Models for evaluating security Unconditional security(perfect secrecy) Adversaries have unlimited computational resources Observation of the ciphertext provides no information to an adversary One time pad Complexity-theoretic security Adversaries have polynomial computational power Asymptotic analysis and usually also worst-case analysis is used Provable security provably secure if the difficulty of defeating crypto system can be shown to be as difficult as solving a well-known number-theoretic problemModels for evaluating security • Unconditional security (perfect secrecy) – Adversaries have unlimited computational resources – Observation of the ciphertext provides no information to an adversary – One time pad • Complexity-theoretic security – Adversaries have polynomial computational power. – Asymptotic analysis and usually also worst-case analysis is used • Provable security – provably secure if the difficulty of defeating crypto system can be shown to be as difficult as solving a well-known number-theoretic problem