1. What is the key size used in AES encryption?
A) 64-bit
B) 128-bit, 192-bit, and 256-bit
C) 512-bit
D) 1024-bit
✅ Answer: B) 128-bit, 192-bit, and 256-bit
📝 Explanation: AES supports key sizes of 128, 192, and 256 bits. Unlike older encryption algorithms like DES, which used a fixed key size (56-bit), AES offers multiple security levels.
2. What type of cipher is AES?
A) Stream cipher
B) Block cipher
C) Hash function
D) Asymmetric cipher
✅ Answer: B) Block cipher
📝 Explanation: AES is a block cipher, meaning it encrypts fixed-size blocks of data (128 bits) at a time, unlike stream ciphers, which encrypt data bit-by-bit.
3. How many rounds of encryption does AES-256 use?
A) 10
B) 12
C) 14
D) 16
✅ Answer: C) 14
📝 Explanation: AES performs different numbers of rounds depending on the key size:
- AES-128 uses 10 rounds
- AES-192 uses 12 rounds
- AES-256 uses 14 rounds
4. What is the block size of AES?
A) 64 bits
B) 128 bits
C) 192 bits
D) 256 bits
✅ Answer: B) 128 bits
📝 Explanation: AES operates on fixed 128-bit blocks regardless of the key size.
5. Which cryptographic component does AES use for key expansion?
A) RSA Key Generation
B) Rijndael Key Schedule
C) Diffie-Hellman Key Exchange
D) SHA-256
✅ Answer: B) Rijndael Key Schedule
📝 Explanation: AES uses the Rijndael Key Schedule to derive round keys from the main encryption key.
6. What is the purpose of the MixColumns step in AES encryption?
A) To add confusion to the encryption process
B) To mix the data at the byte level
C) To ensure diffusion of bits across columns
D) To provide message authentication
✅ Answer: C) To ensure diffusion of bits across columns
📝 Explanation: The MixColumns step applies matrix multiplication over a finite field to spread out the influence of each byte across multiple columns.
7. What is the main weakness of AES when used improperly?
A) Key size is too small
B) It is vulnerable to brute-force attacks
C) Poor implementation can lead to side-channel attacks
D) It has a backdoor
✅ Answer: C) Poor implementation can lead to side-channel attacks
📝 Explanation: While AES is mathematically secure, side-channel attacks (like timing or power analysis attacks) can reveal encryption keys if implemented insecurely.
8. What is the main advantage of AES over DES?
A) AES uses asymmetric encryption
B) AES has a larger key size and more secure design
C) AES is faster than DES
D) AES does not use a block cipher structure
✅ Answer: B) AES has a larger key size and more secure design
📝 Explanation: AES replaced DES (Data Encryption Standard) due to larger key sizes (128, 192, 256 bits vs. 56-bit DES) and stronger security against cryptanalysis.
9. Which AES mode of operation provides authentication as well as encryption?
A) ECB
B) CBC
C) GCM
D) OFB
✅ Answer: C) GCM (Galois/Counter Mode)
📝 Explanation: GCM (Galois/Counter Mode) provides both encryption and authentication, unlike CBC or ECB, which only provide encryption.
10. What is the biggest security risk of using AES in ECB mode?
A) Key reuse
B) Pattern preservation in ciphertext
C) Slow performance
D) Weak diffusion
✅ Answer: B) Pattern preservation in ciphertext
📝 Explanation: ECB (Electronic Codebook) encrypts identical plaintext blocks into identical ciphertext blocks, making it vulnerable to pattern analysis attacks.
11. What is AES primarily used for?
A) Digital signatures
B) Data encryption
C) Key exchange
D) Hashing
✅ Answer: B) Data encryption
📝 Explanation: AES is a symmetric encryption algorithm used for encrypting sensitive data.
12. In AES, what does the ShiftRows operation do?
A) Swaps adjacent bytes
B) Shifts rows of the state matrix
C) XORs the key with data
D) Randomly scrambles the block
✅ Answer: B) Shifts rows of the state matrix
📝 Explanation: ShiftRows rearranges bytes in the AES state matrix to increase diffusion.
13. What is the recommended key size for top security in AES?
A) 128-bit
B) 192-bit
C) 256-bit
D) 512-bit
✅ Answer: C) 256-bit
📝 Explanation: AES-256 offers the highest security level and is used for military-grade encryption.
14. What is the key expansion process in AES called?
A) Key scheduling
B) Key stretching
C) Key exchange
D) Key wrapping
✅ Answer: A) Key scheduling
📝 Explanation: AES uses a key scheduling algorithm to generate round keys from the main key.
15. What makes AES resistant to differential and linear cryptanalysis?
A) Large key size
B) Complex substitution-permutation network
C) Asymmetric key usage
D) Random number generation
✅ Answer: B) Complex substitution-permutation network
📝 Explanation: AES uses a Substitution-Permutation Network (SPN), making it resistant to common cryptanalysis attacks.
16. What is AES encryption used for in Wi-Fi security?
A) WEP
B) WPA2
C) SSL
D) VPN
✅ Answer: B) WPA2
📝 Explanation: WPA2 (Wi-Fi Protected Access 2) uses AES for encrypting wireless communication.
17. Who developed AES?
A) NIST
B) IBM
C) Rijmen and Daemen
D) RSA Labs
✅ Answer: C) Rijmen and Daemen
📝 Explanation: AES was developed by Vincent Rijmen and Joan Daemen and selected by NIST as the official encryption standard.
18. In AES, what is S-Box used for?
A) Key generation
B) Confusion via substitution
C) Key exchange
D) Message authentication
✅ Answer: B) Confusion via substitution
📝 Explanation: The Substitution Box (S-Box) ensures confusion by mapping each byte to a different value.
19. What is the strongest attack known against AES?
A) Brute-force
B) Differential cryptanalysis
C) Side-channel attacks
D) Meet-in-the-middle attack
✅ Answer: C) Side-channel attacks
📝 Explanation: AES is mathematically strong, but side-channel attacks (like timing attacks) can expose keys if implementations are weak.
20. What is the main function of AddRoundKey in AES?
A) Generates a new key
B) XORs the state with the round key
C) Encrypts plaintext
D) Creates randomness
✅ Answer: B) XORs the state with the round key
📝 Explanation: AddRoundKey is where the round key is XORed with the data to increase security.
21. What type of key algorithm does AES use?
A) Symmetric
B) Asymmetric
C) Hybrid
D) Hash-based
✅ Answer: A) Symmetric
📝 Explanation: AES is a symmetric key algorithm, meaning the same key is used for both encryption and decryption.
22. Which country led the standardization process for AES?
A) United States
B) Germany
C) France
D) Japan
✅ Answer: A) United States
📝 Explanation: AES was selected and standardized by NIST (National Institute of Standards and Technology, USA) in 2001.
23. What makes AES superior to DES?
A) It is faster
B) It has a larger key size
C) It is more resistant to brute-force attacks
D) All of the above
✅ Answer: D) All of the above
📝 Explanation: AES is faster, has a larger key size (128, 192, 256 bits vs. 56-bit DES), and is more secure against brute-force attacks.
24. Which AES mode is the most vulnerable to replay attacks?
A) CBC
B) ECB
C) GCM
D) XTS
✅ Answer: B) ECB (Electronic Codebook)
📝 Explanation: ECB mode encrypts identical plaintext blocks into identical ciphertext blocks, making it vulnerable to replay attacks and pattern analysis.
25. What is the purpose of the RCON table in AES?
A) It stores round keys
B) It provides random values for encryption
C) It contains round constants for key expansion
D) It replaces S-Box values
✅ Answer: C) It contains round constants for key expansion
📝 Explanation: RCON (Round Constant Table) is used in key expansion to ensure each round key is unique.
26. What is the output size of AES encryption for a single block?
A) 64 bits
B) 128 bits
C) 192 bits
D) Depends on key size
✅ Answer: B) 128 bits
📝 Explanation: AES always produces 128-bit ciphertext blocks, regardless of the key size.
27. What is the main function of SubBytes in AES?
A) It replaces bytes using an S-Box
B) It adds the round key
C) It shuffles rows in the state matrix
D) It expands the key
✅ Answer: A) It replaces bytes using an S-Box
📝 Explanation: The SubBytes step substitutes each byte in the state matrix with a corresponding value from the AES S-Box, ensuring confusion.
