Tags: jitsi installation, secure, ubuntu, linux
Last update: Jan 2021

Prerequisites

• a fresh installation of Ubuntu 21.04 LTS or Ubuntu 18.04 LTS
• verify all updates are installed by issuing

sudo apt-get update && sudo apt-get dist-upgrade && sudo apt-get autoremove

• If you intend to run Jitsi on your new publicly accessible server, reachable under meet.yourdomain.com, you should change the name of your host accordingly in both files and reboot your machine:

sudo nano /etc/hostname
sudo nano /etc/hosts

• These instructions also take care of the TLS certificate. If the server is meant to be in your own infrastructure, you should ensure that at least ports 80 and 443 can be reached from outside for creating the TLS certificate. That is easily overlooked.

Configuring the firewall

# allow HTTP, HTTPS and UDP datagram for Jitsi communication packets
sudo ufw allow 80/tcp
sudo ufw allow 443/tcp
sudo ufw allow 4443/tcp
sudo ufw allow 10000/udp

# allow SSH if you need it
sudo ufw allow 22/tcp

# enable firewall and check status
sudo ufw enable
sudo ufw status

Installing Jitsi packages

First we need to import the official Jitsi repository:

# importing the repo
echo 'deb https://download.jitsi.org stable/' | sudo tee /etc/apt/sources.list.d/jitsi-stable.list
# importing the encryption key for the repo
wget -qO - https://download.jitsi.org/jitsi-key.gpg.key | sudo apt-key add -

First we install the required packages:

# rebuildung the package list
sudo apt-get update
# installing needed packages
sudo apt-get install apt-transport-https jitsi-meet

You’ll be asked for the hostname of the current installation. Use meet.yourdomain.com.

Afterwards choose to generate a self-signed Let’s Encrypt certificate.

Obtaining a TLS certificate

We will use the builtin shell script for getting our certificate but it depends on the certbox package:

# installing lets encrypt's certbot
sudo apt-get install certbot

For reasons of simplicity and avoiding some common errors we use the prepared script for fetching the encryption certificate automatically. All we have to do is to provide our mail address.

sudo /usr/share/jitsi-meet/scripts/install-letsencrypt-cert.sh

Accessibility test

Point your preferred browser to https://meet.yourdomain.com and verify the installation. The page should be TLS encrypted and not showing any errors.

Avoiding abuse / configuring access control

By default the current installation of Jitsi is allowing everybody to enter our just created virtual meeting room without any access control. If you leave it in that state, anyone can connect to the server and transmit audio and video steams – even if no one else is inside the meeting. This inevitably leads to the server operator sending you an abuse notice and blocking your server due to high traffic. In a self-experiment, I experienced around 3000 GB of data running through my unsecured server. Don’t try this at home 😉

To restrict this abuse we’ll

• enable accounts for us (authenticated users) and for all others (guests)
• allow guests to join only if we have entered a certain room before

Turning on access control
We need to edit 3 files.

1. We turn access control on by changing the authentication type:

sudo nano /etc/prosody/conf.avail/meet.yourdomain.com.cfg.lua

by changing the block:

VirtualHost "meet.yourdomain.com"
    -- enabled = false -- Remove this line to enable this host
    authentication = "anonymous"

to:

VirtualHost "meet.yourdomain.com"
    -- enabled = false -- Remove this line to enable this host
    authentication = "internal_hashed"

and allowing guests to log in by adding at the end of the file:

VirtualHost "guest.meet.yourdomain.com" 
	authentication = "anonymous" 
	c2s_require_encryption = false

2. We edit a file belonging to Jitsi:

sudo nano /etc/jitsi/meet/meet.testprojects.me-config.js

by changing the block:

var config = {
    // Connection
    //

    hosts: {
        // XMPP domain.
        domain: 'meet.yourdomain.com',

        // When using authentication, domain for guest users.
        // anonymousdomain: 'guest.example.com',

to:

var config = {
    // Connection
    //

    hosts: {
        // XMPP domain.
        domain: 'meet.yourdomain.com',

        // When using authentication, domain for guest users.
           anonymousdomain: 'guest.meet.yourdomain.com',

3. We edit the Jicofo element:

sudo nano /etc/jitsi/jicofo/sip-communicator.properties

by adding a second line at the end of the file. From:

org.jitsi.jicofo.BRIDGE_MUC=JvbBrewery@internal.auth.meet.yourdomain.com

to:

org.jitsi.jicofo.BRIDGE_MUC=JvbBrewery@internal.auth.meet.yourdomain.com
org.jitsi.jicofo.auth.URL=XMPP:meet.yourdomain.com

