TL; DR

  • Sym. encryption
  • One Time Pad
  • Shannon: Perfect secrecy
  • History - Badly broken algorithms
  • Stream cipher - OTP in real life
  • Probability basics
  • Pseudo random gen - predictable
  • Negligible / Non-negligible

Topics

Sym. encryption definition

What

  • Sender and receiver use the same secret key for encryption and decryption,
  • or the same encryption scheme,
  • includes DES, AES, TLS, etc

How

  • sym

  • also, 2 time pad is not secure

    2pad

One Time Pad

  • think this as a one time key (different key for every message)
  • has perfect secrecy, since len(K) = len(M) > Shannon: if |K| >= |M|, we say this encryption has perfect secrecy

  • not practical, which is why we have

PRG (Pseudo Random Generator)

Must be unpredictable

  • predictable if

    predictable

  • ie. with an efficient algorithm A,

How secure is secure?

Semantically secure

  • a sym. encryption is semantically secure if,

    adv[A, E] is negligible

to be continued


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