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[[File:Block-Cipher-OFB.png]]
'''Counter (CTR):''' An incrementing counter repeatedly is encrypted to produce a sequence of blocks, which are XORed with the plaintexts to produce the ciphertexts:
[[File:Block-Cipher-CTR.png]]
All four of these modes share the same quality: if the same plaintext block is encrypted multiple times, the result will be different every time. The goal of this attack is to take a target that's using one of these five cipher modes and determine which mode is being used.
= Firmware =
To perform this attack, the SimpleSerial AES XMEGA firmware was modified to allow the target to use all five of these block cipher modes. The <code>encrypt()</code> function takes a new plaintext and produces the next ciphertext:
<pre>
void encrypt(uint8_t* pt)
{
static uint8_t input[16];
static uint8_t output[16];
// Find input
switch(BLOCK_MODE)
{
case ECB:
for(int i = 0; i < 16; i++)
input[i] = pt[i];
break;
case CBC:
for(int i = 0; i < 16; i++)
input[i] = pt[i] ^ ct[i];
break;
case CFB:
for(int i = 0; i < 16; i++)
input[i] = ct[i];
break;
case OFB:
for(int i = 0; i < 16; i++)
input[i] = output[i];
break;
case CTR:
input[0]++;
break;
}
// Encrypt in place
for(int i = 0; i < 16; i++)
output[i] = input[i];
aes_indep_enc(output);
// Use output to calculate new ciphertext
switch(BLOCK_MODE)
{
case ECB:
case CBC:
for(int i = 0; i < 16; i++)
ct[i] = output[i];
break;
case CFB:
case OFB:
case CTR:
for(int i = 0; i < 16; i++)
ct[i] = output[i] ^ pt[i];
break;
}
}
</pre>
Check that all five of these modes match the diagrams above.
This code was compiled five times with five different values of <code>BLOCK_MODE</code>, producing five hex files (one for ECB encryption, one for CBC, etc). All of this code is in the ChipWhisperer repository under <code>chipwhisperer\hardware\victims\firmware\simpleserial-aes-modes\</code>.