Adding XMPP users
Now the access control is configured and we have to add users to XMPP/Prosody in order to authenticate ourselves and to create rooms:

sudo prosodyctl register <username> meet.yourdomain.com <password>

Restart everything
because we have finished configuration:

sudo systemctl restart prosody && sudo systemctl restart jicofo && sudo systemctl restart jitsi-videobridge2

Test your Jitsi installation

Point your browser again to https://meet.yourdomain.com and try to create a conference room. Usually (before our efforts above) a room would be created instantly and you could join. Now you are getting the hint that your room hasn’t been opened yet. Only you can open it by hitting the button to authenticate in order to allow others in.

Customizing your Jitsi installation

If you want to change the default branding you can edit the interface configuration file:

sudo nano /usr/share/jitsi-meet/interface_config.js

You might find the following line/settings useful:

# line 5 changes the page title
APP_NAME: 'Jitsi Meet',

# line 49 changes the logo within the room
DEFAULT_LOGO_URL: 'images/watermark.svg',

# line 50 changes the default name for guests
DEFAULT_REMOTE_DISPLAY_NAME: 'Fellow Jitster',

# line 51 changes the logo on the first page
DEFAULT_WELCOME_PAGE_LOGO_URL: 'images/watermark.svg',

# line 100 changes the animation of generic room names
GENERATE_ROOMNAMES_ON_WELCOME_PAGE: true,

# line 113 changes the logo link
JITSI_WATERMARK_LINK: 'https://jitsi.org',

# line 167 changes the visibility of existing rooms
RECENT_LIST_ENABLED: false,

Credits & further reading

• Aaron Kili: How to Install Jitsi Meet on Debian and Ubuntu (EN)
• Tobias Scheible: Das Design von Jitsi Meet anpassen (DE)

Tags: Linux
Last update: Mar 2018

1. Compare binary files within the console and highlight those areas which differ. Vim and Colordiff have to be installed before.

diff -y <(xxd file.one) <(xxd file.two) | colordiff

2. Using a harddisk device for storing encrypted data:

sudo cryptsetup luksFormat -c aes-xts-plain64 -s 512 -h sha512 -y /dev/sdX
sudo cryptsetup luksOpen /dev/sdX usb-backup
sudo mkfs.btrfs -L backupharddisk /dev/mapper/backup
sudo mount /dev/mapper/backup /usb-backup

3. Mirroring with rsync from /source to /destination. Compression is great when backing up over the Internet.

rsync --compress --compress-level=9 --human-readable -av --progress --delete --log-file=/path/to/log.file /source /destination

4. Wipe a file with zeros:

shred -fvzun 0 file

5. Wipe a directory recursively with zeros:

srm -rvs directory

6. Backup of harddisks into compressed image file. Due to its multithreading capability pigz is much faster than gzip during compression.

sudo dd if=/dev/sdX status=progress bs=1M | pigz > harddisk.img.gz

7. Restore this prior zipped image file and write it directly to disk:

sudo gunzip -c image.gz | sudo dd of=/dev/sdX status=progress bs=1M

8. Recover a Linux Soft-RAID md/5:

sudo mdadm --assemble --scan
sudo cryptsetup luksOpen /dev/md/5 temp
sudo mount /dev/mapper/temp /your_path

9a. Mount a Linux Soft-RAID manually:

sudo mdadm --examine /dev/sdX
sudo mdadm -A -R /dev/md9 /dev/sdX
sudo mount /dev/md9 /openedRaidPartition

9b. And close it again:

sudo umount /openedRaidPartition
sudo mdadm -S /dev/md9

10. List all loadable kernel modules:

find /lib/modules/$(uname -r) -type f -name \*.ko

11. Template for configuring a fresh manjaro xfce installation:

#update system
sudo pacman -Syyu
 
#install zshell
sh -c "$(curl -fsSL https://raw.github.com/robbyrussell/oh-my-zsh/master/tools/install.sh)"
sudo pacman -S powerline-fonts
 
#install virtualbox
sudo pacman -S virtualbox
 
#copy fonts which you need into /usr/share/fonts and run
sudo fc-cache
 
#fixing the Manjaro Linux XFCE text shadow problem on the desktop
xfconf-query -c xfce4-desktop -p /desktop-icons/center-text -n -t bool -s false

