Hackernoon logoLocking and Encrypting Apps with Encfs by@rohit

Locking and Encrypting Apps with Encfs

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@rohitRohit Goswami

For better syntax highlighting and the ability to copy paste the code snippets, view this post on grimoire.science.

One of the prob­lems of a shared sys­tem, is that some­times mul­ti­ple ap­pli­ca­tions are used by mul­ti­ple peo­ple. Normally this would be solved by the ex­cel­lent multi-user sup­port in­her­ent to *nix sys­tems. After all, *nix and de­riv­a­tives were de­signed to be used by mul­ti­ple users at the same time.

However, in some cases, it just makes sense for a sin­gle ap­pli­ca­tion to stay locked un­til a pass­word is sup­plied. In any case, encfs is one of the bet­ter en­cryp­tion meth­ods (as long as us­abil­ity over­shad­ows se­cu­rity) and it’s a good prac­tice to en­crypt ap­pli­ca­tion data any­way. The case for en­cryp­tion varies, but weather it’s to make life harder for hack­ers, or just to stop cloud stor­age providers from sniff­ing around, it’s always a good idea. Even if mul­ti­ple users will not be shar­ing an ac­count, en­cryp­tion pre­vents the root users or admins from get­ting too frisky with your data.

The pop¬≠u¬≠lar gnome-keyring and other se¬≠cu¬≠rity au¬≠then¬≠ti¬≠ca¬≠tion meth¬≠ods are not a good Ô¨Āt for this since most of them are un¬≠locked all at once and with¬≠out be¬≠ing linked to a par¬≠tic¬≠u¬≠lar pro¬≠gram.

Here we will cre¬≠ate a few sim¬≠ple bash scripts to gener¬≠i¬≠cally and Ô¨āex¬≠i¬≠bly lock and en¬≠crypt ap¬≠pli¬≠ca¬≠tions in a way which al¬≠lows for mul¬≠ti¬≠ple users to have their own pri¬≠vate en¬≠crypted in¬≠stances of shared apps. The com¬≠plete scripts are lo¬≠cated in my DotÔ¨Āles.

Requirements

The ba­sic re­quire­ments are com­monly found across most UNIX sys­tems and its de­riv­a­tives in­clud­ing MacOS, Ubuntu and other Debian dis­tros, RPM based sys­tems etc.

I per­son­ally run Arch Linux but that’s just a bi­ased en­dorse­ment.

The re­quire­ments are:

  1. Bash

This is needed for the shell sub­sti­tu­tions in the shell script. Found al­most every­where.

2. EncFS

This han¬≠dles the en¬≠cryp¬≠tion por¬≠tion. Though every¬≠thing cov¬≠ered here will re¬≠quire only the base pro¬≠gram it¬≠self, new users would prob¬≠a¬≠bly ben¬≠e¬≠Ô¨Āt from hav¬≠ing one of the GUI in¬≠ter¬≠faces to encfs as well. I pre¬≠fer Gnome Encfs Manager.

3. An ASKPASS pro­gram

These are most fa­mously known for weird ssh er­rors. However, here we will fo­cus on zen­ity and git as a fall­back.

And that’s it. Other vari­a­tions of this method might use shoes for more GUI good­ness. Other askpass pro­grams and helpers may also be used, like the pop­u­lar x11-ssh-askpass pro­gram.

Program Structure

Our basic structure is simple.

  1. An en­crypted folder is mounted
  2. The ap­pli­ca­tion is run

Additionally we would like the fol­low­ing fea­tures:

  1. Execution with­out the ter­mi­nal (GUI, no ter­mi­nal user queries)
  2. An au­to­mated way of gen­er­at­ing a new stash for ap­pli­ca­tions
  3. A san­i­tized name for the mount points

Preliminaries

Before get­ting to the cre­ation of a script, I like to ex­per­i­ment with the na­tive shell. In this case this sim­ply in­volved check­ing the fol­low­ing:

Shell trials
  • This prompted me to cre¬≠ate the di¬≠rec¬≠tory if it did¬≠n‚Äôt ex¬≠ist, which would not be han¬≠dled prop¬≠erly from within a shell script.

Additionally the MAN page for encfs showed me that sup¬≠port for ex¬≠ter¬≠nal au¬≠then¬≠ti¬≠ca¬≠tion man¬≠agers is granted via the --extpass Ô¨āag.

Implementation

With those pre­lim­i­nar­ies out of the way, it is time to start script­ing. Portions which re­quire the bash shell specif­i­cally will have the she­bang in­cluded.

Always re¬≠mem¬≠ber to start the script with it and to only put it once, right at the top of the Ô¨Āle.

