I have a file that supposedly contains AES-256 encrypted data. The file was created using a proprietary program that claims to use AES-256.
I have tested decrypting the file with an open source program (openssl) and it works, I get my exact original file back.
Given this, is it possible that the proprietary program created a back door into the encrypted file?
In other words, could the program's developer be able to access my original file given the encrypted data (assuming he doesn't know the password I supplied). Does my ability to open it with OpenSSL prove that there isn't some intentional weakness in the way that the proprietary software encrypted it?