I want to store files in a secure manner (i.e. no one should be able to decrypt the contents of the file without the secret key) but I also want a diff tool to be able to show me which lines changed (let's assume that you can't learn anything about the content of the file by seeing which lines have changed).
The idea is to encrypt each line individually and convert the result to hex or BASE64 and then write each line into one file. A line with a certain content should always return the same output, no matter where it is in the file.
Specifically, I want a version control tool like Git or Mercurial to be able to efficiently handle the files as text. At the same time, a (self written) UI should be able to show a decrypted diff without the need of decrypting the data inside of the version control tool (
git diff would show me the line numbers which I could then decrypt in my app to display them myself).
Is that possible? Do you know where I can find more information how to do this?