You need an automatic login from host A / user ‘a’ to Host B / user ‘b’. You don’t want to enter any passwords, maybe because you want to call ssh from a within a shell script.
How to do it
First log in on A as user ‘a’ and generate a pair of authentication keys. Do not enter a passphrase:
a@A:~> ssh-keygen -t rsa
Generating public/private rsa key pair.
Enter file in which to save the key (/home/a/.ssh/id_rsa):
Created directory ‘/home/a/.ssh’.
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/a/.ssh/id_rsa.
Your public key has been saved in /home/a/.ssh/id_rsa.pub.
The key fingerprint is:
Now use ssh to create a directory ~/.ssh as user ‘b’ on B. (The directory may already exist, which is fine):
a@A:~> ssh b@B mkdir -p .ssh
Finally append a’s new public key to b@B:.ssh/authorized_keys and enter b’s password one last time:
a@A:~> cat .ssh/id_rsa.pub | ssh b@B 'cat >> .ssh/authorized_keys'
From now on you can log into B as b from A as a without password:
a@A:~> ssh b@B hostname
ssh-copy-id -i ~/.ssh/id_rsa.pub username@mystery