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Substitution Ciphers
Caesar Cipher
"The Caesar cipher is another example of Roman ingenuity. First described by the Roman historian Suetonius, it was used by Julius Caesar to communicate secretly with his army: If he had anything confidential to say, he wrote it in cipher, that is, by so changing the order of the letters of the alphabet, that not a word could be made out. If anyone wishes to decipher these, and get at their meaning, he must substitute the fourth letter of the alphabet, namely D, for A, and so with the others." – Suetonius, Life of Julius Caesar
Caesar decided on a simple algorithm whereby each letter of the message would be shifted forward 3 places in the alphabet. His cipher looks like this:
| Plaintext | A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z |
| Ciphertex | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | A | B | C |
So our message:
I AM A CODEBREAKER
would read:
L DP D FRGHEUHDHU
Julius Caesar’s original Cipher has a shift of +3, but you could have a shift of anywhere between 1 and 25 (if you shift by 26 you are back to where you started, equally anything over 26 will be the same as those between 1 and 25). A similar cipher was used by Julius Caesar’s nephew Augustus, but with a shift of 1:
"Whenever he wrote in cipher, he wrote B for A, C for B, and the rest of the letters on the same principle, using AA for X." — Suetonius, Life of Augustus
| Alphabet: | A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z |
| Cipher: |
| Click to Shift Cipher |
| Plaintext: | Enter some plain text below: |
| Ciphertext: | The ciphertext should appear here: |
Other Substitution Ciphers