The Internet: Encryption & Public Keys

The padlock in your browser, explained in under seven minutes. Mia Epner, who works on security for a US national intelligence agency, starts with simple scrambled-letter ciphers and builds up to public and private keys, 256-bit encryption and the HTTPS connection behind online banking and checkout pages. It is the broadest security primer here, so it makes a good first watch on the subject. The Diffie-Hellman and 256-bit videos on this list then fill in the two questions it raises: how strangers agree on a secret, and why guessing a key is hopeless.