Data Protection and Security

I

Introduction To Cryptography

I.IV

One-Time Pad

Information Theory and Cipher Security

Information theory is a branch of mathematics which quantifies the concept of information. Among other things, it also concerns with cryptography.

Information-theoretic Security (Perfect Secrecy in terms of entropy):

H(P|C) = H(P)

(The uncertainty of plaintext does not decrease if its ciphertext is observed).

Shannon, the father of information theory, showed that this implies

H(K) <= H(P)

The remaning uncertainty about a key when some ciphertext is known is called key equivocation and can be calculated as

H(K|C) = H(K) + H(P) - H(C)

In an unconditionally secure cipher, H(K|C) never approaches zero.