Caesar shifts every letter of the Plaintext by the same fixed amount — the Shift — so the whole Alphabet slides sideways at once. That makes it a Substitution cipher with a single alphabet: every A becomes, say, D, every B becomes E, and so on for the whole message.
Write your message and keep only the letters (the tool drops spaces and punctuation unless you ask it to keep them).
Choose the shift: a number between 1 and 25. Julius Caesar used 3.
Move every letter forward through the Alphabet by that many positions, wrapping around at the end: after Z comes A.
Read off the letters you landed on — that is the ciphertext.
With a shift of 3 the plaintext letter A lands on D, B on E, and the tail of the alphabet wraps around: X, Y and Z come out as A, B and C. One shift for the whole message is what makes Caesar easy to use — and easy to break.
Numbering A=0 … Z=25, encryption adds the shift and wraps around the alphabet:
cipher = (plain + shift) mod 26
plain = (cipher - shift + 26) mod 26Decryption is the same move backwards: subtract the shift instead of adding it. The tool's swap button does exactly that — it feeds the ciphertext through the second formula instead of the first.
The tool defaults to the English alphabet, but the cipher works with any ordered set of letters: pick a preset or paste your own into the Alphabet field. The arithmetic stays the same; only the alphabet length changes (the formula then wraps at that length instead of 26).
Try it in the toolType your message into the message input.
Drag the rotation slider the Key field to choose the shift — a shift of 3 is the classic choice.
Decide about the two checkboxes: the "Keep spaces" checkbox preserves the word layout and the "Keep case" checkbox preserves capital letters. The tool drops everything else.
Read the encrypted result in the output field.
Paste the cryptogram into the message input.
Press the swap button the swap button to switch to decrypt mode.
Set the rotation slider the Key field to the shift the sender used — you need the exact same number.
Read the original message in the output field.
A shift is a number between 1 and 25, so a Brute force attack needs at most 25 tries — and usually far fewer:
Paste the cryptogram into the message input and switch to decrypt mode with the swap button.
Slide the rotation the Key field through the values and watch the output field: nonsense, nonsense… until a readable word suddenly appears.
If walking through all shifts feels like cheating, use the smarter way: Frequency analysis. Find the most frequent ciphertext letter (often the encrypted E), measure its distance from E, set that as the shift and read one sentence to check.
The Caesar cipher is trivially breakable, and it is worth understanding why: it fails not because of bad arithmetic but because it keeps too much structure. Both classic attacks below need nothing more than the tool.
Try it in the toolEvery letter hides behind the same Shift. Whatever the plaintext statistics were, the Ciphertext keeps them — just wearing different letters. The most common letter of the message is still the most common letter, only shifted; the rhythm of repeated words is still there. A single substitution preserves everything about the text except which letters it uses.
There are only 25 possible shifts (26 if you count "no encryption"). Work through them:
Switch the tool to decrypt mode with the swap button and paste the cryptogram into the message input.
Drag the rotation the Key field from 1 to 25 and read the output field each time.
The readable fragment appears exactly once — when the rotation matches the sender's shift. A human eye finishes the job from there.
A Brute force attack needs no statistics at all, which is why any modern cipher must have a key space far too large to walk through.
Frequency analysis skips the guessing: count the letters of the ciphertext and compare the counts with the language's known profile.
In English, E is the most frequent letter — about 12% of a long text — followed by T, A, O, I and N.
Find the most frequent ciphertext letter; its distance from E is the most likely shift.
Decrypt with that shift and read. If the text is short and stubborn, try the distance to T instead.
Frequency analysis was described by the Arab polymath Al-Kindi in the 9th century, and it breaks every single-alphabet substitution cipher — Caesar is just the simplest of them.
Caesar's failure taught cryptographers the first great lesson: the key must change the text irregularly. Later ciphers kept the shifting idea but made the shift vary letter by letter — the Vigenère cipher is literally "many Caesars", one per key letter, and the Frequency analysis attack above stops working on it until you first recover its key length.