# Computer Science

Suppose Alice has four blocks of plaintext, Po, -P1, P2, -P3· She computes a MAC using key K\, and then CBC encrypts the data using key K2 to obtain C0,C1,C2,C3. Alice sends the IV, the ciphertext, and the MAC to Bob. Trudy intercepts the message and replaces C\ with X so that Bob receives IV, C\$,X, 62,63, and the MAC. Bob attempts to verify the integrity of the data by decrypting (using key K2) and then computing a MAC (using key K1) on the putative plaintext.

a. Show that Bob will detect Trudy’s tampering.

Don't use plagiarized sources. Get Your Custom Essay on
Computer Science
Just from \$13/Page

b. Suppose that Alice and Bob only share a single symmetric key K. They agree to let K1= K and K2 = K (BY, where Y is known to Alice, Bob, and Trudy. Assuming Alice and Bob use the same scheme as above, does this create any security problem?

Suppose that Alice and Bob have access to two secure block ciphers, say, Cipher A and Cipher B, where Cipher A uses a 64-bit key, while Cipher B uses a 128-bit key. Alice prefers Cipher A, while Bob wants the additional security provided by a 128-bit key, so he insists on Cipher B. As a compromise, Alice proposes that they use Cipher A, but they encrypt each message twice, using two independent 64-bit keys. Assume that no shortcut attack is available for either cipher.

Is Alice’s approach as secure as Bob’s? Grab A 14% Discount on This Paper
Pages (550 words)
Approximate price: -
Paper format
• 275 words per page
• 12 pt Arial/Times New Roman
• Double line spacing
• Any citation style (APA, MLA, Chicago/Turabian, Harvard)

Try it now!

## Grab A 14% Discount on This Paper

Total price:
\$0.00

How it works? Fill in the order form and provide all details of your assignment. Proceed with the payment

Choose the payment system that suits you most. 