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A cryptographic hash function is

[a] function that maps a bit string of arbitrary length to a fixed length bit string. Approved hash functions satisfy the following properties:
  1. (One-way) It is computationally infeasible to find any input which maps to any pre-specified output, and
  2. (Collision resistant) It is computationally infeasible to find any two distinct inputs that map to the same output.[1]

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