SSH without password

SSH login without password

You want to use Linux and OpenSSH to automize your tasks. Therefore you need an automatic login from host A / user a to Host B / user b. You don't want to enter any passwords, 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:
3e:4f:05:79:3a:9f:96:7c:3b:ad:e9:58:37:bc:37:e4 a@A

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
b@B's password:

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'
b@B's password:

From now on you can log into B as b from A as a without password:

a@A:~> ssh b@B hostname
B

A note from one of our readers: Depending on your version of SSH you might also have to do the following changes:

* Put the public key in .ssh/authorized_keys2
* Change the permissions of .ssh to 700
* Change the permissions of .ssh/authorized_keys2 to 640


DSA is faster in *signing*, but slower in *verifying*.
A DSA key of the same strength as RSA (1024 bits) generates a smaller signature.
An RSA 512 bit key has been cracked, but only a 280 DSA key. It doesn't matter because with Ssh only authentication is done using RSA or DSA algorithm, and then the "rest" is encoded using a (uh, was it block?) cipher like IDEA, DES, Blowfish, etc, etc after the authentication is done.
While Ssh2 can use either DSA or RSA keys, Ssh1 cannot. Ssh2 will also not use patented cypers like IDEA.