Reference no: EM133280272

Consider CBC to provide confidentiality and CBC-Residue to provide integrity verification where we use one key K1 to generate CBC encryption of a given plaintext P=P1P2...Pn to produce the corresponding ciphertext C= C1C2...Cn and another key K2 to compute the CBC-Residue R from P for integrity verification as C=CBC(K1, P) and R=CBC-Residue(K2, P) and we send C and R to the receiver. Now assume that we want to modify the above mechanism to avoid doing a complete CBC for the second time in CBC-Residue computation as follows: C=CBC(K1,P) and R=AES(K2,Cn) where AES is the encryption algorithm we use in CBC and Cn is the last block of C in CBC(K1, P). In other words, instead of doing a full CBC to produce the CBC-Residue R, we encrypt the last block of CBC (i.e., Cn) using the second key K2 and use it as the CBC-Residue value. We then send C and R to the receiver site. Would this modification make any difference from security point of view? Justify your answer.