To access a service, each user device stores one first key. The user device is connected to a first server. A terminal sends to a second server a connection request. The second server responds with first data relating to a transaction identifier and an associated challenge. The terminal determines a first result depending upon the first data and the first key. The terminal sends to the first server the first result and user device data. The first server identifies a user device based upon the user device data and sends to the device the first result. The device determines the challenge and the transaction identifier based upon the first result and the first key and sends to the second server the challenge and the transaction identifier. The second server verifies whether the data received from the device matches the first data and, if so, authorizes the terminal to connect.