The following notation could be used to describe the operation of Triple DES. Encryption is denoted by C=E^k(P), where C is the cipher text, P is the plaintext message and E^k denotes an encryption operation with key K, while decryption is denoted with P=D^k(C), where D^k denotes a decryption operation with key K. Note that Triple DES performs the following operation for encryption: C=E^k1 (D^k2 (E^k1 (P))). a) Write down the modified version of that expression to show that Triple DES is backwards compatible with DES? b) what is the number of all possible keys for the original DES cipher?.