# Quantum-resistant vs quantum-safe vs post-quantum

> The terms get used interchangeably and loosely. Here is what each one actually means, and which one you can verify.

"Quantum-resistant," "quantum-safe," "quantum-proof," "post-quantum" — crypto marketing uses these as if they're interchangeable, and mostly they are used that way. But only one of them points to something you can actually verify. Getting the vocabulary straight is the fastest way to see through a vague claim, because once you know which words mean something specific, you notice immediately when a project is hiding behind the ones that don't.

## The four terms, ranked by how much they mean

| Term | What it means | Verifiable? |
|---|---|---|
| **Post-quantum** | A specific *class of cryptography* not broken by Shor's algorithm | ✅ Yes — names actual schemes |
| **Quantum-resistant** | Marketing adjective for "uses post-quantum crypto" | ⚠️ Only if tied to a scheme |
| **Quantum-safe** | Same as quantum-resistant; no separate meaning | ⚠️ Only if tied to a scheme |
| **Quantum-proof** | Overclaims — implies proven-unbreakable | ❌ Red flag |

### Post-quantum — the one that means something

**Post-quantum cryptography (PQC)** is a technical category, not a slogan. It refers to algorithms whose security does *not* depend on the two problems [Shor's algorithm](/learn/glossary#shor) solves efficiently — integer factoring and the discrete logarithm. That category has named members you can look up:

- **Hash-based** — XMSS, LMS, SLH-DSA (SPHINCS+)
- **Lattice-based** — ML-DSA (Dilithium), ML-KEM (Kyber), FALCON
- **Code-based** — Classic McEliece
- **Multivariate** — various

When someone says "post-quantum," you can ask *which one* — and there's a real answer. That's what makes it verifiable. See [the NIST post-quantum standards](/learn/nist-post-quantum-standards) for the schemes now formally standardized.

### Quantum-resistant and quantum-safe — the interchangeable adjectives

These two are the same thing. Neither has a formal definition; both are meant to signal "won't be broken by a quantum computer." They're fine words *when backed by a named post-quantum scheme* — "quantum-resistant, using ML-DSA-87" is a real statement. On their own, floating free of any scheme, they're just adjectives, and adjectives are exactly what the misleading projects rely on.

The tell: a genuine project uses "quantum-resistant" as a *label on top of* a disclosed scheme. A marketing-only project uses it *instead of* disclosing one.

### Quantum-proof — the overclaim

Avoid trusting anything marketed as "quantum-proof." "Proof" implies certainty that the scheme can *never* be broken, which no cryptographer can honestly promise. Post-quantum schemes are trusted because they've withstood years of public attack, not because breakage has been mathematically ruled out. A project reaching for "quantum-proof" is either careless with language or overselling — either way, a reason to look harder.

## Why the vocabulary is a filter

Here's the practical value of getting this straight. The moment you separate the *verifiable* term (post-quantum → names a scheme) from the *marketing* terms (quantum-safe/-proof → name nothing), a vague claim exposes itself. Ask one question:

> "Which post-quantum signature scheme do you use to sign transactions?"

A real project answers instantly and specifically — "ML-DSA-87, NIST FIPS 204." A marketing claim deflects, points to a roadmap, or repeats the adjective. The vocabulary doesn't just describe the technology; it separates projects that have it from projects that only say it.

## The bottom line

Don't grade a chain on which adjective it chose — grade it on whether it discloses a specific, standardized, live post-quantum scheme protecting transactions. QRL, for example, doesn't ask you to trust the word "quantum-resistant"; it names [XMSS](/learn/xmss-explained) on Legacy and [ML-DSA-87](/learn/ml-dsa-87-explained) on QRL 2.0, both checkable against public standards. To turn this into a repeatable test for any project, use the [quantum-resistance checklist](/learn/quantum-resistance-checklist).