12. Nice to have Manjaro packages:

gufw (firewall gui)
chromium (browser)
synapse (launcher)
psensor (temperature)
ether (pendrive tool)
file-roller
p7zip zip unzip unrar
tilda (background terminal)

13. Laravel development quickstart:

git clone project.git
cd project
composer install
composer update
cp .env.example .env
php artisan key:generate
php artisan serve --port=8000

14. Restricting sFTP:

Add to /etc/ssh/sshd_config:

Subsystem sftp internal-sftp
 
  Match Group sftpusers
  ChrootDirectory %h
  ForceCommand internal-sftp
  AllowTcpForwarding no
  X11Forwarding no

Adding user and change configuration:

useradd newuser
usermod -s /bin/false newuser
chown root:root /home/newuser
chmod 755 /home/newuser
mkdir /home/newuser/writablefolder
chown newuser:newuser /home/newuser/writeablefolder
chmod 755 /home/newuser/writeablefolder
 
groupadd sftpusers
usermod -G sftpusers newuser
 
service sshd restart
service vsftpd restart

15. Replacing a broken harddisk (/dev/sda) with two partitions with RAID1 (md0 + md1):

#after having replaced the harddisk; copying MBR partition table
sfdisk -d /dev/sdb | sfdisk /dev/sda
#adding partitions to existing RAID-array md0 and md1:
sudo mdadm /dev/md0 -a /dev/sda1
sudo mdadm /dev/md1 -a /dev/sda2
#reinstalling grub on both disks again, leave options empty, select two disks (/dev/sda + /dev/sdb)
sudo dpkg-reconfigure grub-pc

16. Rsync command which doesn’t copy everything every time while preserving times:

rsync -avP source destination
#a = archive mode
#v = verbose
#P = show progress

17. Bash: iterate through all files within a specific folder and count its files

for i in /folderpath /folderpath ; do 
    echo -n $i": " ; 
    (find "$i" -type f | wc -l) ; 
done

18. copying KVM to PROXMOX

# converting image
qemu-img convert -f qcow2 -O raw image.qcow2 image.img

# creating VM in proxmox
qm create 120 --bootdisk scsi0

# importing disk to someStorage (as named in "pvesm status")
# importdisk adds the image as unused disk
qm importdisk 120 someImage.img someStorage

# attaching the image to the VM. still needs to be marked as boot device by hand
qm set 120 --scsi0 someStorage:vm-120-disk-0

19. repair a degraded ZFS pool/raid1 in PROXMOX

# "zpool status" output during normal operation

  pool: rpool
 state: ONLINE
  scan: none requested
config:

	NAME                                             STATE     READ WRITE CKSUM
	rpool                                            ONLINE       0     0     0
	  mirror-0                                       ONLINE       0     0     0
	    ata-SanDisk_SDSSDH3_2T00_193732800276-part3  ONLINE       0     0     0
	    ata-SanDisk_SDSSDH32000G_192970802134-part3  ONLINE       0     0     0

errors: No known data errors

# "zpool status" of degraded state if we unplug /dev/sdb manually

  pool: rpool
 state: DEGRADED
status: One or more devices could not be used because the label is missing or
	invalid.  Sufficient replicas exist for the pool to continue
	functioning in a degraded state.
action: Replace the device using 'zpool replace'.
   see: http://zfsonlinux.org/msg/ZFS-8000-4J
  scan: none requested
config:

	NAME                                             STATE     READ WRITE CKSUM
	rpool                                            DEGRADED     0     0     0
	  mirror-0                                       DEGRADED     0     0     0
	    ata-SanDisk_SDSSDH3_2T00_193732800276-part3  ONLINE       0     0     0
	    ata-SanDisk_SDSSDH32000G_192970802134-part3  UNAVAIL      3   212     0

errors: No known data errors

We see that the disk with ID ata-SanDisk_SDSSDH32000G_192970802134-part3 is not available any more. We have to do the following steps:

• shutdown the server
• plug in a brand new replacement disk
• boot the server up again

You’ll see now another status:

  pool: rpool
 state: DEGRADED
status: One or more devices could not be used because the label is missing or
	invalid.  Sufficient replicas exist for the pool to continue
	functioning in a degraded state.
action: Replace the device using 'zpool replace'.
   see: http://zfsonlinux.org/msg/ZFS-8000-4J
  scan: none requested
config:

