Line data    Source code

       1             : // Copyright (c) 2009-2010 Satoshi Nakamoto
       2             : // Copyright (c) 2009-2022 The Bitcoin Core developers
       3             : // Distributed under the MIT software license, see the accompanying
       4             : // file COPYING or http://www.opensource.org/licenses/mit-license.php.
       5             : 
       6             : #ifndef BITCOIN_RANDOM_H
       7             : #define BITCOIN_RANDOM_H
       8             : 
       9             : #include <crypto/chacha20.h>
      10             : #include <crypto/common.h>
      11             : #include <span.h>
      12             : #include <uint256.h>
      13             : 
      14             : #include <cassert>
      15             : #include <chrono>
      16             : #include <cstdint>
      17             : #include <limits>
      18             : #include <vector>
      19             : 
      20             : /**
      21             :  * Overall design of the RNG and entropy sources.
      22             :  *
      23             :  * We maintain a single global 256-bit RNG state for all high-quality randomness.
      24             :  * The following (classes of) functions interact with that state by mixing in new
      25             :  * entropy, and optionally extracting random output from it:
      26             :  *
      27             :  * - The GetRand*() class of functions, as well as construction of FastRandomContext objects,
      28             :  *   perform 'fast' seeding, consisting of mixing in:
      29             :  *   - A stack pointer (indirectly committing to calling thread and call stack)
      30             :  *   - A high-precision timestamp (rdtsc when available, c++ high_resolution_clock otherwise)
      31             :  *   - 64 bits from the hardware RNG (rdrand) when available.
      32             :  *   These entropy sources are very fast, and only designed to protect against situations
      33             :  *   where a VM state restore/copy results in multiple systems with the same randomness.
      34             :  *   FastRandomContext on the other hand does not protect against this once created, but
      35             :  *   is even faster (and acceptable to use inside tight loops).
      36             :  *
      37             :  * - The GetStrongRand*() class of function perform 'slow' seeding, including everything
      38             :  *   that fast seeding includes, but additionally:
      39             :  *   - OS entropy (/dev/urandom, getrandom(), ...). The application will terminate if
      40             :  *     this entropy source fails.
      41             :  *   - Another high-precision timestamp (indirectly committing to a benchmark of all the
      42             :  *     previous sources).
      43             :  *   These entropy sources are slower, but designed to make sure the RNG state contains
      44             :  *   fresh data that is unpredictable to attackers.
      45             :  *
      46             :  * - RandAddPeriodic() seeds everything that fast seeding includes, but additionally:
      47             :  *   - A high-precision timestamp
      48             :  *   - Dynamic environment data (performance monitoring, ...)
      49             :  *   - Strengthen the entropy for 10 ms using repeated SHA512.
      50             :  *   This is run once every minute.
      51             :  *
      52             :  * On first use of the RNG (regardless of what function is called first), all entropy
      53             :  * sources used in the 'slow' seeder are included, but also:
      54             :  * - 256 bits from the hardware RNG (rdseed or rdrand) when available.
      55             :  * - Dynamic environment data (performance monitoring, ...)
      56             :  * - Static environment data
      57             :  * - Strengthen the entropy for 100 ms using repeated SHA512.
      58             :  *
      59             :  * When mixing in new entropy, H = SHA512(entropy || old_rng_state) is computed, and
      60             :  * (up to) the first 32 bytes of H are produced as output, while the last 32 bytes
      61             :  * become the new RNG state.
      62             : */
      63             : 
      64             : /**
      65             :  * Generate random data via the internal PRNG.
      66             :  *
      67             :  * These functions are designed to be fast (sub microsecond), but do not necessarily
      68             :  * meaningfully add entropy to the PRNG state.
      69             :  *
      70             :  * Thread-safe.
      71             :  */
      72             : void GetRandBytes(Span<unsigned char> bytes) noexcept;
      73             : /** Generate a uniform random integer in the range [0..range). Precondition: range > 0 */
      74             : uint64_t GetRandInternal(uint64_t nMax) noexcept;
      75             : /** Generate a uniform random integer of type T in the range [0..nMax)
      76             :  *  nMax defaults to std::numeric_limits<T>::max()
      77             :  *  Precondition: nMax > 0, T is an integral type, no larger than uint64_t
      78             :  */
      79             : template<typename T>
      80      183719 : T GetRand(T nMax=std::numeric_limits<T>::max()) noexcept {
      81             :     static_assert(std::is_integral<T>(), "T must be integral");
      82             :     static_assert(std::numeric_limits<T>::max() <= std::numeric_limits<uint64_t>::max(), "GetRand only supports up to uint64_t");
      83      183719 :     return T(GetRandInternal(nMax));
      84             : }
      85             : /** Generate a uniform random duration in the range [0..max). Precondition: max.count() > 0 */
      86             : template <typename D>
      87           0 : D GetRandomDuration(typename std::common_type<D>::type max) noexcept
      88             : // Having the compiler infer the template argument from the function argument
      89             : // is dangerous, because the desired return value generally has a different
      90             : // type than the function argument. So std::common_type is used to force the
      91             : // call site to specify the type of the return value.
