Convert a Putty private key to openssh format in Linux

First install putty tools

sudo apt-get install putty-tools


Now, convert the file with:

Putty Private Key to OpenSSH

puttygen [sourcefile.ppk] -O private-openssh -o [Outputfilename]

Putty Public Key to OpenSSH

puttygen [sourcefile.ppk] -O public-openssh -o [Outputfilename]

OpenSSH Private Key to Putty ppk

puttygen [opensshsourcefilename] -O private -o [Outputfilename.ppk]

OpenSSH Public Key to Putty ppk

puttygen [opensshsourcefilename] -O public -o [Outputfilename.ppk]