Add a message queue

This is intended to be a reliable transport between the processors and
coordinator. Since it'll be intranet only, it's written as never fail.

Primarily needs testing and a proper ID.

message-queue/src/messages.rs

@@ -0,0 +1,40 @@
+use transcript::{Transcript, RecommendedTranscript};
+use ciphersuite::{group::GroupEncoding, Ciphersuite, Ristretto};
+
+use serde::{Serialize, Deserialize};
+
+use serai_primitives::NetworkId;
+
+#[derive(Clone, Copy, PartialEq, Eq, Hash, Debug, Serialize, Deserialize)]
+pub enum Service {
+  Processor(NetworkId),
+  Coordinator,
+}
+
+#[derive(Clone, PartialEq, Eq, Debug, Serialize, Deserialize)]
+pub struct QueuedMessage {
+  pub from: Service,
+  pub msg: Vec<u8>,
+  pub sig: Vec<u8>,
+}
+
+#[derive(Clone, Copy, PartialEq, Eq, Debug, Serialize, Deserialize)]
+pub struct Metadata {
+  pub from: Service,
+  pub to: Service,
+  pub from_id: u64,
+}
+
+pub fn message_challenge(
+  from: <Ristretto as Ciphersuite>::G,
+  msg: &[u8],
+  nonce: <Ristretto as Ciphersuite>::G,
+) -> <Ristretto as Ciphersuite>::F {
+  let mut transcript = RecommendedTranscript::new(b"Serai Message Queue v0.1");
+  transcript.domain_separate(b"message");
+  transcript.append_message(b"from", from.to_bytes());
+  transcript.append_message(b"msg", msg);
+  transcript.domain_separate(b"signature");
+  transcript.append_message(b"nonce", nonce.to_bytes());
+  <Ristretto as Ciphersuite>::hash_to_F(b"challenge", &transcript.challenge(b"challenge"))
+}