Rail Fence Cipher Decoder & Encoder

What is the rail fence cipher?

The rail fence cipher, also called the zigzag cipher, hides a message by changing the order of its letters, not the letters themselves. You write the message in a zigzag across a number of rows, the “rails” of the fence, and then read the rows one after the other. With 3 rails, MEETATTHEPARKATNOON becomes MAEKOETTHPRANOETATN.

That makes it a transposition cipher. Ciphers like the Caesar cipher swap each letter for another one (substitution); the rail fence keeps every letter and only moves it. You can see the difference at a glance: a rail fence cipher text has exactly the same letters as the original, just scrambled.

How to use the rail fence cipher encoder and decoder

1. Choose Encode or Decode.
2. Set the number of rails (the key). Both sides have to use the same number.
3. Type or paste your text. The result updates as you type, and the zigzag view in the tool shows where every letter lands on the fence.
4. Tap Copy to save the result.

Do not know the number of rails? The tool can crack it for you: it tries every number of rails from 2 to 10, so you only have to spot the result that reads as real words. The section on cracking below explains why that works so well.

How to encode a rail fence cipher by hand

1. Remove the spaces and write the message in capitals (most puzzles do this, so word lengths do not give the answer away).
2. Draw as many rows as you have rails. Write the first letter on the top rail, the next one rail lower, and keep going down diagonally.
3. When you reach the bottom rail, turn around and go back up; at the top, turn down again.
4. Read the top rail from left to right, then the second rail, and so on down to the bottom.

Here is MEETATTHEPARKATNOON on 3 rails:

Reading the rails in order gives MAEKO, ETTHPRANO and ETATN, which joined together is the cipher text MAEKOETTHPRANOETATN.

The tool treats spaces and punctuation as characters on the fence, just like letters, so WE MEET AT DAWN on 3 rails becomes WEAAEME TDW T N. If you want the classic look, remove the spaces before you encode.

Rail fence cipher with 2, 3, 4 and 5 rails

The pattern repeats every 2 × (rails − 1) letters: down to the bottom and back up to just below the top. That is the cycle. On 3 rails the cycle is 4, which is why you may see a key written as “height 3, cycle 4”. The same message with different numbers of rails:

With 2 rails the cipher is simply every other letter (the 1st, 3rd, 5th…) followed by the rest, which makes it a quick one to do in your head. With 4 rails the same message looks like this:

How to decode a rail fence cipher by hand

Decoding means rebuilding the fence. The trick is to work out how many letters sit on each rail before you place any of them. Take MAEKOETTHPRANOETATN with 3 rails:

1. Count the letters: there are 19.
2. Mark the zigzag. Draw 3 rows with 19 columns and put a dot where each letter would go, going down and up as when encoding. Count the dots on each rail: 5, 9, 5.
3. Cut the cipher text into those lengths: rail 1 gets MAEKO; rail 2 gets ETTHPRANO; rail 3 gets ETATN.
4. Fill each rail with its piece, left to right, on the dots.
5. Read the zigzag down and up again, one letter per column: MEETATTHEPARKATNOON.

Counting the dots is the step people skip, and it is the one that makes everything else fall into place. Graph paper helps a lot.

How to crack a rail fence cipher without the key

The rail fence has very few possible keys. A message of 19 letters can only use 2 to 18 rails (with as many rails as letters, nothing moves), and short messages are almost always written with 2 to 6. So you can simply try them all. Here is MAEKOETTHPRANOETATN decoded with every number of rails from 2 to 8:

Only one row reads as English, so the key was 3. The tool’s crack option does exactly this. A second clue is in the letters themselves: since nothing is substituted, the letter-frequency panel of a rail fence cipher text looks just like normal English (lots of E, T, A and O). If the frequencies look normal but the words do not, you are probably looking at a transposition cipher such as this one.

Variations of the rail fence cipher

• Offset: the zigzag can start partway down the fence instead of on the top rail, which adds a second number to the key. To decode one by hand, draw the zigzag as if it started on the top rail and leave the first few spots empty.
• Reading order: some puzzles read the rails from the bottom up, or read the zigzag in reverse. If no number of rails gives words, reverse the cipher text and try again.
• Combined ciphers: run the result through a substitution cipher such as Atbash or the Vigenère cipher. Mixing transposition and substitution is much stronger than either alone, and it is the idea behind many serious historical ciphers.

Is the rail fence cipher secure?

No. With so few keys, anyone can try them all in a minute, by hand or with this tool. It is still well worth learning, because it shows the second big family of classical ciphers. Transposition is an old idea: the scytale, a strip of material wound around a rod that ancient Greek writers describe, also hid a message by changing the order of its letters. Who first wrote messages in a zigzag is not recorded.

Rail fence cipher in Python

The algorithm is short enough to write in a few lines, which is why it is a classic programming exercise. Encoding appends each character to its rail; decoding works out the zigzag pattern first, cuts the cipher text into one piece per rail, then walks the zigzag again:

def rail_fence_encode(text, rails):
    if rails < 2:
        return text
    rows = [""] * rails
    row, step = 0, 1
    for ch in text:
        rows[row] += ch
        if row == 0:
            step = 1
        elif row == rails - 1:
            step = -1
        row += step
    return "".join(rows)


def rail_fence_decode(cipher, rails):
    if rails < 2:
        return cipher
    # 1. Which rail does each position fall on?
    pattern, row, step = [], 0, 1
    for _ in cipher:
        pattern.append(row)
        if row == 0:
            step = 1
        elif row == rails - 1:
            step = -1
        row += step
    # 2. Cut the cipher text into one piece per rail.
    pieces, start = [], 0
    for r in range(rails):
        count = pattern.count(r)
        pieces.append(list(cipher[start:start + count]))
        start += count
    # 3. Walk the zigzag again, taking the next letter from each rail.
    return "".join(pieces[r].pop(0) for r in pattern)


print(rail_fence_encode("MEETATTHEPARKATNOON", 3))  # MAEKOETTHPRANOETATN

Rail fence cipher practice puzzles

Three messages to decode, with the spaces removed as in most puzzles. Draw the fence on paper first, then check your answer with the decoder.

Rail fence puzzles for classrooms and escape rooms

• Classrooms: have students draw the fence on graph paper, encode a short sentence, and swap with a partner who has to find the number of rails. It teaches patterns, counting and why short keys are weak.
• Escape rooms: paint a fence with three rails on the wall or print the cipher text on a picket sign. The number of rails is the hint, and the decoded text gives the next clue or lock code.
• Layered puzzles: encode a message with the Polybius square, then scramble the digits with a rail fence. Solvers have to take down the fence before the number pairs make sense.

Free and private

This rail fence cipher tool runs entirely in your browser. It is free, needs no sign-up, and nothing you type is uploaded or stored.