I recently found out that active attacks are a threat against AES-256, CBC mode, padded with PKCS#7 (the CBC mode in particular. I should supposedly change to EAX to guard against these attacks).
However, I'm confused about whether such attacks are effective against a particular encrypted local file that a cracker has obtained. (Indeed, if AES-256, CBC, padded with PKCS#7 is secure, a cracker's possession of such a file shouldn't have to be a case for concern in the first place).
In my internet browsing I found out two kinds of active attacks:
- Data tampering: Here, the cracker sends a modified ciphertext to some "system" (e.g. a web site). This could e.g. change a $1 bank transaction into a $1000 transaction.
I can't see how this would be useful against a local file, because there is no "system" to which we send any data. The cracker modifies the local ciphertext and that's it. He has nowhere to send it to.
- Actual decryption of the entire plain text: Here, the cracker will supposedly use something called a "padding oracle". This method relies on the "system" (e.g. a web site) to make a distinction between invalid ciphertext and a valid ciphertext (ciphertext that is decrypted into garbage). For instance, the invalid ciphertext can produce an exception, but the valid ciphertext can produce an "incorrect login".
Again, does this really pose a threat to local encrypted files? There is no "system", no login page etc., that tells us whether the ciphertext is "correct" or "incorrect".
All this confuses me. Are active attacks really useful against particular local encrypted files in CBC mode? In particular, is the "padding oracle" useful against encrypted files that used PKCS#7 padding?