HTW Berlin Fachbereich 4 Internationaler Studiengang Internationale Medieninformatik (Bachelor) Aktuelles - Medien: Kryptographie Winter Term 2010/11

Lab 1: XOR & GCD

1. The one-time pad uses XOR with a random key for encryption. It is a symmetric encryption method, that means that both parties need a copy of the same key.
   1. Produce two copies of a one-time pad with 80 random binary digits. Give one copy to a partner A, and take a one-time pad from another partner B.
   2. Make up a plaintext secret message that contains 10 letters. Encode these letters in ASCII (meaning that you now have 80 binary digits).
   3. Now use one copy of your one-time pad to encode your plaintext using XOR on every binary digit. Write the ciphertext on another piece of paper. Give your partner A the ciphertext.
   4. When B gives you a ciphertext, use their one-time pad to decipher the message.
   5. Compare your results with B, and compare what A calculated with your original message. Report on what happens.
      
      
2. Determine the last two decimal digits of the number 2 ^ 1000!
   
3. Use the extended Euclidian Algorithm to calculate the greatest common divisor of 1309 and 612 by hand. Document your work.
   
   
4. Solve for x:
         612x ≡ 85 mod 1309
   Document your work.
   

Everyone needs to write their own report, as you will have had two different partners. The report is due next Wednesday at 23.00!