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I read the Google paper about POODLE vulnerability (https://www.openssl.org/~bodo/ssl-poodle.pdf) and a lot of other articles.

Each paper/article wrote about that you have to try it in average 256 times to get the next byte in plaintext.

Currently my understanding is that I take a block of my encrypted data (located somewhere in the middle of all blocks) and copy it over the last block which contains the padding. I also understand that with some assumptions it is possible for a hacker to get the whole padding in this final block. And I understand how to calculate the last byte of the copied block if the MAC check succeeds.

However, I don't understand why the MAC check sometimes fails and sometimes not with the same copied encrypted block.

So I have following questions:

  1. Why is the plaintext of the last block (this means for the last byte too) not always the same if I take the same encrypted block? Is this cause of the initial vector I need to get the plaintext?

  2. Why the probability is exactly 1/256?

  • 1) You don't use the same encrypted block - you deliberately alter one byte of it. 2) You are tweaking the single plaintext byte essentially at random, and it needs to match the padding exactly - so there's a 1 in 256 chance of you being right. – paj28 Jun 5 '16 at 20:23
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It is the job of an Initialization Vector to cause the same message to have a different encrypted value when encrypted with the same key. An IV is essentially a nonce used to salt a message.

As for why 1/256, that is the maximum possible values in a byte. A byte is a number that can range from 0 to 255. Including 0, that is 256 values. So a 1 in 256 chance of getting the correct next value of a byte, is the same as if randomly guessing.

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