csrng service implementation

Implements the 'csrng:' service using C++ <random>'s Mersenne Twister, this does make it insecure for cryptographic purposes but it is pointless to attempt to do this regardless as we cannot ensure that the guest will run in a secure environment which cannot be mutated by an attacker. Used by Prison Princess, Pokemon Cafe Mix, Paint your Pet and more.

app/CMakeLists.txt

@@ -217,6 +217,7 @@ add_library(skyline SHARED
         ${source_DIR}/skyline/services/nifm/IGeneralService.cpp
         ${source_DIR}/skyline/services/nifm/IRequest.cpp
         ${source_DIR}/skyline/services/socket/bsd/IClient.cpp
+        ${source_DIR}/skyline/services/spl/IRandomInterface.cpp
         ${source_DIR}/skyline/services/ssl/ISslService.cpp
         ${source_DIR}/skyline/services/ssl/ISslContext.cpp
         ${source_DIR}/skyline/services/prepo/IPrepoService.cpp

app/src/main/cpp/skyline/common.h

@@ -22,6 +22,7 @@
 #include <memory>
 #include <compare>
 #include <variant>
+#include <random>
 #include <sys/mman.h>
 #include <fmt/format.h>
 #include <frozen/unordered_map.h>
@@ -338,6 +339,21 @@ namespace skyline {
         constexpr std::size_t Hash(std::string_view view) {
             return frz::elsa<frz::string>{}(frz::string(view.data(), view.size()), 0);
         }
+
+        namespace detail {
+            static thread_local std::mt19937_64 generator{GetTimeTicks()};
+        }
+
+        /**
+         * @brief Fills an array with random data from a Mersenne Twister pseudo-random generator
+         * @note The generator is seeded with the the current time in ticks
+         */
+        template<typename T> requires (std::is_integral_v<T>)
+        void FillRandomBytes(std::span<T> in) {
+            std::independent_bits_engine<std::mt19937_64, std::numeric_limits<T>::digits, T> gen(detail::generator);
+
+            std::generate(in.begin(), in.end(), gen);
+        }
     }
 
     /**

app/src/main/cpp/skyline/common/uuid.cpp

@@ -1,4 +1,3 @@
-#include <random>
 #include "uuid.h"
 
 namespace skyline {

app/src/main/cpp/skyline/services/serviceman.cpp

@@ -32,6 +32,7 @@
 #include "nfp/IUserManager.h"
 #include "nifm/IStaticService.h"
 #include "socket/bsd/IClient.h"
+#include "spl/IRandomInterface.h"
 #include "ssl/ISslService.h"
 #include "prepo/IPrepoService.h"
 #include "mmnv/IRequest.h"
@@ -95,6 +96,7 @@ namespace skyline::service {
             SERVICE_CASE(nfp::IUserManager, "nfp:user")
             SERVICE_CASE(nifm::IStaticService, "nifm:u")
             SERVICE_CASE(socket::IClient, "bsd:u")
+            SERVICE_CASE(spl::IRandomInterface, "csrng")
             SERVICE_CASE(ssl::ISslService, "ssl")
             SERVICE_CASE(prepo::IPrepoService, "prepo:u")
             SERVICE_CASE(mmnv::IRequest, "mm:u")

app/src/main/cpp/skyline/services/spl/IRandomInterface.cpp

@@ -0,0 +1,17 @@
+// SPDX-License-Identifier: MPL-2.0
+// Copyright © 2021 Skyline Team and Contributors (https://github.com/skyline-emu/)
+
+#include "IRandomInterface.h"
+
+#include <common.h>
+
+namespace skyline::service::spl {
+    IRandomInterface::IRandomInterface(const DeviceState &state, ServiceManager &manager) : BaseService(state, manager) {}
+
+    Result IRandomInterface::GetRandomBytes(type::KSession &session, ipc::IpcRequest &request, ipc::IpcResponse &response) {
+        auto &outBuf{request.outputBuf.at(0)};
+
+        util::FillRandomBytes(outBuf);
+        return {};
+    }
+}

app/src/main/cpp/skyline/services/spl/IRandomInterface.h

@@ -0,0 +1,26 @@
+// SPDX-License-Identifier: MPL-2.0
+// Copyright © 2021 Skyline Team and Contributors (https://github.com/skyline-emu/)
+
+#pragma once
+
+#include <services/base_service.h>
+
+namespace skyline::service::spl {
+    /**
+     * @brief csrng provides cryptographically secure random number generation
+     * @url https://switchbrew.org/wiki/SPL_services#csrng
+     */
+    class IRandomInterface : public BaseService {
+      public:
+        IRandomInterface(const DeviceState &state, ServiceManager &manager);
+
+        /**
+         * @brief Fills an output buffer with random bytes
+         */
+        Result GetRandomBytes(type::KSession &session, ipc::IpcRequest &request, ipc::IpcResponse &response);
+
+        SERVICE_DECL(
+            SFUNC(0x0, IRandomInterface, GetRandomBytes)
+        )
+    };
+}