PCBC (Propagating Cipher Block Chaining) is a variation of CBC, one of the standard DES modes. The encryption function is given below:

for the first block (IV: initial vector).

for the second and subsequent blocks.

a) Write the decryption function for PCBC.

b) Assume that the Ci block is broken during transmission. Which plaintext blocks does this error affect (break) on the receiving side (during decoding)? Explain.

C = Ex(IV OP) C = Ex(C-OP-OP)