system_rng.cpp

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/*
* System RNG
* (C) 2014,2015,2017,2018,2022 Jack Lloyd
* (C) 2021 Tom Crowley
*
* Botan is released under the Simplified BSD License (see license.txt)
*/
 
#include <botan/system_rng.h>
 
#if defined(BOTAN_TARGET_OS_HAS_WIN32)
  #define NOMINMAX 1
  #define _WINSOCKAPI_ // stop windows.h including winsock.h
  #include <windows.h>
#endif
 
#if defined(BOTAN_TARGET_OS_HAS_RTLGENRANDOM)
  #include <botan/internal/dyn_load.h>
#elif defined(BOTAN_TARGET_OS_HAS_CRYPTO_NG)
  #include <windows.h>
  #include <bcrypt.h>
#elif defined(BOTAN_TARGET_OS_HAS_CCRANDOM)
  #include <CommonCrypto/CommonRandom.h>
#elif defined(BOTAN_TARGET_OS_HAS_ARC4RANDOM)
  #include <stdlib.h>
#elif defined(BOTAN_TARGET_OS_HAS_GETRANDOM)
  #include <sys/random.h>
  #include <errno.h>
#elif defined(BOTAN_TARGET_OS_HAS_DEV_RANDOM)
  #include <fcntl.h>
  #include <unistd.h>
  #include <errno.h>
#endif
 
namespace Botan {
 
namespace {
 
#if defined(BOTAN_TARGET_OS_HAS_RTLGENRANDOM)
 
class System_RNG_Impl final : public RandomNumberGenerator
   {
   public:
      System_RNG_Impl() : m_advapi("advapi32.dll")
         {
         // This throws if the function is not found
         m_rtlgenrandom = m_advapi.resolve<RtlGenRandom_fptr>("SystemFunction036");
         }
 
      System_RNG_Impl(const System_RNG_Impl& other) = delete;
      System_RNG_Impl(System_RNG_Impl&& other) = delete;
      System_RNG_Impl& operator=(const System_RNG_Impl& other) = delete;
      System_RNG_Impl& operator=(System_RNG_Impl&& other) = delete;
 
      bool is_seeded() const override { return true; }
      bool accepts_input() const override { return false; }
      void clear() override { /* not possible */ }
      std::string name() const override { return "RtlGenRandom"; }
 
   private:
      void fill_bytes_with_input(std::span<uint8_t> output, std::span<const uint8_t> /* ignored */) override
         {
         const size_t limit = std::numeric_limits<ULONG>::max();
 
         uint8_t* pData = output.data();
         size_t bytesLeft = output.size();
         while (bytesLeft > 0)
            {
            const ULONG blockSize = static_cast<ULONG>(std::min(bytesLeft, limit));
 
            const bool success = m_rtlgenrandom(pData, blockSize) == TRUE;
            if (!success)
               {
               throw System_Error("RtlGenRandom failed");
               }
 
            BOTAN_ASSERT(bytesLeft >= blockSize, "Block is oversized");
            bytesLeft -= blockSize;
            pData += blockSize;
            }
         }
 
   private:
      using RtlGenRandom_fptr = BOOLEAN (NTAPI *)(PVOID, ULONG);
 
      Dynamically_Loaded_Library m_advapi;
      RtlGenRandom_fptr m_rtlgenrandom;
   };
 
#elif defined(BOTAN_TARGET_OS_HAS_CRYPTO_NG)
 
class System_RNG_Impl final : public RandomNumberGenerator
   {
   public:
      System_RNG_Impl()
         {
         auto ret = ::BCryptOpenAlgorithmProvider(&m_prov,
                                                  BCRYPT_RNG_ALGORITHM,
                                                  MS_PRIMITIVE_PROVIDER, 0);
         if(!BCRYPT_SUCCESS(ret))
            {
            throw System_Error("System_RNG failed to acquire crypto provider", ret);
            }
         }
 
      System_RNG_Impl(const System_RNG_Impl& other) = delete;
      System_RNG_Impl(System_RNG_Impl&& other) = delete;
      System_RNG_Impl& operator=(const System_RNG_Impl& other) = delete;
      System_RNG_Impl& operator=(System_RNG_Impl&& other) = delete;
 
