Keychain helps you to manage ssh and GPG keys in a convenient and secure manner. It acts as a frontend to ssh-agent and ssh-add, but allows you to easily have one long running ssh-agent process per system, rather than the norm of one ssh-agent per login session.

This dramatically reduces the number of times you need to enter your passphrase. With keychain, you only need to enter a passphrase once every time your local machine is rebooted. Keychain also makes it easy for remote cron jobs to securely "hook in" to a long running ssh-agent process, allowing your scripts to take advantage of key-based logins.

The latest release of keychain is version 2.6.9, and was released on July 26, 2009. The current version of keychain supports gpg-agent as well as ssh-agent.

Keychain is compatible with many operating systems, including AIX, *BSD, Cygwin, MacOS X, Linux, HP/UX, Tru64 UNIX, IRIX, Solaris and GNU Hurd.

• keychain 2.6.9 source code

Keychain development sources can be found in the keychain git repository. Please use the funtoo-dev mailing list and #funtoo irc channel for keychain support questions as well as bug reports.

You're probably familiar with ssh, which has become a secure replacement for the venerable telnet and rsh commands.

Typically, when one uses ssh to connect to a remote system, one supplies a secret passphrase to ssh, which is then passed in encrypted form over the network to the remote server. This passphrase is used by the remote sshd server to determine if you should be granted access to the system.

However, OpenSSH and nearly all other SSH clients and servers have the ability to perform another type of authentication, called asymmetric public key authentication, using the RSA or DSA authentication algorithms. They are very useful, but can also be complicated to use. keychain has been designed to make it easy to take advantage of the benefits of RSA and DSA authentication.

To use RSA and DSA authentication, first you use a program called ssh-keygen (included with OpenSSH) to generate a key pair -- two small files. One of the files is the public key. The other small file contains the private key. ssh-keygen will ask you for a passphrase, and this passphrase will be used to encrypt your private key. You will need to supply this passphrase to use your private key. If you wanted to generate a DSA key pair, you would do this:

# ssh-keygen -t dsa
Generating public/private dsa key pair.

You would then be prompted for a location to store your key pair. If you do not have one currently stored in ~/.ssh, it is fine to accept the default location:

Enter file in which to save the key (/root/.ssh/id_dsa): /var/tmp/id_dsa

Then, you are prompted for a passphrase. This passphrase is used to encrypt the private key on disk, so even if it is stolen, it will be difficult for someone else to use it to successfully authenticate as you with any accounts that have been configured to recognize your public key.

Note that conversely, if you do not provide a passphrase for your private key file, then your private key file will not be encrypted. This means that if someone steals your private key file, they will have the full ability to authenticate with any remote accounts that are set up with your public key.

Below, I have supplied a passphrase so that my private key file will be encrypted on disk:

Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /var/tmp/id_dsa.
Your public key has been saved in /var/tmp/id_dsa.pub.
The key fingerprint is:
5c:13:ff:46:7d:b3:bf:0e:37:1e:5e:8c:7b:a3:88:f4 root@devbox-ve
The key's randomart image is:
+--[ DSA 1024]----+
|          .      |
|           o   . |
|          o . ..o|
|       . . . o  +|
|        S     o. |
|             . o.|
|         .   ..++|
|        . o . =o*|
|         . E .+*.|
+-----------------+

Here's how you use these files to authenticate with a remote server. On the remote server, you would append the contents of your public key to the ~.ssh/authorized_keys file, if such a file exists. If it doesn't exist, you can simply create a new authorized_keys file in the remote account's ~/.ssh directory that contains the contents of your local id_dsa.pub file.

Then, if you weren't going to use keychain, you'd perform the following steps. On your local client, you would start a program called ssh-agent, which runs in the background. Then you would use a program called ssh-add to tell ssh-agent about your secret private key. Then, if you've set up your environment properly, the next time you run ssh, it will find ssh-agent running, grab the private key that you added to ssh-agent using ssh-add, and use this key to authenticate with the remote server.

