$10.$ Assume we use public key encryption algorithm to construct a one$-$way hash function using RSA with a known arbitrary key. Then process a message consisting of a sequence of blocks as follows:

Encrypt the first block, XOR the result with the second block and encrypt again, and so on$.$ Show that this scheme is not secure by solving the following problem. Given a two$-$block message B$1,$ B$2,$ and its hash, we have RSAH$(B1,B2)=$ RSA$(RSA(B1)o+B2)$

Given an arbitrary block $C1,$ choose $C2$ so that RSAH$(C1,C2)=$RSAH$(B1,B2).$ Thus, the hash function does not satisfy weak collision resistance.

Key distribution and user authentication

$11.$ List and explain three ways in which secret keys can be distributed to two communicating parties.

$12.$ Explain what chain of certificates mean?

$13.$List and explain the steps involved in the SSL Record Protocol, and the SSH transport layer protocol.

Wireless security

$14.$ Briefly describe the four IEEE $802\mathrm{.}11$i phases of operation.

$15.$ For WEP, data integrity and data confidentiality are achieved using the RC$4$ stream encryption algorithm. The transmitter of an MPDU performs the following steps, referred to as encapsulation:

The transmitter selects an initial vector $($IV$)$ value. The IV value is concatenated with the WEP key shared by transmitter and receiver to form the seed, or key input, to $RC4.$ A $32-$bit cyclic redundancy check $($CRC$)$ is computed over all the bits of the MAC data field and appended to the data field. The CRC is a common error$-$ detection code used in data link control protocols. In this case, the CRC serves as

a integrity check value $($ICV$).$ The result of step $3$ is encrypted using RC$4$ to form the ciphertext block. The plaintext IV is prepended to the ciphertext block to form the encapsulated MPDU for transmission.

a$)$ Draw a block diagram that illustrates the encapsulation process.

b$)$ Describe the steps at the receiver end to recover the plaintext and perform the integrity check.

c$)$ Draw a block diagram that illustrates part b$.$