      ~System_RNG_Impl() override
         {
         ::BCryptCloseAlgorithmProvider(m_prov, 0);
         }
 
      bool is_seeded() const override { return true; }
      bool accepts_input() const override { return false; }
      void clear() override { /* not possible */ }
      std::string name() const override { return "crypto_ng"; }
 
   private:
      void fill_bytes_with_input(std::span<uint8_t> output, std::span<const uint8_t> /* ignored */) override
         {
         /*
         There is a flag BCRYPT_RNG_USE_ENTROPY_IN_BUFFER to provide
         entropy inputs, but it is ignored in Windows 8 and later.
         */
 
         const size_t limit = std::numeric_limits<ULONG>::max();
 
         uint8_t* pData = output.data();
         size_t bytesLeft = output.size();
         while (bytesLeft > 0)
            {
            const ULONG blockSize = static_cast<ULONG>(std::min(bytesLeft, limit));
 
            auto ret = BCryptGenRandom(m_prov, static_cast<PUCHAR>(pData), blockSize, 0);
            if(!BCRYPT_SUCCESS(ret))
               {
               throw System_Error("System_RNG call to BCryptGenRandom failed", ret);
               }
 
            BOTAN_ASSERT(bytesLeft >= blockSize, "Block is oversized");
            bytesLeft -= blockSize;
            pData += blockSize;
            }
         }
 
   private:
      BCRYPT_ALG_HANDLE m_prov;
   };
 
#elif defined(BOTAN_TARGET_OS_HAS_CCRANDOM)
 
class System_RNG_Impl final : public RandomNumberGenerator
   {
   public:
      bool accepts_input() const override { return false; }
      bool is_seeded() const override { return true; }
      void clear() override { /* not possible */ }
      std::string name() const override { return "CCRandomGenerateBytes"; }
 
   private:
      void fill_bytes_with_input(std::span<uint8_t> output, std::span<const uint8_t> /* ignored */) override
         {
         if (::CCRandomGenerateBytes(output.data(), output.size()) != kCCSuccess)
            {
            throw System_Error("System_RNG CCRandomGenerateBytes failed", errno);
            }
         }
   };
 
#elif defined(BOTAN_TARGET_OS_HAS_ARC4RANDOM)
 
class System_RNG_Impl final : public RandomNumberGenerator
   {
   public:
      // No constructor or destructor needed as no userland state maintained
 
      bool accepts_input() const override { return false; }
      bool is_seeded() const override { return true; }
      void clear() override { /* not possible */ }
      std::string name() const override { return "arc4random"; }
 
   private:
      void fill_bytes_with_input(std::span<uint8_t> output, std::span<const uint8_t> /* ignored */) override
         {
         // macOS 10.15 arc4random crashes if called with buf == nullptr && len == 0
         // however it uses ccrng_generate internally which returns a status, ignored
         // to respect arc4random "no-fail" interface contract
         if(!output.empty())
            {
            ::arc4random_buf(output.data(), output.size());
            }
         }
   };
 
#elif defined(BOTAN_TARGET_OS_HAS_GETRANDOM)
 
class System_RNG_Impl final : public RandomNumberGenerator
   {
   public:
      // No constructor or destructor needed as no userland state maintained
 
      bool accepts_input() const override { return false; }
      bool is_seeded() const override { return true; }
      void clear() override { /* not possible */ }
      std::string name() const override { return "getrandom"; }
 
   private:
      void fill_bytes_with_input(std::span<uint8_t> output, std::span<const uint8_t> /* ignored */) override
         {
         const unsigned int flags = 0;
 
         uint8_t* buf = output.data();
         size_t len = output.size();
         while(len > 0)
            {
            const ssize_t got = ::getrandom(buf, len, flags);
 
            if(got < 0)
               {
               if(errno == EINTR)
                  continue;
               throw System_Error("System_RNG getrandom failed", errno);
               }
 
            buf += got;
            len -= got;
            }
         }
   };
 
