Random oracle

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A random oracle is a mathematical abstraction used in cryptographic proofs. Random oracles are typically included in proofs when no "real" function (that can be implemented) provides sufficient mathematical properties to satisfy the proof of security. Proofs which make use of random oracles are referred to as secure in the "random oracle model", as opposed to the "standard model". In practice, random oracles are typically used to model cryptographic hash functions in schemes where strong randomness assumptions are needed of the hash function's output. Such proofs indicate that systems or protocols are secure by showing that an attacker must require impossible behavior from the oracle, or solve some other mathematical problem believed hard, in order to break the protocol. Not all uses of cryptographic hash functions require random oracles: schemes which require only the property of collision resistance can be proven secure in the standard model (e.g., the Cramer-Shoup cryptosystem).

When a random oracle is given a query x it does the following:

If the oracle has been given the query x before, it responds with the same value it gave the last time.
If the oracle hasn't been given the query x before, it generates a random response which has uniform probability of being chosen from anywhere in the oracle's output domain.

In the more precise definition formalized by Bellare/Rogaway (1993), the random oracle produces a bit-string of infinite length which can be truncated to the length desired. When a random oracle is used within a security proof, it is made available to all players, including the adversary or adversaries. A single oracle may be treated as multiple oracles by pre-pending a fixed bit-string to the beginning of each query (e.g., queries formatted as "1|x" or "0|x" can be considered as calls to two separate random oracles).

No real function can implement a true random oracle. In fact, certain very artificial protocols have been constructed which are proven secure in the random oracle model, but which are trivially insecure when any real hash function is substituted for the random oracle. Nonetheless, for any more natural protocol a proof of security in the random oracle model gives very strong evidence that an attack which does not break the other assumptions of the proof, if any (such as the hardness of integer factorization) must discover some unknown and undesirable property of the hash function used in the protocol to work. Many schemes have been proven secure in the random oracle model, for example OAEP and PSS.

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References

Mihir Bellare and Phillip Rogaway, Random Oracles are Practical: A Paradigm for Designing Efficient Protocols, ACM Conference on Computer and Communications Security 1993, pp62–73 (PS and PDF).
Ran Canetti, Oded Goldreich and Shai Halevi, The Random Oracle Methodology Revisited, STOC 1998, pp209–218 [1].

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