28. Which AES mode is best for hard drive encryption?
A) ECB
B) CBC
C) XTS
D) OFB
✅ Answer: C) XTS
📝 Explanation: AES-XTS (XEX-based Tweaked Codebook mode with Ciphertext Stealing) is optimized for encrypting disk storage.
29. What is the role of the Initial Vector (IV) in AES CBC mode?
A) It increases performance
B) It prevents identical plaintext blocks from encrypting to identical ciphertext blocks
C) It strengthens the key schedule
D) It replaces the round key
✅ Answer: B) It prevents identical plaintext blocks from encrypting to identical ciphertext blocks
📝 Explanation: The IV (Initialization Vector) adds randomness in AES-CBC, preventing pattern leaks.
30. How many bytes does an AES-256 key contain?
A) 16 bytes
B) 24 bytes
C) 32 bytes
D) 64 bytes
✅ Answer: C) 32 bytes
📝 Explanation: AES-256 has a 256-bit key, which is 32 bytes long (256 ÷ 8 = 32).
31. Which AES mode converts AES into a stream cipher?
A) CBC
B) OFB
C) ECB
D) XTS
✅ Answer: B) OFB (Output Feedback Mode)
📝 Explanation: OFB mode converts AES into a stream cipher, making it useful for real-time encryption.
32. What happens when an AES key is too weak?
A) AES automatically strengthens it
B) It is susceptible to brute-force attacks
C) It becomes faster but less secure
D) It cannot be used
✅ Answer: B) It is susceptible to brute-force attacks
📝 Explanation: Weak keys reduce AES security, making it easier to brute-force.
33. What does the AddRoundKey step do?
A) Generates a new key
B) XORs the round key with the state
C) Replaces bytes using an S-Box
D) Shuffles matrix rows
✅ Answer: B) XORs the round key with the state
📝 Explanation: AddRoundKey is the final step in each AES round, applying the round key using an XOR operation.
34. Which AES mode ensures both encryption and authentication?
A) ECB
B) CBC
C) CFB
D) GCM
✅ Answer: D) GCM (Galois/Counter Mode)
📝 Explanation: GCM provides encryption and authentication (integrity checking), making it a secure choice for network traffic encryption.
35. Which part of AES makes brute-force attacks infeasible?
A) Key size
B) ShiftRows
C) MixColumns
D) Round constants
✅ Answer: A) Key size
📝 Explanation: AES uses large key sizes (128, 192, 256 bits), making brute-force attacks computationally impossible with modern technology.
36. What is the best way to store AES encryption keys securely?
A) Hard-code them in the software
B) Store them in a database in plaintext
C) Use a Hardware Security Module (HSM) or Key Management System (KMS)
D) Encrypt them with AES itself
✅ Answer: C) Use a Hardware Security Module (HSM) or Key Management System (KMS)
📝 Explanation: Secure storage of AES keys is critical, and HSMs/KMS solutions provide secure key management.
37. AES replaced which previous encryption standard?
A) RSA
B) DES
C) Blowfish
D) Twofish
✅ Answer: B) DES
📝 Explanation: AES replaced DES (Data Encryption Standard) due to its small 56-bit key size, which was vulnerable to brute-force attacks.
38. What attack method could exploit an improperly implemented AES?
A) Rainbow table attack
B) Timing attack
C) Reverse engineering
D) Birthday attack
✅ Answer: B) Timing attack
📝 Explanation: Timing attacks analyze the time taken for AES operations to reveal encryption keys.
39. What is the maximum number of rounds in AES?
A) 10
B) 12
C) 14
D) 16
✅ Answer: C) 14
📝 Explanation: AES-256 uses 14 rounds, AES-192 uses 12 rounds, and AES-128 uses 10 rounds.
40. Why was AES chosen over other candidates like Blowfish and Twofish?
A) Simplicity, security, and efficiency
B) It was cheaper
C) It required no keys
D) It used asymmetric encryption
✅ Answer: A) Simplicity, security, and efficiency
📝 Explanation: AES was selected because it was fast, highly secure, and efficient for both software and hardware implementation.
41. Which of the following statements is true about AES?
A) AES is a public-key encryption algorithm
B) AES uses a Feistel structure
C) AES is a symmetric key encryption algorithm
D) AES has a fixed key length of 128 bits
✅ Answer: C) AES is a symmetric key encryption algorithm
📝 Explanation: AES is a symmetric key encryption algorithm, meaning the same key is used for both encryption and decryption. It does not use a Feistel structure (which DES does), and it supports multiple key lengths (128, 192, 256 bits).
42. Which of the following is NOT an AES mode of operation?
A) CFB
B) GCM
C) CTR
D) SHA-256
✅ Answer: D) SHA-256
📝 Explanation: SHA-256 is a hashing algorithm, not an AES mode. AES operates in modes like CFB, GCM, CTR, CBC, and ECB.
43. What does CTR mode stand for in AES encryption?
A) Counter Mode
B) Cipher Transmission Rule
C) Continuous Transformation Rule
D) Critical Transfer Rule
✅ Answer: A) Counter Mode
📝 Explanation: CTR (Counter Mode) turns AES into a stream cipher by using a counter value instead of traditional block encryption.
44. What is the main advantage of using AES over RSA?
A) AES is faster for large data encryption
B) AES uses asymmetric encryption
C) AES does not require a key
D) AES is less secure than RSA
✅ Answer: A) AES is faster for large data encryption
📝 Explanation: AES is a symmetric algorithm, making it much faster than RSA for encrypting large amounts of data.
45. Which mathematical field is fundamental to AES encryption?
A) Graph Theory
B) Linear Algebra
C) Number Theory
D) Finite Field Arithmetic
✅ Answer: D) Finite Field Arithmetic
📝 Explanation: AES operations rely on Galois fields (GF(2^8)), a branch of finite field arithmetic.
46. How does AES handle plaintext that is smaller than the block size?
A) It truncates the plaintext
B) It adds padding
C) It expands the plaintext
D) It generates random bits
✅ Answer: B) It adds padding
📝 Explanation: If the plaintext is smaller than 128 bits, padding is added to ensure the correct block size.
47. What is the key expansion algorithm in AES responsible for?
A) Encrypting the data
B) Creating multiple round keys
C) Generating random numbers
D) Hashing the key
✅ Answer: B) Creating multiple round keys
📝 Explanation: The AES key expansion algorithm derives multiple round keys from the main encryption key.
48. What does the term “confusion” mean in AES cryptography?
A) Mixing data between rounds
B) Making the relationship between the plaintext and ciphertext complex
C) Scrambling plaintext letters
D) Replacing AES keys
✅ Answer: B) Making the relationship between the plaintext and ciphertext complex
📝 Explanation: Confusion ensures that the output ciphertext appears unrelated to the input plaintext, making cryptanalysis difficult.
49. What is the effect of using a weak key in AES?
A) AES automatically strengthens it
B) The encryption may become predictable
C) The key will be rejected by the algorithm
D) The encryption becomes asymmetric
✅ Answer: B) The encryption may become predictable
📝 Explanation: A weak key may result in predictable ciphertext, making AES vulnerable to attacks.
50. Which component of AES ensures “diffusion”?
A) AddRoundKey
B) SubBytes
C) MixColumns
D) Key Schedule
✅ Answer: C) MixColumns
📝 Explanation: MixColumns spreads out the influence of each byte over multiple columns, ensuring diffusion.
51. How does AES prevent known-plaintext attacks?
A) By using a Feistel structure
B) By implementing key expansion and multiple rounds
C) By using RSA instead
D) By reducing key size
✅ Answer: B) By implementing key expansion and multiple rounds
📝 Explanation: AES uses key expansion and multiple rounds to ensure a single plaintext cannot be easily guessed from ciphertext.
52. What is a major weakness of using AES with ECB mode?
A) It is too slow
B) It allows parallel processing
C) Identical plaintext blocks produce identical ciphertext blocks
D) It increases key size
✅ Answer: C) Identical plaintext blocks produce identical ciphertext blocks
📝 Explanation: ECB mode does not use an Initialization Vector (IV), leading to pattern leaks.
