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Chiffre de César

The Caesar cipher is a substitution cipher that shifts each letter a fixed number of positions in the alphabet. Named after Julius Caesar, who reportedly used a shift of 3, it is the simplest classical cipher and the basis for ROT13. While trivially breakable today, it teaches the core concepts of encryption: plaintext, ciphertext, key, and substitution.

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À propos de Caesar Cipher Encoder & Decoder

The Caesar cipher is a substitution cipher that shifts each letter a fixed number of positions in the alphabet. Named after Julius Caesar, who reportedly used a shift of 3, it is the simplest classical cipher and the basis for ROT13. While trivially breakable today, it teaches the core concepts of encryption: plaintext, ciphertext, key, and substitution.

Comment l'utiliser

  1. Enter text to encrypt or decrypt.
  2. Set the shift amount (1–25). Caesar himself used 3; ROT13 uses 13.
  3. Choose encoding direction: encrypt (shift forward) or decrypt (shift backward).
  4. Use brute-force mode to see all 25 possible shifts — useful for decoding without knowing the key.

Formule et méthodologie

Encryption: C = (P + shift) mod 26. Decryption: P = (C − shift + 26) mod 26. ROT13 is self-inverse: ROT13(ROT13(x)) = x. Letter frequency analysis: most common ciphertext letter likely corresponds to E (12.7%), T (9.1%), A (8.2%) in English — enables trivial cracking.

Cas d'usage courants

  • Introduction to cryptography concepts in computer science courses
  • ROT13: traditional Usenet/forum practice for hiding spoilers
  • Puzzle design: escape rooms, treasure hunts, puzzles
  • Demonstrating why simple substitution ciphers are insecure
  • Historical study of Roman and wartime cryptography

Questions fréquentes

Easily — try all 25 possible shifts (brute force takes seconds). For longer texts, frequency analysis: count letter frequencies in the ciphertext and match to known English letter frequencies (E=12.7% is most common). The shift that maps the most frequent ciphertext letter to E is almost certainly correct. Any substitution cipher with a fixed alphabet is breakable this way.
ROT13 is a Caesar cipher with shift 13. Since the English alphabet has 26 letters, applying ROT13 twice returns the original text — it is its own inverse. ROT13 is widely used in online communities to hide spoilers, punchlines, or offensive content that readers must consciously decrypt. It offers zero security — it is purely a courtesy mechanism.

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