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PuTTY Key Generator saves public key in different format than authorized_keys file

Spent quite a few hours today setting up ssh key authentication on my client and server. Most of that time has been dealing with "Server refused our key" message upon connection. I had generated several private/public key pairs, SSH-2 (RSA) and SSH-2 (DSA), then saved them disk with the "Save public key" and "Save private key" buttons. Finally, I copied the public key file to "authorized_keys" in the ".ssh" directory on the server.

It turned out that the public key file that PuTTY Key Generator created is in a different format than the public key shown in the "Public key for pasting into OpenSSH authorized_keys file:" text box. The following is an example of what needs to go into the "authorized_keys" file (although I splitted the key up into multiple lines for easy reading, everything is on a single line).

ssh-rsa
AAAAB3NzaC1yc2EAAAABJQAAAIBifLM0sCq15dbcUJ0ke5nhSywkQoaT5CdBgfM4
Y6RkfqFmQjDRXxiKEPb5wXIFY+gR8YvALRAp4iC1TtenCtqMLrgdtCekqB82u7Wr
lLju7QxYZ1ptljedDkrfPd3tCliPb5l1hebJT5b3wj22x0MKgogjM8+fIw7AgHhN
1b1HCw==
rsa-key-20060124

The following is what ended up in the public key file generated by PuTTY Key Generator with the "Save public key" button. See the discrepancy? Note that the key is separated into four separate lines.

---- BEGIN SSH2 PUBLIC KEY ----
Comment: "rsa-key-20060124"
AAAAB3NzaC1yc2EAAAABJQAAAIBifLM0sCq15dbcUJ0ke5nhSywkQoaT5CdBgfM4
Y6RkfqFmQjDRXxiKEPb5wXIFY+gR8YvALRAp4iC1TtenCtqMLrgdtCekqB82u7Wr
lLju7QxYZ1ptljedDkrfPd3tCliPb5l1hebJT5b3wj22x0MKgogjM8+fIw7AgHhN
1b1HCw==
---- END SSH2 PUBLIC KEY ----

I don't know what is the purpose of generating the public key in this format and which application understands this format. But the easiest way to get the correct public key into the "authorized_keys" file is to copy-and-paste the public key text in the PuTTY Key Generator window.

But there are situations where you have the public key text file, but no PuTTY Key Generator handy. The easiest way is to convert that public key text file into the correct format by hand. Start the string with "ssh-rsa", or "ssh-dss" if the public key is SSH-2 DSA type. Next copy and concatenate the four public key lines into one line and append it after the starting string. Finally, take the string in quotes, behind "Comment:", and append it to the end of the line. That is the public authentication key useable in "authorized_keys" file.

Chieh Cheng
Tue, 24 Jan 2006 17:37:50 -0800

Thanks a ton for this;

I'm a newbie at Unix systems and TLS/SSL, and have had to learn both for firedrills recently:
Been trying to use OpenSSH over PuTTy to find the fingerprint for a certificate - by looking over your article and comparing to my various exported keys, i eventually realized i was exporting the wrong Standard of Key (RFC4716 instead of OpenSSH2) -____- Changed the key out, OpenSSH stoped barfing and grabbed it, got my fingerprint, onwards to glory. Thanks a ton.

Spacebot
Fri, 27 Apr 2012 03:37:09 +0400

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