PuTTYgen and an Ubuntu Server

Recently I used PuTTYgen to create a key pair which I intended to use to connect to one of the Ubuntu servers I have in my home lab. Not paying attention to detail, I uploaded the public key, ran

cat key.pub >> .ssh/authorized_keys

and was surprised to find that I was not able to connect. It turns out the formatting of the key is different in subtle ways. I’m going to cover how I manually modified the key so it would work, and then later found an easier way via ssh-keygen.

Manual Process

First I opened the key in Atom, an alternative to sublime text which I favor (when not using Vim that is 😉 ).

Turns out *Nix systems expect no other formatting or characters apart from the key itself. So I had to remove white spaces, comments and control characters. I manually removed all the words and comments. Then I hit find and searched via regex for carriage return and line feed. \r\n in the picture. I left the replace blank as I wanted to remove everything except for the key and replace it with nothing. You can see where Atom highlighted the space we would be removing.

Then I added ssh-rsa followed by a space at the start of the file. This signals the file encoding being used. Check RFC 4253 secion 6.6 (link) for a detailed, albeit boring, read about why we use ssh-rsa. If I want a comment, I can add a space followed by a comment at the end of the line.

Finally, just to be sure there were no issues with EOL between Windows and Linux, I installed an Atom package called line ending converter. To do this in Atom, I went to settings, chose install and searched community packages for the package. Once it is installed, it can be found under packages at Convert Line Endings to.

So now I have an acceptable key. I ssh to the box via username/password, open .ssh/authorized_keys, paste the key, and I’m good to go.

Or, if you’re the more streamlined type, this whole blog post can be done in one step. But, you wouldn’t learn what’s going on under the hood, and that’s the fun part.

ssh-keygen -i -f putty_key > new_key

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