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What is the purpose of passphrase when I generate public and private keys using ssh-keygen?

I'm fairly new to all the SSH business. I have some basic idea about that using SSH public and private keys allows me to be identified and authorized by remote SSH server without having to have a password transferred over the Internet. But when I generate the keys, it also offers to type in optionally a passphrase. What is this passphrase for? What is the pros and cons to use it?

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To encrypt your keys. So if an attacker get's your private/public key, it can't do anything with them.

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To amplify: Imagine you have the private-key pair on a laptop. You lose the laptop. A thief gets access to your private key and can impersonate you and access all the computers you have access to. A passphrase helps to prevent this. –  RedGrittyBrick Jan 15 '12 at 9:42
    
If I store the private key file on a USB drive which is shared with some friends sometimes, do I need to further protect the file or is the passphrase sufficient? –  Problemaniac Jan 17 '12 at 1:11

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