Every topic, in plain words

Before the math: what cryptography is actually trying to do, and the 2,500-year arms race between codemakers and codebreakers that got us here.

Every public-key algorithm in this catalog leans on the same idea: a calculation that's easy in one direction and effectively impossible to undo in the other.

One-way fingerprints for data, and the mechanism that proves a message is authentic and untampered — without encrypting anything.

A cryptographic hash is too fast for passwords. KDFs deliberately slow things down — and not all of them do it the same way.

128-bit AES, 2048-bit RSA, 256-bit ECC — these numbers aren't comparable at face value. Here's how to actually read them.

Every key, nonce, and IV in this catalog assumes truly unpredictable randomness. When that assumption breaks, everything built on top breaks with it.

Randomness can't just be assumed good or bad — it's measured. This module covers min-entropy, the real NIST test suite for entropy sources, and why a hardware random number generator still needs that testing.

The bridge module: why everything above is called "classical" cryptography, and exactly what a future quantum computer would do to it.

The same key locks and unlocks the data. Fast, simple in concept, and everywhere — from disk encryption to the bulk of every TLS session.

AES's predecessor and the first cipher the world ever standardized. What retired it wasn't a flaw in the design — it was a key that was always too short.

Not every symmetric cipher works in fixed blocks. ChaCha20 generates a keystream instead — and paired with Poly1305, it's AES-GCM's fastest rival.

Four ways to prove a message came from someone holding a shared key and wasn't changed on the way: one built from a hash, one from a block cipher, two from polynomial arithmetic.

A block cipher encrypts 16 bytes. Everything else (long messages, randomness, tamper detection, even keeping a card number a card number) comes from the mode wrapped around it, and that's where most real-world breaks happen.

Two mathematically linked keys — one public, one private — solve the problem symmetric crypto can't: how do you share a secret with someone you've never met?

Textbook RSA is deterministic and malleable. Padding schemes are what actually make RSA encryption and signing safe to use in the real world.

The same public-key guarantees as RSA, with dramatically smaller keys — because the underlying hard problem is different math entirely.

Diffie-Hellman lets two people agree on a secret together. ElGamal uses that same discrete-log idea to let anyone encrypt to a public key — no live handshake required.

Two parties agree on a shared secret over a public channel, without ever transmitting the secret itself — the idea that started public-key cryptography.

A certificate is just a signed statement binding a public key to an identity. Here's what's actually inside one, and how revocation works.

JWTs put a signed claim in every request header. They're everywhere in modern APIs — and a few well-known implementation mistakes keep recurring.

A passkey is what you get when you replace a password with a public-key challenge-response protocol. It fixes reuse, phishing, and breaches in one move.

The protocol behind every remote login and git push combines the same primitives as TLS, arranged slightly differently.

Every padlock icon runs a coordinated handshake combining key exchange, certificates, symmetric encryption, and integrity checks — in under a round trip.

TLS protects data in transit to a server. The Signal Protocol's Double Ratchet goes further — encrypting so not even the server operator can read your messages.

A cryptographic algorithm can be mathematically unbreakable and still leak its secret key through how long it takes to run.

Bitcoin and Ethereum don't invent new cryptography — they compose the same primitives in this catalog into a specific, tamper-evident structure.

How do you protect a master key so no single person holds it, yet any k of a trusted group can reconstruct it together? Split it across a polynomial.

You don't need a working quantum computer today to be at risk today. Anything encrypted now with RSA or ECC can simply be recorded and decrypted later.