Sending from a first user device to a server, through a first channel, a user identifier and a request to get a table. Generating, by the server, a first table including a first character set and being valid. Sending, from the server to the second user device, the first table and a first request to display this latter. Displaying, by or through the second user device, the first table. Selecting, by a user, through the first user device, a character. Sending, from the first user device to the server, through the first channel, the selected character, as submitted data. Determining, by the server, based upon a reference location within the first table, a corresponding reference character, as reference data. Verifying, by the server, for the user, whether the submitted data does match the reference data. And, if yes, succeeding, by the server, in authenticating the user.