M107: Transposition and Substitution

Transposition Ciphers. A transposition is a system of in which each letter of a message changes its position within the message but retains its identity.

1. The discussed last class is a type of transposition cipher. Remind yourself how this method of encryption works by encrypting your full name using a five-row rail fence cipher.

2. Suppose you receive the following from your friend: “MMTLKEEEHDTDEBEORNTYOAEZ” and you had agreed to use a four-row rail fence cipher (so your private was 4).

(a) Determine how many characters there must be in each row of the rail fence using the division algorithm.

(b) Use your previous answer to decode the given ciphertext.

3. Next, suppose you intercepted the the following ciphertext. “TTDELIYFTNHWERFSIFKGEACAEBNEETBYIRNYGRYHETPACTDELSSOHIERINE” Suspecting that the sender used a rail fence cipher, determine what the plaintext was.

Visit http://www.simonsingh.net/The_Black_Chamber/railfencecipher.html to use a utility to decipher the message (using many different possible keys until you find the correct number of rows).

4. Based on your reading, what is wrong with choosing an encryption method given by taking the plaintext and randomly rearranging the letters? Substitution Ciphers. A system of encryption in which each letter of a message is replaced by another character, but retains its position within the message.

1. Write the alphabet below and then directly below that, write the alphabet in reverse.

2. We choose to encrypt using the key you wrote above (replacing each letter in the alphabet by the letter directly below it in the reversed alphabet). Think up a plaintext message and encode it using this method. Pass the ciphertext to your partner.

3. Using your partner’s ciphertext and knowledge of the encryption method, determine the plaintext message.

4. According to The Book, what is the method for decrypting a if the key is unknown?