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On what does the strength of a hash function depend?

Hash Function

  • Strength depends on the length, n, in bits of the hash value
  • Brute force attacks require time proportional to:
  • one-way property – 2 n
  • weak collisions property – 2 n
  • strong collisions property – 2 n/2

    • Example

    Note message can be any size. H is the Hash Function and h is the Hash Value which is a fixed size (e.g. 160 bits)
  • If hash h is 8 bits, n=8
  • 2 8 = 256 combinations)
  • 2 8/2 = 2 4 = 16
  • To break this by brute force using the birthday attack you would need to guess only 16 values to get a 50% chance of being correct.