net: exchange signed .btf descriptors between peers (#32)

Discovery has depended entirely on the Nostr relays. Two connected nodes never
told each other who else existed, so a relay outage left a running network
unable to grow, and a node that found one stale peer had no way to learn there
was anything better. The old `addr` gossip went out with the IRC seed.

Peers now trade signed descriptors over a new "btfpeers" message: our own
first, then peers that actually answered us. The receiver checks each Schnorr
signature against the key the `.btf` address decodes to, so the sender is
trusted for nothing -- forging an entry needs somebody else's secret key. That
leaves flooding with self-generated keys, which the per-message caps and the
"peers that answered" ordering are there to blunt.

Unknown commands are ignored (main.cpp), so nodes without this just drop the
message; nothing here is a protocol break.

The cache from the peer-cache change now keeps each peer's descriptor verbatim
alongside the parsed fields, because that signature is what makes a peer
relayable. Entries without one -- resolved through Nostr, or written before
this -- stay dialable, just not passed on. A dial never clears a descriptor
already on file.

1 files changed, 30 insertions, 0 deletions

diff --git a/src/nostr.cpp b/src/nostr.cpp
index ec02c2e..482a835 100644
--- a/src/nostr.cpp
+++ b/src/nostr.cpp
@@ -342,6 +342,36 @@ std::string BtfLocalAddress()
     return g_nostrKey.BtfAddress();
 }
 
+// This node's own descriptor, signed and self-certifying -- the same blob
+// PublishDescriptor puts on Nostr, but handed straight to a peer instead.
+//
+// Reads strBtfActiveRelay directly rather than going through BtfActiveRelay():
+// only a rendezvous we actually registered at is worth announcing. Handing out
+// a meeting node we are not reachable at would make every peer that receives
+// it burn a dial, and over peer exchange that mistake travels further than it
+// does over Nostr.
+std::string BtfLocalDescriptor()
+{
+    if (!EnsureNostrKey())
+        return "";
+    string meeting;
+    CRITICAL_BLOCK(cs_activeRelay)
+        meeting = strBtfActiveRelay;
+    if (meeting.empty())
+        return "";
+    return btf::SignDescriptor(g_nostrKey.ctx, g_nostrKey.seckey,
+                               g_nostrKey.EncPubHex(), meeting, (uint64_t)GetTime());
+}
+
+// The shared secp256k1 context, so net.cpp can verify descriptors that arrive
+// from peers. NULL if the identity could not be loaded.
+void* BtfSecpContext()
+{
+    if (!EnsureNostrKey())
+        return NULL;
+    return (void*)g_nostrKey.ctx;
+}
+
 int GetDiscoveredPeerCount()
 {
     int n = 0;