How to convert ed25519 private key to putty ppk?

The command in the above answer is just printing public key portion in RFC4716 format.

At some point, ssh-keygen generates openssh private key which doesn't use cipher supported by puttygen.

ssh-keygen doesn't provide option to specify cipher name to encrypt the resulting openssh private key.

There is a workaround: remove the passphrase from the key before importing into puttygen.

$ cp ~/.ssh/id_ed25519 ~/.ssh/id_ed25519-for-putty

$ ssh-keygen -p -f ~/.ssh/id_ed25519-for-putty
Enter old passphrase: <your passphrase>
Enter new passphrase (empty for no passphrase): <press Enter>
Enter same passphrase again: <press Enter>

Then, use puttygen to convert ~/.ssh/id_ed25519-for-putty into .ppk and set the passphrase from puttygen.

Don't forget to shred and remove ~/.ssh_id_ed25519-for-putty afterwards for obvious reason.

You need to export the key into RFC4716 format before importing the key into puttygen

$ ssh-keygen -e -m RFC4716 -f ~/.ssh/id_ed25519 > ~/.ssh/exported_id_ed25519

Then, import the resulting exported_id_ed25519 into puttygen and convert the key into .ppk

Update 2019-03-20: https://www.chiark.greenend.org.uk/~sgtatham/putty/releases/0.71.html supports these keys

At the time of writing this, the puttygen snapshot from https://www.chiark.greenend.org.uk/~sgtatham/putty/snapshot.html seems to support these keys where 0.70 did not. It's not explicitly mentioned in the changelog.

I tested Development snapshot 2019-01-17.53747ad