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IBM developerWorks: OpenSSH key management, Part 3 (Agent forwarding and keychain improvements)

Feb 28, 2002, 21:09 (0 Talkback[s])
(Other stories by Daniel Robbins)

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"In my last article, I've spent some time discussing the security benefits and tradeoffs of running ssh-agent. A few days after the second article appeared on developerWorks, I received an e-mail from Charles Karney of Sarnoff Corporation, who politely informed me of OpenSSH's new authentication agent forwarding abilities, which we'll take a look at in a bit. In addition, Charles emphasized that running ssh-agent on untrusted machines is quite dangerous: if someone manages to get root access on the system, then your decrypted keys can be extracted from ssh-agent. Even though extracting the keys would be somewhat difficult, it is within the skill of professional crackers. And the mere fact that private key theft is possible means that we should take steps to guard against it happening in the first place.

To formulate a strategy to protect our private keys, we must first put the machines we access into one of two categories. If a particular host is well-secured or isolated -- making successful root exploit against it quite unlikely -- then that machine should be considered a trusted host. If, however, a machine is used by many other people or you have some doubts about the security of the system, then the machine should be considered an untrusted host. To guard your private keys against extraction, ssh-agent (and thus keychain) should never be run on an untrusted host. That way, even if the system's security is compromised, there will be no ssh-agent around for the intruder to extract keys from in the first place.

However, this creates a problem. If you can't run ssh-agent on untrusted hosts, then how do you establish secure, passwordless ssh connections from these systems? The answer is to only use ssh-agent and keychain on trusted hosts, and to use OpenSSH's new authentication forwarding abilities to extend passwordless authentication to any untrusted hosts. In a nutshell, authentication forwarding works by allowing remote ssh sessions to contact an ssh-agent running on a trusted system."

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