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Many of the tutorials in this wiki discuss attacks on AES-128 encryption. It turns out that its big brother, AES-256, can be attacked by extending the same attacks. This page discusses AES-256 and how to reuse an AES-128 attack to obtain the key.
= The AES-256 Algorithm =
In AES-128, we used the following steps to encrypt 16 bytes of plaintext:
# Use a 16 byte key to generate a key schedule, which is 176 bytes long (11 words made up of 16 bytes).
# Put the 16 bytes of plaintext into a 4x4 state matrix.
# Combine the first word of the key schedule with the state.
# Apply 10 rounds of transformations to the state, involving the key schedule.
# Retrieve 16 bytes of ciphertext from the state matrix.
The transformations involve several functions which mix together the bytes of the state. These functions are <code>SubBytes()</code>, <code>MixColumns()</code>, and <code>ShiftRows()</code>.
AES-'''<font color=blue>256</font>''' is not much different from AES-128. The encryption process is:
# Use a '''<font color=blue>32</font>''' byte key to generate a key schedule, which is '''<font color=blue>240</font>''' bytes long ('''<font color=blue>15</font>''' words made up of 16 bytes).
# Put the 16 bytes of plaintext into a 4x4 state matrix.
# Combine the first word of the key schedule with the state.
# Apply '''<font color=blue>14</font>''' rounds of transformations to the state, involving the key schedule.
# Retrieve 16 bytes of ciphertext from the state matrix.
Notice that most of this algorithm is the same. Earlier, we could attack a target by examining the output of a substitution box; since AES-256 uses these same S-boxes, we should have no problem finding a sensitive point to attack.
The following code is an example of the AES-256 decryption algorithm, written by [http://www.literatecode.com/ Ilya O. Levin]:
<pre>
aes_addRoundKey_cpy(buf, ctx->deckey, ctx->key);
aes_shiftRows_inv(buf);
aes_subBytes_inv(buf);
for (i = 14, rcon = 0x80; --i;)
{
if( ( i & 1 ) )
{
aes_expandDecKey(ctx->key, &rcon);
aes_addRoundKey(buf, &ctx->key[16]);
}
else aes_addRoundKey(buf, ctx->key);
aes_mixColumns_inv(buf);
aes_shiftRows_inv(buf);
aes_subBytes_inv(buf);
}
aes_addRoundKey( buf, ctx->key);
</pre>
Note that this implementation chooses to expand the key during the decryption process. This order of events isn't a big deal to us - the <code>subBytes()</code> operation will still be visible in a power trace.
= Attacking AES-256 Decryption =
== Attacking Round Key 14 ==
== Attacking Round Key 13 ==
== Recovering the Full Key ==
= The AES-256 Algorithm =
In AES-128, we used the following steps to encrypt 16 bytes of plaintext:
# Use a 16 byte key to generate a key schedule, which is 176 bytes long (11 words made up of 16 bytes).
# Put the 16 bytes of plaintext into a 4x4 state matrix.
# Combine the first word of the key schedule with the state.
# Apply 10 rounds of transformations to the state, involving the key schedule.
# Retrieve 16 bytes of ciphertext from the state matrix.
The transformations involve several functions which mix together the bytes of the state. These functions are <code>SubBytes()</code>, <code>MixColumns()</code>, and <code>ShiftRows()</code>.
AES-'''<font color=blue>256</font>''' is not much different from AES-128. The encryption process is:
# Use a '''<font color=blue>32</font>''' byte key to generate a key schedule, which is '''<font color=blue>240</font>''' bytes long ('''<font color=blue>15</font>''' words made up of 16 bytes).
# Put the 16 bytes of plaintext into a 4x4 state matrix.
# Combine the first word of the key schedule with the state.
# Apply '''<font color=blue>14</font>''' rounds of transformations to the state, involving the key schedule.
# Retrieve 16 bytes of ciphertext from the state matrix.
Notice that most of this algorithm is the same. Earlier, we could attack a target by examining the output of a substitution box; since AES-256 uses these same S-boxes, we should have no problem finding a sensitive point to attack.
The following code is an example of the AES-256 decryption algorithm, written by [http://www.literatecode.com/ Ilya O. Levin]:
<pre>
aes_addRoundKey_cpy(buf, ctx->deckey, ctx->key);
aes_shiftRows_inv(buf);
aes_subBytes_inv(buf);
for (i = 14, rcon = 0x80; --i;)
{
if( ( i & 1 ) )
{
aes_expandDecKey(ctx->key, &rcon);
aes_addRoundKey(buf, &ctx->key[16]);
}
else aes_addRoundKey(buf, ctx->key);
aes_mixColumns_inv(buf);
aes_shiftRows_inv(buf);
aes_subBytes_inv(buf);
}
aes_addRoundKey( buf, ctx->key);
</pre>
Note that this implementation chooses to expand the key during the decryption process. This order of events isn't a big deal to us - the <code>subBytes()</code> operation will still be visible in a power trace.
= Attacking AES-256 Decryption =
== Attacking Round Key 14 ==
== Attacking Round Key 13 ==
== Recovering the Full Key ==