Linux – Generating ssh-keys not creating key in PKCS1 format

in order to connect to an SSH server and authenticate with your public/private keypair you have to first share your public key with the server.

this is done by copying the public key for your private key to the server, and adding it to ~/ssh/authorized_keys either by copy/paste, copying id_rsa.pub to ~/.ssh/authorized_keys on the server or with cat id_rsa.pub >> ~/.ssh/authorized_keys, appending it to the list.

when you connect, the server uses your public key to sign a challenge, and your client uses your private key id_rsa to decrypt the challenge, re-encrypt it with the server's public host key and send it back.

the host verifies that you decrypted the challenge properly, by decrypting your response with its private key, and the client/host establish an encrypted connection, based on the shared data, not on your public/private keys.

at NO POINT in the exchange is your private key, or the host's private key exchanged or revealed to one another. your public key IS stored on the server, but that's why it is a PUBLIC key.

I don't think there is any way to do this via a configuration file. You could set up an alias and put it in a shell initialisation file. This won't stop the user from deleting the alias and running their own command though.

alias ssh-keygen='ssh-keygen -b 3072'

then

$ ssh-keygen -t rsa
Generating public/private rsa key pair.
Enter file in which to save the key (/home/iain/.ssh/id_rsa): /tmp/testkey
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /tmp/testkey.
Your public key has been saved in /tmp/testkey.pub.
The key fingerprint is:
47:3a:03:c8:ac:63:1c:bf:9d:44:1d:4b:b4:0e:66:04 
$ ssh-keygen -lf /tmp/testkey
3072 47:3a:03:c8:ac:63:1c:bf:9d:44:1d:4b:b4:0e:66:04 /tmp/testkey.pub

You could put it in each (existing) user's ~/.bashrc and in /etc/skel/.bashrc so new user's get it too.