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.

5 files changed, 201 insertions, 2 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;
diff --git a/src/net.cpp b/src/net.cpp
index b5d12d0..1d552cf 100644
--- a/src/net.cpp
+++ b/src/net.cpp
@@ -285,6 +285,13 @@ struct CachedBtfPeer
     string meeting;
     string encHex;
     int64  lastSeen;
+    // The peer's own signed descriptor, as it announced itself. Kept verbatim
+    // because that signature is what lets us hand this peer on to somebody
+    // else: the receiver checks it against the key the address decodes to and
+    // never has to take our word for anything. Empty for entries learned
+    // before peer exchange existed, or resolved through Nostr -- those are
+    // still dialable, just not relayable.
+    string desc;
 };
 
 static string BtfPeerCachePath() { return GetAppDir() + "/btfpeers.json"; }
@@ -336,6 +343,8 @@ void LoadCachedBtfPeers(vector<CachedBtfPeer>& out)
             p.meeting  = o["meeting"].get<string>();
             p.encHex   = o["enc"].get<string>();
             p.lastSeen = o.value("seen", (int64)0);
+            if (o.contains("desc") && o["desc"].is_string())
+                p.desc = o["desc"].get<string>();
             if (nNow - p.lastSeen > CACHED_BTF_PEER_TTL) continue; // long gone
             out.push_back(p);
         }
@@ -349,22 +358,35 @@ void LoadCachedBtfPeers(vector<CachedBtfPeer>& out)
 
 static CCriticalSection cs_btfPeerCache;
 
+// strDesc is the peer's own signed descriptor when we have it (it announced
+// itself over peer exchange), "" when we only resolved it through Nostr. An
+// empty one never clears a descriptor already on file: dialing a peer we first
+// learned about by exchange must not cost us the ability to pass it on.
 static void RememberBtfPeer(const string& strBtfAddr, const string& strMeeting,
-                            const unsigned char enc_pub[32])
+                            const unsigned char enc_pub[32],
+                            const string& strDesc = string())
 {
     CRITICAL_BLOCK(cs_btfPeerCache)
     {
         vector<CachedBtfPeer> peers;
         LoadCachedBtfPeers(peers);
 
+        string strKeepDesc = strDesc;
         for (size_t i = 0; i < peers.size(); i++)
-            if (peers[i].btfAddr == strBtfAddr) { peers.erase(peers.begin() + i); break; }
+            if (peers[i].btfAddr == strBtfAddr)
+            {
+                if (strKeepDesc.empty())
+                    strKeepDesc = peers[i].desc;
+                peers.erase(peers.begin() + i);
+                break;
+            }
 
         CachedBtfPeer p;
         p.btfAddr  = strBtfAddr;
         p.meeting  = strMeeting;
         p.encHex   = BytesToHex(enc_pub, 32);
         p.lastSeen = GetTime();
+        p.desc     = strKeepDesc;
         peers.insert(peers.begin(), p); // most recent first
 
         if (peers.size() > MAX_CACHED_BTF_PEERS)
@@ -378,6 +400,8 @@ static void RememberBtfPeer(const string& strBtfAddr, const string& strMeeting,
             o["meeting"] = q.meeting;
             o["enc"]     = q.encHex;
             o["seen"]    = q.lastSeen;
+            if (!q.desc.empty())
+                o["desc"] = q.desc;
             arr.push_back(o);
         }
         FILE* f = fopen(BtfPeerCachePath().c_str(), "w");
@@ -419,6 +443,93 @@ void ThreadReconnectCachedBtfPeers(void* parg)
              nConnected, peers.size());
 }
 
