In the AES encrypt method shown, which mode is used for the cipher?

In the AES encrypt method shown, which mode is used for the cipher?

• A. CTR
• B. CBC
• C. ECB
• D. OFB

Answer: (A)

This question tests how a block cipher like AES is used to generate a keystream and apply it to the plaintext. In CTR mode, AES is run on a unique counter value (often a nonce combined with a counter) to produce a keystream block. Each plaintext block is then XORed with the corresponding keystream block to create ciphertext. The counter is incremented for every block, and encryption and decryption both regenerate the same keystream when the same key and nonce are used, so XOR recovers the original text. This approach has advantages: you don’t need padding, and you can process data in parallel since each keystream block is independent of others, as long as the nonce-counter pair is never reused with the same key. Other modes either chain blocks (making decryption depend on previous ciphertext in CBC) or produce a keystream in a different way (ECB is insecure due to repeating patterns, OFB/CFB feed back previous outputs rather than using a counter). If the shown AES encrypt method uses a counter to derive a keystream and XORs it with the plaintext, that indicates CTR mode.

This question tests how a block cipher like AES is used to generate a keystream and apply it to the plaintext. In CTR mode, AES is run on a unique counter value (often a nonce combined with a counter) to produce a keystream block. Each plaintext block is then XORed with the corresponding keystream block to create ciphertext. The counter is incremented for every block, and encryption and decryption both regenerate the same keystream when the same key and nonce are used, so XOR recovers the original text. This approach has advantages: you don’t need padding, and you can process data in parallel since each keystream block is independent of others, as long as the nonce-counter pair is never reused with the same key. Other modes either chain blocks (making decryption depend on previous ciphertext in CBC) or produce a keystream in a different way (ECB is insecure due to repeating patterns, OFB/CFB feed back previous outputs rather than using a counter). If the shown AES encrypt method uses a counter to derive a keystream and XORs it with the plaintext, that indicates CTR mode.