Although his contribution was not to steganography, Auguste Kerchoffs is one of the best-known names in the field of cryptography. His contributions to modern cryptography deserve very honorable mention. Kerchoffs' book, La Crytographie Militaire, was one of the more revolutionary of its time. His insights differed from his predecessors in that Kerchoffs looked for new answers to problems that new or changing conditions put on cryptography, and he did so brilliantly.
The most notable new problem of the time was to find a form of cryptography that would work well with a new form of communication: the telegraph. Kerchoffs addressed the issue from the point of view of using military cryptography practices. The principles he put forth are still being used today:
The system should be, if not theoretically unbreakable, unbreakable in practice.
Compromise of the system should not inconvenience the correspondents.
The key should be easily remembered without notes and should be easily changeable.
The cryptograms should be transmittable by telegraph.
The apparatus or documents should be portable and operable by a single person.
The system should be easy, neither requiring knowledge of a long list of rules nor involving mental strain.
Kerchoffs is also credited with a cryptography principle that bears his name, which states that if the method used to encipher data is known by an opponent, then security must lie in the choice of the key.