(EN) The invention concerns a method for making secure a commercial transaction carried out by a client (12) using a smart card and a data transmission network (10) such as the Internet, whereby a third party server (16) authorises the commercial transaction with a merchant site (14) to be carried out and comprising the following steps after verification by the third party server of the client's identification data and his PIN code: transmission by the third party server to the client of at least two variables defining an authentication key in a table (or matrix) printed on a material medium available to the client and whereof the two inputs are the two variables; transmission by the client of the authentication key defined by the input variables received from the third party server; and verification by the third party server that the authentication key transmitted by the client actually corresponds to the key defined by the authentication table stored in the third party server database. FIG. 1: 12 CLIENT 14 MERCHANT SITE 16 THIRD PARTY SERVER 20 BANK A INTERNET