ltc: fix private-net IDENTIFIER truncation + independent PREFIX (V36 ship-blocker)

Fixes private-net IDENTIFIER truncation and derives PREFIX independently of network-id. Unblocks c2pool<->p2pool framing parity for the V36 crossing testbed. CI all green (16 checks).

Author: frstrtr

src/c2pool/c2pool_refactored.cpp

@@ -442,6 +442,10 @@ void print_help() {
     std::cout << "                            Nonzero: creates a private sharechain. P2P prefix\n";
     std::cout << "                            and THE metadata will carry this ID on the blockchain.\n";
     std::cout << "                            Genesis shares are created automatically when chain is empty.\n";
+    std::cout << "                            Full 8 bytes (up to 16 hex chars) are used, not truncated.\n";
+    std::cout << "  --prefix HEX              Explicit P2P transport-framing prefix (hex, up to 8 bytes).\n";
+    std::cout << "                            Independent of --network-id; defaults to the compiled\n";
+    std::cout << "                            per-network constant. Must match across all peers.\n";
     std::cout << "  --startup-mode MODE       Sharechain startup behavior:\n";
     std::cout << "                              auto    — wait for peers (60s), then genesis if none (default)\n";
     std::cout << "                              genesis — create new chain immediately, don't wait for peers\n";
@@ -642,7 +646,8 @@ int main(int argc, char* argv[]) {
     std::string coinbase_text;  // --coinbase-text (replaces /c2pool/ tag)
 
     // Private sharechain
-    uint32_t network_id = 0;        // 0 = public p2pool network, nonzero = private
+    uint64_t network_id = 0;        // 0 = public p2pool network, nonzero = private (full 8 bytes)
+    std::string prefix_override_hex; // explicit --prefix; empty = compiled per-network default
 
     // Startup mode: wait (default, p2pool persist=true), genesis, auto
     enum class StartupMode { AUTO, GENESIS, WAIT };
@@ -960,9 +965,14 @@ int main(int argc, char* argv[]) {
             cli_explicit.insert("coinbase_text");
         }
         else if ((arg == "--network-id" || arg == "--chain-id") && i + 1 < argc) {
-            network_id = static_cast<uint32_t>(std::stoul(argv[++i], nullptr, 16));
+            // Full 8-byte identifier: parse as 64-bit, never truncate to 32 bits
+            network_id = std::stoull(argv[++i], nullptr, 16);
             cli_explicit.insert("network_id");
         }
+        else if (arg == "--prefix" && i + 1 < argc) {
+            prefix_override_hex = argv[++i];
+            cli_explicit.insert("prefix");
+        }
         else if (arg == "--startup-mode" && i + 1 < argc) {
             std::string mode = argv[++i];
             if (mode == "genesis") startup_mode = StartupMode::GENESIS;
@@ -2530,11 +2540,14 @@ int main(int argc, char* argv[]) {
             // Private chain: override IDENTIFIER and PREFIX before any P2P or share ops
             if (network_id != 0) {
                 std::ostringstream hex;
-                hex << std::hex << std::setfill('0') << std::setw(8) << network_id;
-                // Pad to 16 hex chars (8 bytes) for full IDENTIFIER
+                hex << std::hex << std::setfill('0') << std::setw(16) << network_id;  // full 8-byte IDENTIFIER, no truncation
+                // setw(16) already produced the full 8-byte IDENTIFIER
                 std::string id_hex = hex.str();
-                while (id_hex.size() < 16) id_hex += "00";
                 ltc::PoolConfig::set_network_id(id_hex);
+                // PREFIX is independent: compiled per-network default unless
+                // explicitly overridden via --prefix (never derived from the ID)
+                if (!prefix_override_hex.empty())
+                    ltc::PoolConfig::set_prefix(prefix_override_hex);
                 LOG_INFO << "[Private] Network ID: " << ltc::PoolConfig::identifier_hex()
                          << " (PREFIX: " << ltc::PoolConfig::prefix_hex() << ")";
             }

src/impl/ltc/config_pool.hpp

@@ -142,7 +142,7 @@ class PoolConfig : protected core::Fileconfig
     static inline std::string override_prefix_hex;
 
     /// Set private network identity. IDENTIFIER is the consensus secret
-    /// (hashed into ref_hash). PREFIX is derived from it for transport framing.
+    /// (hashed into ref_hash). PREFIX is NOT derived from it (see set_prefix).
     /// Call once at startup before any P2P or share operations.
     static void set_network_id(const std::string& network_id_hex) {
         if (network_id_hex.empty() || network_id_hex == "0" || network_id_hex == "00000000")
@@ -154,20 +154,18 @@ class PoolConfig : protected core::Fileconfig
         if (padded.size() > 16) padded = padded.substr(0, 16);
 
         override_identifier_hex = padded;
+    }
 
-        // Derive PREFIX from IDENTIFIER using simple XOR mixing
-        // PREFIX = IDENTIFIER bytes XOR-rotated (fast, deterministic, non-reversible enough
-        // for transport framing — the real security is in IDENTIFIER via ref_hash)
-        auto id_bytes = ParseHex(padded);
-        static const char* HEX = "0123456789abcdef";
-        override_prefix_hex.clear();
-        override_prefix_hex.reserve(16);
-        for (size_t i = 0; i < 8 && i < id_bytes.size(); ++i) {
-            // XOR with rotated byte + constant to ensure PREFIX != IDENTIFIER
-            uint8_t b = id_bytes[i] ^ id_bytes[(i + 3) % id_bytes.size()] ^ 0x5A;
-            override_prefix_hex += HEX[b >> 4];
-            override_prefix_hex += HEX[b & 0x0f];
-        }
+    /// Set an explicit transport-framing PREFIX for a private chain (--prefix).
+    /// Independent of IDENTIFIER; when unset, prefix_hex() falls back to the
+    /// compiled per-network constant. All peers on the chain must agree on it.
+    static void set_prefix(const std::string& prefix_hex_in) {
+        if (prefix_hex_in.empty() || prefix_hex_in == "0")
+            return;
+        std::string padded = prefix_hex_in;
+        while (padded.size() < 16) padded = "0" + padded;
+        if (padded.size() > 16) padded = padded.substr(padded.size() - 16);
+        override_prefix_hex = padded;
     }
 
     static const std::string& identifier_hex() {
@@ -191,7 +189,7 @@ class PoolConfig : protected core::Fileconfig
         if (override_identifier_hex.empty())
             return 0;  // public network
 
-        auto pfx_bytes = ParseHex(override_prefix_hex);
+        auto pfx_bytes = ParseHex(prefix_hex());  // effective prefix (override or compiled default)
         auto id_bytes = ParseHex(override_identifier_hex);
         std::vector<unsigned char> preimage;
         preimage.reserve(pfx_bytes.size() + id_bytes.size());