How to fix 'certificate, issued by ‘/C=US/O=Let's Encrypt/CN=R3’: Issued certificate has expired.'

How to fix 'certificate, issued by ‘/C=US/O=Let's Encrypt/CN=R3’:   Issued certificate has expired.'

This how-to has been tested on Ubuntu 14.xx and higher and may also work on some older Debian OS.

Step 1 - You have to update your operating system to latest minor version:

sudo apt-get update

sudo apt-get dist-upgrade

reboot

Step 2 - Comment out DST_Root_CA_X3.crt

nano /etc/ca-certificates.conf

find 'DST_Root_CA_X3.crt' then put an exclamation mark in front of the line, like this:

!mozilla/DST_Root_CA_X3.crt

Step 3 - Force Update Certificates

sudo update-ca-certificates --fresh

Step 4 - Retest

Your expired Root CA X3 certificate has now been updated!

How to fix NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32

 Run these commands in the terminal:

sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 40976EAF437D05B5
sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 3B4FE6ACC0B21F32

This downloads the missing GPG keys (as mentioned in the original error message) from the Ubuntu GPG keyserver.

You can also add multiple keys with a single command:

apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 3B4FE6ACC0B21F32 40976EAF437D05B5

How to install SFTPGO on Ubuntu 18.xx and higher

 This is a very quick and easy guide how to install SFTPGO on Ubuntu Server 18.xx or higher:

sudo apt update

sudo apt install software-properties-common

sudo add-apt-repository ppa:sftpgo/sftpgo

sudo apt install sftpgo

sudo systemctl status sftpgo

Then, you can access / use SFTP go from:

http://{whatever your IP}:8080

SFTP port is 2022

How to fix apt-get update GPG "NO PUBKEY" error?

Are you getting the following error when executing 'sudo apt-get update'?

W: GPG error: http://security.ubuntu.com trusty-security InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32

W: GPG error: http://archive.ubuntu.com trusty-updates InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32

W: GPG error: http://archive.canonical.com trusty Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32

W: GPG error: http://archive.ubuntu.com trusty Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32

Here is how to fix it:

sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 40976EAF437D05B5

sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 3B4FE6ACC0B21F32

Then you can continue with:

sudo apt-get update