The shebang

Setting Variables

Initially we might simply set an unlock string as follows:

Using a variable for the unlock dialog

Choosing an ASKPASS pro­gram

Since scripts can quickly get clunky with¬≠out in¬≠tend¬≠ing too, we will Ô¨Ārst add a sim¬≠ple vari¬≠able which is suit¬≠able for run¬≠ning the ex¬≠ter¬≠nal au¬≠then¬≠ti¬≠ca¬≠tion.

Zenity Prompt
Zenity implementation

As men­tioned pre­vi­ously, zen­ity is the pret­tier choice, how­ever, it may not be in­stalled every­where. So we need a fall­back.

Git is more or less avail­able every­where, and it just so hap­pens to have a pretty neat askpass tool as well.

Git Askpass Prompt
Git Askpass Implementaion

However, it would be bet­ter to wrap them both up in a way to pick one or the other based on the avail­abil­ity. So, we write a sim­ple test.

The test logic

Honestly the us­age of which in­stead of command -v is a bit con­tro­ver­sial. However, here I went with which sim­ply be­cause it seemed faster. The more portable (POSIX com­pli­ant) ver­sion of the above would use command -v. For more de­tails check this stack ex­change ques­tion.

Creating Mount-points

Gnome Encfs Manager de­faults to re­mov­ing the mount point when the stash is un­mounted, how­ever, this causes a ter­mi­nal in­put de­mand which needed to be sup­pressed, hence the di­rec­to­ries are cre­ated prior to run­ning Encfs.

Variables and directories

Caveats

The above snip­pet does not deal with sit­u­a­tions where:

  • The stash is al¬≠ready mounted
  • The stash does not ex¬≠ist

These are dealt with in the Improvements sec­tion of this doc­u­ment.

Mount and Run

Now we are in a po­si­tion to sim­ply mount our stash and run the pro­gram.

A basic mount and run

Caveats

At this stage the script is not equipped to deal with sit­u­a­tions where:

  • The mount op¬≠er¬≠a¬≠tion fails (wrong pass¬≠word)
  • The con¬≠Ô¨Āg Ô¨Āles are en¬≠crypted

The script runs the pro­gram with­out test­ing the re­sult of the mount, which will lead to much frus­tra­tion and weird er­rors. These edge cases are handled by the code in Handling Authentication.

Improvements

Several im­prove­ments to the ba­sic script cre­ated above are dis­cussed in this sec­tion.

Naming di­rec­to­ries

This is ac­tu­ally not a re­ally im­por­tant bit, how­ever, I wanted the app di­rec­to­ries to start with cap­i­tal let­ters. Also I wanted the en­crypted data to be stored in a hid­den folder.

In any case, this por¬≠tion of the script uses a bash spe¬≠ciÔ¨Āc ex¬≠pan¬≠sion. At this
point we can also make the unlockString a little neater.

Bash substitutions for prettier names

Now that we have the name, we simply modify the directories.

Renaming the directories

Handling mounted di­rec­to­ries

To en¬≠sure that the script is able to even¬≠tu¬≠ally deal with sit¬≠u¬≠a¬≠tions where the com¬≠mand is run in suc¬≠ces¬≠sion, a check is re¬≠quired to Ô¨Āg¬≠ure out if the mount point is cur¬≠rently mounted.

If it is mounted, we will un­mount it.

Testing for mounts

Additionally, for cases where the stash does not yet ex­ist, we will need to cre­ate the other di­rec­tory as well. We shall also kill the ap­pli­ca­tion if the stash is to be mounted (for se­cu­rity). This por­tion was aided by this stack ex­change thread.

Mount handling with directory creation

Handling Authentication

Finally we shall deal with cases where the script ex¬≠e¬≠cutes and the stashes ex¬≠ist, but the pass¬≠word is in¬≠cor¬≠rect. Additionally, we shall deal with man¬≠ag¬≠ing the Ô¨āow of con¬≠trol via the $? vari¬≠able.

Quite sim¬≠ply, the $? vari¬≠able holds the re¬≠sult of the pre¬≠vi¬≠ous com¬≠mand. Hence it can be used to con¬≠trol the Ô¨āow. This was in¬≠spired by the an¬≠swers here.

The results variable and flow of control

Putting it all to­gether

For the lat¬≠est re¬≠vi¬≠sions check my DotÔ¨Āles.

It is also re­pro­duced here as a gist, since Medium can’t handle syntax highlighting.

Future Directions

There ought to be a non-ter¬≠mi¬≠nal way of cre¬≠at¬≠ing the stash for the Ô¨Ārst time. Also, it may be in¬≠ter¬≠est¬≠ing to work on the rules and con¬≠Ô¨Āg¬≠u¬≠ra¬≠tion schemes for a va¬≠ri¬≠ety of ap¬≠pli¬≠ca¬≠tions.

Originally published at grimoire.science.

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