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Cipher decoder for CTFs: Caesar, Vigenère, Morse and more

Encrypt or decrypt Caesar, Vigenère, Atbash, Morse code, A1Z26, Bacon and Rail Fence text. For Caesar, one click shows all 26 shifts and highlights the rows that look like English, so you can find the flag line at a glance.

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Caesar brute force Case and punctuation kept Runs in your browser
Cipher toolkit

Encrypt or decrypt classic ciphers

Pick a cipher, choose Encrypt or Decrypt, and type or paste your text. Output updates as you type.

Mode

Output

What each cipher does and how to spot it

Caesar moves every letter a fixed number of places along the alphabet. ROT13 is Caesar with a shift of 13. Spot it by ciphertext that keeps word lengths and punctuation, with letter frequencies that look like English but rotated. There are only 25 useful shifts, so brute force always works.

Vigenère applies a repeating keyword, so each letter gets a different Caesar shift. Spot it by ciphertext that looks flat, with no dominant letter, but still keeps spaces and punctuation. You need the key, or a statistical attack such as the index of coincidence to find the key length.

Atbash reverses the alphabet (A becomes Z, B becomes Y). It has no key, and applying it twice returns the original text.

Morse code uses dots and dashes. Letters are separated by a space and words by a slash or a wider gap. If a challenge gives only two symbols, also try binary or Bacon.

A1Z26 replaces each letter with its position, A=1 to Z=26. If you see numbers only between 1 and 26, try this first. Numbers above 26 usually mean ASCII decimal or hex instead.

Bacon encodes each letter as five symbols from two choices, such as A and B, or two fonts or cases. Rail Fence is a transposition: it keeps the same letters and writes them in a zigzag over a number of rails, so letter frequencies match English exactly.

Using this in a CTF crypto challenge

A quick triage: letters only with English-like frequencies point to a transposition such as Rail Fence; letters with shifted frequencies point to a substitution such as Caesar or Atbash; flat frequencies with preserved punctuation suggest Vigenère; dots and dashes mean Morse; numbers 1 to 26 mean A1Z26. Flags often keep their format after decoding, so look for the prefix, for example flag{ or CTF{, in the Caesar table.

Worked example

The text Synt{pnrfne} is ROT13 of Flag{caesar}. In the shift table, row 13 is highlighted because it contains "flag". Case and punctuation are kept, so the braces stay where they are. Vigenère with key KEY turns Attack at dawn into Kxrkgi kx bkal; only letters advance the key.

Tips

  • Try the brute-force table before anything else for letter-only ciphertext.
  • Chain ciphers: decode Morse, then run the result through Caesar or A1Z26.
  • Non-letters are passed through unchanged, except in Morse, A1Z26, Bacon and Rail Fence, where the format itself changes.
  • Everything runs locally in your browser. Nothing you paste is uploaded.

When to use this, and when to reach for dCode or CyberChef

This page covers seven classical ciphers and shows every Caesar shift at once, highlighting the row that contains a common word such as flag, ctf or the. That is enough for most beginner crypto challenges. It does not break Vigenère without the key, and it has no substitution solver. For those, dCode has automatic Vigenère key recovery, frequency analysis and hundreds of rarer ciphers, and quipqiup solves general substitution ciphers. Use CyberChef when the cipher sits inside other encodings. Everything here runs in your browser, which matters if the ciphertext came from a private event.

Look up unfamiliar terms in the glossary, browse more CTF tools, or try real challenges on the practice platform.