A serverfault poster had a problem that I thought was a cool challenge. I had so much fun coming up with this answer, I figured I’d share it here as it demonstrates a few handy features of EC2.
The basic need is to get the public ssh key from a keypair that exists inside of EC2. You don’t have access to the private key at the moment (but somebody else does or you will at a different location).
The AWS console and EC2 API do not let you ask for the public ssh key associated with a keypair. However, EC2 does pass the public ssh key to a new EC2 instance when you run it with a specific keypair.
The problem is that we don’t currently have the private key, so we can’t log in to the EC2 instance to get the public key. (Besides, if we did have the private key, we could extract the public key from it directly.)
I proposed creating a user-data script that sends the public ssh key to the EC2 instance console output. You can retrieve the console output without logging in to the EC2 instance.