Problem 1
What are the differences between message confidentiality and message integrity? Can you have confidentiality without integrity? Can you have integrity without confidentiality? Justify your answer
Problem 2
Internet entities (routers, switches, DNS servers, Web servers, user end systems, and so on) often need to communicate securely. Give three specific example pairs of Internet entities that may want secure communication.
Problem 3
From a service perspective, what is an important difference between a symmetric-key system and a public-key system?
Problem 4
Suppose that an intruder has an encrypted message as well as the decrypted version of that message. Can the intruder mount a ciphertext-only attack, a known-plaintext attack, or a chosen-plaintext attack?
Problem 5
Consider an 8-block cipher. How many possible input blocks does this cipher have? How many possible mappings are there? If we view each mapping as a key, then how many possible keys does this cipher have?
Problem 5
Consider the block cipher in Figure 8.5. For a given “key” Alice and Bob would need to keep eight tables, each 8 bits by 8 bits. For Alice (or Bob) to store all eight tables, how many bits of storage are necessary? How does this number compare with the number of bits required for a full-table 64- bit block cipher?
Problem 6
Suppose N people want to communicate with each of N – 1 other people using symmetric key encryption. All communication between any two people, i and j, is visible to all other people in this group of N, and no other person in this group should be able to decode their communication. How many keys are required in the system as a whole? Now suppose that public key encryption is used. How many keys are required in this case?
Problem 7
Suppose n = 10,000, a = 10,023, and b = 10,004. Use an identity of modular arithmetic to calculate in your head (a • b) mod n.
Problem 8
Suppose you want to encrypt the message 10101111 by encrypting the decimal number that corresponds to the message. What is the decimal number?
Problem 8
Consider RSA with p = 5 and q = 11. a. What are n and z? b. Let e be 3. Why is this an acceptable choice for e? c. Find d such that de = 1 (mod z) and d < 160. d. Encrypt the message m = 8 using the key (n, e). Let c denote the corresponding ciphertext. Show all work. Hint: To simplify the calculations, use the fact: [(a mod n) • (b mod n)] mod n = (a • b) mod n
