How does a columnar transposition cipher work?

The plaintext is written into a grid row by row using a fixed number of columns. The columns are then read out top to bottom in the order the keyword letters sort alphabetically, producing the ciphertext.

How does the keyword set the column order?

Number the keyword letters by alphabetical rank: the earliest letter is column 1, the next is column 2, and so on. Duplicate letters keep their left-to-right order. Columns are read in that ranked sequence.

What happens to a partly-filled last row?

When the message does not fill a complete rectangle, the final row is short. The tool tracks exactly which columns are long and which are short so that decryption places every letter back in the right cell.

Does it keep spaces and punctuation?

No. Spaces and punctuation are removed and letters are uppercased before the grid is built, which is standard for classical transposition ciphers.

Is the columnar transposition cipher secure?

Not by modern standards. It hides letter order but preserves letter frequencies, so it can be broken by anagramming and frequency analysis, especially with a known message length.

Columnar Transposition Cipher

The columnar transposition cipher scrambles a message by changing the order of its letters rather than substituting them. Letters are written into a grid and then read out column by column in an order fixed by a keyword. Because nothing is substituted, the ciphertext contains exactly the same letters as the plaintext, just rearranged. This tool encrypts and decrypts instantly in your browser.

How it works

Choose a keyword; its length is the number of columns. Write the cleaned, uppercased plaintext into a grid one row at a time. To find the read order, rank the keyword’s letters alphabetically — the lowest letter marks the first column read, and ties are broken left to right.

To encrypt, read each column from top to bottom in that ranked order and concatenate the results. To decrypt, the tool first works out how many letters belong in each column (long columns versus short columns when the last row is incomplete), slices the ciphertext accordingly, refills the grid, and reads it back row by row.

Example

Encrypt WE ARE DISCOVERED FLEE AT ONCE with the keyword ZEBRAS (6 columns). The keyword ranks as A=1, B=2, E=3, R=4, S=5, Z=6, so columns are read in that order. The result is EVLNACDTESEAROFODEECWIREE. Decrypting with the same keyword restores the original message.

Notes

Transposition ciphers are often combined with substitution ciphers for extra strength — applying a columnar transposition twice (double transposition) was historically a serious hand cipher. On its own, though, it leaks letter frequencies and can be attacked by trying likely column counts.