Add New Package Source

Add Key file such as this:

wget -O- https://duo.com/DUO-GPG-PUBLIC-KEY.asc | 

gpg --dearmor | 

tee /etc/apt/keyrings/duo.gpg

Add new file to sources.list.d with content referencing the new key:

deb [signed-by=/etc/apt/keyrings/duo.gpg] https://pkg.duosecurity.com/Debian bullseye main

Update and install application:

apt update

apt install duo-unix