      92             : {
      93           0 :     assert(max.count() > 0);
      94           0 :     return D{GetRand(max.count())};
      95             : };
      96             : constexpr auto GetRandMicros = GetRandomDuration<std::chrono::microseconds>;
      97             : constexpr auto GetRandMillis = GetRandomDuration<std::chrono::milliseconds>;
      98             : 
      99             : /**
     100             :  * Return a timestamp in the future sampled from an exponential distribution
     101             :  * (https://en.wikipedia.org/wiki/Exponential_distribution). This distribution
     102             :  * is memoryless and should be used for repeated network events (e.g. sending a
     103             :  * certain type of message) to minimize leaking information to observers.
     104             :  *
     105             :  * The probability of an event occurring before time x is 1 - e^-(x/a) where a
     106             :  * is the average interval between events.
     107             :  * */
     108             : std::chrono::microseconds GetExponentialRand(std::chrono::microseconds now, std::chrono::seconds average_interval);
     109             : 
     110             : uint256 GetRandHash() noexcept;
     111             : 
     112             : /**
     113             :  * Gather entropy from various sources, feed it into the internal PRNG, and
     114             :  * generate random data using it.
     115             :  *
     116             :  * This function will cause failure whenever the OS RNG fails.
     117             :  *
     118             :  * Thread-safe.
     119             :  */
     120             : void GetStrongRandBytes(Span<unsigned char> bytes) noexcept;
     121             : 
     122             : /**
     123             :  * Gather entropy from various expensive sources, and feed them to the PRNG state.
     124             :  *
     125             :  * Thread-safe.
     126             :  */
     127             : void RandAddPeriodic() noexcept;
     128             : 
     129             : /**
     130             :  * Gathers entropy from the low bits of the time at which events occur. Should
     131             :  * be called with a uint32_t describing the event at the time an event occurs.
     132             :  *
     133             :  * Thread-safe.
     134             :  */
     135             : void RandAddEvent(const uint32_t event_info) noexcept;
     136             : 
     137             : /**
     138             :  * Fast randomness source. This is seeded once with secure random data, but
     139             :  * is completely deterministic and does not gather more entropy after that.
     140             :  *
     141             :  * This class is not thread-safe.
     142             :  */
     143             : class FastRandomContext
     144             : {
     145             : private:
     146             :     bool requires_seed;
     147             :     ChaCha20 rng;
     148             : 
     149             :     uint64_t bitbuf;
     150             :     int bitbuf_size;
     151             : 
     152             :     void RandomSeed();
     153             : 
     154           3 :     void FillBitBuffer()
     155             :     {
     156           3 :         bitbuf = rand64();
     157           3 :         bitbuf_size = 64;
     158           3 :     }
     159             : 
     160             : public:
     161             :     explicit FastRandomContext(bool fDeterministic = false) noexcept;
     162             : 
     163             :     /** Initialize with explicit seed (only for testing) */
     164             :     explicit FastRandomContext(const uint256& seed) noexcept;
     165             : 
     166             :     // Do not permit copying a FastRandomContext (move it, or create a new one to get reseeded).
     167             :     FastRandomContext(const FastRandomContext&) = delete;
     168             :     FastRandomContext(FastRandomContext&&) = delete;
     169             :     FastRandomContext& operator=(const FastRandomContext&) = delete;
     170             : 
     171             :     /** Move a FastRandomContext. If the original one is used again, it will be reseeded. */
     172             :     FastRandomContext& operator=(FastRandomContext&& from) noexcept;
     173             : 
     174             :     /** Generate a random 64-bit integer. */
     175      177091 :     uint64_t rand64() noexcept
     176             :     {
     177      177091 :         if (requires_seed) RandomSeed();
     178             :         std::array<std::byte, 8> buf;
     179      177091 :         rng.Keystream(buf);
     180      177091 :         return ReadLE64(UCharCast(buf.data()));
     181             :     }
     182             : 
     183             :     /** Generate a random (bits)-bit integer. */
     184      183719 :     uint64_t randbits(int bits) noexcept
     185             :     {
     186      183719 :         if (bits == 0) {
     187        6628 :             return 0;
     188      177091 :         } else if (bits > 32) {
     189      177088 :             return rand64() >> (64 - bits);
     190             :         } else {
     191           3 :             if (bitbuf_size < bits) FillBitBuffer();
     192           3 :             uint64_t ret = bitbuf & (~uint64_t{0} >> (64 - bits));
     193           3 :             bitbuf >>= bits;
     194           3 :             bitbuf_size -= bits;
     195           3 :             return ret;
     196             :         }
     197      183719 :     }
     198             : 
     199             :     /** Generate a random integer in the range [0..range).
