What quantum computing changes
The exposed half of cryptography
Modern cryptography is really two disciplines with two different failure modes. This lesson separates them before quantum computing enters the picture.
7 min read · 7 min 37 sec film
What you'll be able to do
- Distinguish symmetric cryptography and hashing from public-key (asymmetric) cryptography, and state what each relies on for its security.
- List everyday systems — TLS, VPNs, code signing, certificate authorities — that depend on public-key cryptography.
- Explain, in one sentence, why public-key cryptography is the part of today's cryptography that quantum computing changes.
Lesson film
Watch: The exposed half of cryptography
Pull one TLS handshake apart and the two halves of cryptography separate on their own — including the half that has an expiry date.
7 min 37 sec · 10 scenes
7 min 37 sec, narrated by your browser's voice. Captions and a full transcript are below — nothing here needs sound.
Before quantum computing enters this course at all, two things that travel under one name have to be pulled apart.
With the player focused: Space plays or pauses, ← → seek ten seconds, Home restarts, M toggles narration.
Full transcript
1. Two disciplines, one name
2. The two families
3. Inside one handshake
4. The same handshake, in full
5. Where else it lives
6. Remove it and see
7. What each rests on
8. Why the split matters
9. Saying it precisely
10. Takeaway
The written lesson
Cryptography most people rely on every day is really two disciplines wearing one name. Symmetric cryptography and hashing — AES, SHA-2, SHA-3 — protect data using a shared secret or a one-way function. Public-key (asymmetric) cryptography — RSA, Diffie-Hellman, elliptic-curve cryptography (ECC) — lets two parties agree on a secret, or prove identity, without having shared one in advance.
| Cryptography family | Examples | What its security rests on |
|---|---|---|
| Symmetric encryption and hashing | AES, SHA-2, SHA-3 | Brute-force search resistance, or a one-way, collision-resistant construction |
| Public-key (asymmetric) cryptography | RSA, Diffie-Hellman, elliptic-curve cryptography (ECC) | The difficulty of factoring large integers or computing discrete logarithms |
Public-key cryptography is the part that does the invisible work: negotiating a session key when a browser opens an HTTPS connection, authenticating a server certificate, signing a software update, and exchanging keys over SSH or a VPN. Remove it, and none of those systems has a way to agree on a secret or verify who it is talking to.
- TLS and HTTPS handshakes, on effectively every website
- VPN and SSH key exchange
- Code signing and software update verification
- Certificate authorities and the public key infrastructure (PKI) that issues certificates
- Key agreement in encrypted email and messaging
That asymmetry is the reason this course exists. The next two lessons look at each quantum algorithm in turn — what it actually does, and to which family.
A hash function is not a shared-secret cipher. A digest may support integrity checks, signatures, or key derivation, but a bare digest does not keep a message confidential and does not by itself prove who created it. When reviewing a system, label the purpose of each primitive before judging whether it needs a post-quantum replacement.
Work it through
Classify a simple connection
A service uses X25519 to establish a session secret, AES-256-GCM to protect records, an ECDSA server certificate, and SHA-256 in transcript processing. This is a synthetic teaching example.
About 5 min of unscored practice
Your task
- Name the job of each primitive.
- Identify which parts depend on classical public-key assumptions and which part directly protects application bytes.
Type here or copy the template. Your response stays in this page and is not submitted or scored.
Reveal model response and self-check rubric
A different answer can be sound when it states its assumptions and follows the evidence.
X25519 establishes a shared secret; AES-256-GCM encrypts and authenticates records; ECDSA signs the server certificate; SHA-256 supports transcript processing. X25519 and ECDSA use classical elliptic-curve assumptions. Replacing the key exchange would not automatically replace certificate authentication.
Self-check
For each point: 0 = absent or incorrect, 1 = partly supported, 2 = well supported.
- Assign each primitive its actual purpose.
- Distinguish public-key steps from symmetric record protection.
- Avoid claiming a hash provides confidentiality by itself.
Primary reading and claim sources
Publication status checked on the date shown. Draft guidance can change.
- Post-Quantum Cryptography FAQsNIST · Current project guidance · checked 2026-09-26
- Polynomial-Time Algorithms for Prime Factorization and Discrete LogarithmsPeter Shor · Research paper · checked 2026-09-26
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