End-to-End keyserver protocol

The protocol consists of exchanging protobuf3 messages over grpc. The message descriptors are split into three files:

• client.proto -- all messages that are required for operation of a lightweight (stateful or stateless) client. Keyserver entries and server signature structures.
• config.proto -- the client configuration.
• verifier.proto -- everything that a verifier needs to use but a client does not. Pushing signatures, downloading update logs, etc.
• replication.proto -- service-provider internal protocol, included here as a part of the reference implementation. Handles high-availability replication and synchronization of updates to the keyserver state.
• local.proto -- service-provider internal protocol, included here as a part of the reference implementation. The local structures are user for persisting local state at each replica of the service provider.