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CISSP · Question #292

Which of the following is a characteristic of the initialization vector when using Data Encryption Standard (DES)?

The correct answer is B. It can be transmitted in the clear as a random number. In DES (and block cipher modes like CBC), the Initialization Vector (IV) does not need to be secret - it only needs to be unpredictable/random, and can be transmitted in plaintext alongside the ciphertext.

Submitted by alyssa_d· Mar 5, 2026Security Architecture and Engineering

Question

Which of the following is a characteristic of the initialization vector when using Data Encryption Standard (DES)?

Options

  • AIt must be known to both sender and receiver.
  • BIt can be transmitted in the clear as a random number.
  • CIt must be retained until the last block is transmitted.
  • DIt can be used to encrypt and decrypt information.

How the community answered

(45 responses)
  • A
    2% (1)
  • B
    89% (40)
  • C
    7% (3)
  • D
    2% (1)

Why each option

In DES (and block cipher modes like CBC), the Initialization Vector (IV) does not need to be secret - it only needs to be unpredictable/random, and can be transmitted in plaintext alongside the ciphertext.

AIt must be known to both sender and receiver.

The IV does not need to be secretly shared between sender and receiver in advance; it is transmitted openly with the message, unlike the encryption key which must be kept secret and shared securely.

BIt can be transmitted in the clear as a random number.Correct

The IV in DES (particularly in CBC mode) is a random value used to ensure that identical plaintexts produce different ciphertexts. Because it provides randomness rather than secrecy, it does not need to be encrypted and can be safely transmitted in cleartext alongside the ciphertext, as long as it is unique and unpredictable per session.

CIt must be retained until the last block is transmitted.

The IV is only used to XOR with the first plaintext block before encryption in CBC mode; it does not need to be retained until the last block is transmitted, as subsequent blocks use the previous ciphertext block as their IV.

DIt can be used to encrypt and decrypt information.

The IV is not a key and cannot be used to encrypt or decrypt information on its own; encryption and decryption require the secret DES key, not the IV.

Concept tested: DES initialization vector properties and transmission

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

Topics

#cryptography#initialization vector#block ciphers#DES

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