From Dorothy Denning, CRYPTOGRAPHY AND DATA SECURITY, Addison-Wesley 1982,1983, page 8:

"Cryptosystems must satisfy three general requirements: "1. The enciphering and deciphering transformations must be efficient for all keys. "2. The system must be easy to use. "3. The security of the system should depend only on the secrecy of the keys and not on the secrecy of algorithms E [enciphering] or D [deciphering]." --Mike