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# The last XOR combines the first two results, which use both words of the key (<code>k[0]</code> and <code>k[1]</code>). This means that we can only use it if we already know some of the bits in one of these keys (ie: we can only find the first byte of <code>k[0]</code> if we know the first byte of <code>k[1]</code>).
However, this is enough information to recover both of these words. Once we know these 64 bits, the second half of the key can be recovered in exactly the same way.
= Firmware =
The AES SimpleSerial target was adapted to use TEA encryption instead of AES. This new target uses the exact same commands as the original SimpleSerial target:
* <code>kKEY</code> loads KEY as the 128 bit encryption key
* <code>pPLAINTEXT</code> loads PLAINTEXT as the 64 bit plaintext and begins the encryption routine
* <code>rRESPONSE</code> is the reply from the target, where RESPONSE is the 64 bit ciphertext
This firmware is available in <code>chipwhisperer/hardware/victims/firmware/simpleserial-tea</code>. It uses the same build process as the other targets - simply run <code>make</code> to produce the hex file.
= Capture =
== Capture Setup ==
== Sample Outputs ==
= Analysis =
== Searching for Leakage ==
== Attacking the Key ==
= More Ideas =
This guide has walked through a very basic CPA attack on TEA encryption. However, we could take this a lot further...
* This was a very "manual" attack: we had to guide our code through each of the single-byte attacks. It would be good to have a more automatic attack that combines all of the information from all three attack points, rather than picking and choosing bytes to attack.
* We had to use a lower optimization level so that we could see the power signature from the sensitive attack points. Can we avoid this? Maybe it's possible to use a different sampling method to find more leakage.
* A template attack might be a more powerful way to perform this attack. There are several points that we could create a template for, including the three points that we examined in this attack.
