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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

• 

• also, 2 time pad is not secure

  

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

  

• 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