- The system must be substantially, if not mathematically, undecipherable.
- The system must not be required to be secret and can be stolen by the enemy without causing trouble.
- It must be easy to communicate and retain the key without the aid of written notes, it must also be easy to change or modify the key at the discretion of the correspondents.
- The system ought to be compatible with telegraph communication.
- It must be portable, and its use must not require more than one person.
- Finally, given the circumstances in which such system is applied, it must be easy to use and must neither stress the mind or require the knowledge of a long series of rules.
J.-G.-H.-V.-F.-A.-A. Kerckhoffs von Nieuwenhof, “La Cryptographie Militaire” in Journal des sciences militaires, vol. IX, 1883