PWR_Tr4ce [HW]
Overview

PWR_Tr4ce [HW]

br34d br34d
March 31, 2024
3 min read
pwr-trace

Challenge description

PWR_Tr4ce

You’ve been given power traces and text inputs captured from a microcontroller running AES encryption. Your goal is to extract the encryption key.

EXPERIMENT SETUP

  • scope = chipwhisperer lite
  • target = stm32f3
  • AES key length = 16 bytes
Author
Chainfire73
Category
hardware
Flag
ACSC{Pwr!4n4lyz}

Solution

Used a side-channel attack technique, Correlation Power Analysis to solve this challenge.

  • We know the plaintexts (inputs to AES) and we have the power traces.
  • We don’t know the key.
  • For each possible key byte (0-255), we assume how much power the chip should consume at a specific point during encryption.
  • We then find which key guess correlates best with the actual power measurements. The correct key byte gives the strongest correlation.

solve.py

import numpy as np


# Define the AES S-box
AES_SBOX = [0x63, 0x7C, 0x77, 0x7B, 0xF2, 0x6B, 0x6F, 0xC5, 0x30, 0x01, 0x67, 0x2B, 0xFE, 0xD7, 0xAB, 0x76, 0xCA, 0x82, 0xC9, 0x7D, 0xFA, 0x59, 0x47, 0xF0, 0xAD, 0xD4, 0xA2, 0xAF, 0x9C, 0xA4, 0x72, 0xC0, 0xB7, 0xFD, 0x93, 0x26, 0x36, 0x3F, 0xF7, 0xCC, 0x34, 0xA5, 0xE5, 0xF1, 0x71, 0xD8, 0x31, 0x15, 0x04, 0xC7, 0x23, 0xC3, 0x18, 0x96, 0x05, 0x9A, 0x07, 0x12, 0x80, 0xE2, 0xEB, 0x27, 0xB2, 0x75, 0x09, 0x83, 0x2C, 0x1A, 0x1B, 0x6E, 0x5A, 0xA0, 0x52, 0x3B, 0xD6, 0xB3, 0x29, 0xE3, 0x2F, 0x84, 0x53, 0xD1, 0x00, 0xED, 0x20, 0xFC, 0xB1, 0x5B, 0x6A, 0xCB, 0xBE, 0x39, 0x4A, 0x4C, 0x58, 0xCF, 0xD0, 0xEF, 0xAA, 0xFB, 0x43, 0x4D, 0x33, 0x85, 0x45, 0xF9, 0x02, 0x7F, 0x50, 0x3C, 0x9F, 0xA8, 0x51, 0xA3, 0x40, 0x8F, 0x92, 0x9D, 0x38, 0xF5, 0xBC, 0xB6, 0xDA, 0x21, 0x10, 0xFF, 0xF3, 0xD2, 0xCD, 0x0C, 0x13, 0xEC, 0x5F, 0x97, 0x44, 0x17, 0xC4, 0xA7, 0x7E, 0x3D, 0x64, 0x5D, 0x19, 0x73, 0x60, 0x81, 0x4F, 0xDC, 0x22, 0x2A, 0x90, 0x88, 0x46, 0xEE, 0xB8, 0x14, 0xDE, 0x5E, 0x0B, 0xDB, 0xE0, 0x32, 0x3A, 0x0A, 0x49, 0x06, 0x24, 0x5C, 0xC2, 0xD3, 0xAC, 0x62, 0x91, 0x95, 0xE4, 0x79, 0xE7, 0xC8, 0x37, 0x6D, 0x8D, 0xD5, 0x4E, 0xA9, 0x6C, 0x56, 0xF4, 0xEA, 0x65, 0x7A, 0xAE, 0x08, 0xBA, 0x78, 0x25, 0x2E, 0x1C, 0xA6, 0xB4, 0xC6, 0xE8, 0xDD, 0x74, 0x1F, 0x4B, 0xBD, 0x8B, 0x8A, 0x70, 0x3E, 0xB5, 0x66, 0x48, 0x03, 0xF6, 0x0E, 0x61, 0x35, 0x57, 0xB9, 0x86, 0xC1, 0x1D, 0x9E, 0xE1, 0xF8, 0x98, 0x11, 0x69, 0xD9, 0x8E, 0x94, 0x9B, 0x1E, 0x87, 0xE9, 0xCE, 0x55, 0x28, 0xDF, 0x8C, 0xA1, 0x89, 0x0D, 0xBF, 0xE6, 0x42, 0x68, 0x41, 0x99, 0x2D, 0x0F, 0xB0, 0x54, 0xBB, 0x16 ]


def hamming_weight(n):
    """Calculate the Hamming weight of an integer."""
    return bin(n).count("1")


def guess_key_byte(traces, plaintexts, byte_index):
    """Guess a single byte of the key based on power analysis."""
    num_traces = len(traces)
    num_samples = len(traces[0])
    best_correlation = 0
    best_guess = 0


    for guess in range(256):
        # Use the AES S-box in the hypothetical power consumption calculation
        hypothetical_consumption = np.array([hamming_weight(AES_SBOX[pt[byte_index] ^ guess]) for pt in plaintexts])


        # Correlate these hypothetical consumption values with actual traces
        correlation_scores = np.zeros(num_samples)
        for i in range(num_samples):
            sample_values = traces[:, i]
            correlation = np.corrcoef(hypothetical_consumption, sample_values)[0, 1]
            correlation_scores[i] = correlation


        # Find the maximum correlation score for this guess
        max_correlation = max(correlation_scores, key=abs)
        if abs(max_correlation) > best_correlation:
            best_correlation = abs(max_correlation)
            best_guess = guess


    return best_guess


traces = np.load('traces.npy')
plaintexts = np.load('textins.npy')


def main():
    key = []
    for i in range(16):  # AES-128 has 16 bytes in the key
        guess = guess_key_byte(traces, plaintexts, i)
        key.append(guess)
        print(f"Guessed byte {i}: {guess:02x}")


    print("Guessed key:", ''.join(f"{byte:02x}" for byte in key))


if __name__ == "__main__":
    main()