This set tests classical and modern symmetric cryptography: substitution and transposition ciphers such as Caesar, Vigenere, Playfair and rail fence, followed by the structure of DES, Triple DES and AES, Feistel networks, confusion and diffusion, and block cipher modes of operation like ECB, CBC, CFB, OFB, CTR and GCM. It is aimed at university exams, GATE and UGC NET preparation and security interview rounds.
How to practise: read each question and tap the option you think is correct. You will see right away whether you got it, along with a short explanation, and your score is tracked at the bottom of the screen. This set has 18 questions.
Topics covered in this set
- Caesar, Vigenere and Playfair ciphers
- Rail fence and columnar transposition
- Feistel structure and DES parameters
- 3DES and meet-in-the-middle
- AES rounds and transformations
- Block cipher modes: ECB, CBC, CFB, OFB, CTR, GCM
Cyber Security MCQs with answers and explanations
Q1. Using a Caesar cipher with a shift of 3, what is the ciphertext for the plaintext HELLO?
a) KHOOR
b) EBIIL
c) KHPPS
d) IFMMP
Q2. Encrypt ATTACK with the Vigenere cipher using the key KEY (A=0, ..., Z=25).
a) KXRKGO
b) KXRKGI
c) KYRKHI
d) LXSKGI
Q3. How many possible keys does a Caesar (shift) cipher over the 26-letter English alphabet have, excluding the trivial shift of 0?
a) 26
b) 26!
c) 25
d) 13
Q4. Using a 2-rail rail fence cipher, what is the ciphertext of SECURITY?
a) SECURITY
b) SCRTYEUI
c) ECRUTIYS
d) SCRTEUIY
Q5. In the Playfair cipher, if both letters of a digraph lie in the same row of the 5x5 key square, each letter is replaced by:
a) The letter immediately below it
b) The letter immediately to its right, wrapping around the row
c) The letter at the opposite corner of the rectangle
d) The same letter, unchanged
Q6. Why is the Vigenere cipher more resistant to simple frequency analysis than a monoalphabetic cipher?
a) The same plaintext letter can map to different ciphertext letters depending on key position
b) It uses a 5x5 key matrix
c) It permutes letter positions instead of substituting them
d) Its key is always as long as the message
Q7. What are the block size, effective key length and number of rounds of DES?
a) 128-bit block, 56-bit key, 16 rounds
b) 64-bit block, 64-bit key, 10 rounds
c) 64-bit block, 56-bit key, 12 rounds
d) 64-bit block, 56-bit key, 16 rounds
Q8. In a Feistel cipher, which statement is TRUE about the round function F?
a) F must be a bijection for decryption to work
b) F operates on the full block in each round
c) F need not be invertible, because decryption reuses the same structure with subkeys reversed
d) Decryption requires a separate inverse function F^-1
Q9. Double DES with two independent 56-bit keys gives far less than 112 bits of security mainly because of:
a) The meet-in-the-middle attack
b) The birthday paradox
c) Weak keys in DES
d) Differential cryptanalysis of S-boxes
Q10. Triple DES in EDE mode with two keys (K1, K2, K1) is sometimes used for backward compatibility with single DES because:
a) It uses a 128-bit block like AES
b) Decryption is not required in the middle stage
c) Setting all keys equal makes encrypt-decrypt-encrypt collapse to a single DES encryption
d) It eliminates the need for subkey generation
Q11. How many rounds does AES perform with a 256-bit key?
a) 10
b) 14
c) 12
d) 16
Q12. Which AES round transformation is omitted in the final round?
a) SubBytes
b) ShiftRows
c) AddRoundKey
d) MixColumns
Q13. Shannon's property of 'diffusion' in a cipher means that:
a) The relationship between key and ciphertext is made complex
b) The cipher uses only substitution operations
c) Each plaintext bit influences many ciphertext bits, spreading statistical structure
d) The key is as long as the message
Q14. Encrypting a bitmap image in ECB mode often leaves the outline of the image visible. Why?
a) ECB uses a weak key schedule
b) ECB does not encrypt the last block
c) ECB transmits the IV in clear
d) Identical plaintext blocks produce identical ciphertext blocks
Q15. In CBC mode, a single bit error in ciphertext block C(i) during transmission affects decryption of:
a) The whole of P(i) and the corresponding single bit of P(i+1)
b) Only the corresponding bit of P(i)
c) All subsequent plaintext blocks
d) No plaintext blocks, since CBC self-corrects
Q16. Which block cipher mode turns a block cipher into a stream cipher and allows encryption of different blocks in parallel?
a) Cipher Block Chaining (CBC) mode
b) Counter (CTR) mode
c) Cipher Feedback (CFB) mode
d) Electronic Codebook (ECB) mode with padding
Q17. A developer reuses the same nonce with the same key for two messages in CTR mode. What is the main consequence?
a) The key is immediately revealed
b) Decryption fails for the second message
c) The ciphertexts become twice as long
d) The XOR of the two ciphertexts equals the XOR of the two plaintexts, leaking information
Q18. Which mode of operation provides both confidentiality and built-in authentication (AEAD)?
a) OFB
b) GCM
c) CBC
d) ECB
Quick revision notes
- Caesar: C = (P + k) mod 26; only 25 useful keys.
- Vigenere: C(i) = (P(i) + K(i mod m)) mod 26; broken by Kasiski test and index of coincidence.
- Playfair: 5x5 matrix (I/J combined), digraph rules: same row -> right, same column -> down, else rectangle.
- DES: 64-bit block, 56-bit key, 16 Feistel rounds, 8 S-boxes (6 in, 4 out).
- 2DES ~ 2^57 work due to meet-in-the-middle; 3DES two-key gives about 80-112 bits; three-key about 112 bits.
- AES: 128-bit block; 10/12/14 rounds for 128/192/256-bit keys; last round has no MixColumns.
- Modes: ECB (leaks patterns), CBC (IV, sequential encrypt), CFB/OFB/CTR (stream-like), GCM (AEAD).
Frequently asked questions
What is the difference between substitution and transposition ciphers?
A substitution cipher replaces each symbol with another symbol, as in Caesar or Vigenere. A transposition cipher keeps the symbols but rearranges their order, as in rail fence or columnar ciphers. Modern block ciphers combine both ideas over many rounds.
Why is ECB mode considered insecure?
ECB encrypts each block independently with the same key, so identical plaintext blocks always yield identical ciphertext blocks. This leaks structure and repetition in the data, so modes that use an IV or nonce, such as CBC, CTR or GCM, are preferred.
Why was DES replaced by AES?
DES's 56-bit key became small enough to brute-force with dedicated hardware in the late 1990s, and its 64-bit block is also limiting. AES, standardised in 2001, offers 128-bit blocks, key sizes up to 256 bits and efficient implementation in both software and hardware.
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