Remote signer

[wojcik91]

General overview

We propose introducing a remote signer capability to the node, similar to what's available in Cosmos-based blockchains.
We'd leave the current file-based key management system as-is but give node operators a configuration option to offload signing operations to a remote HTTP service.
Such a service could be running on the same machine, on a different server or wherever the user decides to deploy it and it could be implementing various key management schemes, HA approaches etc while being transparent to the node.

Motivation

When initially considering adding HSM support we intended to add a shared Signer trait and implement it for each supported key management solution within the node itself. We believe using a remote signer instead would have some significant advantages:

• implementing additional key management solutions (new HSMs, adding HA, integrating with cloud solutions) would be independent from node development itself
• additional signer services could be implemented by the community
• we would avoid introducing ever-growing complexity to node configuration and codebase
• cloud solutions could be made more broadly available accross different providers (for example a Hetzner VPS could in theory access a signer service deployed on AWS with their HSM)
• node management can be handed off to a third party without exposing private keys
• remote signing capability could be leveraged to implement some HA/fallback solution (for example running multiple nodes with dummy keys and using an external tool to monitor them and only reconfigure master node to use the main remote signer)
• enterprise clients could implement their own signing solutions according to their internal requirements

Additionally, existing prior art in CosmosSDK ecosystem leads us to believe that the additional latency would not be an issue as we've feared, since they are using even shorter block times (4s).

Implementation

From the technical perspective implementing remote signer functionality would require following changes:

• update node configuration options to handle remote signers
• add a Signer trait to allow the node to handle both local keys and remote signers
• define a REST API for remote signer services
• initially implement a basic HTTP server to offload file-based key management to a remote signer
  As the next step we'd propose extending the signer to support some HSM solution.