iso authentication - can't open "sha256.txt.gpg"
 

I've downloaded the iso for cinnamon 18.3 and the required file for shasum check and autorization. The check sum was good and I could download the public key. However when I do the verify I receive an error that "sha256.txt.gpg" can't be opened. I've run all from the same directory and as sudo just to be sure. I've checked to file permisions and they are ok for read and write. Any ideas?

Myself, I don't bother with authentication, if you got the ISO from LinuxMint site and the shasum check is good, use it.
However, if you absolutely desire to verify authentication, delete the sha256.txt.gpg file and download another, maybe it's corrupt.

I've gone further by performing the verify as my normal user - not sudo. I get the message that the key (A25BAE9) is not found. I can see this key in Software Sources. It also shows up (1024R/EEA14886 2016-06-04) when I do sudo apt-key list????

Problem solved. I needed to do the verification as SUDO. The .gpg which I couldn't open as root previously worked fine now.