Let us modify the CBC mode to use a a global counter instead of randomness; deno

Question

Let us modify the CBC mode to use a a global counter instead of randomness; denote the new mode as CBCC^E. The encyption algorithm EK of CBCC^E encrypts a message M in the same way as the encryption algorithm of CBC^E does, except that the Initialization Vector (IV) C[0] is not sampled at random, but assigned with the current value of the counter, that is, C[0] = ctr; afterwards, the global counter is incremented ctr = ctr + 1 and M is encrypted in the same way as CBC^E does. Write down the encryption and decryption algorithms of CBCC^E. Show that CBCC^E is not IND-CPA secure.

Explanation / Answer

Encryptor

Accept Redundant Payload: P1, P2, ...Pn
R0 = group identity (e.g. 0)
Ri = Ri?1

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