The following about hash is true except:

Question

BlackTom AI · Accepted Answer

Hash functions are designed to be one-way: given an input of arbitrary length they produce a fixed-size output (the hash) and are intended to make recovering the original input from the hash computationally infeasible.

1) Relevant concepts:
- Determinism: the same input always yields the same hash.
- Fixed-size output: any-length input maps to a fixed-length digest.
- Preimage resistance: it should be infeasible to compute the original input from its hash.
- Collision resistance and avalanche effect: small input changes produce very different hashes; good hashes behave like pseudorandom functions.

2) Step-by-step reasoning:
- Option a: True. There are many hash algorithms (examples: MD5, SHA-1, SHA-256, SHA-3, BLAKE2, etc.), so multiple hash functions exist.
- Option c: True. Hash functions differ in quality — some have better randomness and security properties (cryptographic hashes) while others are weaker or unsuitable for security.
- Option d: True. A receiver can recompute the hash of received data and compare it to a transmitted hash to verify integrity; matching hashes strongly indicate the message was not altered (assuming secure hash and safe distribution of the expected hash).
- Option b: Claims you can recover original data from a hash via a formula. This contradicts preimage resistance: cryptographic hashes are intentionally non-invertible. In practice one can try brute-force or lookups for small input spaces, but there is no direct formula to recover arbitrary original data from its hash.

3) Calculations: not applicable; the argument is conceptual and relies on definitions of hash properties.

4) Why b is correct as the exception: Because the defining security property of cryptographic hash functions is that you cannot derive the original input from the hash output. Therefore statement b is false.

5) Other options are valid statements about hashes (explained above).

Conclusion: The false statement (the exception) is b. This confirms the provided answer: b.

The following about hash is true except:单项选择题

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