Motivation — why post-quantum cryptography?¶
Prereqs: none
Next: History
Learning goals. Explain what cryptography is for in one breath, why large quantum computers threaten today’s public-key tools, what “store now, decrypt later” means, and why PQC migration started before a world-breaking machine ships.
The job cryptography already does¶
You send a message; the path is not a private room. Cryptography is the toolkit that still lets honest people:
- keep content private,
- notice tampering,
- and sometimes prove who wrote something,
even when the network is untrusted.
Post-quantum cryptography (PQC) is the next generation of that toolkit — designed so large quantum computers should not break the public-key parts we rely on for key setup and signatures.
You do not need a quantum computer to use PQC. You need new algorithms that classical computers can run, but that (we believe) stay hard even for quantum attackers.
Why change anything?¶
Much of today’s web and VPN security uses RSA or elliptic-curve public-key methods so browsers and servers can agree on keys.
A powerful enough quantum computer could break those methods with Shor’s algorithm. That is not a story about “quantum wifi.” It is a story about recorded math puzzles suddenly becoming easy for the attacker.
An adversary can:
- record ciphertext today,
- wait until a large quantum machine exists,
- decrypt the old recording later.
That is store now, decrypt later (SNDL). Long-lived secrets — medical archives, state records, identity keys, firmware roots — are especially exposed. Migration planning started early because recorded traffic lasts, not because a world-breaking machine ships next Tuesday.
Slow-motion timeline¶
| When | What happens |
|---|---|
| Today | Eve records HTTPS / VPN ciphertext under RSA/ECC |
| Years later | A large enough quantum computer exists |
| Then | Eve runs Shor-style attacks on old handshakes |
| Result | Old “secret” traffic may become readable |
Quick self-check¶
Why delay migration until “quantum day”?
Answer
Because traffic recorded *before* that day may still be valuable — and still decryptable later if the public-key layer was RSA/ECC.What about symmetric crypto?¶
Symmetric tools (AES, SHA-2/3, …) are in a different boat. Grover’s algorithm gives a quadratic speedup for some search-style attacks, which usually means “use larger keys / digests,” not “throw the whole design away.”
PQC is mainly about replacing public-key building blocks (key encapsulation, signatures) whose hardness collapses under Shor.
Motivation in one sentence¶
We need cryptography that honest devices can run today, that still protects data decades from now, even if large quantum computers arrive.
Where this curriculum goes next¶
| Next | Why |
|---|---|
| History | How classical PKC, the quantum threat, and NIST PQC fit on one timeline |
| Later: Why cryptography? | Deeper toys for confidentiality / authenticity / keys |
| Later: Quantum threat | Shor vs Grover slogans and family map |
Quick self-check¶
Do you need a quantum computer to deploy ML-KEM-style PQC?