1. Alice has a paper saved on a third-party computer (such as in the cloud).

How can she prevent anyone except herself from reading the content of the paper beside herself?

How can she allow other people to view the paper but she can detect whether the paper has been modified?

2. Computer B requires a user name and a password for every login. User A needs to log on over the Internet. One solution (e.g., subnet) is to send in As user name and password to B in clear text.

What is the security vulnerability of this solution if there is any?

Design a better solution. Draw a diagram only describe the message exchange between A and B, and the content of each message.

3. Party B receives a financial transaction request m from someone claims to be party A. The request is encrypted with a shared security key k which only A and B know. Assume the encryption is unbreakable and the key is never leaked.

Can B be sure that the request is generated by A? Why?

Later user A sues B and claims that he/she did not make the request. Can B prove to court that A is lying? Why?

4. Assume it takes time T to encrypt or decrypt a message m using a 50-bit secret key k algorithm.

How to decrypt the meaning with brutal force? What is the amount of time required?

If the length of the key is now 150-bits, what is the amount of time required to decrypt the message with brutal force? Why?

5. With CBC encryption, message blocks m₁- m₂- m₃- m₄ encrypted into c₁-c₂-c₃-c₄. Now if a hacker intercepts the cipher text and retransmits c₁-c₂-c₃-c₄, what will be the plain text blocks decrypted into? Show all steps of reasoning.

6. Of the four cipher block modes of operations, ECB, CBC, OFB, and CTR, which one is less secure? Why?