Is this not already possible with tools like https://ssh-vault.com/ and (shameless plug) https://sshenc.sh/?
Those tools use the RSA keys to encrypt a symmetric key that is then used to encrypt the data, but the outcome is effectively the same. No?
Those tools are not meant for sender authentication. If you want that you would have to first share the senders pubkey with the recipient, and sign your message with the corresponding privkey.
As I commented in https://news.ycombinator.com/item?id=19953623, I’d love to see another blog post walking folks through why/how the “dangerous” RSA keys are in fact useable for both operations because the textbook RSA concerns aren’t a concern because of X, Y, and Z.
> Goals
> * The option to encrypt to SSH keys, with built-in GitHub .keys support
And lower on the page, it shows the use of built-in command-line syntax for “github:$recipientname”.
It definitely works for keys that aren’t used for SSH, but support for SSH keys seems to be a large part of the rational for the ed25519 support (otherwise it would just be a tool that operated on X25519 keys directly).