Abstract: The encoder/decoder comprises a spherical body on which is provided a plurality of different letters and/or numerals. Connecting means in the form of three circular members sharing the same axis encompass the sphere, two defining two parallel sighting hoops and the third defining a sighting ring. The connecting means are mounted on a support body so that the relative rotation of the connecting means and the body can be effected. In use, the body is rotated until two specified indicia lie usually between the two sighting hoops in a specified order. Then, one or more indicia are read from the sighting ring. The encoder/decoder may be used to protect computer software against copying, and may also be used to hinder unauthorized access to remote computer systems. Thus, the number of individuals using a system can be limited to the number interrogating the user, can determine whether he has in his possession a suitable encoder/decoder such as outlined above.