1. Alice and Bob are sending a message m to each other
a) Give one method to attain confidentiality for m.
b) Give one method to attain integrity but NOT confidentiality for m
2. Computer B needs a user name and password for every login. User A requires 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 the diagram only to describe message exchange and content of each message.
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? (Demonstrate the whole 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? (Demonstrate the entire mathematical steps)
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. Suppose the encryption is absolutely unbreakable.
a) Can B be reasonably sure that message is generated by A? Why?
b) If B shows another person C that B could use the key k to decrypt the message, could C be reasonably sure that 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 could be reasonably sure that the message is indeed generated by A?
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 problems:
a) How to decrypt the message with brutal force? What is the amount of time required?
b) If length of the key is doubled, what is the amount of time needed to decrypt message with brutal force? Why?
6. With CBC encryption, message blocks m1-m2-m3-m4 are encrypted into c1-c2-c3-c4. Now if a hacker intercepts cipher text and retransmits c1-c3-c3-c4, what would be the “plain text” blocks being decrypted into? (Demonstrate all detail calculations)