Add support for reading random data from "bcrypt" on Windows

This fixes #577

Patch inspired by the same, but simplified after reading the docs.

Signed-off-by: Steffen Jaeckel <s@jaeckel.eu>

CMakeLists.txt

@@ -94,6 +94,16 @@ if(CMAKE_SYSTEM_NAME MATCHES "CYGWIN")
     list(APPEND LTC_C_FLAGS -no-undefined)
 endif()
 
+if(MSVC)
+    cmake_push_check_state()
+        check_symbol_exists(BCryptGenRandom bcrypt.h BCRYPT_AVAILABLE)
+    cmake_pop_check_state()
+    if (BCRYPT_AVAILABLE)
+        target_link_libraries(${PROJECT_NAME} PRIVATE Bcrypt)
+        list(APPEND LTC_C_FLAGS -DLTC_WIN32_BCRYPT)
+    endif()
+endif()
+
 # If the user set the environment variables at generate-time, append them
 # in order to allow overriding our defaults.
 # ${LTC_CFLAGS} means the user passed it via sth like:

src/headers/tomcrypt_cfg.h

@@ -300,6 +300,10 @@ typedef unsigned long ltc_mp_digit;
    #define LTC_ALIGN(n)
 #endif
 
+#if defined(_MSC_VER) && defined(_WIN32_WINNT) && _WIN32_WINNT >= 0x0600 && !defined(LTC_WIN32_BCRYPT)
+#   define LTC_WIN32_BCRYPT
+#endif
+
 /* Define `LTC_NO_NULL_TERMINATION_CHECK` in the user code
  * before including `tomcrypt.h` to disable this functionality.
  */

src/misc/crypt/crypt.c

@@ -324,6 +324,9 @@ const char *crypt_build_settings =
 #if defined(LTC_SOBER128)
     "   SOBER128\n"
 #endif
+#if defined(LTC_WIN32_BCRYPT)
+    "   WIN32_BCRYPT\n"
+#endif
 
     "\nPK Crypto:\n"
 #if defined(LTC_MRSA)

src/prngs/rng_get_bytes.c

@@ -82,6 +82,21 @@ static unsigned long s_rng_ansic(unsigned char *buf, unsigned long len,
 
 /* Try the Microsoft CSP */
 #if defined(_WIN32) || defined(_WIN32_WCE)
+#if defined(LTC_WIN32_BCRYPT)
+
+#include <bcrypt.h>
+#pragma comment(lib, "bcrypt.lib")
+
+static unsigned long s_rng_win32(unsigned char *buf, unsigned long len,
+                               void (*callback)(void))
+{
+   LTC_UNUSED_PARAM(callback);
+
+   return BCRYPT_SUCCESS(BCryptGenRandom(NULL, (PUCHAR)buf, (ULONG)len, BCRYPT_USE_SYSTEM_PREFERRED_RNG)) ? len : 0;
+}
+
+#else
+
 #ifndef _WIN32_WINNT
   #define _WIN32_WINNT 0x0501
 #endif
@@ -112,6 +127,7 @@ static unsigned long s_rng_win32(unsigned char *buf, unsigned long len,
 
    return CryptGenRandom(hProv, (DWORD)len, (BYTE *)buf) == TRUE ? len : 0;
 }
+#endif /* Old WIN32 versions */
 #endif /* WIN32 */
 
 /**