problem 1:

a) By using the extended Euclidean algorithm, find out the multiplicative inverse of 504 mod 67.

b) Decrypt the given ciphertext, which has been encrypted by using Caesar cipher:

YMNXJ   CFRNS   FYNTS   UFUJW   NXVZN   YJJFX D

c)

• Construct the Playfair matrix (5 x 5) by using the keyword: cryptology
• Encrypt the given plain text by using the above matrix: billa bong

problem 2:

a) Use the Chinese remainder theorem to evaluate x from the given simultaneous congruence:

x ≡ 1 (mod 2)
x ≡ 2 (mod 3)
x ≡ 3 (mod 5)

b) Compute gcd (14526, 2568).

c) Compare and contrast Advanced Encryption Standard (AES) to Data Encryption Standard (DES). For each of the given elements of DES, point out the comparable element in AES or describe why it is not required in AES:

• XOR of sub key material with the input to the f function
• XOR of the f function output with the left half of the block
• The f function
• Permutation P
• Swapping of halves of the block