	NAME                                             STATE     READ WRITE CKSUM
	rpool                                            DEGRADED     0     0     0
	  mirror-0                                       DEGRADED     0     0     0
	    ata-SanDisk_SDSSDH3_2T00_193732800276-part3  ONLINE       0     0     0
	    14464946144281260868                         UNAVAIL      0     0     0  was /dev/disk/by-id/ata-SanDisk_SDSSDH32000G_192970802134-part3

We need to prepare the new drive and replace it:

# copy partition table from good /dev/sda to new /dev/sdb
sgdisk -R /dev/sdb /dev/sda
sgdisk -G /dev/sdb

# copy the bios and efi boot partitions
dd if=/dev/sda1 of=/dev/sdb1 bs=512
dd if=/dev/sda2 of=/dev/sdb2 bs=512

# replace command
zpool replace rpool ata-SanDisk_SDSSDH32000G_192970802134-part3 /dev/sdb3

Keep in mind: Never use the detach command to remove a disk from a pool, because by adding it again you might be not careful enough to add it in mirror function. The default behaviour is to add a disk in striping mode (raid0)! You’ll have to backup everything and to reinstall a mirrored array again from scratch. Useful commands are:

# show raid status
zpool status

# show all your disk IDs
ls -l /dev/disk/by-id/

Tags: Secure wipe, Linux
Last update: Dec 2017

First we want to fill up our pendrive with zeroes:

sudo cat /dev/zero > /dev/your_pendrive

After completion we can check if our flash drive is really wiped with zeros by installing and using a text-based Hex-editor:

sudo apt-get install ht

Remember: We trust nobody so we check manually if zeros have been written to the drive. If you see weird characters something went wrong.

sudo hte /dev/your_pendrive

Tags: Coreboot, F2A85-M, F2A85-M PRO, A10-5800K
Last Update: Jan 2021

This guide is based on the BIOS UEFI replacement called Coreboot which runs on an ASUS F2A85-M or F2A85-M PRO mainboard with an AMD A10-5800K CPU with integrated graphics.

1. Introduction and aim

This initial idea came up with the uprising problems and vulnerabilities of Intel ME and AMD Secure Technology (PSP). After having researched several alternatives without running those (in)secure technologies I was looking for very specific hardware to meet the requirements on security as mentioned in the pinetree of open and secure classical computing.

The aim is to provide a comprehensive guide which allows the daily user to gain a higher level of security. The daily user shall be empowered to get back the control about every piece of hardware within the computer system. In general hardware is always run by the so-called firmware. Firmware is software which is in the closest control of the underlying hardware to make it work as intended. So firmware is needed by the all BIOS/UEFI computer systems we know. Based on these hardware firmwares the central BIOS/UEFI unit is able to tell other computer parts how to cooperate. The newer the hardware, the worse the availability of open firmware. In fact in most cases the availability of open code is zero, because vendors assume that keeping the source code secret (closed source code) increases the security of the product. Nowadays we’ve learned by many lessons that this conviction is wrong. It’s a fact that every source contains errors which might be exploitable if they are found. The more people access a source code, the better this code is reviewed and the fewer errors are included.

Conclusion: Due to this circumstance sometimes developers have to use vendor’s original binary code (firmware) to get the hardware to work at all. That means that wide limitations are accepted by almost all vendors. Security can only be guaranteed by open source code. This leads to a conflict of objectives: On one hand we want a secure system and on the other hand we don’t want to lose comfort by leaving out the functionality which the vendor has placed within his (closed) firmware.

Maybe you take this into account before you support a certain vendor by buying his hardware the next time.

Since it’s practically almost impossible to use every piece of hardware out there by open code, we need to find the best compromise between functionality and security:

• free and open source code
• running Coreboot or even Libreboot
• running Windows and GNU/Linux
• as little as possible of (closed) firmware
• enough performance to get work done

2. Software we will need

• For booting the system we will use Coreboot instead of the vendor’s BIOS/UEFI. Libreboot is even a “better” option but – as mentioned in other posts – unfortunately very few and old boards are supported.
• As an operating system you can choose one from the recommendations. I prefer using Manjaro GNU/Linux with the free driver stack option which you can choose while booting the installation disk from a pen-drive. In my view this is the best mix between bleeding edge development, security and user comfort.
• If you prefer to build the Coreboot ROM on your own, you need a Linux on which you can compile the Coreboot sources. Ubuntu is usually a good choice.
• If you prefer to extract the VGA BIOS on your own, you will also need to run a live Linux from a pen-drive for extraction.

