Chandan Rajpurohit

An Artist With Technical Skills

1) Which of the following attacks on hardware are invasive in nature?

a. Black box testing.
b. Physical probing.
c. Reverse engineering.
d. Side-channel analysis.

Answer: b, c

Solution: An invasive attack on a hardware device tampers with the device and makes some irreversible changes or damages. In black-box testing or side-channel testing, we are trying to attack the device by looking at the input-output behavior and measuring some side-channel information (power, delay, etc.) respectively. These are non-invasive in nature.

In physical probing, the protecting cover of the device must be broken open to have access to the internal points for signal probing. In reverse engineering, using very sophisticated instruments, we typically remove successive layers of a circuit element and try to learn the design from the photographs obtained. Both these attacks are invasive in nature. The correct options are (b) and (c).

2) Which of the following are true for side channel attacks?

a. They exploit weaknesses in cryptographic algorithms.
b. They exploit weaknesses in algorithm implementations.
c. They work with encrypted messages that are captured in transit.
d. None of these.

Answer: b

Solution: Side-channel attacks do not try to exploit weaknesses in cryptographic algorithms; rather, they try to exploit weaknesses in hardware or software implementations. They do not work with encrypted messages as in conventional attacks. The correct option is (b).

3) Which of the following can be exploited in side-channel attacks?

a. Power consumption
b. Electromagnetic emissions
c. Plaintext and Ciphertext
d. Timing

Answer: a, b, d

Solution: Power consumption, electromagnetic emissions, and timing measurements are typical side channels that are used to attack hardware implementations. The concept of plaintext and ciphertext is not the main focus in side-channel attacks. The correct options are (a), (b) and (d).

4) What is a typical countermeasure used to prevent timing analysis attack on modular exponentiation operation?

a. Include a hardware timer to keep track of the time.
b. Introducing dummy operations to make all branch conditions symmetrical
with respect to time.
c. Include a noise generator in the circuit.
d. None of these.

Answer: b

Solution: Timing analysis attack typically exploits asymmetry in timing behavior in various paths in the program during execution. To have an effective countermeasure, we do not rely on the use of any hardware timer or use of any noise generator. Rather, we can add some dummy operations in the code to make the timing behavior for all branch conditions symmetric. The correct option is (b).

5) Which of the following is/are true for power analysis attacks?

a. Simple power analysis works better than differential power analysis.
b. We observe peaks in the difference waveform during simple power analysis.
c. We compare the power profile against a single measurement for differential
power analysis.
d. None of these.

Answer: d

Solution: Differential power analysis is more sophisticated and effective as compared to simple power analysis. Differential power analysis requires multiple measurements. Peaks in difference waveform are observed in differential power analysis. Hence, none of (a), (b) and (c) are true. The correct option is (d).

6) How does the ring oscillator PUF work?

a. Exploits process variations such that different ring oscillators will have different oscillation frequencies.
b. Selects one path among many paths in a random way.
c. Exploits variations in power-up initial values in SRAM cells.
d. All of these.

Answer: a

Solution: The frequency of a ring oscillator depends on the delays of the gates that constitute the oscillator. Process variations across various hardware implementations make the frequency different. The correct option is (a).

7) What are the characteristics of a PUF?

a. The challenge-response mapping is unclonable.
b. Depends on variations in manufacturing processes.
c. Can be used to generate public and private keys.
d. All of these.

Answer: d

Solution: In a physical unclonable function (PUF), by definition the challenge-response pairs cannot be cloned across devices. The implementation of a PUF exploits process variations during manufacturing. Using PUF, we can have a secure way to generate public-private key pairs. Hence, the correct option is (d).

8) What are the advantages that a PUF can offer?

a. Intrinsic properties of a device are used to generate a key, which is unique.
b. No need to store the key anywhere.
c. The key obtained from the PUF can be deleted after the operation.
d. They are easily available in the market.

Answer: a, b, c

Solution: In a PUF, the key is generated exploiting the uniqueness in the challenge-response pairs of a device. The key is not stored anywhere in the device. After an operation, the key generated using the PUF can be deleted. PUFs are not available in the market. The correct options are (a), (b) and (c).

9) What is a hardware Trojan?

a. It is a form of PUF that can be used for attacking.
b. It is a malicious modification of the circuitry in a chip.
c. The payload of the Trojan is activated when the triggering condition is true.
d. All of these.

Answer: b, c

Solution: A hardware Trojan is not a PUF. It refers to some malicious modification to the hardware, such that whenever some triggering condition becomes true, some unintended operation (called payload) is activated. The correct options are (b) and (c).

10) Which of the following statement(s) is/are false?

a. Detection of hardware Trojans is relatively easy.
b. No single method can detect all types of Trojans.
c. Trojan detection involves a high overhead.
d. None of these.

Answer: a

Solution: Detection of hardware Trojan in a chip is a very difficult task. There are various categories of Trojans, and specific methods have been proposed to try and detect each of them. No single method can detect all Trojan types. The process of detection involved a very high overhead. The correct option is (a).

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