Again, the steps in the previous paragraph is what you'd do if keychain wasn't around to help. If you are using keychain, and I hope you are, you would simply add the following line to your ~/.bash_profile:

eval `keychain --eval id_dsa`

The next time you log in or source your ~/.bash_profile, keychain will start, start ssh-agent for you if it has not yet been started, use ssh-add to add your id_dsa private key file to ssh-agent, and set up your shell environment so that ssh will be able to find ssh-agent. If ssh-agent is already running, keychain will ensure that your id_dsa private key has been added to ssh-agent and then set up your environment so that ssh can find the already-running ssh-agent. It will look something like this:

Note that when keychain runs for the first time after your local system has booted, you will be prompted for a passphrase for your private key file if it is encrypted. But here's the nice thing about using keychain -- even if you are using an encrypted private key file, you will only need to enter your passphrase when your system first boots (or in the case of a server, when you first log in.) After that, ssh-agent is already running and has your decrypted private key cached in memory. So if you open a new shell, you will see something like this:

This means that you can now ssh to your heart's content, without supplying a passphrase.

You can also execute batch cron jobs and scripts that need to use ssh or scp, and they can take advantage of passwordless RSA/DSA authentication as well. To do this, you would add the following line to the top of a bash script:

eval `keychain --noask --eval id_dsa` || exit 1

The extra --noask option tells keychain that it should not prompt for a passphrase if one is needed. Since it is not running interactively, it is better for the script to fail if the decrypted private key isn't cached in memory via ssh-agent.

In the images above, you will note that keychain starts ssh-agent, but also starts gpg-agent. Modern versions of keychain also support caching decrypted GPG keys via use of gpg-agent, and will start gpg-agent by default if it is available on your system. To avoid this behavior and only start ssh-agent, modify your ~/.bash_profile as follows:

eval `keychain --agents ssh --eval id_dsa` || exit 1

The additional --agents ssh option tells keychain just to manage ssh-agent, and ignore gpg-agent even if it is available.

Sometimes, it might be necessary to flush all cached keys in memory. To do this, type:

keychain --clear

Any agent(s) will continue to run.

To improve the security of keychain, some people add the --clear option to their ~/.bash_profile keychain invocation. The rationale behind this is that any user logging in should be assumed to be an intruder until proven otherwise. This means that you will need to re-enter any passphrases when you log in, but cron jobs will still be able to run when you log out.

If you want to stop all agents, which will also of course cause your keys/identities to be flushed from memory, you can do this as follows:

keychain -k all

If you have other agents running under your user account, you can also tell keychain to just stop only the agents that keychain started:

keychain -k mine

The instructions above will work on any system that uses bash as its default shell, such as most Linux systems and Mac OS X.

To learn more about the many things that keychain can do, including alternate shell support, consult the keychain man page, or type keychain --help | less for a full list of command options.

I also recommend you read my original series of articles about OpenSSH that I wrote for IBM developerWorks, called OpenSSH Key Management. Please note that keychain 1.0 was released along with Part 2 of this article, which was written in 2001. keychain has changed quite a bit since then. In other words, read these articles for the conceptual and OpenSSH information, but consult the keychain man page for command-line options and usage instructions :)

• Common Threads: OpenSSH key management, Part 1 - Understanding RSA/DSA Authentication
• Common Threads: OpenSSH key management, Part 2 - Introducing ssh-agent and keychain
• Common Threads: OpenSSH key management, Part 3 - Agent forwarding and keychain improvements

As mentioned at the top of the page, keychain development sources can be found in the keychain git repository. Please use the funtoo-dev mailing list and #funtoo irc channel for keychain support questions as well as bug reports.

Daniel Robbins originally wrote keychain 1.0 through 2.0.3. 1.0 was written around June 2001, and 2.0.3 was released in late August, 2002.

After 2.0.3, keychain was maintained by various Gentoo developers, including Seth Chandler, Mike Frysinger and Robin H. Johnson, through July 3, 2003.

On April 21, 2004, Aron Griffis committed a major rewrite of keychain which was released as 2.2.0. Aron continued to actively maintain and improve keychain through October 2006 and the keychain 2.6.8 release. He also made a few commits after that date, up through mid-July, 2007.

At this point, keychain had reached a point of maturity. From mid-July 2007 through late July 2009, a period of over two years, there have been no new releases. However, a few little tweaks and improvements have been circulating around, so...

In mid-July, 2009, Daniel Robbins migrated Aron's mercurial repository to git and set up a new project page on funtoo.org, and made a few bug fix commits to the git repo that had been collecting in bugs.gentoo.org. Daniel continues to maintain keychain and supporting documentation on funtoo.org, and plans to make regular maintenance releases of keychain as appropriate.

The current release of keychain is currently 2.6.9.