It provides some security against attackers that aren't capable of subtlety. If yank your unencrypted hard drive out and walk away with it in my pocket I can't use your encrypted SSH key to impersonate you to other machines unless I can break the passphrase.

If your HDD is stolen they can plug it into another computer and copy any file they want.

They would get quite a few pseudorandom numbers of mine, aside from a self-compiled Linux kernel. :-)

> aside from a self-compiled Linux kernel.

Likely sitting on an unencrypted /boot, waiting to be replaced by one with a keylogger, am I right? Of course a sane person doesn't trust a system that's been compromised before nuking it and reloading from a clean image.

There is unfortunately relatively little you can do to thwart such an attack, apart from keeping your notebook with you at all/most times.

Though using a USB key for /boot might be an idea, it is a little less clunky than a ThinkPad and since I suspend to RAM most of the time, it could even be practical. Hm.

Can't TPM be used for this? It could verify your /boot with keys external to the disk itself. I'm not sure if somebody has actually built a solution that uses it yet.

https://en.wikipedia.org/wiki/Trusted_Platform_Module

Sure it can, as can its evolution in the form of UEFI's Secure Boot, the problem is everyone wants to label these as technologies to enable lock-in instead of technologies to provide a trusted boot chain to ensure your system isn't compromised.

It could (the same goes for Secure Boot, in some sense). But the three-letter agencies from whom you want to protect yourself here likely have backdoor keys</paranoia>.

Creating multiple SSH keys is an interesting form of obfuscation, but it relies on trimming your bash command history and not using ssh-config files.

> In other words, how much additional security does password-protecting your private key gain you?

Someone took your laptop, they could now have one of two things:

(a) - Just your laptop

(b) - your laptop and every server you're using your key for access to.

b is prevented by the passphrase.

No, I think more like one of three things:

a) just your laptop, b) your laptop and all files, or c) your laptop and all your files, and access to servers as per above.

I like "a" the best, and it's achieved through full-disk encryption. You don't even need a new passphrase for that, considering that you probably already password-protect your computer.