On running the command ssh-add mykey.ppk it asks for passphrase:Enter passphrase for mykey.ppk:But I can see that the key does not have any passphrase and is not encrypted $ head mykey.ppkPuTTY-User-Key-File-2: ssh-rsaEncryption: noneComment: imported-openssh-keyPublic-Lines: 6AAAAB3NzaC1yc2EAAAADAQABAAABAQC8V+PLuklXrfFDZ9GNluXB/L8foOzaEp5sjwaOL1iAxCKDWWsfsmyj9MbhV5r4Z6VGo/0TSimply pressing enter at the prompt does not work. How can I add this key to the agent? PS: I've already heard the sermon on security practices, so you can save your breath :-)
How to add a phrase-less key to ssh agent?
ssh;ssh agent
ssh-agent does not support private keys in PPK format (PuTTY). You need to convert the key to OpenSSH key using PuTTY gen to be able to add it to your ssh-agent.Related question on RaspberryPi.These steps are needed:Load your private key into PuTTYgenGo to Conversions  Export OpenSSH and export your key as mykey.keyAdd your key to your agent using ssh-add mykey.key.On Linux, the equivalent puttygen command is:puttygen mykey.ppk -o mykey.key -O private-openssh