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.
6 min read
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.
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.
Marking a lesson complete only updates this browser.