Caesar Cipher & ROT13
Shift letters by any amount (ROT13 by default), decode, add ROT5 for digits, and brute-force all 26 shifts ranked by English letter frequency.
The shifted text with case and punctuation kept, the effective shift, and on request all 26 candidates with a frequency score and the most English-looking one marked.
Example: Hello, World! with shift 3 → Khoor, Zruog!; ROT13 twice returns the text; Wkh txlfn eurzq ira… brute-forced ranks shift 3 first (The quick brown fox…); ROT18 also turns 007 into 552.
Twenty-six shifts,
ranked by frequency.
What the shift does, how the brute-force table ranks its candidates, and why this is a puzzle, not encryption.
The shift
Each letter A–Z or a–z moves k places along the alphabet and wraps round: with k = 3, x → a. Decoding applies 26 − k. ROT13 is the shift of 13, which is its own inverse; with ROT5 on digits switched on the digits move by 5 as well (ROT18, used on some forums for spoilers). Case, spacing, punctuation and accented letters are left exactly as they are.
Brute force
Because there are only 25 useful shifts, the page can try them all. Each candidate is scored by how closely its letter counts match the frequencies of English text (E 12.7 %, T 9.1 %, A 8.2 % … Z 0.07 %) with a chi-squared statistic; the best fit scores 100 and the others are scaled by how much worse they are. The best guess is right for most English sentences of more than a few words, and meaningless for very short or non-English text — it is a hint to read, not a proof.
Not encryption
A Caesar shift has 25 keys and is broken by this very page in a fraction of a second. Use it for puzzles, spoiler hiding and teaching; for anything that matters use real encryption. Nothing leaves the browser; the same four anonymous usage counts as the rest of the site apply.
SOURCES
- Letter rotation (x + k) mod 26 on A–Z and a–z; ROT5 on 0–9; candidates ranked by chi-squared distance from English letter frequencies (Lewand, Cryptological Mathematics, 2000)
Last reviewed 22 September 2026. How results are checked: How we verify.