53. What is the purpose of an AES IV (Initialization Vector)?
A) To strengthen the encryption key
B) To add randomness to the encryption
C) To increase the encryption speed
D) To generate a new key
✅ Answer: B) To add randomness to the encryption
📝 Explanation: IVs add randomness to the encryption process, preventing identical ciphertext outputs for the same plaintext.
54. Which AES mode is best for file encryption?
A) ECB
B) CBC
C) OFB
D) GCM
✅ Answer: B) CBC (Cipher Block Chaining)
📝 Explanation: CBC mode is widely used for file encryption, as it provides strong security against pattern attacks.
55. What happens if an AES key is exposed?
A) The encrypted data remains secure
B) All encrypted data can be decrypted
C) The key regenerates itself
D) The ciphertext changes automatically
✅ Answer: B) All encrypted data can be decrypted
📝 Explanation: AES security depends entirely on the secrecy of the key—if it is compromised, all encrypted data can be decrypted.
56. How many possible keys exist for AES-256?
A) 2^56
B) 2^128
C) 2^192
D) 2^256
✅ Answer: D) 2^256
📝 Explanation: AES-256 has 2^256 possible keys, making brute-force attacks computationally infeasible.
57. Why is AES more efficient than DES?
A) It requires fewer computational steps
B) It uses smaller key sizes
C) It has more complex rounds with fewer vulnerabilities
D) It operates as a stream cipher
✅ Answer: C) It has more complex rounds with fewer vulnerabilities
📝 Explanation: AES has a stronger structure, more rounds, and larger key sizes, making it more secure and faster than DES.
58. What is the effect of increasing the number of rounds in AES?
A) Increased security and complexity
B) Faster encryption speed
C) Reduced key size
D) No effect on security
✅ Answer: A) Increased security and complexity
📝 Explanation: More rounds in AES provide better security, but at the cost of slightly increased processing time.
59. Which AES mode combines counter mode with authentication?
A) CBC
B) ECB
C) GCM
D) OFB
✅ Answer: C) GCM (Galois/Counter Mode)
📝 Explanation: GCM combines CTR mode for encryption and Galois authentication for integrity checking.
60. Which industry commonly uses AES for securing transactions?
A) Healthcare
B) Finance
C) Government
D) All of the above
✅ Answer: D) All of the above
📝 Explanation: AES is widely used in healthcare, finance, government, and IT security due to its strong encryption properties.
61. In AES encryption, what does the “state” refer to?
A) The encryption key
B) The intermediate data matrix during encryption
C) The final ciphertext
D) The encryption mode
✅ Answer: B) The intermediate data matrix during encryption
📝 Explanation: The state in AES refers to the 128-bit data matrix that undergoes transformation in each round.
62. Why does AES use multiple rounds of encryption?
A) To increase complexity and security
B) To make decryption impossible
C) To generate random keys
D) To improve speed
✅ Answer: A) To increase complexity and security
📝 Explanation: AES uses multiple rounds (10, 12, or 14) to make cryptanalysis attacks infeasible.
63. Which AES component performs non-linear substitution?
A) MixColumns
B) SubBytes
C) AddRoundKey
D) ShiftRows
✅ Answer: B) SubBytes
📝 Explanation: The SubBytes step uses an S-Box (Substitution Box) to introduce non-linearity into AES encryption.
64. What is the main security advantage of AES-256 over AES-128?
A) More encryption rounds
B) Larger key size, making brute-force attacks harder
C) Uses a different algorithm
D) It is faster
✅ Answer: B) Larger key size, making brute-force attacks harder
📝 Explanation: AES-256 has a 256-bit key, making it more resistant to brute-force attacks compared to AES-128.
65. How many possible S-Box values exist in AES?
A) 128
B) 192
C) 256
D) 512
✅ Answer: C) 256
📝 Explanation: AES operates on 8-bit values, meaning its S-Box has 256 entries (2^8 = 256).
66. Which AES mode is best for secure database encryption?
A) ECB
B) CBC
C) XTS
D) GCM
✅ Answer: D) GCM (Galois/Counter Mode)
📝 Explanation: GCM provides both encryption and authentication, making it ideal for secure database encryption.
67. What is the primary weakness of using a static AES key for all encryptions?
A) It increases encryption speed
B) It makes ciphertext predictable
C) It improves security
D) It allows decryption without the key
✅ Answer: B) It makes ciphertext predictable
📝 Explanation: If the same AES key is reused, attackers can correlate ciphertexts, increasing security risks.
68. What is AES-NI?
A) A software-based AES encryption technique
B) A hardware-based AES acceleration feature
C) A type of AES key expansion
D) A cryptanalysis method
✅ Answer: B) A hardware-based AES acceleration feature
📝 Explanation: AES-NI (AES New Instructions) is a hardware feature in modern CPUs that speeds up AES encryption.
69. Which cryptographic property ensures that minor changes in plaintext result in major changes in ciphertext?
A) Confusion
B) Diffusion
C) Key expansion
D) Permutation
✅ Answer: B) Diffusion
📝 Explanation: Diffusion spreads small plaintext changes across multiple ciphertext bits, making pattern recognition difficult.
70. What is the first step of AES encryption?
A) SubBytes
B) AddRoundKey
C) ShiftRows
D) MixColumns
✅ Answer: B) AddRoundKey
📝 Explanation: AES begins with AddRoundKey, where the plaintext is XORed with the initial round key.
71. How many bytes are in an AES-128 key?
A) 8 bytes
B) 16 bytes
C) 24 bytes
D) 32 bytes
✅ Answer: B) 16 bytes
📝 Explanation: AES-128 uses a 128-bit key, which equals 16 bytes (128 ÷ 8 = 16).
72. What role does the S-Box play in AES encryption?
A) It performs key expansion
B) It ensures non-linearity
C) It swaps rows in the state
D) It handles key scheduling
✅ Answer: B) It ensures non-linearity
📝 Explanation: The S-Box (Substitution Box) introduces non-linearity, making AES resistant to linear cryptanalysis.
73. What is the purpose of Key Whitening in encryption?
A) To hide the key during encryption
B) To increase encryption speed
C) To apply an additional layer of security by XORing the key multiple times
D) To compress the key
✅ Answer: C) To apply an additional layer of security by XORing the key multiple times
📝 Explanation: Key Whitening is an additional XOR operation that strengthens encryption.
74. Why is AES widely used in cloud security?
A) It is an asymmetric encryption method
B) It allows faster hashing
C) It provides strong encryption with efficient performance
D) It is the only encryption method available
✅ Answer: C) It provides strong encryption with efficient performance
📝 Explanation: AES provides high security with low computational overhead, making it ideal for cloud security.
75. What does AES use instead of Feistel structure?
A) Lattice-based encryption
B) Substitution-Permutation Network (SPN)
C) Public-key cryptography
D) Elliptic Curve Cryptography
✅ Answer: B) Substitution-Permutation Network (SPN)
📝 Explanation: AES uses an SPN, which applies substitutions and permutations to ensure security.
76. What is the function of a nonce in AES encryption?
A) To generate keys
B) To prevent replay attacks
C) To decrypt ciphertext
D) To hash data
✅ Answer: B) To prevent replay attacks
📝 Explanation: A nonce (number used once) adds randomness, preventing replay attacks in AES encryption.
77. What is the primary advantage of AES-CTR mode?
A) It is resistant to side-channel attacks
B) It supports parallel encryption
C) It provides built-in authentication
D) It does not require a key
✅ Answer: B) It supports parallel encryption
📝 Explanation: AES-CTR mode encrypts data in parallel, making it faster than CBC mode.
78. Why is ECB mode not recommended for securing sensitive data?
A) It increases encryption speed
B) It does not require padding
C) It exposes patterns in the plaintext
D) It requires large keys
✅ Answer: C) It exposes patterns in the plaintext
📝 Explanation: ECB mode encrypts identical blocks identically, exposing plaintext patterns.
79. How does AES achieve high performance in modern hardware?
A) By using longer key sizes
B) By using AES-NI instructions
C) By reducing the number of rounds
D) By avoiding key expansion
✅ Answer: B) By using AES-NI instructions
📝 Explanation: AES-NI accelerates encryption by performing AES operations in hardware.
