Method and device for encrypting a message

A method in a digital pen (hereinafter DP) for encrypting a message for secure transmission of said message from the digital pen (DP) to a receiver (hereinafter ALS, SH, SP), characterized by the steps of obtaining the message by recording images of a surface (2) provided with a pattern (3) position code using an optical detector (8) in the digital pen (DP) and determining at least one position from the images; obtaining (410-420) optical data for generating an encryption key using the optical sensor (8) in the digital pen (DP), said optical data electronically representing values ​​of an optical parameter readable by said optical sensor (8); generating (430-450) exclusively for electronic processing in a processing unit (10) of the digital pen, the encryption key, using said optical data as random seed; and encrypting (480) said message using said encryption key in an encryption algorithm.