Assignment 5

1. How is diffusion achieved in AES and DES?

2. Given the following state array (in hex.) for an AES encryption round,

perform the Sub Bytes step followed by the Shift Rows Step and write the

output array (in hex.)

41 5a 6b 7c

9c 3d 4a 90

12 23 5e 4b

3c 2a 4d 2f

3. Use the output array of Q2 as the input array for an AES decryption round to

perform the Inverse Shift Rows step followed by the Inverse Sub Bytes step

and write the resulting state array.

4. Compute the 0th and the first 4×4 round key matrices w[0,3] and w[4,7]

produced by the key expansion procedure in an AES crypto-system when the

128-bit key is all zeros.

5. During the transmission of C3 (the third cipher block) an error in the 5th bit

occurred. How many plaintext blocks will be affected, if we are using:

a. 16-bit ECB mode? Explain Why?

b. 8-bit CBC mode? Explain Why?

6. What are the limitations of the Cipher Block Chaining method?

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