In the 19th century, a Dutch cryptographer A. Kerckhoff furnished the requirements of a good cryptosystem. Kerckhoff stated that a cryptographic system should be secure even if everything about the system, except the key, is public knowledge. The six design principles defined by Kerckhoff for cryptosystem are −

The cryptosystem should be unbreakable practically, if not mathematically.

Falling of the cryptosystem in the hands of an intruder should not lead to any compromise of the system, preventing any inconvenience to the user.

The key should be easily communicable, memorable, and changeable.

The ciphertext should be transmissible by telegraph, an unsecure channel.

The encryption apparatus and documents should be portable and operable by a single person.

Finally, it is necessary that the system be easy to use, requiring neither mental strain nor the knowledge of a long series of rules to observe.