Hi! I had a question regarding the SSH private key being used when you need to copy it to Windows and generate a .ppk for Putty. I have key based authentication working fine but I had a few questions. First, isn't the id_rsa file supposed to be private(kept on the machine) and not given to anyone? If so why do we copy the id_rsa off the machine instead of the id_rsa.public key to the Windows user? Also why do we need to cat id_rsa.pub into a authorized_keys file in order for the keys to work? Lastly if my .ppk file gets intercepted in the event that I lose my USB key can someone compromise my system?

Thanks.

id_rsa is the private key and for you to keep or convert to Windows .ppk. This is what you use to authenticate yourself against the server's "authorized_keys" where you cat the id_rsa.pub.

If your .ppk falls in the wrong hands and you don't have a passphrase set then yes they can log on to your system.

Encrypt your USB key! It's very unwise to wear unencrypted USB key. See encfs.

If you use a public key encryption, the 'other' party has your public key and you have your private key. In your situation, the other party is your ssh server, so that one will have the public key.

With regards to the last question:
Yes, they can. But a good passphrase will prevent that. My passphrase is about 20 characters. Assuming that there are roughly 72 characters (uppercase, lowercase, numbers and the stuff above the numbers), you have 1.4 e37 possible combinations (72^20). Assume as well that a brute force attack can do 10 e6 attacks per second (feasable with current PCs), it will take 44,447,088,893,843,884,113,135 years (can somebody please pronounce that) to hack.
Even if you only use lower case (26^20 possible combinations), it takes
63,191,745,608,858 years.

So I think that I'm reasonable safe Of course the first hit can be bingo, but it might also be the last one.

Thanks for the reply guys and the tips. I'll take the encryption into consideration and the pass phrases. I have a better understanding of how keys work now.

Thank you.