Explain the following security principles. For each, give one example of how they are used in real$-$life. Do NOT re$-$use the examples in lecture PPT$.$

$($a$)$ Least privilege.

$($b$)$ Privilege separation.s

$($c$)$ Fail$-$safe default.

The following figure shows the encryption diagram for a block cipher mode.

$($a$)$ Draw the decryption diagram.

$($b$)$ For this operation mode, can encryption and decryption be parallelizable and why?

$($c$)$ If one ciphertext block was lost during transmission, what is the impact on decryption?

$($d$)$ What happens if two messages are encrypted with the same key and IV$?$ What can the attacker learn about the two messages just by looking at their ciphertexts?

An attacker is trying to attack the website of a company. Assume that users always visit the company's website using the Diffie$-$Hellman version of TLS$.$ Analyze whether the following statements are true or false and explain why.

$($a$)$ Because Diffie$-$Hellman is vulnerable against man$-$in$-$the$-$middle $($MITM$)$ attacker, a MITM attacker between a user and the company's website server will be able to discover the plaintext of data sent between the user and the company.

$($b$)$ If the attacker has obtained the private key of a certificate authority trusted by users of the company, the attacker can impersonate the company's website to a user.