Let's say you have an encrypted file store of some kind such as an encrypted hard drive, ZIP file, etc. You don't have the private key or password to this file store, so a brute-force attack is not a good option.
But what you DO have is a copy of a file -- a file that you know for certain is on that file store.
Does this help reduce the complexity of an attack? In other words, would I be able to crack the encryption in less time by knowing part of the contents of the encrypted file store?
(Forgive me if this is obvious, encryption is not my specialty.)