Encrypted text file support

[laenion]

Add support for encrypted text files

Motivation: As I keep some private files on my phone I needed some way to read / write encrypted text files on my phone. My original intention was to write a separate program, but seeing that it would have been necessary to implement all the file handling stuff myself I decided to just integrate it into file-manager instead.

Technology: For de- and encryption the "Gibberish AES" library is used (https://github.com/mdp/gibberish-aes), mainly because of it's size and speed. I did not do any security review! The library will decrypt files encrypted with AES-256-CBC, thus making it compatible with OpenSSL (and several other libraries). A password of sufficient length (>=12 characters) should keep a supercomputer busy for several decades 😏

Testing: I'm personally using this feature for several months now, but cleaned it up a bit today for this pull request.

Merging: The big question is if such a feature would be interesting for master, so feel free to reject it if you don't think it suits 😏. In case a merge seems reasonable I'll also change the documentation and translation parts of the source in later commits.

Usage: The text editor now has a new lock icon, indicating if the file will be saved with (locked) or without (unlocked) encryption. You can turn encryption on and off by pressing that icon. If you turn encryption on you will be asked to enter a password, which will be applied when saving the document. If you turn it off the document will (obviously) be saved without encryption.
If you open a document which is encrypted already you will be asked for your password on opening. And if you just want to see the encrypted file you may leave the password field empty.

[j8r]

Nice feature! I believe that someday, crossed with #34 zip feature, we will even be able to encrypt/decrypt archives :D

[elfoxero]

Amazing, it's an interesting feature. Thank you.

[laenion]

Thanks for merging!

@JulR43 I've had a look at the ZIP file specification (https://pkware.cachefly.net/webdocs/casestudies/APPNOTE.TXT). Basically there are two possible ways how a file can be encrypted:

1. The "old" way, using a custom (and insecure) encryption algorithm per file.
2. The "new" (Strong Encryption) way. Unfortunately this part of the specification is proprietary / patented (and thus may not be used without a proper license), and apart from that a plethora of different algorithms are supported in the specification, so I'm not sure if that's really in scope ;-)
3. And then there's the third way: the WinZip encryption, which is not part of the official ZIP standard, is specified here: http://www.winzip.de/aes_info.htm. I guess that's the encryption you meant, isn't it?

In any case: if we want to implement anything of the above this would require some support from the compression library, as required information is stored in the encryption / archive / file header (depending on which encryption it is). Unfortunately JSZip currently doesn't provide access to that information.
It would probably be easier if the compression library supported encrypted files itself in the first place ;-) However JSZip currently has no plans to support encryption (Stuk/jszip#115).

[j8r]

Thanks for our contribution and informations about zip encryption! Finally it isn't so bad that this feature isn't available, because your encrypted text file feature is very useful to protect sensitive data :)