Edit The Source File:

sudo nano /etc/apt/sources.list

Add The Following To The Source List:

deb http://deb.debian.org/debian/ stretch main contrib non-free
deb-src http://deb.debian.org/debian stretch main contrib non-free

And Save

Let's Do A Update:

sudo apt update

Once Done Install Libssl1.0-dev:

sudo apt install libssl1.0-dev