CISSP practice question 2 of 35

What principle requires that changes to the plaintext affect many parts of the ciphertext?

Full set: 1,485 questions

Domain Security Architecture and Engineering

Answer options

A: Diffusion Correct answer

B: Encapsulation

C: Obfuscation

D: Permutation

Correct Answer:

A. Diffusion

Explanation

Diffusion is a fundamental cryptographic principle introduced by Claude Shannon. Its purpose is to obscure the statistical relationship between the plaintext and the ciphertext. This is achieved by ensuring that a single change in a plaintext character or bit results in changes to many parts of the resulting ciphertext. This property, often called the avalanche effect, makes it significantly harder for a cryptanalyst to deduce the key or plaintext by observing patterns, as the influence of each plaintext symbol is spread widely across the ciphertext. Modern block ciphers like AES use operations such as permutation and mixing to achieve strong diffusion.

References

1. Shannon, C. E. (1949). Communication Theory of Secrecy Systems. Bell System Technical Journal, 28(4), 656-715. In Section 5, "Theoretical Secrecy," Shannon introduces diffusion as a method where "the statistical structure of [the message]... is 'dissipated' into long range statistics." (p. 662). https://doi.org/10.1002/j.1538-7305.1949.tb00928.x

2. Stallings, W. (2017). Cryptography and Network Security: Principles and Practice (7th ed.). Pearson. In Chapter 3, Section 3.2, "Feistel Cipher Design Principles," diffusion is defined as the property where "the statistical structure of the plaintext is dissipated into long-range statistics of the ciphertext... by having each plaintext digit affect the value of many ciphertext digits." (p. 71).

3. National Institute of Standards and Technology (NIST). (2001). FIPS PUB 197: Advanced Encryption Standard (AES). Section 5.1.3, "The MixColumns() Transformation," describes a key step in AES that provides diffusion within the cipher, ensuring that the bits from one column are spread to all four columns in the next state.

Question breakdown

Tests: Apply cryptographic design principles (confusion and diffusion)
Difficulty: Easy

Key idea: Diffusion spreads the influence of each plaintext bit across many ciphertext bits, which hides statistical patterns.

How to reach the answer

The clue is 'changes to the plaintext affect many parts of the ciphertext'.
Shannon's two principles are confusion, which obscures the key-to-ciphertext relationship, and diffusion, which spreads plaintext influence.
Spreading plaintext influence is diffusion, and the resulting behavior is called the avalanche effect.
Permutation is one technique for achieving diffusion, not the principle itself.

Each option

ACorrect. Diffusion is the principle that each plaintext change ripples across many ciphertext bits.
BIncorrect. Encapsulation is a networking and object-oriented concept, not a cipher design principle.
CIncorrect. Obfuscation means making code or data hard to understand, not a defined Shannon property of ciphers.
DIncorrect. Permutation (transposition) is a mechanism used to achieve diffusion, but it is not the named principle.

Common trap: Choosing permutation because it is the operation that produces diffusion in ciphers like AES.

In practice: When assessing custom or legacy ciphers, weak diffusion shows up as ciphertext patterns that leak plaintext structure, as in ECB-mode images.

Remember: Diffusion spreads plaintext influence across the ciphertext; confusion hides the key relationship.

Official documentation: CISSP Certification Exam Outline

Drafted with AI assistance and checked against the answer key.

Keep practicing

Take the free CISSP practice test

Ten exam-style questions with answers and explanations, plus the exam facts and study guides.

Start the free practice test