Sign up ×
Cryptography Stack Exchange is a question and answer site for software developers, mathematicians and others interested in cryptography. It's 100% free, no registration required.

What would be the security implications if we reverse CBC encyption? Here is a picture of CBC encryption:enter image description here

Here is a picture of CBC decryption: enter image description here

My guess is that the encryption would work if we ran the decryption process for encryption and encryption process for decryption. However, the effect from the IV is not spread to other blocks other than cipherblock 1. Are there any other potential effects?

share|improve this question

1 Answer 1

If I understand you correctly, you want to use the decryption process of CBC to encrypt, i.e.,

enter image description here

This would not be CPA secure. The problem is, as you already noted, that the IV has no influence on any block but the first one.

I.e. a CPA attacker works as follows: Choose two random messages of length 2 blocks $(m_0^0,m_0^1)$ and $(m_1^0,m_1^1)$ and output them as the challenge messages. Receive back the challenge ciphertext $(c^0,c^1)$. Now, ask $(m_0^0,m_0^1)$ to the encryption oracle and receive the encryption $(c_0^0,c_0^1)$. If $c^1 = c_0^1$ then output $0$, otherwise output $1$.

share|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.