19. The data input system of claim 15 , wherein: said character signal generator is configured such that an error message is generated if a currently received keystroke sequence does not correspond to any of said plurality of predefined keystroke sequences.
15. A data input system for generating character signals from a set of characters, the system comprising: a keyboard comprising a plurality of keys; a key signal detector coupled to said keyboard, said key signal detector being configured for detecting a keystroke of a key from said plurality of keys; and a character signal generator coupled to said key signal detector, said character signal generator being configured for generating a character signal when a keystroke sequence from a plurality of predefined key sequences is detected, wherein said plurality of predefined key sequences is generated using a variable-length prefix-fee encoding.