+//
+// .btf peer exchange
+//
+// Until now discovery 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.
+//
+// What travels here is each peer's own signed descriptor. The receiver checks
+// the Schnorr signature against the key the `.btf` address decodes to, so the
+// sender is trusted for nothing -- forging an entry would need somebody else's
+// secret key. That leaves flooding: valid descriptors for keys the sender
+// generated itself. The caps below bound one message, and the cache prefers
+// peers that actually answered, so a flood costs a Sybil more than it costs us.
+//
+
+void BtfPexCollect(vector<string>& vDescOut)
+{
+    vDescOut.clear();
+
+    // Ours first -- the node on the other end may know nobody but us.
+    string strMine = BtfLocalDescriptor();
+    if (!strMine.empty())
+        vDescOut.push_back(strMine);
+
+    vector<CachedBtfPeer> peers;
+    CRITICAL_BLOCK(cs_btfPeerCache)
+        LoadCachedBtfPeers(peers);
+
+    // Most recently seen first: these answered us, they are not names copied
+    // off a relay listing.
+    foreach(const CachedBtfPeer& p, peers)
+    {
+        if (vDescOut.size() >= MAX_PEX_DESCRIPTORS) break;
+        if (p.desc.empty()) continue;                          // nothing provable to pass on
+        if (p.desc.size() > MAX_PEX_DESCRIPTOR_BYTES) continue;
+        vDescOut.push_back(p.desc);
+    }
+}
+
+int BtfPexAccept(const vector<string>& vDesc)
+{
+    void* ctx = BtfSecpContext();
+    if (!ctx)
+        return 0;
+
+    string strSelf = BtfLocalAddress();
+    int nKept = 0;
+    unsigned int nSeen = 0;
+
+    foreach(const string& strDesc, vDesc)
+    {
+        if (++nSeen > MAX_PEX_DESCRIPTORS) break;
+        if (strDesc.empty() || strDesc.size() > MAX_PEX_DESCRIPTOR_BYTES) continue;
+
+        // The descriptor names the key it belongs to, and taking that key from
+        // the blob is safe precisely because the signature must verify under
+        // it: nobody can produce a valid descriptor for a key they don't hold.
+        unsigned char pubkey[32];
+        try
+        {
+            nlohmann::json v = nlohmann::json::parse(strDesc);
+            if (!v.is_object() || !v.contains("pubkey") || !v["pubkey"].is_string())
+                continue;
+            if (!HexToBytes(v["pubkey"].get<string>(), pubkey, 32))
+                continue;
+        }
+        catch (...) { continue; }   // malformed JSON from an untrusted peer
+
+        btf::Descriptor d;
+        if (!btf::VerifyDescriptor(ctx, strDesc, pubkey, d))
+            continue;
+
+        string strAddr = btf::Address(pubkey);
+        if (strAddr.empty() || strAddr == strSelf)
+            continue;               // ourselves, nothing to learn
+
+        unsigned char enc[32];
+        if (!HexToBytes(d.enc, enc, 32))
+            continue;
+
+        RememberBtfPeer(strAddr, d.meeting_node, enc, strDesc);
+        nKept++;
+    }
+    return nKept;
+}
+
 static CNode* ConnectNodeBtfTail(const string& strBtfAddr, const unsigned char pk[32],
                                   const string& strMeeting, const unsigned char enc_pub[32])
 {
diff --git a/src/net.h b/src/net.h
index f028c3b..7d70721 100644
--- a/src/net.h
+++ b/src/net.h
@@ -29,6 +29,21 @@ CNode* ConnectNodeBtfResolved(const string& strBtfAddr, const string& strMeeting
 void ThreadBtfAccept(void* parg);
 void ThreadBtfConnect(void* parg);
 extern string strBtfConnect;
+
+// --- .btf peer exchange ---------------------------------------------------
+// Peers gossip signed descriptors so discovery survives a relay outage. Both
+// caps apply to a single "btfpeers" message; the generic header limit is 256 MB,
+// which is no protection at all for something a stranger can send unprompted.
+static const unsigned int MAX_PEX_DESCRIPTORS      = 20;
+static const unsigned int MAX_PEX_DESCRIPTOR_BYTES = 1024;
+// Ignore a peer's exchange more often than this. One announcement per side per
+// connection is the intended traffic; anything faster is someone else's idea.
+static const int64        PEX_MIN_INTERVAL         = 60;
+
+// Descriptors to hand a peer: ours first, then peers that answered us.
+void BtfPexCollect(std::vector<std::string>& vDescOut);
+// Verify descriptors a peer sent and remember the good ones. Returns how many.
+int  BtfPexAccept(const std::vector<std::string>& vDesc);
 void AbandonRequests(void (*fn)(void*, CDataStream&), void* param1);
 bool AnySubscribed(unsigned int nChannel);
 void ThreadBitcoinMiner(void* parg);
@@ -467,6 +482,9 @@ public:
     // publish and subscription
     vector<char> vfSubscribe;
 
+    // Last "btfpeers" we accepted from this node, to rate-limit the exchange.
+    int64 nLastPexRecv;
+
 
     CNode(SOCKET hSocketIn, CAddress addrIn, bool fInboundIn=false)
     {
@@ -483,6 +501,7 @@ public:
         fDisconnect = false;
         nRefCount = 0;
         nReleaseTime = 0;
+        nLastPexRecv = 0;
         vfSubscribe.assign(256, false);
 
         // Push a version message
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;
diff --git a/src/nostr.h b/src/nostr.h
index 04149e3..b5eb5e3 100644
--- a/src/nostr.h
+++ b/src/nostr.h
@@ -82,6 +82,14 @@ bool BtfGetIdentity(unsigned char pubkey[32], unsigned char enc_sk[32]);
 // This node's printable .btf address, or "" if the identity failed to load.
 std::string BtfLocalAddress();
 
+// This node's own signed descriptor, ready to hand to a peer over the wire.
+// "" until ThreadBtfAccept has registered at a rendezvous.
+std::string BtfLocalDescriptor();
+
+// The shared secp256k1 context, for btf::VerifyDescriptor (which takes it as
+// void* to keep headers free of secp). NULL if the identity failed to load.
+void* BtfSecpContext();
+
 // Resolve a .btf address via the public discovery relays: fetch the owner's
 // self-certified descriptor and return its meeting node ("host:port") and
 // x25519 public key for the end-to-end channel.