A method for making transactions secure based upon a card-type medium (2) comprising a number (4) and a remote database (5) connected to a telecommunications network. A unique and non-reproducible identifier (3) in the form of a bubble code is physically affixed to the card (2) comprising a number (4) contained either on a magnetic strip, of a bar code, or in an electronic label. One representation of this identifier (6) and or (7) is stored in the remote database (5) at the same number (4) as that present on the medium. This identifier (3) constitutes an unforgeable link between the magnetic strip (1) and the database (5). When a call is made to the remote database by means of the number recorded on the card, a link authentication is performed by comparing the representation of the identifier stored in said database and the actual identifier.