About this tool
The Columnar Transposition Cipher tool is a free, in-browser encoder and decoder for a keyword-driven transposition cipher used historically by militaries and now common in CTFs. The plaintext is written left-to-right into a grid whose width equals the length of the keyword. Each column is then numbered by the alphabetical rank of the keyword letter above it — ties are broken left-to-right — and the ciphertext is produced by reading the columns out in ascending rank order. For the keyword ZEBRAS, for example, the A-column is read first, then B, then E, R, S, Z.
Two variants are supported. In the complete (padded) variant the final short row is filled with a pad character of your choice so every column is the same height. In the incomplete (irregular) variant no padding is added, leaving a ragged last row; decoding then carefully computes each column's true length so the letters land back in the right cells. All computation is done locally in your browser and nothing is uploaded.
Decoding reverses the process: from the ciphertext length and keyword the tool determines how many letters each column holds, assigns the ciphertext blocks to columns in key-rank order, and reads the reconstructed grid back row by row. In the padded variant any trailing pad characters are trimmed from the recovered text. Because transposition changes only letter positions and not their identities, columnar transposition is often layered with a substitution cipher for extra strength.