I just recently concerned myself with cryptography due to Snowden's revelations and there are a few things I cannot really wrap my head around but maybe you can help me with that. I am not a mathematican or a programmer and most of the stuff I know about cryptography comes straigt from The Imitation Game, so pleas excuse my ignorance.
First off: Isn't encryption of data completely pointless if the attacker knows what he is looking for? In the movie, Turing encrypted the German codes because he knew how their weather broadcast started every morning. So, if an attacker knows the exact specifics of a file that is likely tucked away in an encrypted container, couldn't he just modify portions of the file to match the unencrypted file to find out the encryption key? If for example a whistleblower gets hold of a set of documents and his employer wants to proof it but only finds an encrypted disc at his workplace. Would it be easy for this employer to decrypt the encrypted data if he had a cleartext copy of the files he is looking for?
And my second burning question: Truecrypt's documentation says that one shouldn't copy an existing volume because working with two copies of the same container "may aid cryptanalysis". Why exactly is that? As far as I understand those two containers would share a master key but how does that aid cryptanalysis? Let's bring up a few situations here: Copying a halfway full container and working on the data of each individually, adding the same or different data to both containers of cloning an empty container and filling it with completely different data. How does any of that enable cryptanalysis?
