Paper 2005/281

Herding Hash Functions and the Nostradamus Attack

John Kelsey and Tadayoshi Kohno


In this paper, we develop a new attack on Damgård-Merkle hash functions, called the \emph{herding attack}, in which an attacker who can find many collisions on the hash function by brute force can first provide the hash of a message, and later ``herd'' any given starting part of a message to that hash value by the choice of an appropriate suffix. We introduce a new property which hash functions should have--Chosen Target Forced Prefix (CTFP) preimage resistance--and show the distinction between Damgård-Merkle construction hashes and random oracles with respect to this property. We describe a number of ways that violation of this property can be used in arguably practical attacks on real-world applications of hash functions. An important lesson from these results is that hash functions susceptible to collision-finding attacks, especially brute-force collision-finding attacks, cannot in general be used to prove knowledge of a secret value

Available format(s)
Secret-key cryptography
Publication info
Published elsewhere. Unknown where it was published
hash functionsdigital timestampingcollision resistanceDamgaard-Merkle
Contact author(s)
john kelsey @ nist gov
2006-02-18: revised
2005-08-25: received
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Creative Commons Attribution


      author = {John Kelsey and Tadayoshi Kohno},
      title = {Herding Hash Functions and the Nostradamus Attack},
      howpublished = {Cryptology ePrint Archive, Paper 2005/281},
      year = {2005},
      note = {\url{}},
      url = {}
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