How to choose a cipher decoder

Choose a first test from the clue

An alphabet suggests candidates; it does not identify a cipher. Preserve spaces and punctuation until you know their role.

Common puzzle clues and matching tests
Clue	Candidate	First test
A fixed letter shift	Caesar	Try rotations in the ROT13 tool.
A shift of thirteen	ROT13	Apply rotation 13; the same operation reverses it.
A repeated keyword	Vigenere	Enter the supplied key in Vigenere Decoder.
Dots and dashes with separators	Morse	Use Decode in Morse Code Translator and match its delimiters.
A rail count or zigzag clue	Rail fence	Match Rails and Offset.
A multiplier and shift	Affine	Enter the supplied a and b values.
Only E and e	Cetacean	Preserve case and inspect complete sixteen-symbol blocks.

Hexadecimal and Base64 represent bytes using public rules. Neither requires a secret key.

The decoded bytes might be text, ciphertext, or a file. Successful decoding does not establish their meaning.

RC2 belongs to legacy encryption: use it only with the original key and compatible settings.

Count each letter after removing nonletters from a separate analysis copy. Let N be the total letter count.

The unnormalized index of coincidence is Σ fᵢ(fᵢ − 1) / (N(N − 1)).

Uniform independent letters have an expected value of 1/26, approximately 0.03846.

The cited textbook gives typical English a normalized value near 1.75. Dividing by 26 gives approximately 0.0673.

A fixed substitution or transposition preserves letter counts. Repeated-key substitutions can distribute those counts across several alphabets.

Short samples vary widely. Neither reference value proves a method, language, or successful solution.

Suppose the clue gives three rails and a start on the top rail.

Enter WECRLTEERDSOEEFEAOCAIVDEN in Rail Fence Cipher Decoder. Set Rails to 3 and Offset to 0.

The result is WEAREDISCOVEREDFLEEATONCE. Read it as “WE ARE DISCOVERED FLEE AT ONCE”.

The decoder reconstructs row lengths of 7, 12, and 6 characters. It then reads them along the zigzag.

Encode that result with the same settings to reproduce the ciphertext. This checks the transformation against the supplied clue.

With a = 5 and b = 8, Rclla becomes Hello. The decoder calculates the inverse multiplier itself.

The multiplier must be coprime to 26. A wrong valid key can still produce output without an error.

Readable fragments can occur by chance. Judge the complete message against its source and clues.

A reversible transformation can round-trip the wrong interpretation. Preserve the original, settings, and candidate result together.

If candidates remain ambiguous, seek another clue instead of deleting characters until the output looks plausible.