No Public Key

[Thomas George <debianlist@mailfence.com>]

I want to do a new verified instillation of a debian iso. I have the iso 
and SHA512SUMS.sign.txt and SHHA512SUMS.txt and have tried

gpg --verify SHA512SUMS.sign.txt SHA512SUMS.txt with the result No 
Public Key

I thought to skip this step and tried

gpg --verify SHA515SUMS.sign.txt debian-11.5.0-amd64-netinst.iso with 
the result Can't open signed data debian-11.5.0-amd64-netinst.iso

Clearly I am making some elementary mistakes. I have spent fruitless 
hours trying find and use a public key. One source suggested Curl 
ipinfo.io/ip. This outputs an ip address that seams to have nothing to 
do with my problem.

A reference to a step-by-step procedure would be appreciated.

Tom George