![[hw1-1.pdf]] --- # Answers 1. **Non-repudiation** requires two things: * Data integrity * Origin of data Together these two mechanism can guarantee that a message * Has not been tampered with * Comes from a verified source 2. Algorithms 1. **Published** 1. Only reliable way to assess algorithm security 2. prevents backdoors hidden by designers 3. software implementations 4. subject to domestic and international standards 5. large number of implementations lead to low cost and high performance 2. **Secret** 1. smaller number of users, little motivation to break 2. cryptanalysis must include recovering the algorithm 3. unavailable for other countries 3. Key Size $k_{1}=56$ $k_{2}=128$ 1. If you have the ciphertext and have guessed a fragment of the plaintext you can run an exhaustive key search (brute-force attack) 2. Key space is $2^{56}$ 3. See below: $ \frac{2^{56}}{64*10^9} \to 13 days $ 4. Key space is $2^{128}$ 5. See below: $ \frac{2^{128}}{64*10^9} \to 1.69 * 10^{18} centuries $ ![[hw1solution.pdf]]