Question
This question concerns block cipher padding. Suppose the block cipher has a block size of 17 bytes. A certain message ends with a block which
This question concerns block cipher padding. Suppose the block cipher has a block size of 17 bytes. A certain message ends with a block which is 17 bytes. We must reversibly pad out the message to use some block cipher modes, such as CBC (even if the last block is full, which it is in this case). Consider the following padding strategy. Determine the number of padding bytes required. This is a number n which satisfies 1n17 and n+l(P) is a multiple of 17, where l(P) is the length of the unpadded plaintext. Pad the plaintext by appending n bytes, each with value n. Suppose the final (possibly partial) block of the message is "0x000102030405060708090a0b0c0d0e0f10" in hexadecimal. Write out the complete final block in hexadecimal. Please do not put a leading 0x as this has been written already. Because the last block is actually full, the block you add will need to be a completely new block
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