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Random oracle

As explained in Real-World Cryptography

The random oracle model is an idealized security model that treats a hash-like function as a random oracle when analyzing protocols.

A random oracle is an idealized, fictional participant used to analyze protocols when proving security directly from real hash-function properties is difficult. It accepts any input, returns an apparently completely random output, and returns the same output whenever that input is repeated. section 2.2

The model assumes three practical behaviors:

  • Queries produce unpredictable outputs, as though they came from a random source. section 2.2
  • Repeated queries with identical inputs produce identical outputs. section 2.2
  • Security proofs may treat a real hash function as this ideal construction, although replacing the oracle with a real function is not guaranteed to preserve the proof. section 2.2

SHA-3 as an approximation

SHA-3 uses a sponge construction whose permutation is intended to look sufficiently random. Its absorbing and squeezing process therefore behaves closely to a random oracle, supporting the hash-function security properties expected from the model. section 2.5.2

How Real-World Cryptography covers it

How deep it goes
Defines query behavior and practical limitations, then connects the model to SHA-3 without developing a formal proof.
Examples
Uses protocol security proofs and SHA-3’s absorbing-and-squeezing sponge construction as concrete examples.
What it assumes
Assumes familiarity with hash-function inputs and digests while distinguishing collision resistance from related properties.
What you can do after
Evaluate security claims that idealize hash functions and recognize the uncertainty introduced when real functions replace the oracle.
How it connects
Connects hash functions, collision resistance, sponge constructions, permutations, and SHA-3.
Cover of Real-World Cryptography
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Related concepts

This explainer was generated by Manning's knowledge graph from the text of Real-World Cryptography. The section links point to the parts of the book it draws on.