- How 2FA Codes Work: TOTP, HOTP and HMAC Explained
How authenticator apps make 6-digit codes: a shared secret, 30-second time steps, HMAC-SHA-1 and truncation (RFC 6238), plus official test vectors to check. - Public-Key Cryptography Explained, With a Toy RSA Example
How key pairs work, the difference between encrypting and signing, and a small RSA example you can check by hand. Plus where it's used: TLS, SSH and signed software.
Security
Passwords, two-factor authentication, encryption and public-key cryptography in plain terms.