Mixnet integration (#1346)

See #1345, <https://github.com/paritytech/substrate/pull/14207>.

This adds all the necessary mixnet components, and puts them together in
the "kitchen-sink" node/runtime. The components added are:

- A pallet (`frame/mixnet`). This is responsible for determining the
current mixnet session and phase, and the mixnodes to use in each
session. It provides a function that validators can call to register a
mixnode for the next session. The logic of this pallet is very similar
to that of the `im-online` pallet.
- A service (`client/mixnet`). This implements the core mixnet logic,
building on the `mixnet` crate. The service communicates with other
nodes using notifications sent over the "mixnet" protocol.
- An RPC interface. This currently only supports sending transactions
over the mixnet.

---------

Co-authored-by: David Emett <dave@sp4m.net>
Co-authored-by: Javier Viola <javier@parity.io>

substrate/client/mixnet/src/peer_id.rs

@@ -0,0 +1,44 @@
+// This file is part of Substrate.
+
+// Copyright (C) Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: GPL-3.0-or-later WITH Classpath-exception-2.0
+
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with this program. If not, see <https://www.gnu.org/licenses/>.
+
+use libp2p_identity::PeerId;
+use mixnet::core::PeerId as CorePeerId;
+
+/// Convert a libp2p [`PeerId`] into a mixnet core [`PeerId`](CorePeerId).
+///
+/// This will succeed only if `peer_id` is an Ed25519 public key ("hashed" using the identity
+/// hasher). Returns `None` on failure.
+pub fn to_core_peer_id(peer_id: &PeerId) -> Option<CorePeerId> {
+	let hash = peer_id.as_ref();
+	if hash.code() != 0 {
+		// Hash is not identity
+		return None
+	}
+	let public = libp2p_identity::PublicKey::try_decode_protobuf(hash.digest()).ok()?;
+	public.try_into_ed25519().ok().map(|public| public.to_bytes())
+}
+
+/// Convert a mixnet core [`PeerId`](CorePeerId) into a libp2p [`PeerId`].
+///
+/// This will succeed only if `peer_id` represents a point on the Ed25519 curve. Returns `None` on
+/// failure.
+pub fn from_core_peer_id(core_peer_id: &CorePeerId) -> Option<PeerId> {
+	let public = libp2p_identity::ed25519::PublicKey::try_from_bytes(core_peer_id).ok()?;
+	let public: libp2p_identity::PublicKey = public.into();
+	Some(public.into())
+}