Keyed Caesar cipher decoder, solver and encoder

The keyed Caesar cipher is a variant of the Caesar Cipher that provides increased protection. Instead of having all letters in alphabetical order, it starts with a code word (the encryption key). Unused letters are then added after the code word. If the code word is blank, it behaves exactly like an ordinary Caesar Cipher.

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Keyed Caesar cipher


  • The Keyed Caesar cipher is a form of monoalphabetic substitution cipher.
  • The translation alphabet (the letters that are used instead of the ordinary alphabet) starts with the secret key. Each letter is only used once, so if a letter is used twice, only the first occurence is used. Then the translation alphabet is completed with the remaining letters in alphabetical order. Finally, it is shifted like an ordinary Caesar Cipher.
  • If the key is left blank, it behaves like an ordinary Caesar Cipher.
  • To improve obfuscation, spaces can be removed or replaced with a character, such as X, in the plaintext before encryption. Also the shift should not be zero.
  • A keyed Caesar cipher still be broken through frequency analysis.

Caesar cipher, and variants of it, are often used in easy geocaching mystery caches, and easy logic puzzles.

Sample text


The ciphertext above represents "I CAME, I SAW, I CONQUERED." encrypted using a shift of 13 and the key CAESAR.