Hello!
I was going to use a Core I haven’t used in a while for a small project, but I have gradually had issues and followed various troubleshooting guides. When it was was ‘working’, on reset, it would breath white, then flash green, flash cyan, breath cyan (which is what it should do, right) then after a few breaths, flash green forever.
I was trying to flash programs to it in the few seconds the cyan led was breathing, but this didn’t work. I thought it had become corrupted somehow, and have reset the firmware many times. After a firmware reset it would seem to be able to connect to the network once before it would start flashing green forever. Also I used the Particle CLI software to reload the firmware using the command “particle flash --usb tinker”.
It got into a state where it would flash red at the end of the sequence of flashes after pressing the reset button, and apparently this could mean something is up with the security keys. Using the “particle keys doctor deviceID” command would not complete, and left the Core as described in the title - red led followed by rapid yellow flashing indicating that I have definitely messed up the keys!
So I followed a tutorial to generate new public and private keys manually using dfu-util commands. The last step doesn’t work - the command “particle keys send deviceID generated_public_key.pem” gives me the output “Error sending public key to server: Permission Denied”.
Probably I have missed out something I did that I have forgotten, I have spent a day and a half looking through old forum posts, tutorials etc probably losing rationality on the way 
can anybody offer some advice on getting it to work? Thanks