More Definitions unconditional security no matter how much computer power is available, the cipher cannot be broken since the ciphertext provides insufficient information to uniquely determine the corresponding plaintext computational security given limited computing resources(eg time needed for calculations is greater than age of universe), the cipher cannot be broken Unconditional security would be nice, but the only known such cipher is the one-time pad(later) For all reasonable encryption algorithms, have to assume computational security where it either takes too long, or is too expensive, to bother breaking the cipher 9 復大软件学院 LiTLiJT 9 More Definitions • unconditional security – no matter how much computer power is available, the cipher cannot be broken since the ciphertext provides insufficient information to uniquely determine the corresponding plaintext • computational security – given limited computing resources (eg. time needed for calculations is greater than age of universe), the cipher cannot be broken • Unconditional security would be nice, but the only known such cipher is the one-time pad (later). – For all reasonable encryption algorithms, have to assume computational security where it either takes too long, or is too expensive, to bother breaking the cipher