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=== Details of AES-256 CBC ===

The system is using uses the AES algorithm in Cipher Block Chaining (CBC) mode. In general one avoids using encryption 'as-is' (i.e. Electronic Code Book), since it means any piece of plaintext always maps to the same piece of ciphertext. Cipher Block Chaining ensures that if you encrypted the same thing a bunch of times it would always encrypt to a new piece of ciphertext.

You can see another reference on the design of the encryption side, ; we'll be only talking about the decryption side here. In this case AES-256 CBC mode is used as follows, where the details of the AES-256 Decryption block will be discussed in detail later.:

Specifics This diagram shows that the output of the AES-256 Decryption algorithm decryption is no longer used directly as the plaintext. Instead, the output is XORed with a 16 byte mask, which is usually taken from the previous ciphertext. Also, the first decryption block has no previous ciphertext to use, so a secret initialization vector (IV) is used instead. If we are given belowgoing to decrypt the entire ciphertext (including block 0) or correctly generate our own ciphertext, where we'll need to find this IV along with the AES-256 implementation was written by [http://www.literatecode.com/ Ilya Okey. Levin]:

 == Attacking AES-256 ==The system in this tutorial uses AES-256 encryption, which has a 256 bit (32 byte) key - twice as large as the 16 byte key we've attacked in previous tutorials. This section describes how we can use our knowledge of the AES-128 attacks on AES-256. Specifics of the AES-256 decryption algorithm are given below, where this AES-256 implementation was written by [http://www.literatecode.com/ Ilya O. Levin]: <pre>aes_addRoundKey_cpy(buf, ctx-&gt;deckey, ctx-&gt;key);

aes_addRoundKey( buf, ctx-&gt;key);</pre>In an AES-128 implementation there is ten rounds (after 'initially' applying the key), each round applied to the 16-byte state of the AES. With AES-256 the state is still 16 bytes, but is applied over 14 rounds (after initially applying the first part of the key).

Recall that the AES-128 implementation was made up of ten rounds (after 'initially' applying the key), with each round modifying a 16-byte state. In AES-256, the state is still 16 bytes, but the encryption routine includes 14 rounds (after initially applying the first part of the key). Beyond this, much of the AES operation stays the same (<code>subBytes()</code>, <code>mixColumns</code>, etc). In AES-128 , we can target targeted the first output of the S-Box, which is was sufficient to recover the entire encryption key. For AES-256 , we can still use this attack point, but we will only recover 16 bytes of the encryption key, as . This point of the algorithm is shown in the following figure of the initial setup of the decryption algorithm:

<pre>aes_addRoundKey_cpy(buf, ctx-&gt;deckey, ctx-&gt;key);

aes_subBytes_inv(buf);</pre> 

<pre> for (i = 14, rcon = 0x80; --i;)

Which which is shown in this figure:

The critical difference between the initial round and this round is the addition of the mixcols <code>mixColumns</code> operation. This operation takes four bytes of input and generates four bytes of output - any change in a single byte will result in a change of all four bytes of output!

The MixColsMixColumns() operation is a linear function, meaning for example the following applies:

Which This means that, instead of determining the encryption key, we can determine the encryption key modified by the inverse MixCols.:

# Using the known 14th round key, calculate the hypothetical outputs of each S-Box from the 13th round using the ciphertext processed by the 14th round, and determine the 16 bytes of the 13th round key manipulated by inverse mixcolsMixColumns.# Perform the mixcol MixColumns and shift-row ShiftRows operation on the hypothetical key determined above, which will be recovering the 13th round key.