Encryption is the most important part in network security. To encrypt our bash shell script in linux we can use SHC which stands for shell script compiler. By the SCH you can encrypt your bash shell script.
1. Download shc and install it
Download shc and install it as shown below.
# wget http://www.encryptsystem.com/
# 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 linux.sh
#!/bin/bash
echo “Hello How are you Ashish”
$ ./linux.sh
Hello How are you Ashish
3. Encrypt the Shell Script Using shc
Encrypt the linux.sh shell scripting using shc as shown below.
$ ./shc -f linux.sh
This will create the following two files:
$ ls -l linux.sh*
-rwxrw-r–. 1 ashish ashish 149 Mar 27 01:09 linux.sh
-rwx-wx–x. 1 ashish ashish 11752 Mar 27 01:12 linux.sh.x
-rw-rw-r–. 1 ashish ashish 10174 Mar 27 01:12 linux.sh.x.c
linux.sh is the original unencrypted shell script
linux.sh.x is the encrypted shell script in binary format
linux.sh.x.c is the C source code of the random.sh file. This C source code is compiled to create the above encrypted linux.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 linux.sh
linux.sh: Bourne-Again shell script text executable
$ file linux.sh.x
linux.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 linux.sh.x.c
linux.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.
$ ./linux.sh.x
Hello How are you Ashish
Please note that the binary itself is still dependent on the shell (the first line provided in the linux.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 linux.sh.x after 31-Dec-2014 (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 31/12/2014 -f linux.sh
In this example, if someone tries to execute the random.sh.x, after 31-Dec-2014, they’ll get a default expiration message as shown below.
$ ./linux.sh.x
./linux.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 31/12/2014 -m “Contact admin@encryptsystem.com for new version of this script” -f linux.sh
$ ./linux.sh.x
./linux.sh.x: has expired!
Contact admin@encryptsystem.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 linux.sh
shc shll=bash
shc [-i]=-c
shc [-x]=exec ‘%s’ “$@”
shc [-l]=
shc opts=
shc: cc linux.sh.x.c -o linux.sh.x
shc: strip linux.sh.x
shc: chmod go-r linux.x
$ ./linux.sh.x
Hello How are you Ashish
Finally, it is worth repeating again: You should not be encrypting your shell script in the first place. But, if you decided to encrypt your shell script using shc, please remember that a smart person can still generate the original shell script from the encrypted binary that was created by shc.