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The two major operations that can be performed on plaintext for encipherment purposes are transposition and substitution. While in transposition the positions of the units in the message are changed, substitution changes the unit but keeps the position. Monoalphabetic substitution simply means that for each letter, there is only one other letter which represents it. If we choose plaintext 'e' to be represented by 'r' in the ciphertext, 'r' will always represent 'e.' Monoalphabetic substitutions can be quite simple, as Julius Caesar's cipher will show you.
Julius Caesar (that same one murdered on the Ides of March) created his own monoalphabetic cipher system for use in the Roman government. The very simple system substituted each message in the plaintext for a corresponding letter of a ciphertext alphabet. What makes the Caesar substitution so simple is that the ciphertext alphabet is simply a regular alphabet 'moved a few spaces' in either direction of the correct order. Caesar preferred his ciphertext alphabet to be moved 3 spaces to the right, which would mean that plaintext 'a' would become ciphertext 'd', and 'b' would be 'e' and so on. The following example illustrate his simple substitution.
Another very simple substitution scheme is called 'Atbash,' and it is very old. Although it's exact date of origin is unknown, Atbash appears in several Biblical translations. A popular theory claims that the scribes were simply bored and decided to encipher some of their text using the Atbash system. In Atbash substitution, the first letter of the alphabet is represented by the last, and vice versa. The second letter is represented by the second to last letter, and vice versa, etc... The name Atbash comes from the Hebrew alphabet, in which the first, last, second, second to last, etc. are Aleph, Taw, Beth, [A,] SHin, or Atbash. The following example shows a simple Atbash substitution.
So now that we've encoded our plaintext, how do we decode it? Just use the opposite process! To decode a message using the Caesar substitution, we'd simply write out the plaintext/ciphertext alphabets and locate the letter of the ciphertext message on the ciphertext alphabet and then use the corresponding letter from the plaintext alphabet. Continuing this process, the plaintext message would appear.
If you'd like to learn practical monoalphabetic ciphers, though, you'll want to read about a the keyword technique for making a decent system.