Add a MessageContext::DNSResolution to protect against probing

When we make a DNSSEC query with a reply path, we don't want to
allow the DNS resolver to attempt to respond to various nodes to
try to detect (through timining or other analysis) whether we were
the one who made the query. Thus, we need to include a nonce in the
context in our reply path, which we set up here by creating a new
context type for DNS resolutions.

Author: TheBlueMatt

lightning/src/blinded_path/message.rs

@@ -284,6 +284,11 @@ pub enum MessageContext {
 	///
 	/// [`AsyncPaymentsMessage`]: crate::onion_message::async_payments::AsyncPaymentsMessage
 	AsyncPayments(AsyncPaymentsContext),
+	/// Represents a context for a blinded path used in a reply path when requesting a DNSSEC proof
+	/// in a [`DNSResolverMessage`].
+	///
+	/// [`DNSResolverMessage`]: crate::onion_message::dns_resolution::DNSResolverMessage
+	DNSResolver(DNSResolverContext),
 	/// Context specific to a [`CustomOnionMessageHandler::CustomMessage`].
 	///
 	/// [`CustomOnionMessageHandler::CustomMessage`]: crate::onion_message::messenger::CustomOnionMessageHandler::CustomMessage
@@ -402,6 +407,7 @@ impl_writeable_tlv_based_enum!(MessageContext,
 	{0, Offers} => (),
 	{1, Custom} => (),
 	{2, AsyncPayments} => (),
+	{3, DNSResolver} => (),
 );
 
 impl_writeable_tlv_based_enum!(OffersContext,
@@ -428,6 +434,24 @@ impl_writeable_tlv_based_enum!(AsyncPaymentsContext,
 	},
 );
 
+/// Contains a simple nonce for use in a blinded path's context.
+///
+/// Such a context is required when receiving a [`DNSSECProof`] message.
+///
+/// [`DNSSECProof`]: crate::onion_message::dns_resolution::DNSSECProof
+#[derive(Clone, Debug, Hash, PartialEq, Eq)]
+pub struct DNSResolverContext {
+	/// A nonce which uniquely describes a DNS resolution.
+	///
+	/// When we receive a DNSSEC proof message, we should check that it was sent over the blinded
+	/// path we included in the request by comparing a stored nonce with this one.
+	pub nonce: [u8; 16],
+}
+
+impl_writeable_tlv_based!(DNSResolverContext, {
+	(0, nonce, required),
+});
+
 /// Construct blinded onion message hops for the given `intermediate_nodes` and `recipient_node_id`.
 pub(super) fn blinded_hops<T: secp256k1::Signing + secp256k1::Verification>(
 	secp_ctx: &Secp256k1<T>, intermediate_nodes: &[MessageForwardNode],