This type existed before the era of computers, and it has the following two divisions:
- Transposition: It uses permutation. The plaintext is rearranged to another format. The characters are still the same but in different positions.
- Substitution: It uses character substitution, in other words, replacing a character with another one, such as, replacing O with M. The replacement algorithm should be known by the sender and the receiver. The ROT13 and Caesar ciphers are two examples of substitution ciphers.
ROT13 is a substitution cipher where the positions of characters in the plaintext are shifted by 13 places. So, if the plaintext is HELLO, then the ciphertext should be URYYB, as shown:
The Caesar cipher is a substitution ...
