First things first, I have no prior experience with GNU Privacy Guard (GPG). I’ve been aware of it, and familiar with its purpose, but I have never actually used it. Time to change that! In carrying out most of this, I followed a guide here

apt update
apt install gnupg

Create a key pair

gpg --gen-key

Choose (1) RSA and RSA, set a keysize, and determine an expiration
Enter your information, and generate the key

gpg (GnuPG) 1.4.20; Copyright (C) 2015 Free Software Foundation, Inc.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

gpg: directory `/home/user/.gnupg' created
gpg: new configuration file `/home/user/.gnupg/gpg.conf' created
gpg: WARNING: options in `/home/user/.gnupg/gpg.conf' are not yet active during this run
gpg: keyring `/home/user/.gnupg/secring.gpg' created
gpg: keyring `/home/user/.gnupg/pubring.gpg' created
Please select what kind of key you want:
(1) RSA and RSA (default)
(2) DSA and Elgamal
(3) DSA (sign only)
(4) RSA (sign only)
Your selection? 1
RSA keys may be between 1024 and 4096 bits long.
What keysize do you want? (2048) 4096
Requested keysize is 4096 bits
Please specify how long the key should be valid.
0 = key does not expire
= key expires in n days
w = key expires in n weeks
m = key expires in n months
y = key expires in n years
Key is valid for? (0) 0
Key does not expire at all
Is this correct? (y/N) y

You need a user ID to identify your key; the software constructs the user ID
from the Real Name, Comment and Email Address in this form:
"Heinrich Heine (Der Dichter) <>"

Real name: User Name
Email address:
Comment: <comment>
You selected this USER-ID:
"User Name (comment) &lt;;"

Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? o
You need a Passphrase to protect your secret key.

We need to generate a lot of random bytes. It is a good idea to perform
some other action (type on the keyboard, move the mouse, utilize the
disks) during the prime generation; this gives the random number
generator a better chance to gain enough entropy.
We need to generate a lot of random bytes. It is a good idea to perform
some other action (type on the keyboard, move the mouse, utilize the
disks) during the prime generation; this gives the random number
generator a better chance to gain enough entropy.
gpg: /home/user/.gnupg/trustdb.gpg: trustdb created
gpg: key ████████ marked as ultimately trusted
public and secret key created and signed.

gpg: checking the trustdb
gpg: 3 marginal(s) needed, 1 complete(s) needed, PGP trust model
gpg: depth: 0 valid: 1 signed: 0 trust: 0-, 0q, 0n, 0m, 0f, 1u
pub 4096R/4F2220F6 2017-12-06
Key fingerprint = ████ ████ ████ ████ ████ ████ ████ ████ ████ ████
uid User Name (comment) &lt;;
sub █████/███████ 2017-12-06

Create revocation certificate

gpg --output ~/revocation.crt --gen-revoke

Select the reason for the revocation (I chose 0)

gpg --output revocation.crt --gen-revoke
sec █████/████████ 2017-12-06 User Name (comment) &lt;;

Create a revocation certificate for this key? (y/N) y
Please select the reason for the revocation:
0 = No reason specified
1 = Key has been compromised
2 = Key is superseded
3 = Key is no longer used
Q = Cancel
(Probably you want to select 1 here)
Your decision? 0
Enter an optional description; end it with an empty line:
&gt; revocation certificate for key
Reason for revocation: No reason specified
revocation certificate for key
Is this okay? (y/N) y

You need a passphrase to unlock the secret key for
user: "User Name (comment) &lt;;"
4096-bit RSA key, ID ████████, created 2017-12-06

gpg: gpg-agent is not available in this session
ASCII armored output forced.
Revocation certificate created.

Please move it to a medium which you can hide away; if Mallory gets
access to this certificate he can use it to make your key unusable.
It is smart to print this certificate and store it away, just in case
your media become unreadable. But have some caution: The print system of
your machine might store the data and make it available to others!

With that revocation certificate, you can revoke your GPG key if it should become lost or compromised. For this reason, it’s critically important that you protect it just like your private key, otherwise someone could revoke your key without your knowledge.

chmod 600 ~/revocation.crt

A few commands follow:

    • Importing Other Users’ Public Keys
gpg --import name_of_pub_key_file
gpg --keyserver --search-keys search_parameters
    • Fingerprint public key
gpg --fingerprint
    • Sign the key, telling the software to trust it
gpg --sign-key
    • Sending back the signed key
gpg --output ~/signed.key --export --armor
    • They can then import it, adding the signature to their database
gpg --import ~/signed.key
    • To retrieve your public key
gpg --output ~/mygpg.key --armor --export