3. Hardware you will need (mostly second hand)

Hardware used	Notes	$
Mainboard F2A85-M PRO	Both the ASUS F2A85-M or F2A85-M PRO are currently the best choice for a desktop platform, because it is widely supported by Coreboot.	40
CPU AMD A10-5800K	As we found out before AMD’s Piledriver and Steamroller CPU architectures (codenames Trinity, Richland and Kaveri) are the latest ones which does NOT include those (in)secure technologies. The AMD A10-5800K or AMD A10-6800K are the most compatible and powerful CPU/APU you can run on with Coreboot on this mainboard.	40
CPU Cooler Xilence A250PWM	Pick any adequate CPU cooler you want which is supporting the FM2 socket. You can use a good old Arctic Freezer A30 with a silent 120mm fan (~ 30 $). If you want something new you can take the Xilence A250PWM (XC035).	10
Memory Kingston KHX1600C10D3B1K2	DDR3-1866 RAM is the fastest RAM speed our CPU is supporting. The amount depends just on your personal demands. With a minimum of 8 GB we should be good to go. I’ve been successful with Corsair CML16GX3M2A1600C100 and Kingston KHX1600C10D3B1K2; but there are many many others that will work, too.	50
HDD/SSD	We need a SATA hard-drive for the operating system. Think about using a SSD if you want a fast system. Remember to encrypt your partitions and/or files – especially if you are using a SSD.	50
GPU	In the CPU above the graphics are onboard and included within the CPU. So there is no need to buy a dedicated one.	0
Power Supply	As a power supply any standard ATX AC adaptor will work.	30
Case Bitfenix Prodigy M	Choose a computer case according to your personal preferences. I like the Bitfenix Prodigy M. Remember: Not all micro ATX cases are suitable for taking bigger/higher CPU coolers. This one takes and bigger CPU cooler like the Freezer A30 perfectly.	50
Flashtool CH341A	In order to flash the BIOS chip via SPI we need a CH341A USB Programmer.	15
BIOS chip W25Q64FVAIG	I recommend strongly to buy a separate Winbond W25Q64FVAIG or W25Q64BVAIG spare chip in order to flash the Coreboot build onto it. There are people who flash the onboard chip during a running system but this process can fail and in my opinion it’s better to have a fallback – just in case flashing or extraction have failed.	10
	total cost, approx.	295

4. Assembling your computer

I assume you know how to assemble a computer so now it’s a good time to do so. Make sure your DDR3 RAM is placed into the BLUE SLOTS. Otherwise your computer won’t boot up.

5. Extracting the VGA BIOS binary blob

Unfortunately the internal GPU won’t work unless we integrate its binary into the Coreboot ROM.

You can download now the vgabios.zip which contains the vgabios.bin which I have extracted already from an AMD A10-5800K. Or you download it on your own following the Coreboot guide:

Boot up a linux live Distro like Ubuntu from your prepared pendrive.

Check the ID by issueing “lspci -tvnn”. As you can see my ID of the GPU is 1002:9901. Write the ID down. We will need it later to build the Coreboot ROM.

lspci -tvnn
-[0000:00]-+-00.0 Advanced Micro Devices, Inc. [AMD] Family 15h (Models 10h-1fh) Processor Root Complex [1022:1410]
           +-00.2 Advanced Micro Devices, Inc. [AMD] Family 15h (Models 10h-1fh) I/O Memory Management Unit [1022:1419]
           +-01.0 Advanced Micro Devices, Inc. [AMD/ATI] Trinity [Radeon HD 7660D] [1002:9901]
           +-01.1 Advanced Micro Devices, Inc. [AMD/ATI] Trinity HDMI Audio Controller [1002:9902]
...

For enabling to read the ROM we need to set a enablement bit 1 to the corresponding address 0000:00:01.0 which we found above:

echo 1 > /sys/devices/pci0000:00/0000:00:01.0/rom

Now we can extract the VGA BIOS into a binary. Copy the vgabios.bin file onto a pendrive.

cp /sys/devices/pci0000:00/0000:00:01.0/rom vgabios.bin

6. Building the Coreboot ROM

If you want to skip this step or encounter unsolvable errors prohibiting you to generate the ROM you can download one of my compiled ROMs:

• coreboot.rom for AMD A10-5800K & ASUS F2A85M-PRO
• coreboot.rom for AMD A10-5800K & ASUS F2A85M (coming soon)

Since the old method isn’t working anymore you can decide now if you want to prepare Ubuntu Linux on your own or want to download my already prepared Virtual Machine I prepared for you.

