• My Jenkins exists on Amazon EC2 engine.
•  I'm trying to add ubuntu slave via ssh.
• While creating an instance, a <filename>.pem file is generated for storing .ssh keys which is a private key. No separate pub ssh key is there. 
• I can ssh directly using terminal from my master to slave and vice versa using that key file

  ssh -i "/home/ubuntu/.ssh/scarlet.pem" ec2-user@ec2-52-66-241-52.ap-south-1.compute.amazonaws.com

  ssh i "/home/ec2user/.ssh/scarlet.pem" ubuntu@ec2-52-66-103-201.ap-south-1.compute.amazonaws.com

  But when I try to configure my slave via any of the ssh method, it gives me error.

• How I've configured (one of the method)?
  • I've saved slave machine credentials as "ssh key username with private key"
  • Launch method: Launch agents via SSH
  • Hostname: slave machine private ip address, as it is the hostname.
  • Host Key Verification Strategy: Manually trusted key Verification Strategy
• If I try "Manually provided key verification strategy", it gives me bug. see (bug-screenshot attached).

Note:

• I cannot regenerate ssh keys as I'll not be able to access my machines again. 
• I cannot use credentials as username/password as either there is no password or I'm not that much linux expert. They only method offered in amazon.aws seems to be the ssh key (.pem ssh key file)
• I've tried everything whatever mentioned in other jenkins tickets for same issue or on internet, but my issue is not getting resolved.

Please see attachment for log details.

I'm literally blocked right now, need assistance asap.