Which requirement is not necessary for a cryptographic hash function?

Question

Options

• AThe function must have a low probability of producing the same output for different inputs
• BThe function must be efficient to compute
• CThe function must work on a variety of input sizes
• DThe function must be one way

• A
  8% (4)
• B
  2% (1)
• C
  85% (41)
• D
  4% (2)

Explanation

The formal cryptographic requirements for a hash function are: (1) one-way/pre-image resistance - it must be computationally infeasible to reverse; (2) collision resistance - it must have a low probability of producing the same output for different inputs; and (3) efficiency - it must be fast to compute. While hash functions do in practice accept variable-length inputs and produce fixed-length outputs, 'must work on a variety of input sizes' is not listed as a formal cryptographic security requirement. The other three options describe core, formally-defined properties that every secure hash function must satisfy.

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