Cipher Identifier — What Cipher Is This?
Paste a ciphertext to get a ranked list of likely ciphers and encodings — Base64, hex, Morse, Caesar, Vigenère, transposition and more — with reasons and decodes.
The ranking is a statistical guess, not proof. Modern ciphers (AES, RSA) produce random-looking bytes that no tool can identify or break without the key.
| Cipher or encoding | Score | Why |
|---|---|---|
| Caesar / ROT cipher | 93 | Shifting every letter back by 3 gives English. |
| Measure | Value | Meaning |
|---|---|---|
| Letters | 92 | |
| Distinct letters | 26 | |
| Index of coincidence | 0.0492 | English ≈ 0.066, random letters ≈ 0.038 |
| χ² against English letter frequencies (per letter) | 8.102 | below 0.87 = English-like for 92 letters |
| Letter-order score | 0.094 | English ≈ 0.3, letters in random order ≈ 0 (above 0.2 reads as English) |
| Method | Result |
|---|---|
| Best Caesar shift (back 3) | THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG AND THEN RUNS BACK ACROSS THE FIELD TO FIND ITS FRIENDS NEAR THE RIVER |
| Atbash | DPS GCOUM VFIAJ RIZ NCKHE IBSF DPS LWXY TIQ WJT DPSJ FCJE VWUM WUFIEE DPS ROSLT DI ROJT ODE RFOSJTE JSWF DPS FOBSF |
| Best affine key (a = 7, b = 7) | RTA YGIWM HNUKF PUZ XGQJC UVAN RTA BSDO LUE SFL RTAF NGFC HSWM SWNUCC RTA PIABL RU PIFL IRC PNIAFLC FASN RTA NIVAN |
| ROT47 | (zw %){u} t&#+" x#p |)!$' #*w& (zw ~srq v#y s"v (zw" &)"' tsu} su&#'' (zw x{w~v (# x{"v {(' x&{w"v' "ws& (zw &{*w& |
Found a strange string in a puzzle, a geocache or a CTF challenge? This identifier looks at which characters it uses, its length and its letter statistics, and ranks the ciphers and encodings that could have produced it: Base64, Base32, hex, binary, Morse, tap code, Polybius, Baconian, URL encoding, ROT47, Caesar, Atbash, affine, simple substitution, Vigenère, transposition and Playfair. Where a cipher needs no key, it also shows the decoded text.
How to use this tool
- Paste the ciphertext, keeping its spaces and punctuation.
- Read the most likely candidate and why it was chosen.
- Check the decoded text, the candidate list and the letter statistics.
- Open the matching tool (Caesar, Vigenère, substitution solver …) to finish the job.
Formula
IoC = Σ nᵢ(nᵢ − 1) / (N(N − 1)) χ² = Σ (nᵢ − N·pᵢ)² / (N·pᵢ)
nᵢ is how often letter i occurs, N the number of letters and pᵢ the share of that letter in English. English text has an index of coincidence near 0.066 and random letters 0.038: substitution and transposition keep the English value, polyalphabetic ciphers push it towards random. χ² tells whether the letters themselves are English (transposition) or replaced (substitution). A letter-order score (how much more common the text’s letter pairs are than the same letters in random order) decides whether a decoded guess reads as English.
Example
The default text, WKH TXLFN EURZQ IRA…, has English-like statistics but no English letter order; shifting it back by 3 gives "THE QUICK BROWN FOX…", so it is ranked Caesar / ROT cipher first. SGVsbG8gV29ybGQh uses only Base64 characters and decodes to "Hello World!", so Base64 comes first.
Frequently asked questions
Can it identify AES or other modern encryption?
Only that it is not a classical cipher. Modern ciphertext is random-looking bytes, often shown as Base64 or hex; without the key nothing can decrypt it, and the encoding is all that can be recognised.
Why is a short text identified wrongly?
Statistics need data. With fewer than about 50 letters the index of coincidence and frequency tests are unreliable, so several candidates get similar scores.
What is the index of coincidence?
The chance that two letters picked at random from the text are the same. It stays at the English value (about 0.066) when letters are only moved or swapped one-for-one, and drops towards 0.038 when many alphabets are mixed, as in Vigenère.
Last updated:
Page address with these inputs: https://kakao.mobile/tools/cipher-identifier/
Cite or embed this tool
kakaoCalc. (2026, November 8). Cipher Identifier — What Cipher Is This?. https://kakao.mobile/tools/cipher-identifier/
“Cipher Identifier — What Cipher Is This?.” kakaoCalc, 8 Nov. 2026, kakao.mobile/tools/cipher-identifier/.
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