Assume a block cipher which operates on 4-bit blocks written as hexadecimal digits. For a particular key K it implements the following "encryption" on message m: m 0 1 2 3 4 5 5 C 7 e 6 7 8 9 a 2 a 4 9 EK (m) 1 b b C d fd03 a) The ciphertext 188b06 has been encrypted in ECB mode. Explain in one sentence how to decrypt the ciphertext for each block and decrypt the ciphertext (marks are only given for the complete and correct plaintext). (4 marks) b) The ciphertext 301b2 has been encrypted using CBC mode (assume IV=0). Explain in two sentences how to decrypt the ciphertext for each block and decrypt the ciphertext (marks are only given for the complete and correct plaintext). Note: the XOR accelerator table below may be of use. (7 marks) c) Explain TWO advantages that counter mode has over CBC. (4 marks) XOR Accelerator 0 1 2 230 234 4 4 5 6 5 5 67856 4 7 6 6674 5 7 65 4 b 40 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 032 5 4 8 9 1 6 9 1 6 7 5 4 3 cd 321 07 6 89 a 9 98 b a ab89 C e 7 0 1 1 2 3 b ba 98 f def d d C f e f fe d a CO@4 DP 2 0 3 2 30 1 2 1 0 C d e f d C f e f 66945 e d c b a 7 JP 5 fe 89 a b 98 b a b 8 9 e C d dc 8880 a 9 b a b a PPP a e f 0 1 0 2 3 3 2 4 5 5 4 6 30 00 JPP 7 87 6 b a b 8 e d C f 9 f C d b 3 0 S6900 C a d 8 C d e f d e f f f 8 WOP @ H e e 9 9 8 d a b 20 e f C C b 3 4 5 2 1 6 107 6 7 7 6 4 5 5 4 4C2 0 1 2 3 04 f e d d C a b ba 89 20 a 9 8 5 6 7 4 7 6 7 4 5 654 1 2 0 3 2 e f 68 3 3 2 0 1 1 0