Calculator
No key needed: all 26 shifts are tried. The slider shows the top-ranked shift.
Enter some ciphertext containing letters to see all 26 decryptions.
There are only 26 possible shifts, so a Caesar cipher can be broken by trying every one. Select a candidate to decrypt with that shift. Candidates are ranked by pattern score, which measures out of 10 how closely each candidate's letter sequences follow English spelling (ordinary words and names score 7 or more, random letters close to 0), plus a bonus for the share of the text made up of listed words from a list of about 73,000 English words and names, and a small one for English-like letter frequencies. The top-ranked candidate is marked Likely English when such words cover at least half of it, its spelling looks English, and it is well ahead of the second-ranked shift. It is marked Low confidence when the top shifts are nearly tied or nothing looks English, and Best guess otherwise. These labels are a rough guide, not probabilities: a single short word or name can fit several shifts.
See the shift
How your message is transformed
| Position | Input letter | Number | Calculation (mod 26) | Result number | Output letter |
|---|
Type some letters above to see the arithmetic for each one.
Position counts letters only; spaces, digits, and punctuation are not shifted and are left out of the table.
Why does the alphabet wrap around?
What happens to letter frequencies?
A Caesar cipher preserves the frequency pattern of the plaintext; it only shifts which alphabet letters carry those frequencies. If E is the most common letter of the message, then with k = 3 the most common ciphertext letter is H, and every other letter's count moves along with it.
That is why counting letters is enough to recover the shift without trying all 26.
Related topics
The Caesar cipher is a special case of the affine cipher E(x) = ax + b (mod 26) with a = 1 and b = k.