Question at position 4 Which of the following is NOT a property of secure hash functions?Fixed-size outputEfficient computationOne-way behaviorBuilt-in decryption Clear my selection单项选择题
A
Fixed-size output
B
Efficient computation
C
One-way behavior
D
Built-in decryption

Question

Question at position 4 Which of the following is NOT a property of secure hash functions?Fixed-size outputEfficient computationOne-way behaviorBuilt-in decryption Clear my selection

BlackTom AI · Accepted Answer

Secure hash functions produce a fixed-size digest, are fast to compute, and are intentionally one-way (preimage resistant); they do not support decryption or reversal.

1) Relevant knowledge points and concepts
- Fixed-size output: Hash algorithms (e.g., SHA-256) always produce a digest of the same length (256 bits for SHA-256) regardless of input length.  
- Efficient computation: Hash functions must be computable quickly for practical use.  
- One-way behavior (preimage resistance): Given the digest, it should be computationally infeasible to recover the original input. Related properties include second-preimage resistance and collision resistance.  
- Decryption vs. hashing: Decryption is the reversible process associated with encryption algorithms that use keys; hashing is non-reversible and does not involve decryption.

2) Step-by-step reasoning
- Examine each option against the definitions above.
  - Fixed-size output: Matches the hash definition—true property.
  - Efficient computation: Hash functions must be fast for signing, verification, and integrity checks—true property.
  - One-way behavior: Security of hashes depends on being infeasible to invert—true property.
  - Built-in decryption: Decryption implies reversibility and a mechanism (usually a key) to restore plaintext. Hash functions are intentionally irreversible and have no mechanism or key for reversing a digest to the original input.

3) Intermediate points (clarifying why “built-in decryption” is incorrect)
- If a hash had built-in decryption, it would violate one-way behavior and break security guarantees (preimage resistance and collision resistance). Hashes are often used precisely because they cannot be reversed.

4) Why the correct answer is correct
- The three true properties listed are standard, formal characteristics of secure hash functions. "Built-in decryption" contradicts the fundamental one-way property and is therefore not a property of secure hash functions.

5) Why other options are incorrect
- Fixed-size output, Efficient computation, and One-way behavior are all genuine, required attributes of secure hash functions.

Conclusion: The correct choice is "Built-in decryption," which matches the provided answer.

Question at position 4 Which of the following is NOT a property of secure hash functions?Fixed-size outputEfficient computationOne-way behaviorBuilt-in decryption Clear my selection单项选择题

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