[test] Test i2p private key constraints

Github-Pull: bitcoin#28695
Rebased-From: 5cf4d26

src/test/i2p_tests.cpp

@@ -8,6 +8,7 @@
 #include <test/util/logging.h>
 #include <test/util/net.h>
 #include <test/util/setup_common.h>
+#include <util/readwritefile.h>
 #include <util/system.h>
 #include <util/threadinterrupt.h>
 
@@ -45,4 +46,47 @@ BOOST_AUTO_TEST_CASE(unlimited_recv)
     LogInstance().SetLogLevel(prev_log_level);
 }
 
+BOOST_AUTO_TEST_CASE(damaged_private_key)
+{
+    const auto CreateSockOrig = CreateSock;
+
+    CreateSock = [](const CService&) {
+        return std::make_unique<StaticContentsSock>("HELLO REPLY RESULT=OK VERSION=3.1\n"
+                                                    "SESSION STATUS RESULT=OK DESTINATION=\n");
+    };
+
+    const auto i2p_private_key_file = m_args.GetDataDirNet() / "test_i2p_private_key_damaged";
+
+    for (const auto& [file_contents, expected_error] : std::vector<std::tuple<std::string, std::string>>{
+             {"", "The private key is too short (0 < 387)"},
+
+             {"abcd", "The private key is too short (4 < 387)"},
+
+             {std::string(386, '\0'), "The private key is too short (386 < 387)"},
+
+             {std::string(385, '\0') + '\0' + '\1',
+              "Certificate length (1) designates that the private key should be 388 bytes, but it is only "
+              "387 bytes"},
+
+             {std::string(385, '\0') + '\0' + '\5' + "abcd",
+              "Certificate length (5) designates that the private key should be 392 bytes, but it is only "
+              "391 bytes"}}) {
+        BOOST_REQUIRE(WriteBinaryFile(i2p_private_key_file, file_contents));
+
+        CThreadInterrupt interrupt;
+        i2p::sam::Session session(i2p_private_key_file, CService{}, &interrupt);
+
+        {
+            ASSERT_DEBUG_LOG("Creating persistent SAM session");
+            ASSERT_DEBUG_LOG(expected_error);
+
+            i2p::Connection conn;
+            bool proxy_error;
+            BOOST_CHECK(!session.Connect(CService{}, conn, proxy_error));
+        }
+    }
+
+    CreateSock = CreateSockOrig;
+}
+
 BOOST_AUTO_TEST_SUITE_END()