How it works
ssh-vault will randomly create a password and use it for encrypting the contents of the vault using a symmetric cipher AES256|ChaCha20Poly1305, the password later will be encrypted using the SSH public key (RSA or Ed25519) of the receiver.
When creating a vault, the option -u user will fetch the receiver ssh public keys from GitHub or any other server provide server
Example fetchinig keys from GitLab
ssh-vault create -u https://gitlab.com/alice.keys
This helps to create a vault very straight forward, since both the sender and receiver don't need to exchange their keys before in order to share sensitive data, therefore this helps to work asynchronously.