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A long time ago, I read that the definition of "cryptographic hash function" is "collision-resistant one-way function". (A similar definition shows up in the FIPS standards for SHA-1 etc.)

But this is not a sufficient property for many of the modern uses of cryptographic hash functions, like Bitcoin. What they really want is for hash functions to act like a "random oracle".

Of course, no deterministic function can be random.

I am taking Dan Boneh's Coursera cryptography class. Most cryptographic primitives we cover assume some sort of secret, and then the definition of "secure" involves probabilities over the space of possible secrets. For example, a secure RNG has a secret seed, and if I (a) pick a seed at random and generate a long pseudo-random string, and (b) pick a long truly random string, then no efficient algorithm should be able to tell them apart with noticeably better than even odds. (With those odds computed over the space of keys and truly random strings.)

In the intro lecture to hash functions (see here or here if you do not want to register), Boneh gives the following definition of "collision resistant" hash function:

A function $H$ is collision resistant if for all (explicit) "effective" algorithms $A$, $$\mathrm{ADV}_\mathrm{CR}[A,H] = \mathrm{Pr}[A\ \mathrm{outputs\ a\ collision\ for\ } H]$$ is "negligible".

Now, obviously, a very simple algorithm exists to output a collision for, say, SHA-256; we just do not know what that algorithm is. And there is no indication in this definition of the space over which we are calculating the probability.

So my question is twofold:

  1. What is the formal definition of "explicit" algorithm? (I assume this is meant to rule out algorithms that we know exist but cannot explicitly describe.)

  2. Over what space is the probability being computed in Boneh's definition?

To clarify: I am aware of the dictionary definition of "explicit". I am asking for a formal definition. I know how to translate a statement like "There exists an algorithm such that..." into (say) the language of ZF set theory, at least in principle. I do not know how to translate "...but nobody knows what it is" into any formal language at all. So I am wondering how a mathematician would think about this sort of definition.

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(1) Quoting a dictionary: stated clearly and in detail, leaving no room for confusion or doubt. This is subtle, but we don't worry about adversaries that exist in theory, we worry about adversaries we are at least somewhat likely to run into. (2) The adversary's random tape. –  K.G. Oct 6 '13 at 20:36
eprint.iacr.org/2006/281.ps $\;$ –  Ricky Demer Oct 6 '13 at 20:52
Yep, @RickyDemer's citation pretty handily explains your confusion, I believe, within the first few pages. In short, the issue is that a formal treatment of hash functions typically involves keying them; without doing so, you run into the problem your question describes. –  Reid Oct 6 '13 at 21:29
@RickyDemer: Yes, that appears to be exactly what I am looking for (see update to question). If you can summarize it in an answer I will accept it; otherwise I will take a crack at doing so myself when I can find some time. Thanks. –  Nemo Oct 7 '13 at 15:07
@user11206 print X ; print X0 is very short and very fast for any (short) X and X0. We know such a program exists; we just do not know how to find it efficiently. This is not just mincing words, if you care about precise (i.e. formal) reasoning... I am still not convinced I have ever seen a formal definition of "secure hash function". –  Nemo Jan 3 at 5:59
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1 Answer

What Dan Boneh says is not a formal definition as you want it. Let me quote Rogaway on this:

In cryptographic practice, a collision-resistant hash-function (also called a collision-free or collision-intractable hash-function) maps arbitrary-length strings to fixed-length ones; it’s an algorithm $H:\{0,1\}^*\rightarrow \{0,1\}^n$ for some fixed $n$ (we momentarily assume a message space of $\{0,1\}^*$. But in cryptographic theory, a collision-resistant hash-function is always keyed; now $H:\,\mathcal{K}\times\{0,1\}^*\rightarrow \{0,1\}^n$, where each $K\in\mathcal{K}$ names a function $H_K = H(K,\cdot)$ (This formalization assumes a concrete-security framework.) In this case $H$ can be thought of as a collection or family of hash functions $\mathcal{H} = \{H_K:\:K\in\mathcal{K}\}$, each key (or index) $K\in\mathcal{K}$ naming one. (Note that we call $K$ a key but it is not secret; it will be chosen from $\mathcal{K}$ and then made public.)

Why should theoretical treatments be keyed when practical constructions are not? The traditional answer is that a rigorous treatment of collision resistance for unkeyed hash-functions just doesn’t work. At issue is the fact that for any function $H:\{0,1\}^*\rightarrow \{0,1\}^n$ there is always a simple and compact algorithm that outputs a collision: the algorithm that has one “hardwired in.” That is, by the pigeonhole principle there must be distinct strings $X$ and $X_0$ of length at most $n$ such that $H(X) = H(X_0)$, and so there’s a short and fast program that outputs such an $(X, X_0)$. The difficulty, of course, is that us human beings might not know any such pair $(X, X_0)$, so no one can actually write the program down.

Because of the above, what is meant when someone says that a hash function $H:\{0,1\}^*\rightarrow \{0,1\}^n$ is collision resistant cannot be that there is no efficient adversary that outputs a collision in $H$. What is meant is that there is no efficient algorithm known to man that outputs a collision in $H$. But such a statement would seem to be unformalizable — outside the realm of mathematics. One cannot hope to construct a meaningful theory based on what humankind currently does or does not know. Regarding a hash function like SHA-1 as a random element from a family of hash functions has been the traditional way out of this quandary.

Rogaway then demonstrates that even though it is not possible to formally define the collision resistance, it is possible to formally define the requirement for a collision resistant hash function by a higher-level protocol or a construction. This is done by a reduction that shows that if someone can break the protocol in a certain way, then he is able to find a collision for the underlying hash function. This allows to consider the protocol secure as long as collisions have not been explicitly found.

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