Hybrid encryption: why combine classical and post-quantum cryptography

When a new cryptographic algorithm appears, a tension shows up: classical algorithms such as X25519 or Ed25519 have resisted attacks for years, but are vulnerable to a future quantum computer; post-quantum ones such as ML-KEM or ML-DSA resist quantum attacks, but are newer and less tested. Hybrid encryption resolves the tension: use both at once . The idea in one sentence Combine a classical and a post-quantum algorithm so that the system only breaks if both fail simultaneously . A classical attacker would have to break the post-quantum algorithm; a quantum attacker would have to break the classical one and the post-quantum one. You gain security against the future without betting everything on a young algorithm. Two places to apply it Key exchange (encrypting for a recipient). You combine: X25519 — classical key exchange, fast and heavily tested. ML-KEM-1024 — NIST's post-quantum key encapsulation mechanism, at its highest level. The two resulting keys are mixed with a context-bound derivation function (HKDF), so that neither one alone is enough. Digital signatures (authenticity). You combine: Ed25519 — classical signature. ML-DSA-87 — NIST post-quantum signature. The message is accepted only if both signatures verify — an AND combiner. One principle that never breaks There is a golden rule in cryptography, Kerckhoffs's principle : a system must be secure even if the attacker knows its entire design; security lives in the key , not in hiding the format. A good hybrid system uses public, audited primitives — XChaCha20-Poly1305 to encrypt, Argon2id to derive keys from passwords, HKDF to separate domains — and never invents its own cryptography . How Quipu applies it Quipu is a free library implementing exactly this approach for data at rest : hybrid X25519 + ML-KEM-1024 encryption, hybrid Ed25519 + ML-DSA-87 signatures, and only verified primitives underneath. It targets NIST security level 5 (CNSA 2.0) and is open source, so anyone can review how it works. An honest