This crate contains the type Letter. It is supposed to be a container for signed data.
You can create signed data either from a ed25519 key or using a Certificate.
For example, when you generate a ephermeral keypair, you sign your public key by creating a
Letter and send that over the network. The other end can then validate the
Letter and knows that you own that certificate, and if the other end trusts that certificate
(for example, by signing your certificate with the master keypair), it knows, that the
sent public key is really yours.