Encrypting and Decrypting Files and Directories on Linux Using mcrypt command

Encryption is a crucial aspect of securing sensitive information on your Linux system. One of the tools you can use for this purpose is mcrypt, which provides symmetric encryption and decryption capabilities. This guide will walk you through the process of using mcrypt to encrypt and decrypt files and directories on Linux, covering a range of options and examples.

In this tutorial you will learn:

• How to use mcrypt to encrypt files
• How to use mcrypt to decrypt files
• How to apply compression with encryption using mcrypt
• Different encryption algorithms and modes available in mcrypt

Software Requirements and Linux Command Line Conventions

Category	Requirements, Conventions or Software Version Used
System	Linux operating system
Software	mcrypt
Other	Basic understanding of Linux command line
Conventions	# – requires given linux commands to be executed with root privileges either directly as a root user or by use of sudo command $ – requires given linux commands to be executed as a regular non-privileged user

Installing mcrypt

To use mcrypt on your Linux system, you first need to install it. Below are the installation instructions for Debian, Ubuntu, CentOS, and RHEL.
For debian and ubuntu, use the following commands:

Debian and Ubuntu

$ sudo apt update
$ sudo apt install mcrypt

CentOS and RHEL

First, you need to enable the EPEL repository, then install mcrypt.

$ sudo yum install epel-release
$ sudo yum install mcrypt

For CentOS 8 and RHEL 8, use the dnf package manager:

$ sudo dnf install epel-release
$ sudo dnf install mcrypt

Creating a Sandbox Environment

To ensure a safe practice environment, create a sandbox directory where you can test the encryption and decryption examples without affecting important data. Use the following commands:

$ mkdir ~/encryption_sandbox
$ cd ~/encryption_sandbox
$ touch file1.txt file2.txt
$ mkdir dir1 dir2
$ echo "This is a test file" > file1.txt
$ echo "This is another test file" > file2.txt
$ echo "File in directory 1" > dir1/file_in_dir1.txt
$ echo "Another file in directory 1" > dir1/another_file_in_dir1.txt
$ echo "File in directory 2" > dir2/file_in_dir2.txt

This setup creates a directory named encryption_sandbox containing two text files and two subdirectories, each with their own text files, providing a controlled environment for practicing encryption and decryption.

Encryption Examples

Encrypting files and directories using mcrypt involves various options and configurations. Note that by default, mcrypt will prompt you to supply a password twice (once for encryption and once for confirmation) unless you use the -k option to specify a password directly. Here are seven examples to demonstrate its usage:

1. Basic File Encryption: Encrypting a single file using the default settings.

   $ mcrypt file1.txt

   This command encrypts file1.txt and creates an encrypted file named file1.txt.nc.

   

2. File Encryption with Specified Algorithm: Encrypting a file using a specific encryption algorithm.

   $ mcrypt -a blowfish file1.txt

   This command uses the Blowfish algorithm to encrypt file1.txt, outputting file1.txt.nc.

3. File Encryption with Compression: Encrypting and compressing a file using gzip before encryption.

   $ mcrypt -z file2.txt

   This command compresses file2.txt with gzip and then encrypts it, resulting in file2.txt.nc.

4. Encrypting a Directory: Encrypting all files in a directory by first compressing the directory and then encrypting it.

   $ tar cz dir1/ | mcrypt -k abc123 > dir1.tar.gz.nc

   This command compresses dir1 into a tar.gz file and then encrypts it using the password abc123, resulting in dir1.tar.gz.nc.

5. File Encryption with a Keyfile: Using a keyfile for encryption instead of a passphrase.

   $ mcrypt -f keyfile.txt file1.txt

   This command reads the encryption key from keyfile.txt and uses it to encrypt file1.txt.

   

6. File Encryption with OpenPGP Format: Encrypting a file using the OpenPGP file format.

   $ mcrypt --openpgp file2.txt

   This command encrypts file2.txt in OpenPGP format, creating file2.txt.nc.

7. File Encryption with a Specific Key Size: Encrypting a file with a specified key size.

   $ mcrypt -s 32 file1.txt

   This command encrypts file1.txt using a key size of 32 bytes, resulting in file1.txt.nc.

   ---

   ---

Decryption Examples

Decrypting files encrypted with mcrypt is straightforward. Here are seven examples to demonstrate how to decrypt various types of encrypted data:

1. Basic File Decryption: Decrypting a file encrypted with the default settings.

   $ mcrypt -d file1.txt.nc

   This command decrypts file1.txt.nc, outputting the original file1.txt.

2. File Decryption with Specified Algorithm: Decrypting a file using a specific encryption algorithm.

   $ mcrypt -d -a blowfish file1.txt.nc

   This command uses the Blowfish algorithm to decrypt file1.txt.nc, outputting the original file1.txt.

3. Decryption of Compressed File: Decrypting a file that was compressed before encryption.

   $ mcrypt -d file2.txt.nc

   This command decrypts file2.txt.nc and decompresses it, outputting the original file2.txt.

4. Decrypting a Directory: Decrypting and extracting a directory that was compressed and encrypted.

   $ mcrypt -d dir1.tar.gz.nc
   $ tar -xzvf dir1.tar.gz

   This command decrypts dir1.tar.gz.nc and then the next command extracts the contents, restoring dir1.

5. File Decryption with a Keyfile: Using a keyfile for decryption instead of a passphrase.

   $ mcrypt -d -f keyfile.txt file1.txt.nc

   This command reads the decryption key from keyfile.txt and uses it to decrypt file1.txt.nc.

6. File Decryption with OpenPGP Format: Decrypting a file encrypted in OpenPGP format.

   $ mcrypt -d --openpgp file2.txt.nc

   This command decrypts file2.txt.nc which was encrypted in OpenPGP format, outputting the original file2.txt.

7. File Decryption with a Specific Key Size: Decrypting a file with a specified key size.

   $ mcrypt -d -s 32 file1.txt.nc

   This command decrypts file1.txt.nc using a key size of 32 bytes, resulting in the original file1.txt.

Conclusion

Encrypting and decrypting files and directories on Linux using mcrypt is a powerful way to protect your sensitive data. This guide has provided various examples of how to use mcrypt for both encryption and decryption, with and without compression, using different algorithms and formats. By following these examples, you can effectively secure your data against unauthorized access.