Which of the following statements about cryptographic systems is/are true?
A. If the sender and receiver each use a different key, the system is referred to as public-key encryption.
B. A stream cipher processes the input one block of elements at a time, producing an output block for each input block.
C. All encryption algorithms are based on two general principles: substitution, in which elements in the plaintext are rearranged; and transposition, in which each element in the plaintext is mapped into another element.
D. A block cipher processes the input elements continuously.