I'm afraid this is quite a common question, but searching several times through the questions did not prove useful, and I'm really eager of knowledge here :D

I understand that keeping SSH private key well secured (strong passphrase, backup ecc.) is crucial, but I'm wondering about how much having it stolen is dangerous. An eventual attacker, once got our private key, could log into every server which has our public key in authorized_keys, right? But besides that, are there other things that he could use the key for?

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    Isn't this bad enough? – M'vy Jun 10 '15 at 14:47
  • Depends what else is the the key used for. Best place to store your key is a smartcard or something similar. – mikky Jun 10 '15 at 15:06
  • @M'vy sure that is bad, I was just curious :D – Pie3Thousands Jun 10 '15 at 15:09
  • @mikky Can SSH private keys be used for something else? – Pie3Thousands Jun 10 '15 at 15:10
  • Technically yes, why not. It's (usually) just an RSA key that can be used in any RSA-enabled application. – mikky Jun 10 '15 at 15:17

Because even if you've secured your private key with a strong passphrase, the passphrase is the only protection of the key.

And since the encrypted key is only one part of an equation (your passphrase is the other half) it only needs enough computation power to bruteforce "crack" that key.

So, the key is as valuable as the passphrase to it.

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You could try drawing a mitigation tree where you can visualize the effects of a compromised key on the service environment. Also, you might want to take into account the physical safety of the key itself.

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