An update, still no solution.
Firstly, the file command recognises it as an SSH key, while ssh-keygen doesn’t.
Screenshot: Imgur: The magic of the Internet
Secondly, I have tried to follow the steps outlined here:
The output of running these commands was not successful:
Openssl commands: Imgur: The magic of the Internet
Trying to convert to pkcs8: Imgur: The magic of the Internet
ssh and ssh-add are in the same directory: Imgur: The magic of the Internet
ldd does link to libcrypto.so: Imgur: The magic of the Internet
In the comments it was suggested that it’s a Windows vs Linux line endings problem. I haven’t used Windows for this lab, I’ve been on Kali the entire time, but decided to give it a go anyway.
Both sed and dos2unix didn’t yield the desired result.
Running puttygen “would perform no useful action”: Imgur: The magic of the Internet
Another command has been suggested due to the key being invalid:
chmod 600 id_rsa
ssh-keygen -p -N "" -m pem -f id_rsa
Output: Imgur: The magic of the Internet
(mine is with key.txt because I tried to copy it again and save it into a different file)