Is it possible to get hold of an RSA private key (pem file) simply by having (root) access to a server that authorizes ssh access using that that private key. For example, can the entries in .ssh/authorized_keys be used to generated the RSA private keys for which they are associated with.

The reason I ask is because I need to disable ssh access to a server with a particular key file. I've edited the authorized_keys file manually and removed the entry associated with that key. That worked and ssh access is no longer possible with that key file, but before I did that, I had created a new private key for ssh access.

I'm concerned that someone with that disabled key file may have been able to acquire the new RSA private key before I was able to delete the old one.

Is that possible?

closed as off-topic by random Sep 9 '15 at 15:31

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For practical purposes: no, it's not possible. More strictly speaking: it's possible, but not feasible. The attacker would need immense computing power or major mathematical breakthroughs.

The entry in your authorized_keys file is a public key. The whole point of public-key cryptography is that your public key can be public because an attacker can't use it to derive your private key.

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