puttygen: error loading x.ppk: PuTTY key format too new

Putty saves ssh key in file with .ppk extension. To use the key on Linux systems, you need to convert it to openssh format. When converting ppk file to openssh format, i got error “PuTTY key format too new”.

boby@sok:~$ puttygen key.ppk -O private-openssh -o 2.pem
puttygen: error loading `key.ppk': PuTTY key format too new
boby@sok:~$

This error is because the puttygen command is older version. Ubuntu 20.04 comes with older version of puttygen command provided by putty-tools.

The solution is to remove putty-tools package, then install latest version of puttygen from source code.

Remove putty-tools with

sudo apt remove putty-tools

Download and install latest puttygen from

https://www.chiark.greenend.org.uk/~sgtatham/putty/latest.html

As of writing this, latest version of putty is version 0.76

mkdir ~/src
cd ~/src
wget https://the.earth.li/~sgtatham/putty/latest/putty-0.76.tar.gz
tar -xvf putty-0.76.tar.gz
cd putty-0.76/
./configure
make
sudo cp puttygen /usr/bin/

Once installed, you can check version with

boby@sok:~/src/putty-0.76$ puttygen --version
puttygen: Release 0.76
Build platform: 64-bit Unix
Compiler: gcc 9.3.0
Source commit: 1fd7baa7344bb38d62a024e5dba3a720c67d05cf
boby@sok:~/src/putty-0.76$ 

You will be able to convert newer version of ppk files to openssh format now.

See Convert PPK file to PEM