IFN642 Applied Cryptography and Network Security 1 | P a g e Symmetric Stream Ciphers Exercise – Exploring CrypTool Templates CrypTool provides templates for many of the basic combinations of primitives. If there is a primitive you are interested in, the templates section if often a good point of reference. To start this practical we are going to explore and modify several templates around XOR and RC4 stream ciphers. Activity 1. XOR Cipher Open the XOR Cipher template (Cryptography / Classical / XOR Cipher). Try running the program by pressing the Play button, in the ribbon menu. a) What are the functionalities of StringEncoder and StringDecoder? b) What happens when the key length does not match the plaintext length? c) What happens to ciphertext when the same plaintext is repeated? IFN642 Applied Cryptography and Network Security 2 | P a g e Activity 2. RC4 Stream Cipher Open and run RC4 Cipher Template (Modern / Symmetric / RC4 Cipher). Try running the program by pressing the Play button, in the ribbon menu. a) What are the differences between RC4 Encryption and RC4 Decryption? (if any) b) What happens when the key length does not match the plaintext length? Why? (Try 4-byte, 5 byte and 20-byte keys) c) What happens to ciphertext when the same plaintext is repeated? How is it different than XOR Cipher? Activity 3. Random Number Generation Open following templates and try running them by pressing the Play button, in the ribbon menu. 1. Linear Congruential Generator (under Mathematics). Please refer to lecture slides and discuss within your group the following questions: how does it generate numbers? What is the cycle length? 2. Linear Feedback Shift Register (LFSR) (under Mathematics). Please refer to lecture slides and discuss within your group the following questions: How does it generate numbers? What is a tap sequence? What is the seed? Please change the tap sequence to something else and run it again. 3. Non-Linear Feedback Shift Register (NLFSR) (under Mathematics). Please refer to lecture slides and discuss within your group the following questions: How does it generate numbers? Please change the tap sequence to something else and run it again. IFN642 Applied Cryptography and Network Security 3 | P a g e Assessment Challenge – The Stream Cipher Task 1. Using three LFSRs, develop the generator of A5/1 Encryption. Please refer to lecture slides for the layout of A5/1. (Hint: Create a new Cryptool workspace from the startcenter. You may like to check Component/Tools and Component/Protocols for required components. ) Task 2: Modify XOR Cipher Template to implement A5/1. Modify the template to work with your generator from task1. Preparing your results At the end of this practical please prepare a report, which should consist of a copy of the source (.cwm) file that you had at the end of task 2, and a 2 page pdf with a screen shot and text explaining how it is done. Add these files to Assessment 2 portfolio.