How to fix apt update EXPKEYSIG 1DB7590E83C8F643

Problem:

When running sudo apt update you see the following error message:

Err:14 https://ocean.surfshark.com/debian stretch InRelease
  The following signatures were invalid: EXPKEYSIG 1DB7590E83C8F643 Surfshark package maintainer <[email protected]>

Solution

Install the updated Surfshark package by pasting the following into your shell (as shown on the Surfshark help page):

curl -f https://downloads.surfshark.com/linux/debian-install.sh --output surfshark-install.sh
cat surfshark-install.sh
sh surfshark-install.sh

After that, the error should be fixed and you can run

sudo apt update

again.