Add an implementation of mutual message exchange

This adds a new crate, `mutual-message-exchange` which allows two
parties to maintain a list of keys with which they want to exchange
messages and exchange one message at the cost of an extra
half-round-trip.

This is anticipated for use in BOLT12, where extra data can be
included in a BOLT12 `Invoice` which allows a mutually-trusting
sender to include a message in the onion, while any
non-mutually-trusting entities will not learn anything about the
recipient (subject to the use of blinded paths).

A full write-up of this protocol is available as
[bLIP 31](lightning/blips#31).

Cargo.toml

@@ -2,14 +2,15 @@
 resolver = "2"
 
 members = [
-    "lightning",
-    "lightning-block-sync",
-    "lightning-invoice",
-    "lightning-net-tokio",
-    "lightning-persister",
-    "lightning-background-processor",
-    "lightning-rapid-gossip-sync",
-    "lightning-custom-message",
+	"lightning",
+	"lightning-block-sync",
+	"lightning-invoice",
+	"lightning-net-tokio",
+	"lightning-persister",
+	"lightning-background-processor",
+	"lightning-rapid-gossip-sync",
+	"lightning-custom-message",
+	"mutual-message-exchange",
 ]
 
 exclude = [

mutual-message-exchange/Cargo.toml

@@ -0,0 +1,21 @@
+[package]
+name = "mutual-message-exchange"
+version = "0.1.0"
+authors = ["Matt Corallo"]
+license = "MIT OR Apache-2.0"
+repository = "https://github.com/lightningdevkit/rust-lightning/"
+description = """
+An implementation of a trivial mutual authentication scheme where a message can be exchanged iff two parties mutually trust each other (out of individual trusted-keys lists).
+"""
+edition = "2021"
+
+[package.metadata.docs.rs]
+rustdoc-args = ["--cfg", "docsrs"]
+
+[features]
+
+[dependencies]
+bitcoin_hashes = { version = "^0.12.0", default-features = false }
+secp256k1 = { version = "^0.27.0", default-features = false, features = ["alloc"] }
+
+[dev-dependencies]

mutual-message-exchange/src/chacha20.rs

@@ -0,0 +1 @@
+../../lightning/src/crypto/chacha20.rs

mutual-message-exchange/src/chacha20poly1305rfc.rs

@@ -0,0 +1 @@
+../../lightning/src/crypto/chacha20poly1305rfc.rs