Secret-key cryptography The following is a simple encryption/decryption scheme for character strings in ASCII code: • The plaintext and the ciphertext are divided into blocks of n ASCII characters. • The encryption/decryption key K is a string of n ASCII characters. • The encryption and decryption functions are both the bitwise exclusive OR function, XOR, applied to the blocks of ASCII characters and the key: E(P, K) = XOR(P, K) and D(C, K) = XOR(C, K), where P is a block of n plaintext characters, C is a block of n ciphertext characters, and K is the key.
b) If one encryption takes 0.01 μs, determine the time needed to try all possible keys on a ciphertext of 30 characters.
c) How long would the key have to be to increase the search time to more than one month?