A method for authenticating a user, comprising: - sending, from a first user device to the server, through a first communication channel, a first message including an identifier relating to the user and a request to get a challenge table, as challenge user authentication data; - generating, by the server, a first challenge table including a first set of characters, the first challenge table being valid; - getting, by the server, an identifier relating to a second user device; - sending, thanks to the second user device identifier, from the server to the second user device, through a second communication channel, a second message including the first challenge table and a first request to display the first challenge table; - displaying, by or through the second user device, the first challenge table; - generating, by the server, a second challenge table including a second set of characters, the second challenge table being invalid; - sending, from the server to the first user device, through the first communication channel, a third message including the second challenge table and a second request to display the second challenge table; - displaying, by or through the first user device, the second challenge table; - selecting, by a user, through a man machine interface relating to the first user device, at least one corresponding displayed character, as at least one selected character; - sending, from the first user device to the server, through the first communication channel, a fourth message, as a challenge response message, including the at least one selected character, as submitted user authentication data; - getting, by the server, at least one predetermined reference location within the first challenge table, as a reference table; - determining, by the server, based upon the at least one predetermined reference location within the first challenge table, at least one corresponding reference character, as reference user authentication data; - verifying, by the server, for the user, whether the submitted user authentication data does or does not match the reference user authentication data; and - succeeding, by the server, in authenticating the user only if the submitted user authentication data matches the reference user authentication data. - sending, from a first user device to the server, through a first communication channel, a first message including an identifier relating to the user and a request to get a challenge table, as challenge user authentication data; - generating, by the server, a first challenge table including a first set of characters, the first challenge table being valid; - getting, by the server, an identifier relating to a second user device; - sending, thanks to the second user device identifier, from the server to the second user device, through a second communication channel, a second message including the first challenge table and a first request to display the first challenge table; - displaying, by or through the second user device, the first challenge table; - generating, by the server, a second challenge table including a second set of characters, the second challenge table being invalid; - sending, from the server to the first user device, through the first communication channel, a third message including the second challenge table and a second request to display the second challenge table; - displaying, by or through the first user device, the second challenge table; - selecting, by a user, through a man machine interface relating to the first user device, at least one corresponding displayed character, as at least one selected character; - sending, from the first user device to the server, through the first communication channel, a fourth message, as a challenge response message, including the at least one selected character, as submitted user authentication data; - getting, by the server, at least one predetermined reference location within the first challenge table, as a reference table; - determining, by the server, based upon the at least one predetermined reference location within the first challenge table, at least one corresponding reference character, as reference user authentication data; - verifying, by the server, for the user, whether the submitted user authentication data does or does not match the reference user authentication data; and - succeeding, by the server, in authenticating the user only if the submitted user authentication data matches the reference user authentication data.