6a. Preparing Ubuntu 20.04 LTS for building Coreboot

Get yourself a fresh copy of Ubuntu 20.04 LTS. Supply your Ubuntu with needed packages:

sudo apt-get install git build-essential gnat flex bison libncurses5-dev wget zlib1g-dev python

Let’s clone the Coreboot repository and change into that folder:

git clone https://review.coreboot.org/coreboot
cd coreboot

Now copy your vgabios.bin from chapter 5 into the root directory of your Coreboot folder. Alternatively you can use my extracted vgabios.bin from a 5800K:

wget https:/freundschafter.com/download/coreboot/vgabios.bin

Now you can set up the Coreboot environment for building the Coreboot ROM, which we’ll transfer to our BIOS chip.

6b. Setup your settings for the Coreboot build

Run the configuration wizard with:

make nconfig

Set up the following parameters:

• Submenu Mainboard: Choose ASUS as vendor.
• Submenu Mainboard: Choose F2A85-M or F2A85-M PRO as model.
• Submenu Mainboard: Choose the DDR3 memory voltage. Refer to your vendor’s website if you are unsure. Possible values are 1.35 / 1.5 / 1.65 V. Some tester said 1.5 V is the failsafe value if you can’t figure it out.

• Submenu Chipset: Enable Hudson XHCI Controller
• Submenu Chipset: Add XHCI firmware. The default path is already set correctly.

• Submenu Devices: Add your VGA BIOS image (your extracted one, or mine) and make sure the PCI ID is matching yours by typing in the right values. For our CPU 5800K the value is always the same: 1002,9901.

Save your configuration by hitting “Save”, leave the filename “.config” and exit the tool.

Build the Compiler. CPUS=4 means it will use 4 cores to compile. Adjust this value to meet your number of cores. Be patient, this process takes some time. With my older notebook I had to wait around 10-20 minutes.

make crossgcc-i386 CPUS=4

Finally we are able to build the Coreboot ROM:

make

You will find the generated coreboot.rom in a subfolder called build.

7. Flashing the Coreboot ROM

• Place your spare chip on the USB programmer and plug it into a USB port on your Linux machine.
• Install flashrom under Ubuntu by

sudo apt-get install flashrom

Flash your spare chip now with your generated coreboot.rom:

sudo flashrom -n -w build/coreboot.rom --programmer ch341a_spi

If you get the following notifications your chip is ready:

Found Winbond flash chip "W25Q64.V" (8192 kB, SPI).
Erase/write done.
Verifying flash... VERIFIED.

Replace your old BIOS chip by the new flashed one during your system is switched off. Be careful not to bend a pin. So-called DIP8 pliers might help you to release the chip from the board.

Just in case you mess something up you can download a fresh vendor’s copy of the original BIOS/UEFI image and flash it again:

• ASUS F2A85-M: BIOS image v6508
• ASUS F2A85-M PRO: BIOS image v6601

8. Operating System

Now, equipped with Coreboot, you can choose to install any operating system you wish. Since I am very familiar with it I’d go with something Debian-based: PureOS. If you want to follow me, you’ll notice that after installation of PureOS 9.x your screen will remain black due to the lack of AMD graphics firmware packages within PureOS. (Actually it’s a good sign because now you see that binary blobs like the AMD firmware really weren’t loaded during bootup). To fix this we need to install a package into our PureOS installation:

8a. Prepare the pendrive

Get a pendrive formatted with FAT and use another computer to prepare it. Download the firmware.zip archive from https://cdimage.debian.org/cdimage/unofficial/non-free/firmware/testing/current/ and extract firmware-amd-graphics*.deb from it. If you use Linux you can use the following lines:

wget https://cdimage.debian.org/cdimage/unofficial/non-free/firmware/testing/current/firmware.zip
unzip firmware.zip firmware-amd-graphics*.deb

Copy this .deb package into your pendrive, unmount it and plug it into your secure computer system.

8b. Installing the AMD graphics firmware

First login into the system on another TTY by pressing Crtl + Alt + F3 by providing your user and password. Afterwards we will create a new folder as mounting point for your pendrive, mount it, and install the .deb package. You will need the identifier for your pendrive partition. You can find it out by using “lsblk” for instance. In my case it’s /dev/sdb1.

sudo mkdir /pendrive
sudo mount /dev/sbd1 /pendrive
cd /pendrive
sudo apt install ./fireware-amd-graphics*.deb
cd /
sudo umount /dev/sdb1

After having installed the graphics firmware you can now unplug your pendrive, reboot your system by issuing “reboot” and enjoy PureOS welcoming you.