Encrypt and decrypt text using the Caesar cipher. Choose any shift value from 1–25, or use the classic ROT13 (shift 13). Includes brute-force mode to crack an unknown Caesar cipher automatically.
Type or paste your text into the input box. Set the shift value from 1 to 25 using the slider or number input. Shift 13 is ROT13, the most common variant. Only alphabetical characters are shifted — numbers, spaces and punctuation are preserved as-is.
Click Encrypt to shift characters forward by the chosen amount, or Decrypt to shift them backward. Decrypting with the same shift that was used to encrypt recovers the original text. For ROT13, encrypt and decrypt use the same shift.
If you have a Caesar-encrypted message but don't know the shift, click Brute Force to see all 25 possible decryptions at once. Scan the results for the one that produces readable text — the correct shift is highlighted automatically using English frequency analysis.
The Caesar cipher is one of the oldest and simplest encryption techniques, named after Julius Caesar who allegedly used it for personal correspondence. It is a substitution cipher where each letter in the plaintext is shifted a fixed number of positions along the alphabet. With a shift of 3, A becomes D, B becomes E, and Z wraps around to C. It provides no meaningful security by modern standards but is historically significant and useful for teaching cryptography concepts.
ROT13 (Rotate 13) is the Caesar cipher with a shift of 13. Because the English alphabet has 26 letters, applying ROT13 twice returns the original text — it is its own inverse. ROT13 is used on internet forums and communities (particularly Usenet and Reddit) to obscure spoilers, punchlines, or offensive content so readers must actively choose to decode it. It provides no security.
The Caesar cipher is trivially insecure. There are only 25 possible shifts, so any encrypted message can be cracked by trying all possibilities in seconds (brute force). Even without brute force, frequency analysis (comparing letter frequencies in the ciphertext to known language frequencies) easily recovers the shift. The Caesar cipher has not been used for real security since at least the 9th century when Al-Kindi described frequency analysis.
A substitution cipher replaces each character with a different character (or group of characters) — the Caesar cipher, Vigenère cipher, and ROT13 are substitution ciphers. A transposition cipher rearranges the positions of characters without changing them — rail fence and columnar transposition are examples. Modern encryption combines many layers of substitution, transposition, and key mixing to achieve security.
In English text, the most common letter is E (about 12.7% frequency), followed by T, A, O, I, N. After encrypting with Caesar cipher, the most frequent letter in the ciphertext corresponds to E in the plaintext. By finding which letter appears most often in the ciphertext and calculating how many positions it is shifted from E, you recover the shift without needing to try all possibilities.
This tool operates on the standard 26-letter Latin alphabet (A–Z). Non-Latin characters (accented letters, Cyrillic, Arabic, CJK etc.) are passed through unchanged — only ASCII letters are shifted. For ciphers supporting other alphabets or Unicode character ranges, a Vigenère cipher or custom substitution table would be needed.
