Day 2: Symmetric Encryption

Resources (optional)

• one-time pad: video

Lecture Topics

• notes
• one-time pad
  • the two-time pad is insecure
  • limitations
• pseudo-random generators
• stream ciphers

Problems

Reminders:

• You can run tests for a function by running python main.py FUNCTION_NAME from the shell.

We’ll be working in the main.py file. The cryptoy.py library contains the PRG function, cryptoy.prg(seed, n_bytes) that you’ll use for the stream cipher.

• done together:

  • bytes_to_bitstring
  • bitstring_to_bytes

• required:

  • random_bytes
  • xor
  • bitstring_xor
  • one_time_pad_encrypt
  • one_time_pad_decrypt
  • stream_cipher_encrypt
  • stream_cipher_decrypt

• bonus:

  • the “daisy chain” PRG:
    • prg1
    • daisy_chain_prg
  • binary formatting and parsing
    • int_to_binary, binary_to_int
  • cryptanalyzing the many-time pad
    • many_time_pad_cryptanalysis