substitution-cipher

Substitution cipher is a method of cryptography.
This program encrypt a message by replacing every letter with another letter. (HW from online course CS50)
To decrypt the message, the receiver of the message would need to know the key (a mapping of every alphabet).
Reverse process: translate ciphertext (the encrypt text) into plaintext (original one).

For example, a key might be the string "POMAFTHRLZGECYJIUWSKDVBNQX"
This 26-character key means that A (the first letter of the alphabet) should be converted into P (the first character of the key), Z should be converted into X (the last character of the key), and so forth.
A message like HELLO, then, would be encrypted as RFEEJ.