Security procedure specifically designed to legitimize transactions and avoid Internet fraud, put into practice with the use of a Multi-key card that contains a card identification code, user NICKs and a variable number of hidden PINs that serve to enable a single operation and then become invalidated. The procedure contemplates the possibility of the user using the Multi-key card by means of the Web or by means of a Call Center, in both cases in the expectation of the authentication of his identity by the Authorization Center. The Center has an isolated database not available on line where all the sensitive data is safely stored to avoid any type of falsification. In this manner, not only the system that operates on line but the Call Center Operator of can check the information supplied by the user (card code+NICK+PIN) in the isolated database and then authorize or deny the operation, in accordance with the result of the process of the authentication of identity.