etc1tool software package provides ETC1 conversion tool, you can install in your Debian 9 (Stretch) by running the commands given below on the terminal,
$ sudo apt-get update $ sudo apt-get install etc1tool
etc1tool is installed in your system.
Make ensure the etc1tool package were installed using the commands given below,
$ sudo dpkg-query -l | grep etc1tool *
You will get with etc1tool package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Development |
| Package | etc1tool |
| Version | 6.0.1+r55-1 |
| Description | ETC1 conversion tool |
| Sub Section | devel |