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Open toolCaesar Cipher Decoder
Select Decode, Encode, or Brute-force mode, set your alphabet shift key (1 to 25), and paste your text in the Caesar Cipher Decoder below to transform messages or inspect all 25 rotations at once.
Free to useNo sign-upRuns in your browser
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Enter the inputs, then select Run Caesar cipher.
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How Caesar Cipher Decoder works
The Caesar Cipher Decoder rotates uppercase (A–Z) and lowercase (a–z) ASCII letters modulo 26 while leaving spaces, numbers, and punctuation untouched.
Decryption: Plain_index = (Cipher_index − Shift + 26) mod 26; Encryption: Cipher_index = (Plain_index + Shift) mod 26.
In Brute-force mode, the tool evaluates all 25 possible shift keys and ranks the most likely plaintext using English vowel and high-frequency consonant weights.
Caesar Cipher Decoder Quick Reference & Input Guide
Review the input parameters, allowed value ranges, and worked reference cases for the Caesar Cipher Decoder below before running your calculation.
| Parameter / Scenario | Default / Sample Value | Rule or Allowed Range | Output / Usage Note |
|---|---|---|---|
| Input: Operation mode | decode | Options: Decode (shift letters backward by key), Encode (shift letters forward by key), Brute-force all 25 shifts (auto-rank best English match) | Required workspace parameter |
| Input: Shift key (1 to 25, ROT13 = 13) | 3 | Range: 1 to 25 | Required workspace parameter |
| Input: Ciphertext or plaintext | Khoor, Zruog! Wklv lv d Fdhvdu fl... | ASCII letters A-Z and a-z are shifted; spaces, digits, and punctuation remain unchanged. | Required workspace parameter |
| Worked Case 1: Baseline run (Decode shift = 3) | Preloaded / test values | Decode: P = (C − k) mod 26; Encode: C = (P + k) mod 26 (applied independently to A-Z and a-z) | Inputting 'Khoor, Zruog! Wklv lv d Fdhvdu flskhu whvw.' in Decode mode with shift 3 shifts each letter 3 places backward (K→H, h→e, o→l) to reveal 'Hello, World! This is a Caesar cipher test.' across 34 alphabetic characters. |
| Worked Case 2: Unknown shift recovery (Brute-force all 25 shifts) | Preloaded / test values | Decode: P = (C − k) mod 26; Encode: C = (P + k) mod 26 (applied independently to A-Z and a-z) | Switching to Brute-force mode on 'Gur dhvpx oebja sbk whzcf bire gur ynml qbt.' tests shifts 1 through 25, automatically identifies shift -13 (ROT13) as the top English match, and outputs 'The quick brown fox jumps over the lazy dog.' plus the full 25-line table. |
How to use Caesar Cipher Decoder step by step
Choose your operation mode, set the shift key (or select Brute-force), and run the decoder.
Choose Decode, Encode, or Brute-force mode
Select Decode when you know the shift key used to create the ciphertext, Encode to create a shifted message, or Brute-force when the shift key is unknown.
Set the shift key from 1 to 25
Enter a whole number between 1 and 25 for the alphabet offset (classical Caesar shift is 3; ROT13 is 13).
Paste text and export the decoded output
Paste your text into the box, click Run Caesar cipher, and use Copy result or Download result.txt to save the output.
Worked examples for Caesar Cipher Decoder
Test a classical right-shift-3 ciphertext and a ROT13 puzzle.
Baseline run (Decode shift = 3)
Inputting 'Khoor, Zruog! Wklv lv d Fdhvdu flskhu whvw.' in Decode mode with shift 3 shifts each letter 3 places backward (K→H, h→e, o→l) to reveal 'Hello, World! This is a Caesar cipher test.' across 34 alphabetic characters.
Unknown shift recovery (Brute-force all 25 shifts)
Switching to Brute-force mode on 'Gur dhvpx oebja sbk whzcf bire gur ynml qbt.' tests shifts 1 through 25, automatically identifies shift -13 (ROT13) as the top English match, and outputs 'The quick brown fox jumps over the lazy dog.' plus the full 25-line table.
Check letter case and non-alphabetic symbols
Digits (0–9), punctuation, and line breaks are never shifted by a standard 26-letter Caesar cipher. If digits in your puzzle are also shifted, decode the letters first and shift the digits modulo 10 separately.
How to interpret the Caesar Cipher Decoder output
Review the primary decoded string, character counts, and brute-force candidate table.
In Decode and Encode modes, the result box displays the exact transformed string so you can copy it directly. In Brute-force mode, the headline shows the best-scoring plaintext candidate, while the downloadable text box lists all 25 shifts side by side so you can spot non-English words or short acronyms.
Input limits, assumptions, and related tools
Understand the scope of monoalphabetic shift ciphers before analyzing unknown ciphertext.
A Caesar cipher uses a single fixed alphabet offset for every letter in the message. If brute-forcing all 25 shifts produces gibberish on every line, the ciphertext likely uses a polyalphabetic cipher (such as Vigenere) or a keyed substitution alphabet.
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Open toolCaesar Cipher Decoder: common questions
Answers about using Caesar Cipher Decoder and understanding its results.
How does a Caesar cipher decoder work?
A Caesar cipher shifts each letter of the alphabet forward by a fixed key k during encryption. Decoding reverses the process by shifting each ciphertext letter backward by k positions around the 26-letter alphabet (wrapping A back to Z).
What if I do not know the shift key used to encrypt the message?
Switch the Operation mode to 'Brute-force all 25 shifts'. Because the English alphabet has only 25 non-trivial shifts, the tool generates all 25 candidate plaintexts at once and highlights the shift with the highest English letter-frequency score.
Is ROT13 a Caesar cipher?
Yes. ROT13 is a Caesar cipher with a shift key of 13. Because 13 is exactly half of the 26-letter alphabet, encoding and decoding with shift 13 produce the exact same result.
Does my text stay private when using Caesar Cipher Decoder?
Yes. All character shifting and brute-force analysis run locally in your browser with no server requests. Use Copy result or Download result.txt to save the output.
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