CPA attack for AES256

Embedded Security Side Channel Power Analysis
1

I am trying to perform a CPA attack on CW305 with AES-256.

The default ChipWhisperer demo/notebooks are for AES-128.

I would like to know:

  • What changes are needed in the CPA analyzer for AES-256?

  • Can the same leakage models used for AES-128 be used for AES-256?

  • Any example or guidance for adapting the CW305 AES-128 demo to AES-256?

2

We explain that in this notebook: Extending AES-128 Attacks to AES-256.ipynb.

As for the CW305 AES demo, you should be able to simply change the size_i input of aes_core.v. On the Python API side you’ll have to modify CW305.py to send the larger key.

3

I have successfully modified the CW305 AES demo from AES-128 to AES-256 on FPGA side, and capture is working properly. I am able to collect traces without issues.

However, I am facing problems during the CPA attack stage. The default leakage model used in the demo:

attack = cwa.cpa(project, cwa.leakage_models.last_round_state_diff)

does not seem to work correctly for AES-256.

I suspect this may be because the default leakage model and attack configuration are mainly designed for AES-128. Since AES-256 uses 14 rounds and a different key schedule, the last-round leakage model may no longer align properly.

4

We don’t have out-of-the-box support for AES256 leakage models. We may add it in the future - for now you can either consider coding it yourself (see e.g. this notebook where we teach how to build a leakage model from scratch for AES-128: chipwhisperer-jupyter/courses/sca101/Lab 4_2 - CPA on Firmware Implementation of AES (MAIN).ipynb at fe7e2aa1585798c3a5a6b247f231abe2e2b4a2cd · newaetech/chipwhisperer-jupyter · GitHub), or use a third-party library like SCARED or LASCAR.

5

Dear Sir,

I am able to recover the Phase-1 (Round-14) key using the referenced AES-256 CPA attack methodology.

However, I am unable to recover the Phase-2 key. I have implemented the leakage model based on your inputs.

Could you please suggest how I can verify that the Phase-2 CPA attack, leakage model, and key schedule implementation are correct?

Regards,
Jogendra

phase1_1
phase1_1936×371 43 KB

phase2_2
phase2_2924×347 40.3 KB

6

What’s phase 2? That’s not something we reference in our notebooks.

7

chipwhisperer-jupyter/courses/sca201/Extending AES-128 Attacks to AES-256.ipynb at fe7e2aa1585798c3a5a6b247f231abe2e2b4a2cd · newaetech/chipwhisperer-jupyter · GitHub, In this link you are saying Getting second half of the key you have mentioned in the procedure. 1st half of the key i am getting peg values are zero. but in the second half of the key i am getting higher PGE values.

8

I have done the coding for 2nd half of the key using your shared document as below..“”"

def leakage(self, pt, ct, guess, bnum):

K14_recovered must be defined in the cell above before this runs

state = [ct[i] ^ K14_recovered[i] for i in range(16)] # undo AddRoundKey(K14)
state = self.inv_shiftrows(state) # undo ShiftRows
state = self.inv_subbytes(state) # undo SubBytes — ATTACK POINT
state = self.inv_mixcolumns(state) # undo MixColumns
state = self.inv_shiftrows(state) # undo Round 13 ShiftRows
return self.inv_sbox(state[bnum] ^ guess[bnum]) # predict leakage


def process_known_key(self, inpkey):
    K13 = key_schedule_rounds(list(inpkey), 0, 13)
    return list(inv_shiftrows(list(inv_mixcolumns(K13))))

leak_model_p2 = cwa.leakage_models.new_model(AES256_Round13_Model)
attack2 = cwa.cpa(project, leak_model_p2)
9

Try to attack via first and second addRoundKey. It is much easier. Use HW(pt ^ key) leakage model.

10

Hi, can you share the any document pl.. or any procedure document can you share

11

I would recommend to study what you are doing first otherwise all your activity will be useless.

The side channel attacks are what you should do by yourself without templates and stereotypes.