rail-fence

Challenge Description

This was an interesting challenge that involves a message encoded using the rail fence cipher.

Rail Fence Cipher

Essentially, the plaintext is written diagonally in a zig-zag format, according the number of “rails” in the “fence”. The ciphertext is the result of reading off the “rails” horizontally.

For more information about this cipher, refer to this page. This is the same page provided in the challenge description.

Ciphertext

We begin by download the file message.txt using wget, followed by running cat to see the ciphertext that we need to decode:

Ta _7N6D49hlg:W3D_H3C31N__A97ef sHR053F38N43D7B i33___N6

Decoding the ciphertext

Using an online Rail Fence Cipher decoder, we can specify the number of rails to decode the ciphertext and obtain the flag.

Flag

picoCTF{WH3R3_D035_7H3_F3NC3_8361N_4ND_3ND_4A76B997}