# Is it possible to narrow down the possible keys used for AES CBC encryption, knowing a given plaintext and its ciphertext, where IV=0? [duplicate]

I'm brute forcing a ciphertext with a given dictionary to figure out which key was used. However, it's been hinted at that there is a way to narrow down the dictionary to a smaller subset of possibilities.

I know the plaintext, its ciphertext, and the IV (0). It was encrypted using AES-128-CBC and no salting was used during encryption.

At first I thought I was onto something because of: $$C_j = E_k(P_j \oplus IV)=E_k(P_j)$$ for IV = 0. But I was dead in my tracks right after. Is this situation even plausible or am I barking up the wrong tree?

• CBC mode doesn't use a salt. Password based key derivation often uses a salt. Now a password may well be vulnerable against e.g. dictionary attacks. – Maarten Bodewes Oct 19 '19 at 3:32
• Funny, I could not find a direct dupe, but this one might come close or be a dupe... – Maarten Bodewes Oct 19 '19 at 3:35
• Modern block ciphers like AES are resistant to key recovery attack. Maybe post the original homework text so that we can find out what exactly is being hinted? – DannyNiu Oct 19 '19 at 5:16