# what is the minimum number of keys one would probably use in his brute force attack?

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.

• 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. – SAI Peregrinus Dec 2 '17 at 22:47
• thanks. its 10 bit but we have to answer assuming that the hacker does not know. – CyberMad Dec 2 '17 at 23:56
• 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? – Squeamish Ossifrage Dec 3 '17 at 0:38
• 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? – CyberMad Dec 3 '17 at 0:58
• 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. – Squeamish Ossifrage Dec 3 '17 at 1:10

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.