Application: Two ciphers

Let $A$ be a finite set, which we call the alphabet, and let $M$ be the set of all finite sequences of elements of $A$ , which we call the message space. A cipher is a mapping

$$E:M\rightarrow E,$$ (1.4)

called encryption, and an inverse mapping

$$D:M\rightarrow M,$$

called a decryption, which satisfy $D(E(m))=m$ for all $m\in M$ . The messages in the range of $E$ are called the message text and the domain of $E$ is called the cipher text.