I got an encryption related question. There were a few files stored unencrypted on my Dropbox account. I've moved them together with a couple of other, more sensitive files, to a Boxcryptor encrypted subdirectory (with encrypted filenames enabled).
The file sizes of encrypted files are the same as of the unencrypted files. So the people at Dropbox (or at any other storage provider) could quite easily match encrypted and unencrypted files by their file size regardless of the encrypted filename.
Now to my question:
Can the encryption key be broken if someone possesses an encrypted and an unencrypted version of the same file (or of a couple of files) and render the whole encryption useless?