How to Encrypt Your Bash Shell Script on Linux Using SHC

 In a Linux system, how can I encrypt my bash shell script? I don't want people with execute access to be able to examine the shell script and obtain the password because it contains a password. Is it possible to encrypt the shell script I wrote?

First off, encrypting your shell script is not a recommended practice. It would be incredibly helpful if you could fully document your shell script so that anyone who examines it knows exactly what it does. It is best to find an alternative way to construct the shell script without encrypting it if it contains important data, such as a password.

Having said that, the SHC utility can be used, as detailed below, if you are still determined to encrypt a shell script. Please be aware that regular users cannot access the encrypted shell script that shc produced. Nevertheless, the original shell script can be extracted from the encrypted binary that shc produced by someone who knows how this works.

Shell script compiler is referred to as SHC.

1. Download shc and install it

Download shc and install it as shown below.

# wget http://www.datsi.fi.upm.es/~frosal/sources/shc-3.8.7.tgz
# tar xvfz shc-3.8.7.tgz
# cd shc-3.8.7
# make

Verify that shc is installed properly.

$ ./shc -v
shc parse(-f): No source file specified

shc Usage: shc [-e date] [-m addr] [-i iopt] [-x cmnd] [-l lopt] [-rvDTCAh] -f script

2. Create a Sample Shell Script

Create a sample bash shell script that you like to encrypt using shc for testing purpose.

For testing purpose, let us create the following random.sh shell script which generates random numbers. You have to specify how many random numbers you like to generate.

$ vi random.sh
#!/bin/bash

echo -n "How many random numbers do you want to generate? "
read max

for (( start = 1; start <= $max; start++ ))
do
  echo -e $RANDOM
done

$ ./random.sh
How many random numbers do you want to generate? 3
24682
1678
491

3. Encrypt the Shell Script Using shc

Encrypt the random.sh shell scripting using shc as shown below.

$ ./shc -f random.sh

This will create the following two files:

$ ls -l random.sh*
-rwxrw-r--. 1 ramesh ramesh   149 Mar 27 01:09 random.sh
-rwx-wx--x. 1 ramesh ramesh 11752 Mar 27 01:12 random.sh.x
-rw-rw-r--. 1 ramesh ramesh 10174 Mar 27 01:12 random.sh.x.c

• random.sh is the original unencrypted shell script
• random.sh.x is the encrypted shell script in binary format
• random.sh.x.c is the C source code of the random.sh file. This C source code is compiled to create the above encrypted random.sh.x file. The whole logic behind the shc is to convert the random.sh shell script to random.sh.x.c C program (and of course compile that to generate the random.sh.x executable)

$ file random.sh
random.sh: Bourne-Again shell script text executable

$ file random.sh.x
random.sh.x: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.18, stripped

$ file random.sh.x.c
random.sh.x.c: ASCII C program text

4. Execute the Encrypted Shell Script

Now, let us execute the encrypted shell script to make sure it works as expected.

$ ./random.sh.x
How many random numbers do you want to generate? 3
7489
10494
29627

Please note that the binary itself is still dependent on the shell (the first line provided in the random.sh. i.e /bin/bash) to be available to execute the script.

5. Specifying Expiration Date for Your Shell Script

Using shc you can also specify an expiration date. i.e After this expiration date when somebody tries to execute the shell script, they'll get an error message.

Let us say that you don't want anybody to execute the random.sh.x after 31-Dec-2011 (I used last year date for testing purpose).

Create a new encrypted shell script using "shc -e" option to specify expiration date. The expiration date is specified in the dd/mm/yyyy format.

$ ./shc -e 31/12/2011 -f random.sh

In this example, if someone tries to execute the random.sh.x, after 31-Dec-2011, they'll get a default expiration message as shown below.

$ ./random.sh.x
./random.sh.x: has expired!
Please contact your provider

If you like to specify your own custom expiration message, use -m option (along with -e option as shown below).

$ ./shc -e 31/12/2011 -m "Contact admin@thegeekstuff.com for new version of this script" -f random.sh

$ ./random.sh.x
./random.sh.x: has expired!
Contact admin@thegeekstuff.com for new version of this script

6. Create Redistributable Encrypted Shell Scripts

Apart from -e, and -m (for expiration), you can also use the following options:

• -r will relax security to create a redistributable binary that executes on other systems that runs the same operating system as the one on which it was compiled.
• -T will allow the created binary files to be traceable using programs like strace, ltrace, etc.
• -v is for verbose

Typically you might want to use both -r and -T option to craete a redistributable and tracable shell encrypted shell script as shown below.

$ ./shc -v -r -T -f random.sh
shc shll=bash
shc [-i]=-c
shc [-x]=exec '%s' "$@"
shc [-l]=
shc opts=
shc: cc  random.sh.x.c -o random.sh.x
shc: strip random.sh.x
shc: chmod go-r random.sh.x

$ ./random.sh.x
How many random numbers do you want to generate? 3
28954
1410
15234    

In conclusion, it is important to emphasize once more: Initially, you shouldn't have encrypted your shell script. However, should you choose to encrypt your shell script with shc, keep in mind that a knowledgeable individual can still produce the original shell script from the encrypted binary that shc produced.