Shanon Perfect Secrecy

Definition: A Cipher (E,D) over (K, M, C) has perfect secrecy if

for all m_0, m_1 in M (m_0 length = m_1 length) and for all cipher texts in C the probability Pr[E(k,m_0) = c] = Pr[E(k,m_1) = c]

Meaning: If an attack sees our cipher text c , the probably that the original message was m_0 or m_1 is the same. So the attack has no idea what the original message would have been!