 
#elif defined(BOTAN_TARGET_OS_HAS_DEV_RANDOM)
 
// Read a random device
 
class System_RNG_Impl final : public RandomNumberGenerator
   {
   public:
      System_RNG_Impl()
         {
#ifndef O_NOCTTY
#define O_NOCTTY 0
#endif
 
         /*
         * First open /dev/random and read one byte. On old Linux kernels
         * this blocks the RNG until we have been actually seeded.
         */
         m_fd = ::open("/dev/random", O_RDONLY | O_NOCTTY);
         if(m_fd < 0)
            throw System_Error("System_RNG failed to open RNG device", errno);
 
         uint8_t b;
         const size_t got = ::read(m_fd, &b, 1);
         ::close(m_fd);
 
         if(got != 1)
            throw System_Error("System_RNG failed to read blocking RNG device");
 
         m_fd = ::open("/dev/urandom", O_RDWR | O_NOCTTY);
 
         if(m_fd >= 0)
            {
            m_writable = true;
            }
         else
            {
            /*
            Cannot open in read-write mode. Fall back to read-only,
            calls to add_entropy will fail, but randomize will work
            */
            m_fd = ::open("/dev/urandom", O_RDONLY | O_NOCTTY);
            m_writable = false;
            }
 
         if(m_fd < 0)
            throw System_Error("System_RNG failed to open RNG device", errno);
         }
 
      System_RNG_Impl(const System_RNG_Impl& other) = delete;
      System_RNG_Impl(System_RNG_Impl&& other) = delete;
      System_RNG_Impl& operator=(const System_RNG_Impl& other) = delete;
      System_RNG_Impl& operator=(System_RNG_Impl&& other) = delete;
 
      ~System_RNG_Impl() override
         {
         ::close(m_fd);
         m_fd = -1;
         }
 
      bool is_seeded() const override { return true; }
      bool accepts_input() const override { return m_writable; }
      void clear() override { /* not possible */ }
      std::string name() const override { return "urandom"; }
 
   private:
      void fill_bytes_with_input(std::span<uint8_t> output, std::span<const uint8_t> /* ignored */) override;
      void maybe_write_entropy(std::span<const uint8_t> input);
 
   private:
      int m_fd;
      bool m_writable;
   };
 
void System_RNG_Impl::fill_bytes_with_input(std::span<uint8_t> output, std::span<const uint8_t> input)
   {
   maybe_write_entropy(input);
 
   uint8_t* buf = output.data();
   size_t len = output.size();
   while(len)
      {
      ssize_t got = ::read(m_fd, buf, len);
 
      if(got < 0)
         {
         if(errno == EINTR)
            continue;
         throw System_Error("System_RNG read failed", errno);
         }
      if(got == 0)
         throw System_Error("System_RNG EOF on device"); // ?!?
 
      buf += got;
      len -= got;
      }
   }
 
void System_RNG_Impl::maybe_write_entropy(std::span<const uint8_t> entropy_input)
   {
   if(!m_writable || entropy_input.empty())
      return;
 
   const uint8_t* input = entropy_input.data();
   size_t len = entropy_input.size();
   while(len)
      {
      ssize_t got = ::write(m_fd, input, len);
 
      if(got < 0)
         {
         if(errno == EINTR)
            continue;
 
         /*
         * This is seen on OS X CI, despite the fact that the man page
         * for macOS urandom explicitly states that writing to it is
         * supported, and write(2) does not document EPERM at all.
         * But in any case EPERM seems indicative of a policy decision
         * by the OS or sysadmin that additional entropy is not wanted
         * in the system pool, so we accept that and return here,
         * since there is no corrective action possible.
         *
         * In Linux EBADF or EPERM is returned if m_fd is not opened for
         * writing.
         */
         if(errno == EPERM || errno == EBADF)
            return;
 
         // maybe just ignore any failure here and return?
         throw System_Error("System_RNG write failed", errno);
         }
 
      input += got;
      len -= got;
      }
   }
 
#endif
 
}
 
RandomNumberGenerator& system_rng()
   {
   static System_RNG_Impl g_system_rng;
   return g_system_rng;
   }
 
}