Generic Attacks

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1 Preimage attacks

The resistance of a hash function to collision and (second) preimage attacks depends in the first place on the length n of the hash value. Regardless of how a hash function is designed, an adversary will always be able to find preimages or second preimages after trying out about 2n different messages. In case an adversary is given 2k distinct target hashes, preimages can be found after trying about 2n-k different messages. This is first described in the thesis of Merkle, pages 12-13.

Ralf C. Merkle - Secrecy, authentication, and public key systems
Phd Thesis, , 1979
[Bibtex]
Author : Ralf C. Merkle
Title : Secrecy, authentication, and public key systems
In : Phd Thesis, -

2 Collision attacks

As independently observed by Merkle and Yuval in 1979, finding collisions requires a much smaller number of trials: about 2n/2, as described subsequently. As a result, hash functions producing less than 160 bits of output are currently considered inherently insecure. Moreover, if the internal structure of a particular hash function allows collisions or preimages to be found more efficiently than what could be expected based on its hash length, then the function is considered to be broken.

Ralph C. Merkle - A Certified Digital Signature
In Proceedings of CRYPTO, LNCS 435, pp. 218-238, Springer, 1989
[Electronic Edition] [Bibtex]
Author : Ralph C. Merkle
Title : A Certified Digital Signature
In : In Proceedings of CRYPTO -

Gideon Yuval - How to Swindle Rabin
Cryptologia3:187-189, 1979
[Bibtex]
Author : Gideon Yuval
Title : How to Swindle Rabin
In : Cryptologia -

2.1 The birthday attack

The birthday attack (also calledsquare-root attack) is a generic attack which considers a hash function as black box. Therefore, a birthday attack is successful for every hash function. For any message m we can compute the n-bit hash value y = h(m). Since at least a fraction 2-n of the pairs (m,m*) satisfies h(m) = h(m*), one can expect to find a colliding message pair after trying about 2n, arbitrary message pairs. Nevertheless, it follows from the birthday paradox that one can check 2n pairs with only 2n/2 evaluations of h. A birthday attack works as follows:

  • Pick any message m and compute h(m).
  • Update list L. Check if h(m) is in the list L.
    • if (h(m),m) is already in L, a colliding message pair has been found.
    • else save the pair (h(m),m) in the list L and go back to step 1.

From the birthday paradox we know that we can expect to find a matching entry, after performing about 2n/2 hash evaluations. Note that in a birthday attack an attacker has full control over the messages. Hence, as pointed out by Yuval, this method enables an attacker to construct meaningful collisions.


The simple algorithm discussed above has memory requirements O(2n / 2). Pollard's rho-method, based on distinguished points, can be used to greatly reduce memory requirements at little extra cost. Also, various cycle finding algorithms are of use for collision search. In practice, generic collision search might not run on a single processor, but rather in parallel on many processors. Van Oorschot and Wiener discuss a method to achieve linear runtime gains.

Paul C. van Oorschot, Michael J. Wiener - Parallel Collision Search with Cryptanalytic Applications
J. Cryptology12(1):1-28, 1999
[Electronic Edition] [Bibtex]
Author : Paul C. van Oorschot, Michael J. Wiener
Title : Parallel Collision Search with Cryptanalytic Applications
In : J. Cryptology -
[Abstract]

Michael J. Wiener - The Full Cost of Cryptanalytic Attacks
J. Cryptology17(2):105-124, 2004
[Electronic Edition] [Bibtex]
Author : Michael J. Wiener
Title : The Full Cost of Cryptanalytic Attacks
In : J. Cryptology -
[Abstract]

3 Attacks in the quantum setting

Results of quantum complexity theorists as well as newly invented algorithms suggest that even with the power of hypothetic quantum computers applied against commonly used hash functions, no exponential improvement over classical computers is possible. Here we briefly discuss hash function related aspects of this work.

One of the celebrated results in quantum algorithms is Grover's from 1996: The search for a particular element in an unordered database of size r takes at most O(r1/2), an actual algorithm is provided. Matching lower bounds exist for this problem as well (see Boyer et al. and Zalka). This algorithm is of wide interest as it does not rely on a particular structure of the elements in the search space.

This result has already direct implications on hash functions: There, the search for a preimage or a second preimage is at most as hard as a search in an unordered database, hence security against these types of generic attacks is lowered from 2n to 2n/2 in the quantum setting.

How about collision attacks in the quantum setting? Here, the fact that many collisions exist and the problem is to find a single one indeed leads to (both asymptotically and for commonly used finite dimensions) faster algorithms. An actual quantum algorithm for the collision problem is due to Brassard, H{\o}yer, and Tapp from 1997. This combination of Grover's algorithm with the birthday effect yields a runtime of 2n/3 for a hash function with n bit output size. The algorithm requires Θ(n1 / 3logn) classical bits of memory. To the best of the author's knowledge, no time/memory tradeoffs are known. Is this the best one can do? Nontrivial lower bounds for the collision problem were an open problem for some time. Only in 2001, Aaronson proved a query complexity of Ω(n1 / 5). Subsequent work of Shi improved this bound to Ω(n1 / 4) and finally to Ω(n1 / 3). As this constitutes a tight bound, indeed one can not do better than Brassard et al. That is, given our current axiomatic assumptions about the nature of quantum mechanics.