Now for the good stuff. In order to encrypt messages, do the following:

gpg --encrypt --sign --armor -r name_of_file

If you want to be able to read the message (you made it so theoretically you don’t need to do this) then add a second -r recipient
And in order to decrypt messages, it’s pretty simple:

gpg file_name.asc

There’s no point in having a key if other people don’t have your public key. An easy way to enable that is by submitting it to public key repositories, like below. At the time of writing, seemed to be having some issues, so I instead uploaded to The nice thing is these keyservers exchange their entries with one another, so it’ll make it to a number of places eventually. A solid list of keyservers can be found here.

gpg --list-keys
pub 4096R/311B1F84 2013-10-04
uid Test User &lt;;
sub 4096R/8822A56A 2013-10-04

The 311B1F84 portion is your key’s ID

gpg --send-keys --keyserver key_id

Now in the event that your key gets compromised or lost, you can use the revocation certificate that you created previously like so

gpg import revocation.crt
gpg --key-server --send-keys key_id

List of keyservers: link

Now there’s a whole lot more to GPG than what I’ve written, and the link I posted at the beginning of this post is a good primer on a lot of it. My purpose is to get GPG up and running, and I think that that has been achieved!

As a side note, I’ll also mention Their whole deal seems to be lowering the bar to using GPG and simplifying the process of verifying peoples’ online identities. You can use it to encrypt and decrypt messages from a GUI, and can chat with other users while the encryption process happens in the background. They have also recently added a ‘teams’ functionality which seems to be a response to Slack, except since everyone has GPG keys, everything is encrypted. Check it out, and give me a follow while you’re at it.

Securing SSH

SSH is probably the best way to interact with the Raspberry Pi, especially if it is a headless machine. As a way in to your machine, it can also present a security hazard and needs to be properly configured.

nano /etc/ssh/sshd_config

And add / change the following

PermitRootLogin no
PubKeyAuthentication yes
AuthorizedKeysFile %h/.ssh/authorized_keys
PasswordAuthentication no
PermitEmptyPasswords no
X11Forwarding no
IgnoreRhosts yes
MaxAuthTries 3

If necessary, create the directory and file for your authorized keys and ensure that only you can read it.

mkdir ~/.ssh
touch ~/.ssh/authorized_keys
chmod 600 ~/.ssh/authorized_keys

You can test your configuration with ssh -t. When ready, restart the SSH daemon with service ssh restart. I suggest allowing password authentication until you are certain that your public key works. I also recommend protecting your key with a password. Sure, a password-less key is still more secure than a simple password, but if anyone else gains access to your key then they can access your machine without issue. A password on your key will help to prevent this and allow you to revoke the lost keys (hopefully) before that password can be cracked. And the failed authentication attempts will be a good indicator that the key may have been compromised.

Generate your keys either in BASH or with PuTTYGen (which is what I used), insert the public key into the authorized_keys file (ensuring to use the proper format), and make sure that it works.

Raspberry Pi – Portknocking with knockd

56 Raspberry Pi – Portknocking with knockd Security through Obscurity is not Security. That being said, obscuring your SSH port via port knocking can be a great way to protect your SSH service from drive-bys and script kiddies. It is no replacement for a proper service configuration, but when paired with a good config can greatly improve your security. Also, it’s just kind of cool; who doesn’t wish they had a secret passage in their home?

Port knocking, at its core, is pretty simple. The daemon listens for connection attempts, and if attempts are made to the correct ports, in the correct order, in the specified amount of time, then it executes a command. Typically the command is to either add or delete a firewall rule. I followed this guide to get it up and running, but it is deprecated, and therefore there were some issues. All of the solutions are contained herein. First, we need to set up our firewall rules per this guide, ensuring that we don’t lock ourselves out of SSH access until we are certain that everything works.

apt install knockd
touch /var/log/knockd.log
nano /etc/knockd.conf
    LogFile = /var/log/knockd.log
    interface = wlan0

    sequence = 7303,40303,33528
    tcpflags = syn
    seq_timeout = 15
    start_command = /sbin/iptables -I INPUT 4 -s %IP% -p tcp --dport 22 -j ACCEPT
    cmd_timeout = 10
    stop_command = /sbin/iptables -D INPUT -s %IP% -p tcp --dport 22 -j ACCEPT

Next edit

nano /etc/default/knockd

and change the following line


IMPORTANT! This is where we run into some issues. The Debian version of knockd will not start at boot, no matter what you do. In theory, the above modification to /etc/default/knockd should make this happen, but it doesn’t. After countless Google searches, I found a patch in a Debian bug post. The patch follows:

nano /lib/systemd/system/knockd.service

Append the following:


Finally, run the following and knockd will start at boot

systemctl enable knockd.service

Start knockd and test it out. If you have any issues, you can run the following to try to debug your problems

knockd -D -v
service knockd start

In order to carry out the port knocking, I chose to write a Python script, since I knew that that would work on both my Linux and Windows machines since they all have Python installed. As long as Scapy is installed then it’s good to go. See the script here.

