Question 1. Alice and Bob are sending a message m to each other

a) Give one method to achieve confidentiality for m.

b) Give one method to achieve integrity but NOT confidentiality for m

Question 2. Computer B requires a user name and a password for every login. User A needs to log on over the Internet. One solution is to send in the user name and the password in clear text.

a) What is the security vulnerability of this solution?

b) Design a better solution. Draw a diagram only to describe the message exchange and the content of each message.

Question 3. A stream cipher is used to encrypt. The plain text m1 is 10101010, the pseudo random k is: 01101011

a) What is the cipher text c1? (Show the entire mathematical steps)

b) If another plain text m2 11100010 is encrypted with the same pseudo random number k andgenerate cipher text c2, what useful information about m1 and m2 can you extract from c1 and c2? (Show the entire mathematical steps)

Question 4. B receives a financial transaction request message m from someone claims to be A. The message is encrypted with a shared security key k which only A and B know. Assume the encryption is absolutely unbreakable.

a) Can B be reasonably sure that the message is generated by A? Why?

b) If B shows another person C that B can use the key k to decrypt the message, can C be reasonably sure that the message is generated by A? Whey?

c) Design a different protocol so that when B receives a message m from the real A, both B and C can be reasonably sure that the message is indeed generated by A?

Question 5. If the time to encrypt a message m using a 64-bit secret key k algorithm is T, use the big-O notation to answer the following questions:

a) How to decrypt the message with brutal force? What is the amount of time required?

b) If the length of the key is doubled, what is the amount of time required to decrypt the message with brutal force? Why?

Question 6. With CBC encryption, message blocks m1-m2-m3-m4 are encrypted into c1-c2-c3-c4. Now if a hacker intercepts the cipher text and retransmits c1-c3-c3-c4, what will be the "plain text" blocks being decrypted into?