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Modern Steam Ciphers

Modern stream ciphers use an initial seed key to generate an infinite pseudorandom keystream

Pseudo Randomness​

There are three types of random number generators

  • True Random Number Generator (TRNGs) - Impractical at scale
  • Pseudorandom Number Generators (PRNGs) - insecure
  • Cryptographically Secure Random Number Generators (CSPRNGs) - Are modern stream ciphers

LFSR​

  • A linear-feedback shift register is a register of bits whose positions shift to the right
  • Usually comprised of flip-flops, the last bit represents the output

Mathematical Representation​

  • Usually represent m-bit LFSRs using polynomials of degree m

Attacking LFSRs​

  • Suppose an attacker knows 2m2m plaintext bits
  • Step 1 - Calculate key bits
  • Step 2 - Reconstruct the LFSR

Trivium​

LFSRs are much more cryptographically secure if we combine more than one together in a non-linear way

  • 3 LFSRs
  • Feedback between each with non-linear AND gates
  • Initialises the LSFRs with an 80-bit key and 80-bit random value

ChaCha20​

  • ChaCha is a stream cipher written by Daniel Berstein
  • Modificaiton of a previous cipher, Salsa
  • Very lightweight, using only add, xor and rotate operations
  • One of two ciphers in TLS1.3

Vulnerabilities​

  • Stream ciphers give us confidentiality, but not integrity
  • Must include another mechanism