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Math · Intermediate

Cryptography

For most of history, secret codes were broken by clever counting. Crack a cipher with letter frequencies, see why the Vigenère cipher held out for centuries, meet the one-time pad (provably unbreakable), discover how two strangers can agree on a secret key while everyone listens in, and build RSA, the public-key system behind digital signatures and secure websites.

7 lessons~40 min0/7 complete
Start courseHelpful background: Number Theory
1

Classical Ciphers

Codes and how they fall.

  1. Frequency AnalysisLetters leave fingerprints
  2. The Vigenère CipherThe "indecipherable" cipher
2

Modern Cryptography

Perfect secrecy and public keys.

  1. The One-Time PadA provably unbreakable code
  2. Sharing a Secret in PublicDiffie–Hellman key exchange
  3. Fingerprints for DataHash functions and the birthday attack
3

Public-Key Cryptography

RSA and digital signatures.

  1. RSAA lock anyone can close
  2. Digital SignaturesProving who sent a message