gpgv1.4-udeb software package provides minimal signature verification 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 gpgv1.4-udeb
gpgv1.4-udeb is installed in your system.
Make ensure the gpgv1.4-udeb package were installed using the commands given below,
$ sudo dpkg-query -l | grep gpgv1.4-udeb *
You will get with gpgv1.4-udeb package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | debian-installer udeb packages |
| Package | gpgv1.4-udeb |
| Version | 1.4.21-1+b1 |
| Description | minimal signature verification tool |
| Sub Section | Debian-installer |