Give better error messages when missing SSL files

While copying my data dir to another drive, I missed copying the rpc_ssl.key file b/c of the file permissions.
This change will give a much more clear, descriptive error in that scenario.

contrib/epee/src/net_ssl.cpp

@@ -38,6 +38,7 @@
 #include "misc_log_ex.h"
 #include "net/net_helper.h"
 #include "net/net_ssl.h"
+#include "file_io_utils.h" // to validate .crt and .key paths
 
 #undef MONERO_DEFAULT_LOG_CATEGORY
 #define MONERO_DEFAULT_LOG_CATEGORY "net.ssl"
@@ -356,6 +357,15 @@ boost::asio::ssl::context ssl_options_t::create_context() const
   }
 
   CHECK_AND_ASSERT_THROW_MES(auth.private_key_path.empty() == auth.certificate_path.empty(), "private key and certificate must be either both given or both empty");
+
+  const bool private_key_exists = epee::file_io_utils::is_file_exist(auth.private_key_path);
+  const bool certificate_exists = epee::file_io_utils::is_file_exist(auth.certificate_path);
+  if (private_key_exists && !certificate_exists) {
+    ASSERT_MES_AND_THROW("private key is present, but certificate file '" << auth.certificate_path << "' is missing");
+  } else if (!private_key_exists && certificate_exists) {
+    ASSERT_MES_AND_THROW("certificate is present, but private key file '" << auth.private_key_path << "' is missing");
+  }
+
   if (auth.private_key_path.empty())
   {
     EVP_PKEY *pkey;
@@ -392,7 +402,12 @@ boost::asio::ssl::context ssl_options_t::create_context() const
 
 void ssl_authentication_t::use_ssl_certificate(boost::asio::ssl::context &ssl_context) const
 {
+  try {
     ssl_context.use_private_key_file(private_key_path, boost::asio::ssl::context::pem);
+  } catch (const boost::system::system_error&) {
+    MERROR("Failed to load private key file '" << private_key_path << "' into SSL context");
+    throw;
+  }
   ssl_context.use_certificate_chain_file(certificate_path);
 }
 
@@ -590,7 +605,15 @@ boost::system::error_code store_ssl_keys(boost::asio::ssl::context& ssl, const b
     const boost::filesystem::path key_file{base.string() + ".key"};
     file.reset(std::fopen(key_file.string().c_str(), "wb"));
     if (!file)
+    {
+      if (epee::file_io_utils::is_file_exist(key_file.string())) {
+        MERROR("Permission denied to overwrite SSL private key file: '" << key_file.string() << "'");
+      } else {
+        MERROR("Could not open SSL private key file for writing: '" << key_file.string() << "'");
+      }
+
       return {errno, boost::system::system_category()};
+    }
     boost::filesystem::permissions(key_file, boost::filesystem::owner_read, error);
     if (error)
       return error;

src/rpc/core_rpc_server.cpp

@@ -350,13 +350,24 @@ namespace cryptonote
 
     bool store_ssl_key = !restricted && rpc_config->ssl_options && rpc_config->ssl_options.auth.certificate_path.empty();
     const auto ssl_base_path = (boost::filesystem::path{data_dir} / "rpc_ssl").string();
-    if (store_ssl_key && boost::filesystem::exists(ssl_base_path + ".crt"))
+    const bool ssl_cert_file_exists = boost::filesystem::exists(ssl_base_path + ".crt");
+    const bool ssl_pkey_file_exists = boost::filesystem::exists(ssl_base_path + ".key");
+    if (store_ssl_key)
     {
+      // .key files are often given different read permissions as their corresponding .crt files.
+      // Consequently, sometimes the .key file wont't get copied, while the .crt file will.
+      if (ssl_cert_file_exists != ssl_pkey_file_exists)
+      {
+        MFATAL("Certificate (.crt) and private key (.key) files must both exist or both not exist at path: " << ssl_base_path);
+        return false;
+      }
+      else if (ssl_cert_file_exists) { // and ssl_pkey_file_exists
         // load key from previous run, password prompted by OpenSSL
         store_ssl_key = false;
         rpc_config->ssl_options.auth =
           epee::net_utils::ssl_authentication_t{ssl_base_path + ".key", ssl_base_path + ".crt"};
       }
+    }
 
     auto rng = [](size_t len, uint8_t *ptr){ return crypto::rand(len, ptr); };
     const bool inited = epee::http_server_impl_base<core_rpc_server, connection_context>::init(