A method and a corresponding device for generating true random numbers for use in encryption of a message for secure transmission of said message from a sending device to a receiver, or for authentication of a sent message. An optical image represented by optical data is obtained by an optical sensor and processed in order to improve the stochasticity of the optical data. The thus processed data is then used for generating random numbers which can be used as an encryption key, or for generating an encryption key, for subsequent use in an encryption algorithm for encrypting the message. The method is implemented in a digital pen (DP) to be used in an information management system.