The bottom line here is as follows. Ignoring practical problems with implementations of large, stable quantum computers, and still requiring a (what now became standard) 128-bit security level gives rise to the following minimal output sizes for hash functions. If only oneway-ness but not collision resistance is required then 256-bits would be enough, for collision resistance at least 384 bits are needed. Of course, these blackbox results might not be the end of the story, dedicated quantum cryptanalytic techniques have not been considered yet. Fast quantum algorithms to compute median and mean values by Grover, and other basic building blocks seem to be an interesting starting point.

Lov K. Grover - A Fast Quantum Mechanical Algorithm for Database Search
In Proceedings of Proceedings of the Twenty-Eighth Annual ACM Symposium on the Theory of Computing (STOC), Philadelphia, Pennsylvania, USA, May 22-24, 1996, , pp. 212--219, ACM, 1996
[Electronic Edition] [Bibtex]
Author : Lov K. Grover
Title : A Fast Quantum Mechanical Algorithm for Database Search
In : In Proceedings of Proceedings of the Twenty-Eighth Annual ACM Symposium on the Theory of Computing (STOC), Philadelphia, Pennsylvania, USA, May 22-24, 1996 -
[Abstract]

Michel Boyer, Gilles Brassard, Peter H\oyer, Alain Tapp - Tight bounds on quantum searching
Fortschritte der Physik46(PP-1996-11):493-505, 30,, 1998
[Electronic Edition] [Bibtex]
Author : Michel Boyer, Gilles Brassard, Peter H\oyer, Alain Tapp
Title : Tight bounds on quantum searching
In : Fortschritte der Physik -
[Abstract]

Christof Zalka - Grover’s quantum searching algorithm is optimal
Physical Review A60(4):2746-–2751, 1999
[Electronic Edition] [Bibtex]
Author : Christof Zalka
Title : Grover’s quantum searching algorithm is optimal
In : Physical Review A -
[Abstract]

Gilles Brassard, Peter H\oyer, Alain Tapp - Quantum Cryptanalysis of Hash and Claw-Free Functions
In Proceedings of LATIN '98: Theoretical Informatics, Third Latin American Symposium, Campinas, Brazil, April, 20-24, 1998, Proceedings, LNCS 1380, pp. 163--169, Springer, 1998
[Electronic Edition] [Bibtex]
Author : Gilles Brassard, Peter H\oyer, Alain Tapp
Title : Quantum Cryptanalysis of Hash and Claw-Free Functions
In : In Proceedings of LATIN '98: Theoretical Informatics, Third Latin American Symposium, Campinas, Brazil, April, 20-24, 1998, Proceedings -
[Abstract]

Scott Aaronson - Quantum lower bound for the collision problem
In Proceedings of Proceedings on 34th Annual ACM Symposium on Theory of Computing (STOC), May 19-21, 2002, Montr{\'e}al, Qu{\'e}bec, Canada, , pp. 635-642, ACM, 2002
[Electronic Edition] [Bibtex]
Author : Scott Aaronson
Title : Quantum lower bound for the collision problem
In : In Proceedings of Proceedings on 34th Annual ACM Symposium on Theory of Computing (STOC), May 19-21, 2002, Montr{\'e}al, Qu{\'e}bec, Canada -
[Abstract]

Yaoyun Shi - Quantum Lower Bounds for the Collision and the Element Distinctness Problems
In Proceedings of 43rd Symposium on Foundations of Computer Science (FOCS 2002), 16-19 November 2002, Vancouver, BC, Canada, Proceedings, , pp. 513-519, IEEE Computer Society, 2002
[Electronic Edition] [Bibtex]
Author : Yaoyun Shi
Title : Quantum Lower Bounds for the Collision and the Element Distinctness Problems
In : In Proceedings of 43rd Symposium on Foundations of Computer Science (FOCS 2002), 16-19 November 2002, Vancouver, BC, Canada, Proceedings -
[Abstract]

Scott Aaronson, Yaoyun Shi - Quantum lower bounds for the collision and the element distinctness problems
J. ACM51(4):595-605, 2004
[Electronic Edition] [Bibtex]
Author : Scott Aaronson, Yaoyun Shi
Title : Quantum lower bounds for the collision and the element distinctness problems
In : J. ACM -
[Abstract]

Lov K. Grover - A Framework for Fast Quantum Mechanical Algorithms
In Proceedings of Proceedings of the Thirtieth Annual ACM Symposium on the Theory of Computing (STOC), Dallas, Texas, USA, May 23-26, 1998, , pp. 53--62, ACM, 1998
[Electronic Edition] [Bibtex]
Author : Lov K. Grover
Title : A Framework for Fast Quantum Mechanical Algorithms
In : In Proceedings of Proceedings of the Thirtieth Annual ACM Symposium on the Theory of Computing (STOC), Dallas, Texas, USA, May 23-26, 1998 -
[Abstract]

Lov K. Grover - Rapid sampling though quantum computing
In Proceedings of Proceedings of the Thirty-Second Annual ACM Symposium on Theory of Computing (STOC), May 21-23, 2000, Portland, OR, USA, , pp. 618-626, ACM, 2000
[Electronic Edition] [Bibtex]
Author : Lov K. Grover
Title : Rapid sampling though quantum computing
In : In Proceedings of Proceedings of the Thirty-Second Annual ACM Symposium on Theory of Computing (STOC), May 21-23, 2000, Portland, OR, USA -
[Abstract]
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