Random oracle

A random oracle is perfect cryptographic hash function, although not used in practice it is often used to build a random oracle model in theoretical papers on cryptography to prove the security of a cryptographic system.

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.