**Chapter 12**
1. Suppose that just prior to heading into the S-boxes, you have the following string of bits 010100000110101100111101000110110011000011110101. What will the result be immediately after going through the S-boxes? That is, complete the example begun earlier in this chapter.
2. Suppose that just prior to heading into the S-boxes, you have the following string of bits
000110010010111011010110110100111000110110000111. What will the result be immediately after going through the S-boxes?
3. If the result immediately after passing through the S-boxes in some particular round is 10111001101101011100110101001010, can you uniquely determine the bit string that entered the S-boxes in that round?
4. The initial permutation applied in DES is a far from random reordering of the bits. Produce terse computer code to accomplish this permutation.
5. If we were to add a 17^{th} round, what would that round’s key look like?
6. Suppose you consider an n bit key to be just barely secure, for a specific value of n. That is, you don’t believe a brute force attack is realistic, but you wouldn’t be comfortable with a key any smaller than n bits. Moore’s law predicts that the number of transistors that can be fit per square inch on integrated circuits doubles every two years. If we also factor in the fact that transistors are becoming faster, we get the result that processing speed doubles every 18 months. Because of limits imposed by physics, this cannot continue forever, but suppose it continues for the next ten years. How many bits should your key be increased by, if you want it to remain just barely secure ten years from now?
7. Complete the encipherment of the Richard Feynman quote using Levine’s method.
8. Show how the plaintext could be recovered from the ciphertext you found in exercise 4. Apply your technique to the first three ciphertext blocks to verify that it works.
9. In Cipher Block Chaining mode, must the Initialization Vector be kept secret or can it be revealed without compromising the message?
10. In Cipher Feedback mode, must the Initialization Vector be kept secret or can it be revealed without compromising the message?
11. Using the same Initialization Vector for two different messages (under the same encryption key) in Cipher Feedback mode would be bad. Why?
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