I use OpenSSL with aes-256-cbc to encrypt a 30 GB tar file containing a recent backup of my hard drive. Now suppose somebody wants to run a brute-force attack on this file, trying out all possible password combinations. Does OpenSSL have to scan through the entire 30 GB of data in order to say whether the password matches or whether it doesn't match or can OpenSSL determine really quickly whether the specified password is the correct one?
In other words: Does it take longer to run a brute-force attack using OpenSSL on a 30 GB file than on a 100 KB file?
it depends, but - generally - no. AES is a block cipher, so you're breaknig a thing by 256 bit blocks in your case. And 100KB is more than enough to guess a type of "what's inside", by MIME, for example, after that a task is a way more simplier