Question

Symmetric Cipher (Note: the decrypted message is NOT an English word). (1) (16') Decrypt the ciphertext "cdefghijklmnop" using the rail fence cipher with depth 5 (give detailed steps).

Answer 1

Decrypting 'cdefghijklmnop' using a rail fence cipher involves reconstructing the zigzag pattern of the cipher's rails, with the depth of 5. After mapping the ciphertext onto the pattern, the plaintext 'pndlijekmjcfgh' is read off each rail left to right.

Step-by-step explanation:

Decrypting Ciphertext using the Rail Fence Cipher with Depth 5

To decrypt the ciphertext "cdefghijklmnop" using the rail fence cipher with a depth of 5, follow these steps:

1. Write the ciphertext in a zigzag pattern across 5 lines (rails) to mimic the original encryption process. Since the decrypted message is not an English word, we will rely solely on the pattern.
2. On a separate set of lines, denote the zigzag pattern of the rail fence by assigning letters to their respective positions. You will not fill in these letters immediately, as they serve as a guide. The pattern looks like this:

1 x x x
2 x x x x x
3 x x x
4 x x x x x
5 x x

1. Now, fill in the ciphertext letters in a zigzag manner, following the pattern determined by the skeleton. Starting from the top rail to the bottom and then back up, attempt to match the pattern.
2. The fill-in process reveals the transposition pattern. Begin to write out the extracted text by reading each rail from left to right. This results in the plaintext being revealed: "pndlijekmjcfgh".

As indicated in the Zimmerman Telegram, this is an example of a Transposition Cipher, where the order of letters in plaintext is changed according to a rule. Here, the rail fence cipher uses the depth of 5 as a guide for creating and decrypting the cipher.