IFN648 Applied Cryptography

Problem solving task:

The problems you will solve in this task relate to the cryptanalysis of classic ciphers.

• Download the Ciphers.zip file and extract to find the text file corresponding to your student number.  
  • The files are labelled based on student numbers; the last five digits in your student number will be the prefix, and the suffix will be the file type: txt.
  • For example: for student number 01234567 the appropriate file is34567.txt.
• In your file you will find four ciphertexts, labelled Q1 Caesar Cipher, Q2 Transposition Cipher, Q3 Vin cipher, Q4 Hill Cipher. Youwill attempt to recover the corresponding four plaintexts.

Cryptanalysis of your four ciphertexts are indicated below. Structure your report so that it is clear to the marker which ciphertext you are discussing, explain your process clearly to demonstrate your understanding and be sure to include the specific items requested for each of the ciphers, as follows:

Ciphertext 1 – Caesar cipher

26 character alphabet A-Z, punctuation has been removed and spaces have been retained but not encrypted.

1. Create a frequency analysis chart for your ciphertext (include a screenshot of this chart in your report).
2. Refer to yourfrequency chart and identify the ciphertext character that most likely represents the plaintext character ‘e’. Justify your answer.
3. Determine the key used with the Caesar cipher. Explain how you obtained this key.
4. Provide the plaintext corresponding to the ciphertext you were given.
5. Create a frequency analysis chart for your ciphertext.
6. Refer to the frequency chart and identify the ciphertext character that represents the plaintext character ‘e’. Justify your answer.
7. Determine the key used with the transposition cipher. Explain how you obtained this key.
8. Provide the plaintext corresponding to the ciphertext you were given, including appropriate punctuation.

Ciphertext 2 – Row/column Transposition Cipher

26 character alphabet,spaces have been shown as * and punctuation has been removed. Text begins at the start of a sentence but may end mid-word.

Ciphertext 3 – Vigenere Cipher

26 character alphabet, non-alphabet characters such as spaces and punctuation have been removed. Text begins at the start of a sentence, but may end mid-word.

1. Provide the output of Kasiski’s Test for factors up to 20.  
2. Refer to the output of Kasiski’s Test and identify a probable key length.
3. Describe the process involved in breaking the Vigenere cipher once the key length is determined.
4. Apply this process to determine the key used with the Vigenere cipher. Clearly explain how you obtained the key.
5. Provide the plaintext corresponding to the ciphertext you were given, including appropriate punctuation.

Ciphertext 4 – HillCipher, m = 2

26 character alphabet, non-alphabet characters such as spaces and punctuation have been removed. Text begins at the start of a sentence.

1. The Hill cipher is polygraphic, with block size m. For m = 2, explain how a known-plaintext attack is performed, and how much known plaintext is required for the attack to succeed.  
2. For a ciphertext-alone attack on a Hill cipher,some plaintext must be guessed.Assuming the plaintext is in English,list some common digrams. Justify your choice of items.
3. The position of a common plaintext word within a message may be unknown, so many positions may be tested for a common word, in a process called ‘crib dragging’. Given the knowledge that your plaintext contains the word about, apply crib dragging to obtain the encryption key matrix .
4. Compute the corresponding decryption matrix.
5. Provide the plaintext corresponding to the ciphertext you were given, including appropriate punctuation.