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I set up my Linux server to accept connections from my home PC by adding the public key generated by PuTTYgen to the authorized_keys file.

Now I'm trying to connect to the same machine, but this time from another Ubuntu machine. I have to use the same private key (for a weird reason, don't ask..) and I don't quite get how to install it on my client Ubuntu.

Do I have to convert it to some other format?

24

I am not sure if your private key will work in ubuntu, but its worth a shot. just copy the keys to /home/yourName/.ssh/ name the private key as id_rsa, and the public key as id_rsa.pub.

If that is not working, then you can create you own ssh key-pair using ssh-keygen and copy the new public key to the server, and ssh as follows

ssh -i ~/.ssh/id_rsa_ubuntu.pub <hostName>

I haven't played with it, but I hear that ssh-agent can also be used to manage ssh-keys.

  • 3
    This does work in ubuntu. However, make sure to not over-write an existing id_rsa key if you already have one (by copying the new key on top of it) unless you know you don't use the old one. – David Oneill Nov 29 '10 at 17:35
  • 5
    This answer completely lacks information, how do you get the id_rsa file (by converting the key in PuTTYgen) – Martin Prikryl Feb 12 '15 at 12:15
38

By coincidence, I just had to do this. You do need to convert the keys to OpenSSH format. The command for doing that is:

ssh-keygen -i -f puttygen_key > openssh_key

then you can copy the contents of openssh_key in to .ssh/authorized_keys just as with a normal SSH key.

The -i option is the one that tells ssh-keygen to do the conversion. The -f option tells it where to find the key to convert.

This works for unencrypted keys. The public key is unencrypted, but the private one is probably encrypted. I'm not sure if it there's a way to unencrypt the private key, convert it, and then recrypt it. It may well be easier to use new keys as the other answer suggests (and I'd recommend using ssh-agent though that's orthogonal to the current issue).

28

Andrew Stacey explained how to convert the keys to OpenSSH format on Linux.

If you want to do the same on Windows, follow these steps:

  1. Start PuTTYGen.
  2. Click on "Load".
  3. Select your private key and enter the passphrase.
  4. From the 'Conversions' menu, select "Export OpenSSH key".
  5. Choose destination filename.
  • 1
    I just wanted to say that this worked for me on my Mac OS X, even though I know this is an Ubuntu section. I converted my generated private key in Windows as described above, then copied it in my mac to ~/.ssh/id_rsa. chmod id_rsa 600 to keep Mac from complaining about it. – Richard Jul 1 '14 at 14:16
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How to re-use your Putty key pairs in Ubuntu as OpenSSH keys:

apt-get install putty-tools #Install Putty tools in Linux
cd /my-putty-keys
puttygen mykey-sec.ppk ‐O private‐openssh ‐o my‐openssh‐key.sec
ssh-keygen -i -f mykey-pub.ppk > my-openssh-key.pub

Since purpose of step 4 is to add your public key to *~./ssh/authorized_keys*, so you can use your Putty secret key like this instead of doing 4 as an intermediate step:

puttygen ‐L mykey-sec.ppk >> $HOME/.ssh/authorized_keys
  • 1
    about step 4: if the private key name was mykey-sec.ppk, the source file name in this case would usually be mykey-sec.pub. – Kinjal Dixit Sep 4 '11 at 4:56
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    Naming a public key (presumably in RFC 4716 format) mykey-pub.ppk is pretty confusing, as .ppk file has a completely different format and is used for key pairs (not just public key). – Martin Prikryl Feb 12 '15 at 12:17
  • That last puttygen line worked perfectly for me after installing putty-tools on debian 9. The other lines and answers in here didn't work. – Tim Apr 10 '18 at 4:55
  • Weirdly I was getting puttygen: unknown output type `private‐openssh' and puttygen: cannot handle more than one input file so I copied the exact characters from the source code and renamed my public key to something else entirely – CPHPython Oct 14 '18 at 16:33
  • as a heads up, if you try to run the commands in this answer and get "puttygen: cannot handle more than one input file", it's likely because the dash character in "private‐openssh" is not pasted in as an actual ASCII dash so puttygen doesn't recognize it. Instead, type out the word yourself and it should work. – Matt Styles Mar 27 at 17:08
9

PuTTY/PuTTYgen uses its own proprietary format of key pair. It won't work on Linux, where OpenSSH format of keys prevails.

  • In PuTTYgen, you can directly see (and copy + paste) a public key in the format used by the OpenSSH authorized_keys file.

  • You can use the button Save public key to save the public key in the .pub format (RFC 4716). On Linux the file is typically named id_rsa.pub (or id_dsa.pub). But that's typically not needed.

  • Use the Conversions > Export OpenSSH key to export the private key in the OpenSSH format. On Linux the file is typically named id_rsa (or id_dsa) and is stored in .ssh folder.

See the official Using PuTTYgen, the PuTTY key generator.

You can also use a Linux version of PuTTYgen to do the conversion. Linux version is command-line, contrary to Windows version.

puttygen mykey.ppk ‐O private‐openssh ‐o id_rsa

See Linux puttygen man page.

2

I found one more clear solution.

On puttygen create a key, then navigate to Top menu - Conversion and click export openssh key

File content will start and end with

-----BEGIN RSA PRIVATE KEY----- 

-----END RSA PRIVATE KEY-----
  • for root user Copy that key file to /root/.ssh/ as id_rsa or id_dsa

  • for other user Copy that key file to /home/user/.ssh/ as id_rsa or id_dsa

Note :

  1. No need to edit authorized_keys.
  2. I am using amazon linux
  3. File permission 0600
1

** Be careful and make sure you have console access to the box because if you don't do it right, you won't be able to ssh in again until you fix it from the console.

The process is much easier than you think. Load the public / private key pair you generated in puttygen again. In puttygen, you'll notice a window in the middle of the screen which says: "Public key for pasting into Open SSH authorized_keys file:".

highlight the entire contents of the box and press control-c to copy it.

SSH into your linux box and paste it into the "/home/username/.ssh/authorized_keys" file. I prefer to use nano and just right click to paste it in. Make sure it all stays on one line.

Modify your /etc/sshd_config file as needed and restart your sshd service: "service ssh restart"

If you need a sample sshd_config file, let me know and I can post mine.

I've done this on Ubuntu 8.04, 10.04 and 12.04 LTS server and it works slick.

0

I am not sure if this thread is still active, but I stumbled upon a similar problem with Windows 10 anniversary edition which now support Ubuntu kernel. I use to use Putty before for connecting to Linux machine. For generating id_rsa in linux format, use puttykeygen and load your putty private key then click on conversion and choose the second option.

Putty KeyGenerator conevrsions

open the newly generated key file and copy all it's contents, make sure your content starts with : -----BEGIN RSA PRIVATE KEY----- and ends with -----END RSA PRIVATE KEY-----

vi id_rsa inside your ~/.ssh directory and paste the copied contents, this is required because linux otherwise will not understand the contents of the file.

That's all, try ssh to the remote server, it should work.

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