Wednesday, September 11, 2013

Notes - One-Time Pads

The following are notes from Introduction to Cryptography with Coding Theory.
  • One-Time Pad is an unbreakable cryptosystem developed by Gilbert Vernam and Joseph Mauborgne 1918
  • Key is a random sequence of 0,1 same length as the message and are added together with XOR aka exclusive or
    • Example
      • plaintext  00101001
      • key         10101100
      • ciphertext 10000101
  • Decryption uses the same key, add teh key onto the ciphertext to return the plaintext
    • If used on the alphabet, the key is a random sequence of shifts between 0 to 25
    • unbreakable for ciphertext only attack
    • if we only have a piece of plaintext, the random generation means we know nothing about the rest of the key
    • Issues
      • truly random generation
      • trusted courier
      • key is very long, dangerous to reuse
  • Trivia
    • hotline between washington and USSR was thought to be a one-time pad
    • Variation of one time pad
      • satellite produce and broadcast several random sequences of bits at a rate fast enough that no computer can store more than a very small faction of the outputs
      • Alice wants to send message to bob
      • use RSA to agree on a method to sample bits from the random bit streams, use these bits to generate a key for a one time pad
      • Attacker Eve cannot decrypt because by the time she knows about the message the stream has disappeared

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