We found we needed to change some certificate formats at work to support a specific 3rd party. Googling found this excellent cheat-sheet for working with
There’s a lot of useful stuff here, ones we needed were:
Check if one of our private keys was in a compatible format
openssl rsa -in privateKey.key -check
It wasn’t - we used PuttyGen to export as a SSL format pem file private key.
Actions -> Load Key Converstions (Menu) -> Export OpenSSH Key
Generate the matching public key in SSL format:
openssl rsa -in key.pem -pubout -out pubkey.pem