Many of us use the excellent OpenSSH (see Resources later in this article) as a secure, encrypted
replacement for the venerable
commands. One of OpenSSH's more intriguing features is its ability to
authenticate users using the RSA and DSA authentication protocols, which
are based on a pair of complementary numerical keys. As one of its main
appeals, RSA and DSA authentication promise the capability of establishing
connections to remote systems without supplying a password. While
this is appealing, new OpenSSH users often configure RSA/DSA the quick and
dirty way, resulting in passwordless logins, but opening up a big security
hole in the process.
What is RSA/DSA authentication?
SSH, specifically OpenSSH (a completely free implementation of SSH), is an
incredible tool. Like
ssh client can be used to log in to a remote machine. All
that's required is for this remote machine to be running
ssh server process. However, unlike
ssh protocol is very secure. It uses
special algorithms to encrypt the data stream, ensure data stream
integrity and even perform authentication in a safe and secure way.
ssh is really great, there is a certain
ssh functionality that is often ignored,
dangerously misused, or simply misunderstood. This component is OpenSSH's
RSA/DSA key authentication system, an alternative to the standard secure
password authentication system that OpenSSH uses by default.
OpenSSH's RSA and DSA authentication protocols are based on a pair of specially generated cryptographic keys, called the private key and the public key. The advantage of using these key-based authentication systems is that in many cases, it's possible to establish secure connections without having to manually type in a password.
While the key-based authentication protocols are relatively secure,
problems arise when users take certain shortcuts in the name of
convenience, without fully understanding their security implications. In
this article, we'll take a good look at how to correctly use RSA and DSA
authentication protocols without exposing ourselves to any unnecessary
security risks. In my next article, I'll show you how to use
ssh-agent to cache decrypted private keys, and introduce
ssh-agent front-end that offers a
number of convenience advantages without sacrificing security. If you've
always wanted to get the hang of the more advanced authentication features
of OpenSSH, then read on.
How RSA/DSA keys work
Here's a quick general overview of how RSA/DSA keys work. Let's start with
a hypothetical scenario where we'd like to use RSA authentication to allow
a local Linux workstation (named localbox) to open a remote shell
on remotebox, a machine at our ISP. Right now, when we try to
connect to remotebox using the
ssh client, we get
the following prompt:
% ssh drobbins@remotebox drobbins@remotebox's password:
Here we see an example of the
sshdefault way of handling authentication. Namely, it asks for the
password of the drobbins account on remotebox. If we
type in our password for remotebox,
ssh uses its
secure password authentication protocol, transmitting our password over to
remotebox for verification. However, unlike what
telnet does, here our password is encrypted so that it can
not be intercepted by anyone sniffing our data connection. Once
remotebox authenticates our supplied password against its
password database, if successful, we're allowed to log on and are greeted
with a remotebox shell prompt. While the
authentication method is quite secure, RSA and DSA authentication open up
some new possibilities.
However, unlike the
ssh secure password authentication, RSA
authentication requires some initial configuration. We need to perform
these initial configuration steps only once. After that, RSA
authentication between localbox and remotebox will be
totally painless. To set up RSA authentication, we first need to generate
a pair of keys, one private and one public. These two keys have some very
interesting properties. The public key can be used to encrypt a message,
and only the holder of the private key can decrypt it. The public key can
only be used for encryption, and the private key can only be used
for decryption of a message encoded by the matching public key.
The RSA (and DSA) authentication protocols use the special properties of
key pairs to perform secure authentication, without needing to transmit
any confidential information over the network.
To get RSA or DSA authentication working, we perform a single one-time
configuration step. We copy our public key over to
remotebox. The public key is called "public" for a reason.
Since it can only be used to encrypt messages for us, we don't
need to be too concerned about it falling into the wrong hands. Once our
public key has been copied over to remotebox and placed in a
special file (~/.ssh/authorized_keys) so that remotebox's
sshd can locate it, we're ready to use RSA authentication to
log onto remotebox.
To do this, we simply type
ssh drobbins@remotebox at
localbox's console, as we always have. However, this time,
ssh lets remotebox's
sshd know that it
would like to use the RSA authentication protocol. What happens next is
rather interesting. Remotebox's
sshd generates a
random number, and encrypts it using our public key that we copied over
earlier. Then, it sends this encrypted random number back to the
ssh running on localbox. In turn, our
ssh uses our private key to decrypt this random
number, and then sends it back to remotebox, saying in effect
"See, I really do hold the matching private key; I was able to
successfully decrypt your message!" Finally,
that we should be allowed to log in, since we hold a matching private key.
