Caesar Cipher Solver

The Caesar cipher replaces each letter with the letter a fixed number of places further along the alphabet. With a shift of 3, A becomes D, B becomes E and “attack” becomes “dwwdfn”. It is named after Julius Caesar, who used it for military letters, and it is the first cipher almost everyone learns, both to use and to break. This tool does both.

Cracking without the key

A Caesar cipher has only 25 useful keys, so the fastest attack is to try them all. Paste the cipher text and the solver decrypts it with every shift from 1 to 25. Each attempt is scored on how English it looks, by comparing its letter frequencies with normal English, where e, t and a are common and q, x and z are rare, and by counting common words such as the, and and to. The list is sorted with the most likely answer first and highlighted.

Reading the list

On a sentence or more, the right shift is almost always at the top and reads clearly. With very short texts, a few letters or a single word, the statistics are weak and another shift can score slightly higher. That is why every shift is shown: glance down the list and the readable one stands out. Each candidate shows its shift and the key letter, the letter that turns into A, which is how puzzles often state Caesar keys (“key D”).

Encrypting and decrypting with a known shift

Switch to “Encrypt or decrypt with a shift” to work with a key you know. Choose the shift and direction; capitals stay capitals and punctuation stays in place. For encryption you can also choose letters only, or the classic style of five-letter groups that hides word lengths, as real cipher clerks did.

Why it is so weak

The cipher has only 25 keys and keeps letter frequencies intact: whatever letter is most common in the cipher text is probably e. Both weaknesses make it breakable by hand in minutes and by computer instantly. That makes it ideal for teaching and puzzles, and useless for protecting anything. ROT13 is simply the Caesar cipher with a shift of 13.

Using it in class and games

Teachers use the Caesar cipher to introduce keys, brute force and frequency analysis. Escape rooms and treasure hunts give players a cipher wheel or the shift as a clue. Students checking homework can encrypt with the shift they used and compare. For a stronger classroom challenge, move on to a keyword cipher such as Vigenère.

Private

Encryption and solving happen in your browser; nothing you paste is uploaded.

A worked example

Paste “Phhw ph dw wkh rog eulgjh dw plgqljkw” into the solver. Shift 3 comes first: “Meet me at the old bridge at midnight”, scoring high because meet, the and at are common words and the letter frequencies match English. The second-best shift reads as gibberish, so the answer is clear. Shift 3 also shows the key letter D, meaning cipher D stands for plain A, the way Caesar’s own key is usually described. Switch to encrypt with shift 3 to reproduce the cipher and confirm.

Related tools

Try the self-inverse shift in the ROT13 Decoder, use a keyword with the Vigenère Cipher, reverse the alphabet with the Atbash Cipher, or multiply as well as shift with the Affine Cipher.