SSCP Systems Security Certified PractitionerCryptographyHard
A cryptanalyst is attempting to break a new block cipher. They have observed that when the same plaintext block is encrypted multiple times with the same key, it always produces the same ciphertext block. This observation indicates a lack of which important cryptographic property that helps obscure patterns in encrypted data?
- ADiffusion
- BAvalanche effect
- CInitialization Vector (IV) usage
- DConfusion
Show answer & explanationAnswer & explanation
Correct answer: C. Initialization Vector (IV) usage
If the same plaintext block always produces the same ciphertext block with the same key, it indicates that the cipher is not using an Initialization Vector (IV) or nonce. An IV ensures that even identical plaintext blocks produce different ciphertext blocks, preventing pattern analysis and enhancing semantic security.
Why the other options are wrong
- A. Diffusion spreads the influence of a single plaintext bit over many ciphertext bits. This is a property of the cipher's internal operation, but the issue described (identical plaintext yields identical ciphertext) specifically points to the absence of an IV.
- B. The avalanche effect is a strong form of diffusion and confusion where a small change in plaintext or key results in a significant change in ciphertext. The observed issue is more fundamental than a weak avalanche effect.
- D. Confusion aims to make the relationship between the key and ciphertext as complex as possible. While related to obscuring patterns, the direct cause described is not a lack of confusion itself.
Initialization Vector (IV)
A non-secret, random or pseudo-random number used with a secret key in data encryption to ensure that identical plaintexts produce different ciphertexts.
- Crucial for modes of operation like CBC, CTR.
- Must be unique for each encryption with the same key.
- Prevents pattern detection and replay attacks.
Memory trick: IVs are like fresh paint for each block: same house, different look every time.