Information Security System 2011S 1

Information Security System HW#1 (Due 4-6-11) 2011-I

1. Text (Problems 2.2) Consider a very simple symmetric block encryption algorithm, in which 32-bits blocks of plaintext are encrypted using 64-bit key. Encryption is defined as C = (P⨁K) ⊞ K whereC =ciphertext; K = secret key; K0 = leftmost 64 bits of K; K1 = rightmost 64 bits of K, ⊕ = bitwise exclusive OR; and ⊞ is addition mod 264 . a) Show the decryption equation. That is, show the equation for P as a function of C, K0 and

K1. b) Suppose and adversary has access to two sets of plaintexts and their corresponding

ciphertexts and wishes to determine K. We have the two equations: C = (P⨁K) ⊞ K; C = (P⨁K) ⊞ K First, derive an equation in one unknown (e.g., K0). Is it possible to proceed further to solve for K0?

2. Text (Problems 2.9) = (3rd edition: Problems 2.2)

3. Text (Problems 2.10) = (3rd edition: Problems 2.3)

4. Text (Problems 2.12) = (3rd edition: Problems 2.4)

5. Text (Problems 2.16) = (3rd edition: Problems 2.6)

6. Text (Problems 2.17) = (3rd edition: Problems 2.7)

7. Text (Problems 2.18) = (3rd edition: Problems 2.8)

8. Using S-DES, decrypt the string (10100010) using the key (0111111101). Show intermediate

results after each function (IP, fK, SW, fK, IP-1). Then decode the first 4bits of the plaintext

string to a letter and the second 4 bits to another letter where we encode A through P in base 2

(i.e. A=0000, B=0001, … , P=1111). Hint: As a midway check, after the application of SW,

the string should be (00010011).

★ (option-extra credit) Programming:

The secret key of RC4 is 7-byte (1, 2, 3, 4, 5, 6, 7). Find the first 20 byte key stream.