80. Which cryptographic attack is most relevant to AES?
A) Man-in-the-Middle (MITM)
B) Differential Cryptanalysis
C) Brute-force attack
D) Rainbow Table Attack
✅ Answer: C) Brute-force attack
📝 Explanation: AES is designed to resist cryptanalysis attacks, meaning brute-force is the only viable attack, but it is infeasible due to large key sizes.
81. What type of attack could potentially break AES if quantum computers become powerful enough?
A) Side-channel attack
B) Grover’s algorithm attack
C) Brute-force attack
D) Replay attack
✅ Answer: B) Grover’s algorithm attack
📝 Explanation: Grover’s algorithm in quantum computing could reduce AES’s key search time, but AES-256 would still be secure against it.
82. Which AES operation ensures that encrypted blocks are uniquely transformed?
A) SubBytes
B) MixColumns
C) AddRoundKey
D) Key Scheduling
✅ Answer: C) AddRoundKey
📝 Explanation: AddRoundKey XORs the round key with the state, ensuring that each encryption round is uniquely affected by the key.
83. What property of AES ensures that changing one bit in plaintext significantly alters the ciphertext?
A) Confusion
B) Diffusion
C) Permutation
D) Key Scheduling
✅ Answer: B) Diffusion
📝 Explanation: Diffusion ensures that small changes in plaintext result in large changes in ciphertext, making attacks more difficult.
84. What is the main disadvantage of AES-CBC mode?
A) It leaks plaintext length
B) It allows parallel encryption
C) It requires an IV
D) It is vulnerable to padding oracle attacks
✅ Answer: D) It is vulnerable to padding oracle attacks
📝 Explanation: Padding oracle attacks can exploit errors in AES-CBC decryption to reveal information about the plaintext.
85. How does AES ensure its resistance against linear cryptanalysis?
A) It uses large key sizes
B) It applies multiple rounds of non-linear transformations
C) It encrypts data in small blocks
D) It generates random ciphertext
✅ Answer: B) It applies multiple rounds of non-linear transformations
📝 Explanation: AES uses S-Boxes and permutation steps to break linear relationships, making linear cryptanalysis ineffective.
86. What is the primary purpose of AES in TLS (Transport Layer Security)?
A) Encrypting HTTP headers
B) Encrypting the communication channel
C) Authenticating servers
D) Verifying digital signatures
✅ Answer: B) Encrypting the communication channel
📝 Explanation: In TLS, AES is used to encrypt data transmitted between a client and server to maintain confidentiality.
87. How does AES-GCM provide authentication?
A) It uses message padding
B) It includes a Message Authentication Code (MAC)
C) It uses double encryption
D) It requires multiple keys
✅ Answer: B) It includes a Message Authentication Code (MAC)
📝 Explanation: AES-GCM includes a Galois Message Authentication Code (GMAC), ensuring both encryption and authentication.
88. Which of the following statements about AES key scheduling is true?
A) It generates subkeys for each round
B) It uses a Feistel network
C) It outputs a single round key
D) It requires an external random number generator
✅ Answer: A) It generates subkeys for each round
📝 Explanation: AES key scheduling expands the original key into multiple round keys, used in each encryption round.
89. What is the maximum theoretical attack speed against AES-256 using brute force?
A) 2^64 operations
B) 2^128 operations
C) 2^192 operations
D) 2^256 operations
✅ Answer: D) 2^256 operations
📝 Explanation: A brute-force attack on AES-256 would require 2^256 operations, which is computationally infeasible.
90. Which AES mode is commonly used in secure messaging applications?
A) ECB
B) CBC
C) GCM
D) XTS
✅ Answer: C) GCM
📝 Explanation: AES-GCM is widely used in secure messaging apps like Signal and WhatsApp due to its encryption + authentication features.
91. Why is AES-128 still considered secure despite AES-256 being available?
A) AES-128 uses a different encryption algorithm
B) AES-128 has never been cracked by brute-force
C) AES-128 is faster while still being secure
D) AES-128 is more resistant to side-channel attacks
✅ Answer: C) AES-128 is faster while still being secure
📝 Explanation: AES-128 is more efficient than AES-256 and remains secure against all known attacks.
92. Which of the following attacks is a major concern for improperly implemented AES?
A) Differential cryptanalysis
B) Side-channel attacks
C) Known-plaintext attack
D) Meet-in-the-middle attack
✅ Answer: B) Side-channel attacks
📝 Explanation: Side-channel attacks (timing, power, electromagnetic analysis) can leak information about the AES key if implemented poorly.
93. What is the role of ShiftRows in AES?
A) It scrambles key bits
B) It rotates rows of the state matrix
C) It adds diffusion to the encryption process
D) Both B and C
✅ Answer: D) Both B and C
📝 Explanation: ShiftRows shifts row positions in AES’s state matrix, improving diffusion.
94. What is the impact of using AES-NI on encryption performance?
A) It reduces encryption time
B) It increases key size
C) It changes the encryption algorithm
D) It slows down encryption
✅ Answer: A) It reduces encryption time
📝 Explanation: AES-NI (Advanced Encryption Standard New Instructions) accelerates encryption by offloading AES computations to hardware.
95. Which AES mode is considered “stateless”, making it ideal for parallel encryption?
A) ECB
B) CBC
C) CTR
D) OFB
✅ Answer: C) CTR (Counter Mode)
📝 Explanation: CTR mode treats AES as a stream cipher, making it stateless and suitable for parallel encryption.
96. How does AES differ from DES?
A) AES has a larger block size
B) AES uses a more secure key expansion process
C) AES does not use a Feistel structure
D) All of the above
✅ Answer: D) All of the above
📝 Explanation: AES differs from DES in block size (128 bits vs. 64 bits), key expansion, and its use of an SPN structure instead of Feistel.
97. What is the effect of increasing the key size in AES?
A) It improves security but increases computational cost
B) It makes encryption weaker
C) It reduces the number of rounds
D) It makes decryption faster
✅ Answer: A) It improves security but increases computational cost
📝 Explanation: A larger key size (256-bit) makes AES more secure but slightly slower.
98. What is the role of MixColumns in AES decryption?
A) It substitutes bytes using an inverse S-Box
B) It reverses the diffusion introduced in MixColumns during encryption
C) It applies the encryption key
D) It rearranges the rows
✅ Answer: B) It reverses the diffusion introduced in MixColumns during encryption
📝 Explanation: During decryption, MixColumns is reversed to undo the diffusion effects.
99. What ensures AES is resistant to differential cryptanalysis?
A) The use of S-Boxes
B) High non-linearity and key expansion
C) Random padding
D) Public-key encryption
✅ Answer: B) High non-linearity and key expansion
📝 Explanation: Non-linearity in AES’s S-Box and strong key expansion make it resistant to differential cryptanalysis.
100. Why is AES widely used in VPN encryption?
A) It provides strong security with low computational overhead
B) It does not require a key
C) It does not encrypt traffic
D) It is the only available encryption method
✅ Answer: A) It provides strong security with low computational overhead
📝 Explanation: AES is fast, secure, and efficient, making it ideal for VPN encryption.
101. Which AES mode is considered the most secure for encrypting real-time network communications?
A) ECB
B) CBC
C) GCM
D) OFB
✅ Answer: C) GCM
📝 Explanation: AES-GCM (Galois/Counter Mode) is preferred for real-time network encryption because it provides both encryption and authentication.
102. Why does AES use an S-Box in the SubBytes step?
A) To add confusion
B) To increase diffusion
C) To generate random numbers
D) To speed up encryption
✅ Answer: A) To add confusion
📝 Explanation: The S-Box (Substitution Box) makes it difficult for attackers to correlate plaintext with ciphertext, ensuring confusion in AES encryption.
103. Which AES key size is the most computationally efficient while remaining secure?
A) 128-bit
B) 192-bit
C) 256-bit
D) 512-bit
✅ Answer: A) 128-bit
📝 Explanation: AES-128 is the most efficient while still being secure, as AES-256 requires more computational power.
104. What is the best method for securely generating AES keys?
A) Use a weak password
B) Store keys in plain text
C) Use a cryptographically secure random number generator (CSPRNG)
D) Generate keys using a hash function
✅ Answer: C) Use a cryptographically secure random number generator (CSPRNG)
📝 Explanation: AES keys should be generated using CSPRNG to ensure high entropy and unpredictability.
