An introduction to SSH2

Learn about the differences between SSH1 and SSH2 and why you should consider upgrading.

What you will learn from this tip: The differences between SSH1 and SSH2, and why you should consider the upgrade.


Secure Shell (SSH) provides a secure means of connecting computers over an inherently insecure medium. One of the most common uses of SSH is to facilitate remote login sessions between an authorized user and a trusted host over the Internet. With the release of SSH2, users have the benefit of added functionality and security.

When comparing SSH2 to SSH1, it's important to note that SSH2 is not simply SSH1 with new protocols plugged in. It's a complete rewrite of the original protocol and has now incorporated built-in protections against a number of known vulnerabilities in SSH1.

The major differences between SSH1 and SSH2 fall into two main categories: technical and licensing. Technically speaking, SSH2 uses different encryption and authentication algorithms. SSH1 offers four encryption algorithms (DES, 3DES, IDEA and Blowfish), while SSH2 dropped support for DES and IDEA, but added three new algorithms. SSH1 also utilized the RSA authentication algorithm, while SSH2 switched to the Digital Signature Algorithm (DSA). These changes were designed to both circumvent intellectual property issues surrounding the use of IDEA and RSA, and increase the base level of security in SSH2 by utilizing stronger algorithms.

SSH2 also provides added functionality. Most notably, it adds the sftp program to the SSH suite. This program uses the SSH protocol's encryption technology to provide a secure means for file transfer between remote systems. It's also rapidly approaching acceptance as an industry standard by the Internet Engineering Task Force's Secure Shell Working Group.

In the early days of SSH, users were able to freely download and use SSH1 despite the fact that it contained patented technologies. With the advent of SSH2, the creators restricted the license. The firm that holds the license, SSH Communications Security, re-released the SSH client as SSH Tectia and no longer directly offers free downloads. Those who qualify for a non-commercial license can download an older version of SSH2 (SSH 3.2) from a variety of Web sites. Search for "SSH noncommercial" on Google and you'll find a variety of sites that allow users to download older versions of SSH for Unix, Windows and Macintosh systems.

The open source community objected to this change in the SSH licensing scheme and offers an alternative, OpenSSH. An offshoot of the OpenBSD project, OpenSSH provides the same functionality as SSH2 without conflicting with any intellectual property restrictions. It may be downloaded from OpenSSH (www.openssh.org).

So, what does all of this mean for security practitioners? First, if you're currently using SSH1, you probably want to consider upgrading to SSH2 technology. Why? Consider these reasons:

  • SSH1 is no longer supported by an active development community. This limits the availability of future upgrades and defenses against new threats.
  • SSH1 has documented vulnerabilities, including susceptibility to a variant of the cryptographic man-in-the-middle attack.

Unless you have a specific reason not to upgrade, it's a prudent idea to do so.

Second, if you're not currently using any secure means to facilitate remote connections to your systems, you should look into implementing SSH2 or a similar technology immediately. It's important to remember that tools like telnet and ftp have NO built-in security. They pass usernames and passwords in plain text and greatly undermine the security of your network.

SSH2 provides security professionals with a powerful means to facilitate secure connections. Don't forget the importance of educating your user base on the proper use of these tools.


Related information:

About the author
Mike Chapple, CISSP, currently serves as Chief Information Officer of the Brand Institute, a Miami-based marketing consultancy. He previously worked as an information security researcher for the U.S. National Security Agency. His publishing credits include the TICSA Training Guide from Que Publishing, the CISSP Study Guide from Sybex and the upcoming SANS GSEC Prep Guide from John Wiley. He's also the author of the About.com Guide to Databases.

This was last published in February 2005

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