GSLC Question #352: Real Exam Question with Answer & Explanation

The correct answer is C. DES-CBC. DES-CBC (Cipher Block Chaining) mode uses an IV that is XORed with the first plaintext block before encryption, effectively marking the boundary where encrypted data begins in the output.

Question

Which of the following uses a variable-length Initialization Vector (IV), where the encrypted data begins?

Options

ADES-OFB
BDES-ECB
CDES-CBC
DDES-CFB

Explanation

DES-CBC (Cipher Block Chaining) mode uses an IV that is XORed with the first plaintext block before encryption, effectively marking the boundary where encrypted data begins in the output.

Common mistakes.

A. DES-OFB (Output Feedback) mode uses a fixed-length IV equal to the block size and generates a keystream independently of the plaintext, with no concept of a variable-length IV marking the ciphertext start.
B. DES-ECB (Electronic Codebook) mode does not use an IV at all - each 64-bit block is encrypted independently using only the key.
D. DES-CFB (Cipher Feedback) mode uses a fixed-length IV equal to the DES block size and feeds ciphertext segments back into the encryption process, but does not support a variable-length IV in its standard definition.

Concept tested. DES cipher block chaining mode and IV usage

Reference. https://csrc.nist.gov/publications/detail/sp/800-38a/final

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