Cryptography is the math that turns "I have a secret" into "I can prove that secret without revealing it, transmit it without being overheard, and verify that nobody altered it on the way." None of the rest of cybersecurity works without it.
The three subdomains map to the three problems cryptography solves. Symmetric encryption protects data when both parties already share a secret. Asymmetric cryptography solves the bootstrapping problem: how do two parties who have never met agree on a key without an eavesdropper hearing? Hash functions make fingerprints that catch tampering, anchor signatures, and ground every modern integrity check.
Pick a subdomain below, start at 02.A.01 and walk straight through, or jump to the interactive labs at the bottom of this page.
Symmetric Encryption
The classical model: Alice and Bob share a secret key and use it to encrypt and decrypt the same way. Fast, well-understood, and the workhorse of every TLS session once the handshake completes. The catch is that both parties have to obtain the same key somehow without an eavesdropper intercepting it, which is the entire reason asymmetric cryptography had to be invented. Hands-on Caesar, substitution, Vigenere, and XOR challenges live in the labs section below.
Asymmetric Cryptography
Public-key cryptography. Every party has two keys: one private, one public. Encrypt with the public key and only the holder of the matching private key can decrypt. Sign with the private key and anyone with the public key can verify. The math is harder than symmetric crypto and the operations are slower, but the result solves problems symmetric crypto cannot: key distribution at scale, digital signatures, and the entire model of TLS, SSH, and code signing. Hands-on PKI labs (sign/verify, encrypt/decrypt, key picker) are in the labs section below.
Hashing
Hash functions take any input and produce a fixed-size fingerprint that looks like random noise but is fully deterministic. Change one bit and the entire output transforms. Hashes are not encryption, have no key, and cannot be reversed by design. They are the integrity proofs of file downloads, the storage form of passwords, the anchor under every digital signature, the address of every git commit and Docker image, and the structural backbone of blockchains.
Interactive Labs
Reference pages are not enough. Each subdomain has companion lab pages that put the math on a keyboard. Each lab runs in the browser, no install required, and uses real cryptographic primitives where the browser exposes them.