Quantum vs Crypto: What Breaks, What Survives
The goal is not to panic. The goal is to understand what is actually at stake.
This is part of a series marking World Quantum Day 2026. Start there for context. Or read on — this piece stands on its own.
The threat model in plain terms
Quantum computers do not break all cryptography. They break specific mathematical problems that classical computers find intractable — problems that happen to underpin most of the public-key infrastructure the internet is built on.
The key algorithm to understand is Shor's algorithm, published in 1994. It can factor large integers and compute discrete logarithms exponentially faster than any known classical algorithm. This is not a theoretical edge case. It is a direct attack on:
- RSA — the most widely deployed public-key encryption standard
- ECC (Elliptic Curve Cryptography) — the basis of HTTPS, SSH keys, and nearly all blockchain protocols
- Diffie-Hellman key exchange — foundational to TLS, VPNs, and secure communications
If a sufficiently powerful quantum computer runs Shor's algorithm against your RSA-2048 key, it breaks it. Not slowly. It breaks it.
What breaks
| Algorithm | Used for | Quantum threat |
|---|---|---|
| RSA-2048 / RSA-4096 | HTTPS, email, code signing | Broken by Shor's algorithm |
| ECDSA / ECDH | TLS, SSH, Bitcoin, Ethereum | Broken by Shor's algorithm |
| Diffie-Hellman | VPNs, secure key exchange | Broken by Shor's algorithm |
| Ed25519 | SSH keys, JWT signing | Broken — also elliptic curve |
The uncomfortable truth: the private key behind your Ethereum wallet, your SSH server, and your HTTPS certificate are all vulnerable to a large-scale quantum computer running Shor's algorithm.
What survives
Not everything is equally exposed. Grover's algorithm is the other relevant quantum attack — it gives a quadratic speedup for brute-force search. This weakens symmetric encryption but does not break it.
| Algorithm | Quantum threat | Recommendation |
|---|---|---|
| AES-256 | Grover halves effective key size to 128-bit | Still safe — just use AES-256, not AES-128 |
| SHA-256 / SHA-3 | Weakened but not broken | Still safe with longer digests |
| Argon2 / bcrypt | Weakened but not broken | Still safe for passwords |
The practical takeaway: symmetric cryptography is quantum-resistant with key size adjustments. Asymmetric public-key cryptography is not.
The post-quantum transition: what NIST decided
In August 2024, NIST finalized the first three post-quantum cryptography standards:
- ML-KEM (formerly CRYSTALS-Kyber) — for key encapsulation / key exchange
- ML-DSA (formerly CRYSTALS-Dilithium) — for digital signatures
- SLH-DSA (formerly SPHINCS+) — hash-based signature scheme
These are based on mathematical problems — lattice problems and hash functions — that are believed to resist both classical and quantum attacks. Cloudflare, Google, and Signal have already started deploying them in parallel with existing standards.
This is the migration path. It is not optional, and it is not fast.
The "harvest now, decrypt later" problem
There is a subtle attack that makes this urgent today even if quantum computers are still years from cracking production keys at scale: HNDL (Harvest Now, Decrypt Later).
State actors and sophisticated adversaries are intercepting and storing encrypted traffic now, betting they will have quantum capability to decrypt it later. Anything with a long confidentiality window — health data, financial records, state secrets, long-lived blockchain transactions — is already at risk under this model.
This is not hypothetical. The NSA recommended transitioning away from ECC and RSA for classified systems by 2030.
What teams should actually do today
Short term (now):
- Audit your cryptographic dependencies. Know which libraries and protocols use RSA or ECC.
- Move to AES-256 over AES-128 for symmetric encryption.
- Watch the NIST post-quantum migration guidance.
Medium term (next 1–2 years):
- Start hybrid deployments: run post-quantum algorithms alongside classical ones. Major TLS libraries support this now.
- Prioritize long-lived data and credentials — these are most exposed.
Web3 specifically:
- Read the next piece in this series. The situation there deserves its own treatment.
The bottom line
Quantum computers are not breaking your systems today. But the cryptographic foundations most of us rely on were not designed with quantum adversaries in mind, and the migration to post-quantum standards takes years.
The teams that start now will be ready. The teams that wait for a confirmed threat will be scrambling.
Intel tracks these developments daily. The post-quantum transition is already in motion — and it is moving faster than most software teams have noticed.
This is part of the Hyperdrift Quantum Day series. Read the context piece, the qubit explainer, or the Web3 provocation.
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