Rail Fence Cipher Encoder & Decoder

The Rail Fence is a transposition cipher: the letters of a message are written diagonally down and back up across a fixed number of rails, then read off one row at a time. Nothing is substituted — every letter of the plaintext is still there, in a different order — so the key is simply how many rails you used, and decoding with the same number puts everything back.

That "nothing is substituted" is the property that gives a transposition cipher away instantly. Count the letters in the ciphertext and the frequencies match ordinary text: E is still the most common letter, the distribution still looks like the language it came from. A substitution cipher shifts that curve onto different symbols; a transposition leaves it exactly where it was. So the first thing an analyst learns from a rail-fence message is that it is a rail fence, or something like one, before trying a single key.

The key space is the other half of the problem, and it is tiny. The number of rails can only run from two up to just under the message length, so a hundred-character message has fewer than a hundred possible keys and every one of them can be tried and read in the time it takes to scroll. Two rails is the degenerate case: it simply separates the odd-numbered letters from the even-numbered ones. The zigzag also repeats with a period of twice the rail count minus two, which is what a solver looks for when the rail count is unknown.

None of that makes it useless, just not secret. It is a genuinely good puzzle cipher, it is where most people meet the idea that rearranging can hide meaning as well as replacing can, and it composes: applying a transposition after a substitution defeats the frequency analysis that would have broken either one alone, which is roughly the insight that classical cryptography spent a few centuries working out. Use it for a puzzle, a lesson or a game — not for anything you would mind a stranger reading.