Convert ssh-keygen generated keys for use in PuTTY
By default, the keys (with passphases) generated by ssh-keygen cannot be used by PuTTY. This article describes the steps to convert ed25519 keys to a form that PuTTY understands.
Remove passphase from the key
When loading the key (with passphase) generated by ssh-keygen into PuTTY, an error is shown, saying:
Couldn't load private key (unrecognized cipher name)
Assume the private key is test.ed25519, make a copy and remove the passphase.
cp test.ed25519 test.ed25519.putty ssh-keygen -p -f test.ed25519.putty
When prompted for new passphase, leave the field empty and press Enter.
Import and convert the key
Now the new key (test.ed25519.putty) can be loaded into PuTTY.
Enter the key passphase and save the private key. The PPK file can be used in PuTTY (and WinSCP).