OPAQUE

As explained in Real-World Cryptography

OPAQUE is an asymmetric password-authenticated key exchange protocol that uses an oblivious pseudorandom function to protect a password-derived encrypted key record.

Passwords are convenient but often weak, reused, and exposed by database breaches. OPAQUE lets a user authenticate to a known server without revealing the real password, using an asymmetric password-authenticated key exchange. section 11.4

The name comes from oblivious PAKE: an oblivious pseudorandom function (OPRF) helps turn the password into a strong symmetric key while keeping the password input hidden from the party holding the OPRF key. section 11.2.2

Figure 11.4 An oblivious PRF (OPRF) is a construction that allows one party to compute a PRF over the input of another party without learning that input. To do this, Alice first generates a random blinding factor, then blinds her input with that before sending it to Bob. Bob uses his secret key to compute the PRF over the blinded value, then sends the blinded output to Alice who can unblind it. The result does not depend on the value of the blinding factor.

Follow the OPRF construction: one participant blinds an input, the other applies a keyed pseudorandom function without learning that input, and the first participant unblinds the result. section 11.2.2

How Real-World Cryptography covers it

How deep it goes

Explains OPRF blinding and the registration/login flow without supplying implementation-level details.

Examples

Uses Alice’s registration and login flow to illustrate key generation, encrypted key storage, and password-based recovery.

What it assumes

Assumes familiarity with PRFs while introducing OPRFs through blinding, keyed evaluation, and unblinding.

What you can do after

Evaluate password authentication designs that avoid storing or revealing users’ real passwords to servers.

How it connects

Connects OPAQUE with OPRFs, asymmetric PAKEs, CPace, SRP, SSO, and TLS 1.3.

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Related concepts

• oblivious pseudorandom functionuses · An oblivious pseudorandom function is a two-party protocol that lets one party obtain a pseudorandom-function output on a secret input without revealing the input to the key holder or the key to the input holder.
• asymmetric password-authenticated key exchangekind of · An asymmetric password-authenticated key exchange is a password-authenticated key exchange in which the server does not learn the user's password and typically stores a verifier derived from it.

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.