Rail Fence Cipher CTF Walkthrough – Decryption Guide

 

Rail Fence

CTF's Author: Will Hong

Description

A type of transposition cipher is the rail fence cipher, which is described here. Here is one such cipher encrypted using the rail fence with 4 rails. Can you decrypt it?Download the message here.Put the decoded message in the picoCTF flag format, picoCTF{decoded_message}.

Hint is-Once you've understood how the cipher works, it's best to draw it out yourself on paper.

The challenge gives us a type of transposition cipher called a rail fence cipher. The description reads:

"A type of transposition cipher is the rail fence cipher, which is described here. Here is one such cipher encrypted using the rail fence with 4 rails. Can you decrypt it?"

We will download the file and get the encrypted text.

The Rail Fence Cipher is a form of transposition cipher. Instead of replacing characters with others (like in substitution ciphers), it reorders the characters in a zig-zag pattern across several "rails" (rows), then concatenates the letters row-by-row to create the ciphertext.

To decrypt a rail fence cipher:

1. Determine the zig-zag path for 4 rails.
2. Mark positions where each character will go.
3. Place characters from the cipher into the zig-zag positions.
4. Read off the characters in zig-zag order to reconstruct the plaintext.

We’ll write a Python script to automate this decryption.

def rail_fence_decrypt(ciphertext, num_rails):

    # Create the rail matrix

    rail = [['\n' for _ in range(len(ciphertext))] for _ in range(num_rails)]

 

    # Step 1: Mark zig-zag path

    dir_down = None

    row, col = 0, 0

    for _ in range(len(ciphertext)):

        if row == 0:

            dir_down = True

        elif row == num_rails - 1:

            dir_down = False

        rail[row][col] = '*'

        col += 1

        row += 1 if dir_down else -1

 

    # Step 2: Fill the rails with the ciphertext

    index = 0

    for i in range(num_rails):

        for j in range(len(ciphertext)):

            if rail[i][j] == '*' and index < len(ciphertext):

                rail[i][j] = ciphertext[index]

                index += 1

 

    # Step 3: Read the message in zig-zag order

    result = []

    row, col = 0, 0

    for _ in range(len(ciphertext)):

        if row == 0:

            dir_down = True

        elif row == num_rails - 1:

            dir_down = False

        if rail[row][col] != '\n':

            result.append(rail[row][col])

            col += 1

        row += 1 if dir_down else -1

 

    return ''.join(result)

Flag is -picoCTF{WH3R3_D035_7H3_F3NC3_8361N_4ND_3ND_D00AFDD3}