1  /*************************************************************************
  2   * Copyright (c) 2020-2021 Elichai Turkel                                *
  3   * Distributed under the CC0 software license, see the accompanying file *
  4   * EXAMPLES_COPYING or https://creativecommons.org/publicdomain/zero/1.0 *
  5   *************************************************************************/
  6  
  7  /*
  8   * This file is an attempt at collecting best practice methods for obtaining randomness with different operating systems.
  9   * It may be out-of-date. Consult the documentation of the operating system before considering to use the methods below.
 10   *
 11   * Platform randomness sources:
 12   * Linux   -> `getrandom(2)`(`sys/random.h`), if not available `/dev/urandom` should be used. http://man7.org/linux/man-pages/man2/getrandom.2.html, https://linux.die.net/man/4/urandom
 13   * macOS   -> `getentropy(2)`(`sys/random.h`), if not available `/dev/urandom` should be used. https://www.unix.com/man-page/mojave/2/getentropy, https://opensource.apple.com/source/xnu/xnu-517.12.7/bsd/man/man4/random.4.auto.html
 14   * FreeBSD -> `getrandom(2)`(`sys/random.h`), if not available `kern.arandom` should be used. https://www.freebsd.org/cgi/man.cgi?query=getrandom, https://www.freebsd.org/cgi/man.cgi?query=random&sektion=4
 15   * OpenBSD -> `getentropy(2)`(`unistd.h`), if not available `/dev/urandom` should be used. https://man.openbsd.org/getentropy, https://man.openbsd.org/urandom
 16   * Windows -> `BCryptGenRandom`(`bcrypt.h`). https://docs.microsoft.com/en-us/windows/win32/api/bcrypt/nf-bcrypt-bcryptgenrandom
 17   */
 18  
 19  #if defined(_WIN32)
 20  /*
 21   * The defined WIN32_NO_STATUS macro disables return code definitions in
 22   * windows.h, which avoids "macro redefinition" MSVC warnings in ntstatus.h.
 23   */
 24  #define WIN32_NO_STATUS
 25  #include <windows.h>
 26  #undef WIN32_NO_STATUS
 27  #include <ntstatus.h>
 28  #include <bcrypt.h>
 29  #elif defined(__linux__) || defined(__APPLE__) || defined(__FreeBSD__)
 30  #include <sys/random.h>
 31  #elif defined(__OpenBSD__)
 32  #include <unistd.h>
 33  #else
 34  #error "Couldn't identify the OS"
 35  #endif
 36  
 37  #include <stddef.h>
 38  #include <limits.h>
 39  #include <stdio.h>
 40  
 41  
 42  /* Returns 1 on success, and 0 on failure. */
 43  static int fill_random(unsigned char* data, size_t size) {
 44  #if defined(_WIN32)
 45      NTSTATUS res = BCryptGenRandom(NULL, data, size, BCRYPT_USE_SYSTEM_PREFERRED_RNG);
 46      if (res != STATUS_SUCCESS || size > ULONG_MAX) {
 47          return 0;
 48      } else {
 49          return 1;
 50      }
 51  #elif defined(__linux__) || defined(__FreeBSD__)
 52      /* If `getrandom(2)` is not available you should fallback to /dev/urandom */
 53      ssize_t res = getrandom(data, size, 0);
 54      if (res < 0 || (size_t)res != size ) {
 55          return 0;
 56      } else {
 57          return 1;
 58      }
 59  #elif defined(__APPLE__) || defined(__OpenBSD__)
 60      /* If `getentropy(2)` is not available you should fallback to either
 61       * `SecRandomCopyBytes` or /dev/urandom */
 62      int res = getentropy(data, size);
 63      if (res == 0) {
 64          return 1;
 65      } else {
 66          return 0;
 67      }
 68  #endif
 69      return 0;
 70  }
 71  
 72  static void print_hex(unsigned char* data, size_t size) {
 73      size_t i;
 74      printf("0x");
 75      for (i = 0; i < size; i++) {
 76          printf("%02x", data[i]);
 77      }
 78      printf("\n");
 79  }
 80  
 81  #if defined(_MSC_VER)
 82  // For SecureZeroMemory
 83  #include <Windows.h>
 84  #endif
 85  /* Cleanses memory to prevent leaking sensitive info. Won't be optimized out. */
 86  static void secure_erase(void *ptr, size_t len) {
 87  #if defined(_MSC_VER)
 88      /* SecureZeroMemory is guaranteed not to be optimized out by MSVC. */
 89      SecureZeroMemory(ptr, len);
 90  #elif defined(__GNUC__)
 91      /* We use a memory barrier that scares the compiler away from optimizing out the memset.
 92       *
 93       * Quoting Adam Langley <agl@google.com> in commit ad1907fe73334d6c696c8539646c21b11178f20f
 94       * in BoringSSL (ISC License):
 95       *    As best as we can tell, this is sufficient to break any optimisations that
 96       *    might try to eliminate "superfluous" memsets.
 97       * This method used in memzero_explicit() the Linux kernel, too. Its advantage is that it is
 98       * pretty efficient, because the compiler can still implement the memset() efficiently,
 99       * just not remove it entirely. See "Dead Store Elimination (Still) Considered Harmful" by
100       * Yang et al. (USENIX Security 2017) for more background.
101       */
102      memset(ptr, 0, len);
103      __asm__ __volatile__("" : : "r"(ptr) : "memory");
104  #else
105      void *(*volatile const volatile_memset)(void *, int, size_t) = memset;
106      volatile_memset(ptr, 0, len);
107  #endif
108  }