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what is the minimum number of keys one would probably use in his brute force attack, Given info- The Algorithm uses a blocking technique where the ciphertext from one block becomes the key to the next block. Size of the block is not known.

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  • $\begingroup$ There is insufficient information here to give a concrete answer. An attacker only has to brute force the first block, but to know how long that will take requires knowledge of the block and key sizes. $\endgroup$ – SAI Peregrinus Dec 2 '17 at 22:47
  • $\begingroup$ thanks. its 10 bit but we have to answer assuming that the hacker does not know. $\endgroup$ – CyberMad Dec 2 '17 at 23:56
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    $\begingroup$ Did you just ask crypto.se to solve problems you took a screenshot of from your homework without even bothering to enter the text into the question? $\endgroup$ – Squeamish Ossifrage Dec 3 '17 at 0:38
  • $\begingroup$ I solved until 4 :D but stuck in number five! It's not my homework just practice questions! Is it not allowed to take screenshots? $\endgroup$ – CyberMad Dec 3 '17 at 0:58
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    $\begingroup$ The problem isn't the screenshot; the problem is the lack of any demonstrated effort put into asking us the question, as if you just forwarded it on to us as your personal homework-solving service. $\endgroup$ – Squeamish Ossifrage Dec 3 '17 at 1:10
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This is a homework problem, so I won't give a full answer. Hopefully this helps enough to let you solve it.

Part 4: This is asking for a worst case. There are 40 bits in the message. Thus $2^{40}$ possible combinations. Therefore in the absolute worst case he'd have to perform $2^{40}-1$ decryptions (-1 because it can then be determined that the last possible key is the true key). Thus the answer must be lower than this, if you get a larger number you've done something wrong.

Part 5: There are 40 bits in the message. The total message must be a multiple of the block size, so the block size can be 1, 2, 4, 8, 10, or 40 bits. On average it takes $2^{n-1}$ tries to brute force a length-n block. The attacker can start assuming a 1-bit block length, then try a 2-bit, then 4-bit, etc. Since the true key is 10 bits he'll never reach the 40-bit length.

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