Handle secret key encryption

See https://download.libsodium.org/doc/advanced/ed25519-curve25519.html to get a secret key pubkey from a signing key pubkey

And see https://download.libsodium.org/doc/public-key_cryptography/authenticated_encryption.html for encryption using a secret key pubkey (don't forgot the nonce !)

[EDIT] To simplify encryption, we use the box_seal methods of libnacl. No nonce involved, message is just encrypted, not signed in the process, involving only the pubkey to encrypt.