Decrypt AES-128 with key file but missing IV?

Question

I want to decrypt a file that has been encrypted using AES-128 in CBC mode using OpenSSL. I got the “.key” file – which is 32 digits – but when I try to decrypt with OpenSSL, the program asks me for “-iv” and I don't know the IV of that file so it won't decrypt. Is there is any other way to decrypt the file without knowing the IV?

I tried with another file of which I have the correspondent IV and it works, and when I don't supply that IV, the decryption fails too.

By the way: is there anyway to know what type of encryption was used to encrypt a file?

Answer 1

First of all, I suggest you to try use as IV first 16 bytes of encrypted file. Because in general IV is the first block of ciphertext.

But if that doesn't work, then – of course – you can decrypt all message except first block. Just use first block as IV, and start to decrypt from second block. That will work because CBC does not provide integrity, and you can decrypt parts of the message picking any part of the ciphertext.

To better understand how this works, I also suggest you to read about CBC.