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 pop­u­lar gnome-keyring and other se­cu­rity au­then­ti­ca­tion meth­ods are not a good fit for this since most of them are un­locked all at once and with­out be­ing linked to a par­tic­u­lar pro­gram.

Here we will cre­ate a few sim­ple bash scripts to gener­i­cally and flex­i­bly lock and en­crypt ap­pli­ca­tions in a way which al­lows for mul­ti­ple users to have their own pri­vate en­crypted in­stances of shared apps. The com­plete scripts are lo­cated in my Dotfiles.

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 han­dles the en­cryp­tion por­tion. Though every­thing cov­ered here will re­quire only the base pro­gram it­self, new users would prob­a­bly ben­e­fit from hav­ing one of the GUI in­ter­faces to encfs as well. I pre­fer 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:

• This prompted me to cre­ate the di­rec­tory if it did­n’t ex­ist, which would not be han­dled prop­erly from within a shell script.

Additionally the MAN page for encfs showed me that sup­port for ex­ter­nal au­then­ti­ca­tion man­agers is granted via the --extpass flag.

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 re­mem­ber to start the script with it and to only put it once, right at the top of the file.

Setting Variables

Initially we might simply set an unlock string as follows:

Choosing an ASKPASS pro­gram

Since scripts can quickly get clunky with­out in­tend­ing too, we will first add a sim­ple vari­able which is suit­able for run­ning the ex­ter­nal au­then­ti­ca­tion.

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.

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.

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.

Caveats

The above snip­pet does not deal with sit­u­a­tions where:

• The stash is al­ready mounted
• The stash does not ex­ist

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.

Caveats

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

• The mount op­er­a­tion fails (wrong pass­word)
• The con­fig files are en­crypted

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 por­tion of the script uses a bash spe­cific ex­pan­sion. At this
point we can also make the unlockString a little neater.

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

Handling mounted di­rec­to­ries

To en­sure that the script is able to even­tu­ally deal with sit­u­a­tions where the com­mand is run in suc­ces­sion, a check is re­quired to fig­ure out if the mount point is cur­rently mounted.

If it is mounted, we will un­mount it.

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.

Handling Authentication

Finally we shall deal with cases where the script ex­e­cutes and the stashes ex­ist, but the pass­word is in­cor­rect. Additionally, we shall deal with man­ag­ing the flow of con­trol via the $? vari­able.

Quite sim­ply, the $? vari­able holds the re­sult of the pre­vi­ous com­mand. Hence it can be used to con­trol the flow. This was in­spired by the an­swers here.

Putting it all to­gether

For the lat­est re­vi­sions check my Dotfiles.

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

Future Directions

There ought to be a non-ter­mi­nal way of cre­at­ing the stash for the first time. Also, it may be in­ter­est­ing to work on the rules and con­fig­u­ra­tion schemes for a va­ri­ety of ap­pli­ca­tions.

Originally published at grimoire.science.