[fuzz] Occasional valid magic bytes for transport serialization test

Before commit:
Unable to deserialize : 0%
Wrong message start   : ~45.62%
Header too large      : ~14.5%
Wrong checksum        : ~38.13%
Invalid message type  : ~1.78%

304820	NEW    cov: 1440 ft: 4452 corp: 92/12551b lim: 2237 exec/s: 3386 rss: 486Mb L: 47/1111 MS: 1 ChangeByte-
1416181	REDUCE cov: 1442 ft: 5681 corp: 125/59Kb lim: 4096 exec/s: 3522 rss: 535Mb L: 2164/4049 MS: 1 EraseBytes-

After commit:
Unable to deserialize : 0%
Wrong message start   : ~39.6%
Header too large      : ~30.85%
Wrong checksum        : ~25.54%
Invalid message type  : ~4.01%

302684	NEW    cov: 1454 ft: 3936 corp: 84/7056b lim: 2424 exec/s: 4146 rss: 477Mb L: 65/1108 MS: 3 CopyPart-CrossOver-CMP- DE: "\x0e\x00\x00\x00"-
1383925	REDUCE cov: 1454 ft: 4828 corp: 102/14573b lim: 4096 exec/s: 3954 rss: 534Mb L: 116/4050 MS: 2 EraseBytes-ChangeByte-

1 files changed, 18 insertions, 2 deletions

diff --git a/src/test/fuzz/p2p_transport_serialization.cpp b/src/test/fuzz/p2p_transport_serialization.cpp
index c29d8e7039..edee5aeef7 100644
--- a/src/test/fuzz/p2p_transport_serialization.cpp
+++ b/src/test/fuzz/p2p_transport_serialization.cpp
@@ -29,8 +29,24 @@ FUZZ_TARGET_INIT(p2p_transport_serialization, initialize_p2p_transport_serializa
     FuzzedDataProvider fuzzed_data_provider{buffer.data(), buffer.size()};
 
     auto checksum_assist = fuzzed_data_provider.ConsumeBool();
-    int header_random_bytes_count = checksum_assist ? CMessageHeader::CHECKSUM_OFFSET : CMessageHeader :: HEADER_SIZE;
-    auto mutable_msg_bytes = fuzzed_data_provider.ConsumeBytes<uint8_t>(header_random_bytes_count);
+    auto magic_bytes_assist = fuzzed_data_provider.ConsumeBool();
+    std::vector<uint8_t> mutable_msg_bytes;
+
+    auto header_bytes_remaining = CMessageHeader::HEADER_SIZE;
+    if (magic_bytes_assist) {
+        auto msg_start = Params().MessageStart();
+        for (size_t i = 0; i < CMessageHeader::MESSAGE_SIZE_SIZE; ++i) {
+            mutable_msg_bytes.push_back(msg_start[i]);
+        }
+        header_bytes_remaining -= CMessageHeader::MESSAGE_SIZE_SIZE;
+    }
+
+    if (checksum_assist) {
+        header_bytes_remaining -= CMessageHeader::CHECKSUM_SIZE;
+    }
+
+    auto header_random_bytes = fuzzed_data_provider.ConsumeBytes<uint8_t>(header_bytes_remaining);
+    mutable_msg_bytes.insert(mutable_msg_bytes.end(), header_random_bytes.begin(), header_random_bytes.end());
     auto payload_bytes = fuzzed_data_provider.ConsumeRemainingBytes<uint8_t>();
 
     if (checksum_assist && mutable_msg_bytes.size() == CMessageHeader::CHECKSUM_OFFSET) {