105. How does AES prevent differential cryptanalysis attacks?
A) Through high key entropy
B) By using strong diffusion techniques
C) By reducing round complexity
D) By making the ciphertext larger than the plaintext
✅ Answer: B) By using strong diffusion techniques
📝 Explanation: AES ensures strong diffusion by using MixColumns and ShiftRows operations, preventing differential cryptanalysis.
106. What does AES-CCM stand for?
A) Counter with Cipher Block Chaining
B) Counter with CBC-MAC
C) Chained Counter Mode
D) Continuous Counter Mode
✅ Answer: B) Counter with CBC-MAC
📝 Explanation: AES-CCM (Counter with CBC-MAC) combines CTR mode encryption with CBC-MAC authentication, ensuring both confidentiality and integrity.
107. What property of AES ensures that an attacker cannot infer the encryption key from the ciphertext?
A) Avalanche effect
B) Hashing
C) Key expansion
D) Data compression
✅ Answer: A) Avalanche effect
📝 Explanation: AES ensures that a small change in plaintext results in a drastically different ciphertext, making it impossible to infer the key.
108. In AES, how are round keys derived from the original encryption key?
A) Using Feistel rounds
B) By XORing the original key with the plaintext
C) Through key expansion using Rijndael’s key schedule
D) By applying a hash function
✅ Answer: C) Through key expansion using Rijndael’s key schedule
📝 Explanation: AES uses the Rijndael key schedule to generate multiple round keys from the main key.
109. How does AES-GCM improve upon AES-CTR?
A) It adds authentication to detect tampering
B) It reduces key size
C) It removes the need for an IV
D) It eliminates ciphertext expansion
✅ Answer: A) It adds authentication to detect tampering
📝 Explanation: AES-GCM adds message authentication (GMAC), which detects tampering and prevents forgery.
110. What is the key difference between AES and Blowfish?
A) AES is a symmetric algorithm, but Blowfish is asymmetric
B) AES has a fixed block size, while Blowfish has a variable block size
C) AES is older than Blowfish
D) Blowfish uses fewer rounds than AES
✅ Answer: B) AES has a fixed block size, while Blowfish has a variable block size
📝 Explanation: AES uses a fixed 128-bit block size, whereas Blowfish uses a variable block size (32-448 bits).
111. Why is AES-GCM recommended for encrypting web traffic over HTTPS?
A) It has no padding vulnerabilities
B) It provides both encryption and authentication
C) It is faster than RSA
D) It does not require a key
✅ Answer: B) It provides both encryption and authentication
📝 Explanation: AES-GCM is used in TLS 1.2 and TLS 1.3 to provide authenticated encryption for HTTPS.
112. What is the purpose of the Inverse S-Box in AES decryption?
A) To reverse the SubBytes step
B) To generate round keys
C) To shuffle ciphertext blocks
D) To reduce encryption time
✅ Answer: A) To reverse the SubBytes step
📝 Explanation: The Inverse S-Box is used in decryption to reverse the SubBytes transformation applied during encryption.
113. Why is AES preferred over RSA for encrypting large files?
A) AES is an asymmetric encryption algorithm
B) AES is faster for bulk encryption
C) RSA does not require keys
D) AES produces shorter ciphertext
✅ Answer: B) AES is faster for bulk encryption
📝 Explanation: AES is a symmetric cipher, making it significantly faster than RSA for encrypting large files.
114. Which AES operation provides security against linear cryptanalysis?
A) SubBytes
B) ShiftRows
C) MixColumns
D) Key Expansion
✅ Answer: A) SubBytes
📝 Explanation: SubBytes (S-Box substitution) provides non-linearity, making AES resistant to linear cryptanalysis.
115. What makes AES more resistant to brute-force attacks than DES?
A) AES uses asymmetric encryption
B) AES uses larger key sizes
C) AES operates on smaller blocks
D) AES is based on a Feistel structure
✅ Answer: B) AES uses larger key sizes
📝 Explanation: AES uses 128, 192, and 256-bit keys, whereas DES used only 56-bit keys, making AES much harder to brute-force.
116. How does AES ensure that the same plaintext does not produce the same ciphertext in CBC mode?
A) By using a hash function
B) By XORing plaintext with the previous ciphertext block
C) By using a second encryption key
D) By changing the key expansion process
✅ Answer: B) By XORing plaintext with the previous ciphertext block
📝 Explanation: In CBC mode, each plaintext block is XORed with the previous ciphertext block, preventing pattern repetition.
117. How does AES compare to RSA in terms of key exchange?
A) AES is used for key exchange
B) AES does not require key exchange
C) AES requires an asymmetric key exchange method like RSA
D) AES uses a longer key size than RSA
✅ Answer: C) AES requires an asymmetric key exchange method like RSA
📝 Explanation: Since AES is symmetric, it needs RSA, Diffie-Hellman, or ECC to securely exchange the encryption key.
118. How does AES prevent dictionary attacks?
A) By using a static IV
B) By using key stretching techniques
C) By using larger block sizes
D) By using encryption rounds
✅ Answer: B) By using key stretching techniques
📝 Explanation: AES uses key stretching techniques like PBKDF2, bcrypt, or scrypt to prevent dictionary attacks.
119. What is the primary role of an IV in AES encryption?
A) To act as an additional key
B) To ensure ciphertext randomness
C) To authenticate the encryption
D) To compress data
✅ Answer: B) To ensure ciphertext randomness
📝 Explanation: The IV (Initialization Vector) ensures that repeated plaintext encrypts to different ciphertexts, preventing pattern detection.
120. What makes AES-CTR mode unique?
A) It encrypts blocks in a chain
B) It treats AES as a stream cipher
C) It does not use an S-Box
D) It does not require a key
✅ Answer: B) It treats AES as a stream cipher
📝 Explanation: AES-CTR (Counter Mode) encrypts blocks independently, allowing parallel processing and making it function like a stream cipher.
121. In AES, what ensures that identical plaintext blocks encrypt to different ciphertexts in CBC mode?
A) A unique key for each block
B) A unique IV (Initialization Vector)
C) A fixed key schedule
D) A random S-Box
✅ Answer: B) A unique IV (Initialization Vector)
📝 Explanation: In CBC mode, an Initialization Vector (IV) is used to ensure that identical plaintext blocks result in different ciphertexts, preventing pattern analysis.
122. What makes AES resistant to brute-force attacks?
A) Large key sizes
B) Random block sizes
C) Single encryption round
D) Asymmetric key generation
✅ Answer: A) Large key sizes
📝 Explanation: AES supports 128-bit, 192-bit, and 256-bit keys, making brute-force attacks computationally infeasible.
123. What is the function of the “Round Constant” (RCON) in AES?
A) It provides an additional encryption key
B) It prevents pattern attacks
C) It ensures that each round key is unique
D) It compresses the encryption key
✅ Answer: C) It ensures that each round key is unique
📝 Explanation: The Round Constant (RCON) is used in the key expansion process to ensure that each round key is unique.
124. How does AES differ from 3DES in terms of efficiency?
A) AES is slower but more secure
B) AES is faster and more secure
C) 3DES is faster but less secure
D) AES uses asymmetric encryption
✅ Answer: B) AES is faster and more secure
📝 Explanation: AES is more efficient than 3DES, as 3DES applies three rounds of DES, making it slower.
125. Which AES mode is preferred for encrypting disk storage?
A) ECB
B) CBC
C) XTS
D) OFB
✅ Answer: C) XTS
📝 Explanation: AES-XTS (XEX-based Tweaked Codebook Mode with Ciphertext Stealing) is optimized for disk encryption.
126. What is a potential risk when using AES without proper key management?
A) Increased encryption speed
B) Unbreakable encryption
C) Key exposure leading to decryption of all data
D) Increased randomness in ciphertext
✅ Answer: C) Key exposure leading to decryption of all data
📝 Explanation: If an AES encryption key is compromised, all encrypted data using that key can be decrypted.
127. Which of the following is NOT a valid AES key size?
A) 128-bit
B) 192-bit
C) 256-bit
D) 512-bit
✅ Answer: D) 512-bit
📝 Explanation: AES supports only 128-bit, 192-bit, and 256-bit keys. There is no 512-bit AES.