Thus, the fact that we hold a matching private key grants us access to
There are two important observations about the RSA and DSA authentication. The first is that we really only need to generate one pair of keys. We can then copy our public key to the remote machines that we'd like to access and they will all happily authenticate against our single private key. In other words, we don't need a key pair for every system we'd like to access. Just one pair will suffice.
The other observation is that our private key should not fall into the wrong hands. The private key is the one thing that grants us access to our remote systems, and anyone that possesses our private key is granted exactly the same privileges that we are. Just as we wouldn't want strangers to have keys to our house, we should protect our private key from unauthorized use. In the world of bits and bytes, this means that no one should be able to read or copy our private key.
Of course, the
ssh developers are aware of the private keys'
importance, and have built a few safeguards into
ssh-keygen so that our private key is not abused. First,
ssh is configured to print out a big warning message if our
key has file permissions that would allow it to be read by anyone but us.
Secondly, when we create our public/private key pair using
ssh-keygen will ask us to enter a
passphrase. If we do, our private key will be encrypted using this
passphrase, so that even if it is stolen, it will be useless to anyone who
doesn't happen to know the passphrase. Armed with that knowledge, let's
take a look at how to configure
ssh to use the RSA and DSA
ssh-keygen up close
The first step in setting up RSA authentication begins with generating a
public/private key pair. RSA authentication is the original form of
ssh key authentication, so RSA should work with any version
of OpenSSH, although I recommend that you install the most recent version
available, which was openssh-2.9_p2 at the time this article was written.
Generate a pair of RSA keys as follows:
% ssh-keygen Generating public/private rsa1 key pair. Enter file in which to save the key (/home/drobbins/.ssh/identity): (hit enter) Enter passphrase (empty for no passphrase): (enter a passphrase) Enter same passphrase again: (enter it again) Your identification has been saved in /home/drobbins/.ssh/identity. Your public key has been saved in /home/drobbins/.ssh/identity.pub. The key fingerprint is: a4:e7:f2:39:a7:eb:fd:f8:39:f1:f1:7b:fe:48:a1:09 drobbins@localbox
ssh-keygen asks for a default location for the key, we
hit enter to accept the default of /home/drobbins/.ssh/identity.
ssh-keygen will store the private key at the above path, and
the public key will be stored right next to it, in a file called
Also note that
ssh-keygen prompted us to enter a passphrase.
When prompted, we entered a good passphrase (seven or more hard-to-predict
ssh-keygen then encrypted our private key
(~/.ssh/identity) using this passphrase so that our private key will be
useless to anyone who does not know it.
The quick compromise
When we specify a passphrase, it allows
ssh-keygen to secure
our private key against misuse, but it also creates a minor inconvenience.
Now, every time we try to connect to our drobbins@remotebox
ssh will prompt us to enter
the passphrase so that it can decrypt our private key and use it for RSA
authentication. Again, we won't be typing in our password for the
drobbins account on remotebox, we'll be typing in
the passphrase needed to locally decrypt our private key. Once our private
key is decrypted, our
ssh client will take care of the rest.
While the mechanics of using our remote password and the RSA passphrase
are completely different, in practice we're still prompted to type a
"secret phrase" into
# ssh drobbins@remotebox Enter passphrase for key '/home/drobbins/.ssh/identity': (enter passphrase) Last login: Thu Jun 28 20:28:47 2001 from localbox.gentoo.org Welcome to remotebox! %
Here's where people are often mislead into a quick compromise. A lot of the
time, people will create unencrypted private keys just so that they don't
need to type in a password. That way, they simply type in the
ssh command, and they're immediately authenticated via RSA
(or DSA) and logged in.
# ssh drobbins@remotebox Last login: Thu Jun 28 20:28:47 2001 from localbox.gentoo.org Welcome to remotebox! %
However, while this is convenient, you shouldn't use this approach without fully understanding its security impact. With an unencrypted private key, if anyone ever hacks into localbox, they'll also get automatic access to remotebox and any other systems that have been configured with the public key.