Raspberry Pi – iptables for knockd

54 Raspberry Pi – iptables for knockd I’m sure that there are more secure ways to configure iptables, what follows is a basic configuration to get knockd up and running (the guide to which can be found here rel=”noopener” target=”_blank”). All of the below came from following the linked guide at DigitalOcean.

Our iptables configuration will block most traffic, while still allowing local, established, and port 80 connections. Our first rule will allow local traffic which the machine uses to talk to itself

iptables -A INPUT -i lo -j ACCEPT

You’ll notice that the rule is being applied to the INPUT chain, to the lo or loopback interface (where you find local traffic), and has been configured to ACCEPT rather than DENY the traffic.

Next we want to ensure that all established connections and traffic related to them are allowed. I’m a bit hazy on the specifics of what we’re doing in this rule, so I’ll need to look up the particulars later.

iptables -A INPUT -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT

This rule is pretty important, because without it we would lose our active SSH connection (which we are presumably using rather than hooking our pi up to a monitor) when we enter our deny all rule. Next, since we have a web service running, we’ll make a rule to ensure that that is accessible.

iptables -A INPUT -p tcp --dport 80 -j ACCEPT

Pretty straight forward stuff. Now that we know that all traffic we want to allow has been accounted for, we will add our final rule. It’s important to note that as we add each of these rules, they get appended to the bottom of that list. When iptables checks the firewall rules to determine whether or not to allow traffic, it starts at the top of the list and works towards the bottom until it finds a rule that tells it what to do. So if traffic doesn’t match any of the above, iptables will check it against our final rule which will tell it to drop the traffic.

iptables -A INPUT -j DROP

Go ahead and view the rules with the following command

iptables -L --line-numbers
Chain INPUT (policy ACCEPT)
num  target     prot opt source               destination
1    ACCEPT     all  --  anywhere             anywhere
2    ACCEPT     all  --  anywhere             anywhere             ctstate RELATED,ESTABLISHED
3    ACCEPT     tcp  --  anywhere             anywhere             tcp dpt:http
4    DROP       all  --  anywhere             anywhere

This is a useful way to look at them because each rule is in a more orderly format, and has a line number attached. If you ever wanted to delete a rule or insert one at a specific point in the list, you would need to know which line number you needed. With this command, you can easily determine that. An alternative is

iptables -S
-A INPUT -i lo -j ACCEPT
-A INPUT -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A INPUT -p tcp -m tcp --dport 80 -j ACCEPT

Unfortunately, iptables has an annoying tendency to flush your rules on shutdown if you don’t make them persistent. We’ll do this by installing iptables-persistent.

apt install iptables-persistent

At this point, I learned that the service that we need to start is no longer iptables-persistent, but netfilter-persistent. That’s what I get for following a deprecated guide. I don’t know if we could avoid installing iptables-persistent and just install netfilter-persistent, which comes with it, but I’m not going to bother trying to figure it out. It works. So now just start the service and we’re good to go, the rules will remain persistent over power cycles.

service netfilter-persistent start

Save the current rules to persistence:

netfilter-persistence save

With that, we’re ready to move on to setting up knockd.

Raspberry Pi – iptables Script

While we don’t have many rules in our firewall, it can end up being convenient to have a script on-hand so that we can automatically restore the rules that we want. I copied the structure from these guys and filled my own rules in (I left some of their intriguing bits in as comments for the time being).

nano /root/
# Set up iptables baseline

# Flush rules to work from a clean slate
iptables -F

# Set default policies for INPUT, FORWARD, and OUTPUT chains
#iptables -P INPUT DROP
#iptables -P FORWARD DROP
#iptables -P OUTPUT ACCEPT

# Allow access for localhost
iptables -A INPUT -i lo -j ACCEPT

# Accept packets belonging to established and related connections
iptables -A INPUT -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT

# Add access from other computers on our network
#iptables -A INPUT -s <IP Address 1> -j ACCEPT
#iptables -A INPUT -s <IP Address n> -j ACCEPT

# Open port for HTTP
iptables -A INPUT -p tcp --dport 80 -j ACCEPT

# Deny All
iptables -A INPUT -j DROP

# List iptables chains
iptables -L --line-numbers
chmod 700 /root/

Don’t forget that last bit. Not only does it allow us to execute the script, but probably more importantly it hides our firewall configuration from unauthorized eyes. Again, obscurity is NOT security, but it can help augment security.