Add -blocksxor boolean option

src/node/blockmanager_args.cpp

@@ -16,6 +16,7 @@
 namespace node {
 util::Result<void> ApplyArgsManOptions(const ArgsManager& args, BlockManager::Options& opts)
 {
+    if (auto value{args.GetBoolArg("-blocksxor")}) opts.use_xor = *value;
     // block pruning; get the amount of disk space (in MiB) to allot for block & undo files
     int64_t nPruneArg{args.GetIntArg("-prune", opts.prune_target)};
     if (nPruneArg < 0) {

src/node/blockstorage.cpp

@@ -19,6 +19,7 @@
 #include <pow.h>
 #include <primitives/block.h>
 #include <primitives/transaction.h>
+#include <random.h>
 #include <reverse_iterator.h>
 #include <serialize.h>
 #include <signet.h>
@@ -1144,9 +1145,43 @@ FlatFilePos BlockManager::SaveBlockToDisk(const CBlock& block, int nHeight)
     return blockPos;
 }
 
+static auto InitBlocksdirXorKey(const BlockManager::Options& opts)
+{
+    // Bytes are serialized without length indicator, so this is also the exact
+    // size of the XOR-key file.
+    std::array<std::byte, 8> xor_key{};
+
+    if (opts.use_xor && fs::is_empty(opts.blocks_dir)) {
+        // Only use random fresh key when the boolean option is set and on the
+        // very first start of the program.
+        FastRandomContext{}.fillrand(xor_key);
+    }
+
+    const fs::path xor_key_path{opts.blocks_dir / "xor.dat"};
+    if (fs::exists(xor_key_path)) {
+        // A pre-existing xor key file has priority.
+        AutoFile xor_key_file{fsbridge::fopen(xor_key_path, "rb")};
+        xor_key_file >> xor_key;
+    } else {
+        // Create initial or missing xor key file
+        AutoFile xor_key_file{fsbridge::fopen(xor_key_path, "wbx")};
+        xor_key_file << xor_key;
+    }
+    // If the user disabled the key, it must be zero.
+    if (!opts.use_xor && xor_key != decltype(xor_key){}) {
+        throw std::runtime_error{
+            strprintf("The blocksdir XOR-key can not be disabled when a random key was already stored! "
+                      "Stored key: '%s', stored path: '%s'.",
+                      HexStr(xor_key), fs::PathToString(xor_key_path)),
+        };
+    }
+    LogInfo("Using obfuscation key for blocksdir *.dat files (%s): '%s'\n", fs::PathToString(opts.blocks_dir), HexStr(xor_key));
+    return std::vector<std::byte>{xor_key.begin(), xor_key.end()};
+}
+
 BlockManager::BlockManager(const util::SignalInterrupt& interrupt, Options opts)
     : m_prune_mode{opts.prune_target > 0},
-      m_xor_key{},
+      m_xor_key{InitBlocksdirXorKey(opts)},
       m_opts{std::move(opts)},
       m_block_file_seq{FlatFileSeq{m_opts.blocks_dir, "blk", m_opts.fast_prune ? 0x4000 /* 16kB */ : BLOCKFILE_CHUNK_SIZE}},
       m_undo_file_seq{FlatFileSeq{m_opts.blocks_dir, "rev", UNDOFILE_CHUNK_SIZE}},