     200             :      * Precondition: range > 0.
     201             :      */
     202      183719 :     uint64_t randrange(uint64_t range) noexcept
     203             :     {
     204      183719 :         assert(range);
     205      183719 :         --range;
     206      183719 :         int bits = CountBits(range);
     207      183719 :         while (true) {
     208      183719 :             uint64_t ret = randbits(bits);
     209      183719 :             if (ret <= range) return ret;
     210             :         }
     211             :     }
     212             : 
     213             :     /** Generate random bytes. */
     214             :     template <typename B = unsigned char>
     215             :     std::vector<B> randbytes(size_t len);
     216             : 
     217             :     /** Fill a byte Span with random bytes. */
     218             :     void fillrand(Span<std::byte> output);
     219             : 
     220             :     /** Generate a random 32-bit integer. */
     221           0 :     uint32_t rand32() noexcept { return randbits(32); }
     222             : 
     223             :     /** generate a random uint256. */
     224             :     uint256 rand256() noexcept;
     225             : 
     226             :     /** Generate a random boolean. */
     227           0 :     bool randbool() noexcept { return randbits(1); }
     228             : 
     229             :     /** Return the time point advanced by a uniform random duration. */
     230             :     template <typename Tp>
     231           0 :     Tp rand_uniform_delay(const Tp& time, typename Tp::duration range)
     232             :     {
     233           0 :         return time + rand_uniform_duration<Tp>(range);
     234             :     }
     235             : 
     236             :     /** Generate a uniform random duration in the range from 0 (inclusive) to range (exclusive). */
     237             :     template <typename Chrono>
     238           0 :     typename Chrono::duration rand_uniform_duration(typename Chrono::duration range) noexcept
     239             :     {
     240             :         using Dur = typename Chrono::duration;
     241           0 :         return range.count() > 0 ? /* interval [0..range) */ Dur{randrange(range.count())} :
     242           0 :                range.count() < 0 ? /* interval (range..0] */ -Dur{randrange(-range.count())} :
     243           0 :                                    /* interval [0..0] */ Dur{0};
     244             :     };
     245             : 
     246             :     // Compatibility with the UniformRandomBitGenerator concept
     247             :     typedef uint64_t result_type;
     248           0 :     static constexpr uint64_t min() { return 0; }
     249           0 :     static constexpr uint64_t max() { return std::numeric_limits<uint64_t>::max(); }
     250           0 :     inline uint64_t operator()() noexcept { return rand64(); }
     251             : };
     252             : 
     253             : /** More efficient than using std::shuffle on a FastRandomContext.
     254             :  *
     255             :  * This is more efficient as std::shuffle will consume entropy in groups of
     256             :  * 64 bits at the time and throw away most.
     257             :  *
     258             :  * This also works around a bug in libstdc++ std::shuffle that may cause
     259             :  * type::operator=(type&&) to be invoked on itself, which the library's
     260             :  * debug mode detects and panics on. This is a known issue, see
     261             :  * https://stackoverflow.com/questions/22915325/avoiding-self-assignment-in-stdshuffle
     262             :  */
     263             : template <typename I, typename R>
     264           0 : void Shuffle(I first, I last, R&& rng)
     265             : {
     266           0 :     while (first != last) {
     267           0 :         size_t j = rng.randrange(last - first);
     268           0 :         if (j) {
     269             :             using std::swap;
     270           0 :             swap(*first, *(first + j));
     271           0 :         }
     272           0 :         ++first;
     273             :     }
     274           0 : }
     275             : 
     276             : /* Number of random bytes returned by GetOSRand.
     277             :  * When changing this constant make sure to change all call sites, and make
     278             :  * sure that the underlying OS APIs for all platforms support the number.
     279             :  * (many cap out at 256 bytes).
     280             :  */
     281             : static const int NUM_OS_RANDOM_BYTES = 32;
     282             : 
     283             : /** Get 32 bytes of system entropy. Do not use this in application code: use
     284             :  * GetStrongRandBytes instead.
     285             :  */
     286             : void GetOSRand(unsigned char* ent32);
     287             : 
     288             : /** Check that OS randomness is available and returning the requested number
     289             :  * of bytes.
     290             :  */
     291             : bool Random_SanityCheck();
     292             : 
     293             : /**
     294             :  * Initialize global RNG state and log any CPU features that are used.
     295             :  *
     296             :  * Calling this function is optional. RNG state will be initialized when first
     297             :  * needed if it is not called.
     298             :  */
     299             : void RandomInit();
     300             : 
     301             : #endif // BITCOIN_RANDOM_H