Information-Theoretic Security

• Plaintext space.

  • Set of all possible plaintext messages (M).

• Ciphetext space.

  • Set of all possible ciphertext values (C).

• Key space.

  • Set of all possible key values for a given algorithm (K).

• Perfect security.

  • Given cj ∈ Ci p(mi, kj) = p (mi)

  • #K ≥ #M

  • Vernam cipher (one-time pad).

• The cipher cannot be broken.

  • Even by adversaries with unlimited computing power.