Side Channel Attacks

Side Channel Attacks

• Algorithms are mathematically robust but their circuit implementations have flaws.
• Physical attacks exploit the hardware implementation of algorithms.
• Circuits have current consumption and computation time linked to handled data.
• These parameters can be measured to obtain information about secrets in "Side Channel Attacks".

Instruction Power Differences

• Different operations during plaintext encryption cause complexity differences.
• These complexity differences can lead to varying power consumption.
• Power consumption can leak the position of an AES Sub bytes operation.
• Power consumption is measured to differentiate between assembly instructions.
• The 'capture_trace' function returns a 5000-sample trace by generating a plaintext/key pair and getting the trace wave back.
• The simpleserial-base.c file in the firmware directory dictates target execution and trace capture.

Modifying the Firmware

• Comment out the 'simpleserial_put' function call.
• Rebuild the firmware by navigating to the simpleserial-base- lab2 directory.
• Command to rebuild firmware: make PLATFORM=$1 CRYPTO_TARGET=NONE
• Command to capture a trace and check for pattern: wave1 = capture_trace()
• Use cw.plot(wave1) for trace plotting

Instructions Impact on Power Trace

• Multiple instructions with varying complexity can be introduced to check influence on power consumption.
• Introducing addition operations within get_pt function, between triggers changes the firmware.
• After rebuilding and uploading with the new firmware, the target adds variable A to the variable.
• Next step involves capturing a new trace and comparing it to the old one.

Introducing a Loop into Execution

• Looping A+=2 20 times is another way to add 2 to the variable A 20 times.
• C code can be changed by introducing a loop.
• The captured trace should be plotted and compared to the previous addition implementation.
• A loop takes more time to execute than a full loop enrolling due to assembly instructions like JMP and CMP.

Expensive Operations: Exponential

• Exponential operations execute for a longer time since they are defined as repeated multiplications.
• Repeated multiplications are repeated addition operations.

Power Analysis for Password Bypass

• Different traces can be used to break a password by determining when a device is performing certain operations.
• The device performs a password check.
• A basic power analysis demonstrates how to perform this.

Trace Capturing - Password Guess

• Target is reset before trace capturing, serial buffer read with "in waiting" and "read" functions, mandatory scope armed.
• Password guess sent, target trace captured, trace returned with get_last_trace call.
• Slight delays exist between traces when comparing random characters.
• Delays can be due to the check algorithm executing immediately on wrong characters and trying to find a second, correct character.

Testing Combinations

• All traces plotted via looping through possible characters to find outliers.
• Code:
  • For chars in tqdm('abcdefghijklmnopqrstuvwxyz0123456789'):
    • trace = cap_pass_trace ( c + "\n")
    • fig *= cw.plot(trace [0:1000])

Attack Automation

• Automate attack interpreter character guess outcomes.
• Outlier trace checked when looping characters.
• Compute trace difference of each guess with a wrong guess.
• Time factor represents delay between traces.
• Spikes can occur at certain sample points
• Automation code:
  • ref_trace = cap_pass_trace("\x00\n") [0:1000]
  • plot = cw.plot({})

Full Attack

• Required the guess trace to be updated, otherwise the forst character trace will always be compared when the updated reference trace comes out.
• The comparison would be between the foruth character and the first trace.
• For each correct guess "bee" is guessed as a passowrd.
• Since the target goes to idle after an incorrect guess the referecne trace is updated per the correct guess

Conclusion

• Devices with poorly hidden operations are easier to break.
• Power can be used to determine timing information can be useful for hackers.
• Wave-form differences can be used to find the correct password characters.

Large Hamming Weight Swings

• Power consumption can vary based on device firmware which indicates whether the correct character is guessed.
• Factor impacts power consumption, data manipulation via reducing brute-force scope/dividing attack into 5 parts.
• Process:

Gathering Traces

• Used ktp.next to generate a plaintext/key pair.
• Target's key set before entering the trace gathering loop.
• Inside the loop, armed the scope which is mandatory for capturing.
• Least Significant Byte set to either 0x00 or 0xFF, based on bit’s value: LSB=1 sets byte to 0xFF, or to 0x00.
• The altered text sent before trace capturing.

Recovering AES Key From A Single Bit Of Data

• AES takes the round key as round 0, not the key schedule itself
• If the master key can be atacked and round key can be obtained then master key can be obtained.
• A master key is 128 bits therefore encryption takes place, so its hard to brute force without inside knowledge
• internal key is XORed with bitwise blocks in key addition by using S-Boxes.

Brute Force

• Assumed the attacker is on the device plaintext and wants to read one bit from it
• This 1 bit stream belongs to Oth round and Sbox substitution.
• With this 1 bit leak the attacker has to brute force 8 bits by 8 bits and reduce the key size of 2^8 to 16.

DPA On Firmware Implementation Of AES

• Directly attacks the AES.
• Directly reads one bitstream from the board.
• Attacks the S Box by using differential oower analysis,
• Using this we have to chose the intermediate value, measures traces and use the leakage models

Measuring Traces

• We used known plaintext in traces to get the cipher key bit
• By doing this we can bruteforce to break ciphered key blocks
• The power consumption is determined by the value of guess byte
• A successful guess will mean the most likely key is correct.

Power and Hamming Weight Relationship

• Relationship exists between manipulating bit value to the target, recovering whole AES keys using this property.
• Power also relies to the hamming weight, recovering traces and the location of a cryptographic implementation.