128. Why is AES considered quantum-resistant for now?
A) It uses an S-Box
B) It has multiple rounds
C) Grover’s algorithm only halves the key strength, making AES-256 still secure
D) It is an asymmetric algorithm
✅ Answer: C) Grover’s algorithm only halves the key strength, making AES-256 still secure
📝 Explanation: While Grover’s algorithm can reduce AES key security, AES-256 remains resistant, as a 256-bit key is still too large to brute-force.
129. What is the main advantage of using AES in CTR mode?
A) It allows parallel processing
B) It eliminates the need for an encryption key
C) It reduces the key expansion process
D) It uses asymmetric encryption
✅ Answer: A) It allows parallel processing
📝 Explanation: AES-CTR (Counter Mode) allows parallel encryption and decryption, making it faster.
130. What is the effect of using a weak AES key?
A) It strengthens encryption
B) It increases decryption time
C) It makes brute-force attacks easier
D) It reduces ciphertext length
✅ Answer: C) It makes brute-force attacks easier
📝 Explanation: Weak keys make AES encryption vulnerable to brute-force attacks.
131. What is the primary purpose of AES in the Secure Sockets Layer (SSL) and TLS?
A) Encrypting communication between a client and server
B) Authenticating user identities
C) Hashing passwords
D) Generating random numbers
✅ Answer: A) Encrypting communication between a client and server
📝 Explanation: AES is used in SSL/TLS to encrypt communications, ensuring data confidentiality.
132. What does AES stand for?
A) Advanced Encryption System
B) Asymmetric Encryption Standard
C) Advanced Encryption Standard
D) Automatic Encryption System
✅ Answer: C) Advanced Encryption Standard
📝 Explanation: AES stands for Advanced Encryption Standard, replacing DES as the encryption standard.
133. Why is AES used in mobile device encryption?
A) It does not require a key
B) It is optimized for low-power environments
C) It generates new keys automatically
D) It does not require software support
✅ Answer: B) It is optimized for low-power environments
📝 Explanation: AES is fast and efficient, making it suitable for mobile device encryption.
134. Which of the following can break AES encryption?
A) Differential cryptanalysis
B) Side-channel attacks
C) Meet-in-the-middle attack
D) AES is unbreakable
✅ Answer: B) Side-channel attacks
📝 Explanation: While AES is mathematically secure, side-channel attacks (such as timing attacks) can expose keys.
135. What does AES use to generate subkeys?
A) A cryptographic hash function
B) Rijndael key schedule
C) Elliptic curve mathematics
D) Random bit shifting
✅ Answer: B) Rijndael key schedule
📝 Explanation: AES uses the Rijndael key schedule to generate subkeys for each encryption round.
136. Why is ECB mode not suitable for encrypting large amounts of data?
A) It does not support large key sizes
B) It preserves patterns in the plaintext
C) It requires multiple encryption keys
D) It is too fast
✅ Answer: B) It preserves patterns in the plaintext
📝 Explanation: ECB mode encrypts identical plaintext blocks into identical ciphertext blocks, exposing patterns.
137. Which AES mode is commonly used in cloud storage encryption?
A) ECB
B) CBC
C) GCM
D) XTS
✅ Answer: C) GCM
📝 Explanation: AES-GCM is commonly used in cloud encryption because it provides both encryption and authentication.
138. What is the effect of using AES-NI?
A) It makes AES encryption faster
B) It weakens encryption
C) It replaces the S-Box
D) It reduces the key size
✅ Answer: A) It makes AES encryption faster
📝 Explanation: AES-NI (Advanced Encryption Standard New Instructions) is a hardware feature that accelerates AES encryption.
139. Which industry regulation requires AES encryption for data protection?
A) HIPAA
B) PCI DSS
C) GDPR
D) All of the above
✅ Answer: D) All of the above
📝 Explanation: AES is required by HIPAA (healthcare), PCI DSS (payment security), and GDPR (data protection).
140. What is the default block size of AES?
A) 64 bits
B) 128 bits
C) 256 bits
D) 512 bits
✅ Answer: B) 128 bits
📝 Explanation: AES operates on fixed 128-bit blocks, regardless of the key size.
141. Why is AES considered more secure than older encryption algorithms like DES?
A) AES uses a Feistel network
B) AES supports larger key sizes and more complex transformations
C) AES is based on asymmetric encryption
D) AES requires two keys for encryption and decryption
✅ Answer: B) AES supports larger key sizes and more complex transformations
📝 Explanation: AES is superior to older algorithms like DES because it supports 128-bit, 192-bit, and 256-bit keys, uses a Substitution-Permutation Network (SPN), and has better diffusion and confusion mechanisms.
142. What is the primary purpose of MixColumns in AES encryption?
A) To add confusion
B) To improve diffusion by mixing bytes across columns
C) To generate encryption keys
D) To replace bytes using the S-Box
✅ Answer: B) To improve diffusion by mixing bytes across columns
📝 Explanation: MixColumns applies matrix multiplication to spread the influence of each byte across multiple columns, improving diffusion.
143. What is a major advantage of AES-XTS mode over AES-CBC?
A) AES-XTS encrypts entire files
B) AES-XTS does not require an IV
C) AES-XTS prevents data corruption in individual sectors of disk storage
D) AES-XTS does not use the MixColumns transformation
✅ Answer: C) AES-XTS prevents data corruption in individual sectors of disk storage
📝 Explanation: AES-XTS is optimized for disk encryption, ensuring that encryption errors in one sector do not affect others.
144. How does AES achieve resistance against algebraic attacks?
A) By using a linear transformation
B) By using a strong key expansion mechanism
C) By relying on prime number factorization
D) By reducing the number of rounds
✅ Answer: B) By using a strong key expansion mechanism
📝 Explanation: AES key expansion generates round keys using a non-linear Rijndael key schedule, making it resistant to algebraic attacks.
145. What is the main risk of using the same IV repeatedly in AES-GCM?
A) It reduces encryption speed
B) It makes ciphertext deterministic and vulnerable to replay attacks
C) It increases memory usage
D) It changes the block size
✅ Answer: B) It makes ciphertext deterministic and vulnerable to replay attacks
📝 Explanation: If an IV is reused in AES-GCM, attackers can detect patterns in the ciphertext and potentially break encryption.
146. What is the key benefit of AES-NI (Advanced Encryption Standard New Instructions)?
A) It improves encryption speed by offloading operations to hardware
B) It replaces key scheduling
C) It reduces the need for encryption rounds
D) It increases key size automatically
✅ Answer: A) It improves encryption speed by offloading operations to hardware
📝 Explanation: AES-NI provides hardware acceleration for AES encryption, significantly improving performance.
147. How does AES prevent known-plaintext attacks?
A) By using key expansion and multiple rounds of encryption
B) By changing the key for each encryption operation
C) By encrypting each byte individually
D) By using asymmetric encryption
✅ Answer: A) By using key expansion and multiple rounds of encryption
📝 Explanation: Key expansion and multiple transformation rounds ensure that plaintext-ciphertext relationships are difficult to exploit.
148. Why is AES commonly used in VPN encryption?
A) AES is a public-key algorithm
B) AES provides a balance between security and performance
C) AES requires no key exchange
D) AES uses different keys for encryption and decryption
✅ Answer: B) AES provides a balance between security and performance
📝 Explanation: AES is fast, secure, and efficient, making it a great choice for VPN encryption.
149. Which component of AES contributes most to the avalanche effect?
A) MixColumns
B) AddRoundKey
C) SubBytes
D) ShiftRows
✅ Answer: C) SubBytes
📝 Explanation: The SubBytes operation, using the S-Box, ensures that a small change in plaintext leads to a large change in ciphertext (avalanche effect).
150. What is the major disadvantage of AES when implemented incorrectly?
A) AES becomes a stream cipher
B) AES encryption can be reversed easily
C) AES may become vulnerable to side-channel attacks
D) AES uses excessive computational power
✅ Answer: C) AES may become vulnerable to side-channel attacks
📝 Explanation: Poor implementations can expose AES to side-channel attacks like timing analysis and power analysis.
151. What is the best way to securely store AES encryption keys?
A) Hardcode them in software
B) Store them in plaintext in a database
C) Use a Hardware Security Module (HSM) or Key Management System (KMS)
D) Encrypt them with AES itself
✅ Answer: C) Use a Hardware Security Module (HSM) or Key Management System (KMS)
📝 Explanation: HSMs and KMS solutions provide secure storage and management of encryption keys.
