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, 31 insertions, 0 deletions

diff --git a/src/main.cpp b/src/main.cpp
index a75bf54..bb317d2 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -1877,6 +1877,14 @@ bool ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv)
             pfrom->PushMessage("getblocks", CBlockLocator(pindexBest), uint256(0));
         }
 
+        // Introduce ourselves and hand over the .btf peers that answered us.
+        // Everything in here is self-certifying, so this is safe to send to a
+        // node we know nothing about, and safe for it to act on.
+        vector<string> vPexOut;
+        BtfPexCollect(vPexOut);
+        if (!vPexOut.empty())
+            pfrom->PushMessage("btfpeers", vPexOut);
+
         if (LogAcceptsCategory("net")) printf("version addrMe = %s\n", addrMe.ToString().c_str());
     }
 
@@ -1888,6 +1896,29 @@ bool ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv)
     }
 
 
+    else if (strCommand == "btfpeers")
+    {
+        // Peer exchange. Nothing here is taken on trust: BtfPexAccept checks
+        // every descriptor's signature against the key its .btf address
+        // decodes to, and drops the rest.
+        int64 nNow = GetTime();
+        if (nNow - pfrom->nLastPexRecv < PEX_MIN_INTERVAL)
+        {
+            if (LogAcceptsCategory("net"))
+                printf("btfpeers: ignoring, %s is sending them too fast\n",
+                       pfrom->addr.ToString().c_str());
+            return true;
+        }
+        pfrom->nLastPexRecv = nNow;
+
+        vector<string> vDesc;
+        vRecv >> vDesc;
+        int nKept = BtfPexAccept(vDesc);
+        LogPrint("net", "btfpeers: kept %d of %zu descriptor(s) from %s\n",
+                 nKept, vDesc.size(), pfrom->addr.ToString().c_str());
+    }
+
+
     else if (strCommand == "inv")
     {
         vector<CInv> vInv;