Algorithm Uses and Properties
-  
Algorithms reversibly converting data (plaintext) into unintelligible form (ciphertext).
-  E(P) = C, and then  D(C) = P
-  Only knowledge of "secret" allows recovery.
-  Kerckhoffs's doctrine (19th century):
not based on the secrecy of the algorithm.
-  E(P, EK) = C, and then  D(C, DK) = P
 
-  Building blocks for services/protocols that provide other assurances such as
-  confidentiality
-  integrity
-  authentication
-  non-repudiation