152. Why is AES considered more efficient for mobile devices?
A) It has fewer encryption rounds
B) It requires no key exchange
C) It is optimized for low-power environments
D) It is an asymmetric algorithm
✅ Answer: C) It is optimized for low-power environments
📝 Explanation: AES is optimized for efficiency, making it ideal for mobile encryption.
153. What is the best way to generate strong AES keys?
A) Use user-generated passwords
B) Use a cryptographically secure random number generator (CSPRNG)
C) Use a weak key to reduce computational load
D) Generate keys from a hash function
✅ Answer: B) Use a cryptographically secure random number generator (CSPRNG)
📝 Explanation: AES keys should be generated using CSPRNGs to ensure high entropy and randomness.
154. What is a major drawback of AES-ECB mode?
A) It requires additional authentication
B) It exposes plaintext patterns
C) It is slower than other modes
D) It requires longer encryption keys
✅ Answer: B) It exposes plaintext patterns
📝 Explanation: AES-ECB encrypts identical plaintext blocks into identical ciphertext blocks, making it insecure.
155. Why is AES-GCM commonly used in Wi-Fi security?
A) It is the only mode available
B) It provides both encryption and authentication
C) It does not require an IV
D) It is faster than all other modes
✅ Answer: B) It provides both encryption and authentication
📝 Explanation: AES-GCM is used in WPA3 Wi-Fi security because it provides integrity (authentication) and confidentiality (encryption).
156. What is a nonce in AES encryption?
A) A cryptographic hash
B) A unique value used only once
C) A key expansion function
D) A compression technique
✅ Answer: B) A unique value used only once
📝 Explanation: A nonce (Number used once) ensures that encryption outputs remain unique.
157. How many possible keys exist for AES-192?
A) 2^56
B) 2^128
C) 2^192
D) 2^256
✅ Answer: C) 2^192
📝 Explanation: AES-192 has 2^192 possible keys, making it resistant to brute-force attacks.
158. How is AES used in password-based encryption (PBE)?
A) It encrypts passwords directly
B) It hashes passwords
C) It derives encryption keys from passwords using key stretching
D) It replaces hashing algorithms
✅ Answer: C) It derives encryption keys from passwords using key stretching
📝 Explanation: Key stretching algorithms (e.g., PBKDF2, bcrypt, Argon2) generate AES keys from user passwords.
159. What is the biggest concern when implementing AES in IoT devices?
A) Key management
B) Large computational overhead
C) Lack of hardware support
D) Slow decryption
✅ Answer: A) Key management
📝 Explanation: IoT devices often lack secure key management, making them vulnerable.
160. Which mode of AES provides confidentiality but NOT integrity?
A) ECB
B) CBC
C) CTR
D) GCM
✅ Answer: C) CTR
📝 Explanation: CTR mode provides encryption (confidentiality) but not authentication (integrity).
161. What is the primary reason AES is used in modern cryptographic protocols?
A) It is an asymmetric algorithm
B) It provides strong encryption with efficient performance
C) It does not require a key
D) It replaces hash functions
✅ Answer: B) It provides strong encryption with efficient performance
📝 Explanation: AES is fast, secure, and efficient, making it a preferred choice in modern cryptographic protocols like TLS, VPNs, and Wi-Fi encryption.
162. In AES encryption, which transformation ensures that bits from one plaintext block affect multiple ciphertext bits?
A) SubBytes
B) MixColumns
C) AddRoundKey
D) Key Expansion
✅ Answer: B) MixColumns
📝 Explanation: MixColumns ensures diffusion, spreading the influence of each input byte across the ciphertext.
163. What is the role of the AES decryption process?
A) It reverses the encryption process using inverse transformations
B) It generates new encryption keys
C) It compresses the ciphertext
D) It hashes the plaintext
✅ Answer: A) It reverses the encryption process using inverse transformations
📝 Explanation: Decryption reverses encryption by applying inverse SubBytes, ShiftRows, MixColumns, and AddRoundKey.
164. Which attack is possible if an AES implementation leaks execution time information?
A) Brute-force attack
B) Side-channel attack
C) Dictionary attack
D) Meet-in-the-middle attack
✅ Answer: B) Side-channel attack
📝 Explanation: Timing attacks analyze how long encryption takes to infer key-related information.
165. Which AES key size provides the best balance between security and performance?
A) 128-bit
B) 192-bit
C) 256-bit
D) 512-bit
✅ Answer: A) 128-bit
📝 Explanation: AES-128 provides strong security while being faster and more efficient than AES-256.
166. Why is AES more secure than RC4?
A) AES uses symmetric encryption while RC4 is asymmetric
B) AES is resistant to known vulnerabilities in RC4
C) AES requires fewer keys
D) AES has a larger block size
✅ Answer: B) AES is resistant to known vulnerabilities in RC4
📝 Explanation: RC4 has known weaknesses like biased output and key reuse issues, while AES remains secure.
167. What is the primary advantage of AES-CTR mode?
A) It ensures message integrity
B) It allows parallel encryption of blocks
C) It requires no key exchange
D) It uses smaller keys
✅ Answer: B) It allows parallel encryption of blocks
📝 Explanation: CTR mode converts AES into a stream cipher, allowing parallel encryption.
168. Which AES mode should NOT be used for encrypting large files due to pattern exposure?
A) CBC
B) CTR
C) ECB
D) GCM
✅ Answer: C) ECB
📝 Explanation: ECB (Electronic Codebook) mode encrypts identical plaintext blocks into identical ciphertext blocks, revealing patterns.
169. Why does AES use a fixed block size of 128 bits?
A) To ensure uniform encryption strength
B) To allow for key expansion
C) To optimize for asymmetric encryption
D) To make brute-force attacks easier
✅ Answer: A) To ensure uniform encryption strength
📝 Explanation: AES’s fixed 128-bit block size ensures consistent security and performance.
170. Which AES mode provides both encryption and integrity verification?
A) CBC
B) ECB
C) GCM
D) CTR
✅ Answer: C) GCM
📝 Explanation: AES-GCM provides both encryption and authentication, making it ideal for secure communications.
171. What is the primary benefit of AES over Twofish?
A) AES has been widely adopted as a standard
B) AES uses larger key sizes
C) AES does not require key exchange
D) AES has fewer encryption rounds
✅ Answer: A) AES has been widely adopted as a standard
📝 Explanation: AES was standardized by NIST, making it more widely used than Twofish.
172. Why is a cryptographically secure random number generator (CSPRNG) required for AES keys?
A) To make key storage easier
B) To ensure key unpredictability and resistance to attacks
C) To allow easy key sharing
D) To make AES work with asymmetric encryption
✅ Answer: B) To ensure key unpredictability and resistance to attacks
📝 Explanation: CSPRNGs ensure AES keys are random, making brute-force and key prediction attacks infeasible.
173. Which AES mode is preferred for encrypting video streaming data?
A) ECB
B) CBC
C) GCM
D) CTR
✅ Answer: D) CTR
📝 Explanation: AES-CTR mode is preferred for streaming because it allows parallel encryption and fast processing.
174. What is a potential weakness of AES-GCM if implemented incorrectly?
A) It requires multiple keys
B) It is vulnerable if IVs are reused
C) It cannot be used with passwords
D) It does not support large file encryption
✅ Answer: B) It is vulnerable if IVs are reused
📝 Explanation: Reusing IVs in AES-GCM can lead to key recovery attacks.
175. What is the primary reason AES replaced DES?
A) AES has a larger block size and stronger key schedule
B) AES is an asymmetric encryption algorithm
C) AES requires fewer encryption rounds
D) AES requires no key exchange
✅ Answer: A) AES has a larger block size and stronger key schedule
📝 Explanation: AES has a 128-bit block size and a more secure key schedule, while DES used only 56-bit keys, making it vulnerable.
176. How does AES achieve confusion in encryption?
A) By applying multiple encryption keys
B) By using an S-Box for substitution
C) By reordering ciphertext bits
D) By reducing key length
✅ Answer: B) By using an S-Box for substitution
📝 Explanation: Confusion is introduced using the S-Box, which substitutes bytes to obscure relationships between plaintext and ciphertext.
