Cipher Calculator | Caesar, Vigenère, Atbash & Brute-Force Decoder
Encode and decode text with four classical ciphers: Caesar (any shift), Vigenère (any keyword), ROT13, and Atbash. Includes letter frequency analysis comparing plaintext and ciphertext distributions, and a brute-force Caesar cracker that shows all 26 possible decryptions ranked by English letter frequency score.
What Is the Cipher Calculator | Caesar, Vigenère, Atbash & Brute-Force Decoder?
Classical ciphers substitute or rearrange characters to conceal meaning. Caesar shift rotates each letter by a fixed amount — Julius Caesar reportedly used shift 3. Vigenère uses a keyword to vary the shift at each position, making simple frequency analysis harder. ROT13 is its own inverse. Atbash mirrors the alphabet. The brute-force Caesar cracker tries all 26 possible shifts and ranks them by how closely their letter frequencies match English text.
Formula
Caesar: E(x) = (x + k) mod 26
Vigenère: Eᵢ = (pᵢ + kᵢ) mod 26
ROT13: shift = 13 (self-inverse)
Atbash: E(x) = 25 − x (A↔Z, B↔Y, …)
Non-alphabetic characters are preserved unchanged
How to Use
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Select a cipher: Caesar, Vigenère, ROT13, or Atbash
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Enter your plaintext (encode) or ciphertext (decode)
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For Caesar: set the shift amount (0–25)
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For Vigenère: enter a keyword (letters only)
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Toggle Encode / Decode as needed
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Click Process — view result and letter frequency chart
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For unknown Caesar shift: use Brute Force to rank all 26 shifts
Select a cipher. Enter your text. For Caesar, set the shift (0–25). For Vigenère, enter a keyword. Toggle between Encode and Decode. Click Process. For brute-force Caesar decoding, enter ciphertext and click Brute Force to see all 26 shifts ranked by English frequency score.
Example Calculation
Example: Caesar cipher, shift 3
Plaintext: HELLO WORLD
H+3=K, E+3=H, L+3=O, L+3=O, O+3=R → KHOOR
W+3=Z, O+3=R, R+3=U, L+3=O, D+3=G → ZRUOG
Ciphertext: KHOOR ZRUOG
Decode shift 3: K−3=H, H−3=E, O−3=L, O−3=L, R−3=O → HELLO ✓
Frequently Asked Questions
How does the brute-force cracker work?
The cracker tries all 26 Caesar shifts (0–25) on the ciphertext. For each shift, it computes a chi-squared-like score comparing the letter frequencies of the decrypted text to the known frequencies of English (E≈12.7%, T≈9.1%, A≈8.2%, …). The shift with the lowest score is most likely to be English and appears first in the ranked list.
Why is Vigenère harder to break than Caesar?
Caesar has only 26 possible keys. Vigenère uses a keyword that varies the shift at each position, so the same plaintext letter can encrypt to different ciphertext letters. This defeats single-letter frequency analysis. However, Vigenère is vulnerable to the Kasiski test and index-of-coincidence analysis when the keyword is short.
What is ROT13?
ROT13 is a Caesar cipher with shift 13. Because the alphabet has 26 letters, ROT13 applied twice returns the original text. It is commonly used online to obscure puzzle answers or spoilers without requiring a separate key. It provides zero cryptographic security.
What is Atbash?
Atbash is an ancient Hebrew cipher that replaces each letter with its mirror in the alphabet: A↔Z, B↔Y, C↔X, etc. Like ROT13, it is its own inverse. Atbash is used in the Bible (e.g., Babel written as Sheshach in Jeremiah). Numerically, E(x) = 25−x for 0-indexed letters.
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