I know what you're thinking. Passwordless authentication, despite being a
bit risky does seem really appealing. I totally agree. But there is a
better way! Stick with me, and I'll show you how to gain the
benefits of passwordless authentication without compromising your private
key security. I'll show you how to masterfully use
(the thing that makes secure passwordless authentication possible
in the first place) in my next article. Now, let's get ready to use
ssh-agent by setting up RSA and DSA authentication. Here
RSA key pair generation
To set up RSA authentication, we'll need to perform the one-time step of generating a public/private key pair. We do this by typing:
Accept the default key location when prompted (typically ~/.ssh/identity
and ~/.ssh/identity.pub for the public key), and provide
ssh-keygen with a secure passphrase. Once
ssh-keygen completes, you'll have a public key as well as a
passphrase-encrypted private key.
RSA public key install
Next, we'll need to configure remote systems running
use our public RSA key for authentication. Typically, this is
done by copying the public key to the remote system as follows:
% scp ~/.ssh/identity.pub drobbins@remotebox:
Since RSA authentication isn't fully set up yet, we'll be prompted to enter our password on remotebox. Do so. Then, log in to remotebox and append the public key to the ~/.ssh/authorized_keys file like so:
% ssh drobbins@remotebox drobbins@remotebox's password: (enter password) Last login: Thu Jun 28 20:28:47 2001 from localbox.gentoo.org Welcome to remotebox! % cat identity.pub >> ~/.ssh/authorized_keys % exit
Now, with RSA authentication configured, we should be prompted to enter our
RSA passphrase (rather than our password) when we try to
connect to remotebox using
% ssh drobbins@remotebox Enter passphrase for key '/home/drobbins/.ssh/identity':
Hurray, RSA authentication configuration complete! If you weren't prompted
for a passphrase, here are a few things to try. First, try logging in by
ssh -1 drobbins@remotebox. This will tell
ssh to only use version 1 of the ssh protocol, and may be
required if for some reason the remote system is defaulting to DSA
authentication. If that doesn't work, make sure that you don't have a line
RSAAuthentication no in your /etc/ssh/ssh_config.
If you do, comment it out by pre-pending it with a "#". Otherwise, try
contacting the remotebox system administrator and verifying that
they have enabled RSA authentication on their end and have the appropriate
settings in /etc/ssh/sshd_config.
DSA key generation
While RSA keys are used by version 1 of the
ssh protocol, DSA
keys are used for protocol level 2, an updated version of the
ssh protocol. Any modern version of OpenSSH should be able to
use both RSA and DSA keys. Generating DSA keys using OpenSSH's
ssh-keygen can be done similarly to RSA in the following
% ssh-keygen -t dsa
Again, we'll be prompted for a passphrase. Enter a secure one. We'll also be prompted for a location to save our DSA keys. The default, normally ~/.ssh/id_dsa and ~/.ssh/id_dsa.pub, should be fine. After our one-time DSA key generation is complete, it's time to install our DSA public key to remote systems.
DSA public key install
Again, DSA public key installation is almost identical to RSA. For DSA, we'll want to copy our ~/.ssh/id_dsa.pub file to remotebox, and then append it to the ~/.ssh/authorized_keys2 on remotebox. Note that this file has a different name than the RSA authorized_keys file. Once configured, we should be able to log in to remotebox by typing in our DSA private key passphrase rather than typing in our actual remotebox password.
Right now, you should have RSA or DSA authentication working, but you
still need to type in your passphrase for every new connection. In my next
article, we'll see how to use
ssh-agent, a really nice system
that allows us to establish connections without supplying a
password, but also allows us to keep our private keys encrypted on disk.
I'll also introduce
keychain, a very handy
ssh-agent front-end that makes
more secure, convenient, and fun to use. Until then, check out the handy
resources below to keep yourself on track.
- Be sure to visit the home of OpenSSH development.
- Take a look at the latest OpenSSH source tarballs and RPMs.
- Check out the OpenSSH FAQ.
is an excellent
sshclient for Windows machines.
- You may find O'Reilly's SSH, The Secure Shell: The Definitive Guide to be helpful. The authors' site contains information about the book, a FAQ, news, and updates.
- Browse more Linux resources on developerWorks.
- Browse more Open source resources on developerWorks.