177. What happens if an AES key is too predictable?
A) Encryption becomes faster
B) It becomes vulnerable to brute-force and cryptanalysis attacks
C) It generates stronger ciphertext
D) It improves diffusion
✅ Answer: B) It becomes vulnerable to brute-force and cryptanalysis attacks
📝 Explanation: Predictable keys weaken AES security, making it easier for attackers to break encryption.
178. How does AES ensure data integrity in encryption?
A) By hashing the plaintext before encryption
B) By adding an authentication tag in AES-GCM mode
C) By using asymmetric encryption
D) By modifying the key expansion process
✅ Answer: B) By adding an authentication tag in AES-GCM mode
📝 Explanation: AES-GCM includes an authentication tag, ensuring data integrity and authenticity.
179. Why is AES used in government and military applications?
A) It is faster than all other encryption algorithms
B) It is standardized by NIST and provides strong security
C) It does not require keys
D) It is based on public-key cryptography
✅ Answer: B) It is standardized by NIST and provides strong security
📝 Explanation: AES is approved by NIST and widely used in government and military systems for top-level security.
180. What is the most effective way to protect AES encryption keys?
A) Store them in plaintext in a database
B) Use a hardware security module (HSM)
C) Hardcode them in software
D) Generate a new key for each encryption
✅ Answer: B) Use a hardware security module (HSM)
📝 Explanation: HSMs provide secure storage and management of AES encryption keys.
181. Which of the following AES modes is NOT recommended for encrypting structured data such as databases?
A) CBC
B) ECB
C) GCM
D) XTS
✅ Answer: B) ECB
📝 Explanation: ECB mode is insecure for structured data because it encrypts identical plaintext blocks into identical ciphertext blocks, exposing patterns.
182. What is the main function of the AddRoundKey step in AES encryption?
A) To mix data between rounds
B) To apply XOR with the round key
C) To shuffle ciphertext blocks
D) To perform byte substitution
✅ Answer: B) To apply XOR with the round key
📝 Explanation: The AddRoundKey step ensures that each round key influences the ciphertext by performing XOR with the round key.
183. What is the primary challenge of key management in AES encryption?
A) AES keys are too large to store
B) Secure storage and distribution of keys
C) AES does not require key management
D) AES keys expire too quickly
✅ Answer: B) Secure storage and distribution of keys
📝 Explanation: Key management involves securely storing, distributing, and rotating encryption keys to maintain security.
184. What property of AES makes it difficult to predict ciphertext output?
A) Non-linearity in S-Box transformations
B) Asymmetric key exchange
C) Reversible hash functions
D) Public-key cryptography
✅ Answer: A) Non-linearity in S-Box transformations
📝 Explanation: The S-Box introduces non-linearity, making AES encryption resistant to cryptanalysis attacks.
185. Why is AES considered a symmetric key algorithm?
A) It uses one key for encryption and another for decryption
B) It uses the same key for both encryption and decryption
C) It uses public and private key pairs
D) It does not require a key
✅ Answer: B) It uses the same key for both encryption and decryption
📝 Explanation: Symmetric encryption means the same key is used for encryption and decryption, as in AES.
186. What is the primary reason AES replaced DES?
A) AES uses a Feistel structure
B) AES supports larger key sizes and is more secure
C) AES requires less computational power
D) AES uses fewer encryption rounds
✅ Answer: B) AES supports larger key sizes and is more secure
📝 Explanation: DES had a 56-bit key, which is vulnerable to brute-force attacks, whereas AES supports 128, 192, and 256-bit keys.
187. What is a possible attack if an AES key is weak or reused?
A) Chosen-ciphertext attack
B) Brute-force attack
C) Differential cryptanalysis
D) All of the above
✅ Answer: D) All of the above
📝 Explanation: A weak or reused key makes AES vulnerable to brute-force attacks, chosen-ciphertext attacks, and cryptanalysis techniques.
188. How does AES provide forward secrecy in protocols like TLS?
A) By using ephemeral key exchange methods
B) By using static keys for all sessions
C) By encrypting without keys
D) By storing keys in plaintext
✅ Answer: A) By using ephemeral key exchange methods
📝 Explanation: Forward secrecy ensures that past encrypted sessions cannot be decrypted if a key is compromised, using ephemeral keys.
189. Why is AES-256 recommended for military and government encryption?
A) It is the fastest encryption method
B) It provides the highest security against brute-force attacks
C) It does not require key storage
D) It uses quantum cryptography
✅ Answer: B) It provides the highest security against brute-force attacks
📝 Explanation: AES-256 is highly resistant to brute-force attacks and is recommended for government and military use.
190. What makes AES encryption secure against meet-in-the-middle attacks?
A) Its use of a large keyspace and multiple rounds
B) Its reliance on public-key cryptography
C) Its ability to use different keys for encryption and decryption
D) Its dependence on prime number factorization
✅ Answer: A) Its use of a large keyspace and multiple rounds
📝 Explanation: AES uses multiple rounds and large key sizes, making meet-in-the-middle attacks computationally infeasible.
191. Why is it important to use a cryptographic salt with AES?
A) To make brute-force attacks harder
B) To increase encryption speed
C) To reduce key size
D) To simplify key management
✅ Answer: A) To make brute-force attacks harder
📝 Explanation: Salts ensure that even if the same plaintext is encrypted multiple times, the ciphertext will be different, making brute-force attacks difficult.
192. What is a major advantage of AES over RSA for encrypting large amounts of data?
A) AES is a public-key encryption algorithm
B) AES is significantly faster for bulk data encryption
C) AES provides digital signatures
D) AES does not require key exchange
✅ Answer: B) AES is significantly faster for bulk data encryption
📝 Explanation: AES is a symmetric cipher, making it much faster than RSA, which is better suited for key exchange.
193. Why is AES widely used in mobile applications?
A) It requires no computational power
B) It is optimized for efficiency and speed
C) It does not require key management
D) It operates without encryption keys
✅ Answer: B) It is optimized for efficiency and speed
📝 Explanation: AES is fast and efficient, making it ideal for mobile encryption.
194. Which AES mode is best for encrypting real-time communication data?
A) ECB
B) CBC
C) GCM
D) XTS
✅ Answer: C) GCM
📝 Explanation: AES-GCM provides both encryption and authentication, making it ideal for real-time communication protocols.
195. What is the role of the Key Expansion step in AES?
A) It generates new encryption keys
B) It derives multiple round keys from the main key
C) It compresses the encryption key
D) It increases ciphertext length
✅ Answer: B) It derives multiple round keys from the main key
📝 Explanation: AES expands the original key to create multiple round keys, ensuring stronger encryption.
196. How does AES ensure diffusion in encryption?
A) Through key expansion
B) By using the MixColumns transformation
C) By hashing the ciphertext
D) By randomly selecting encryption rounds
✅ Answer: B) By using the MixColumns transformation
📝 Explanation: The MixColumns step ensures that changes in one byte affect multiple bytes, increasing diffusion.
197. What is the main advantage of AES-CFB mode?
A) It supports parallel encryption
B) It converts AES into a self-synchronizing stream cipher
C) It uses asymmetric key exchange
D) It reduces encryption speed
✅ Answer: B) It converts AES into a self-synchronizing stream cipher
📝 Explanation: CFB (Cipher Feedback Mode) turns AES into a stream cipher while retaining block cipher security.
198. Why should AES keys never be hardcoded in applications?
A) It increases key size
B) It exposes the encryption key to attackers
C) It slows down encryption
D) It improves performance
✅ Answer: B) It exposes the encryption key to attackers
📝 Explanation: Hardcoding AES keys makes them vulnerable to reverse engineering and key exposure.
199. What is the effect of using AES-NI in modern processors?
A) It speeds up AES encryption and decryption
B) It weakens AES security
C) It replaces S-Box transformations
D) It requires fewer encryption rounds
✅ Answer: A) It speeds up AES encryption and decryption
📝 Explanation: AES-NI (Advanced Encryption Standard New Instructions) accelerates AES processing by offloading encryption operations to hardware.
200. What is the default key size used in AES when no specific key length is mentioned?
A) 64-bit
B) 128-bit
C) 192-bit
D) 256-bit
✅ Answer: B) 128-bit
📝 Explanation: When no key size is explicitly stated, AES defaults to 128-bit encryption.