1. Homework #1
J. H. Wang
Oct. 2, 2013

2. Homework #1 Chap.1: Chap.2: Due: two weeks (Oct. 16, 2013)
Review question 1.2
Chap.2:
Review question 2.4, 2.8
Problems: 2.3, 2.11, 2.13
Due: two weeks (Oct. 16, 2013)

3. Chap. 1
Review question 1.2: What is the difference between passive and active security threats?
Chap. 2
Review question 2.4: What is the difference between a block cipher and a stream cipher?
Review question 2.8: Why is the middle portion of 3DES a decryption rather than an encryption?

4. Problem 2.3: Perhaps the simplest “serious” symmetric block encryption algorithm is the Tiny Encryption Algorithm (TEA). TEA operates on 64-bit blocks of plaintext using a 128-bit key. The plaintext is divided into two 32-bit blocks (L0, R0), and the key is divided into four 32-bit blocks (K0, K1, K2, K3). (…to be continued)

5. Encryption involves repeated application of a pair of rounds, defined as rounds i and i+1: Li=Ri-1 Ri=Li-1+F(Ri-1, K0, K1, i) mod 264 Li+1=Ri Ri+1=Li+F(Ri, K2, K3, i+1) mod 264 where F is defined as F(M, Kj, Kk, i) = ((M<<4)+Kj mod 264) ((M>>5)+Kk mod 264) (M+i mod 264) And where the logical shift of x by y bits is denoted by x<<y, the logical right shift of x by y bits is denoted by x>>y, and i is a sequence of predetermined constants. (… to be continued…)

6. (a) Comment on the significance and benefit of using the sequence of constants.
(b) Illustrate the operation of TEA using a block diagram or flow chart type of depiction.
(c) If only one pair of rounds is used, then the ciphertext consists of the 64-bit block (L2, R2). For this case, express the decryption algorithm in terms of equations.
(d) Repeat part (c) using an illustration similar to that used for part (b).

7. Problem 2.11: Alice and Bob agree to communicate privately via using a scheme based on RC4, but they want to avoid using a new secret key for each transmission. Alice and Bob privately agree on a 128-bit key k. To encrypt a message m consisting of a string of bits, the following procedure is used:
Choose a random 80-bit value v
Generate the ciphertext c=RC4(v||k) m
Send the bit string (v||c)
Suppose Alice uses this procedure to send a message m to Bob. Describe how Bob can recover the message m from (v||c) using k.
If an adversary observes several values (v1||c1), (v2 ||c2), … transmitted between Alice and Bob, how can he/she determine when the same key stream has been used to encrypt two messages?

8. Problem 2.13: Is it possible to perform encryption operations in parallel on multiple blocks of plaintext in CBC mode? How about decryption?

9. Homework Submission
For hand-written exercises, please hand in your homework in class (paper version)

10. Thanks for Your Attention!