client/server biometric control operation, regardless of whether the payments or transactions are made directly at a point of sale or indirectly. A client/server-type system authenticates the identity of a smart card holder, based on the use of the smart card and on the identification of the holder by biometric data which are stored on an extended memory smart card and compared with biometric data from an external, remote database that is secured and operated by the third-party authentication server or by the financial institution, but which, for reasons of security and performance, are not transferred over the telecommunication network.