Creating a new RNG and replacing rand().

[ Merge of http://go/wvgerrit/84607 ]
[ Merge of http://go/wvgerrit/84608 ]

The primary goal is to replace the use of `rand()` with the random
number generators provided with the C++11 standard.

This simplified generator wraps some of the technical aspects of the
<random> library and provides an interface for uniformly distributed
integers.

As part of the `rand()` purge in the CDM, all uses of the C random int
function in `core()` have been removed.  Places that previously used
`rand()` now use `CdmRandom` facilities.

Test: Linux unittest and Android unittest
Bug: 130680365
Change-Id: Ica383870536ed462dbb80e630c2d66845e38b937

libwvdrmengine/cdm/util/include/cdm_random.h

@@ -0,0 +1,109 @@
+// Copyright 2019 Google LLC. All Rights Reserved. This file and proprietary
+// source code may only be used and distributed under the Widevine Master
+// License Agreement.
+#ifndef WVCDM_CORE_CDM_RANDOM_H_
+#define WVCDM_CORE_CDM_RANDOM_H_
+
+#include <mutex>
+#include <random>
+#include <string>
+
+namespace wvcdm {
+
+// CdmRandomGenerator is a thread safe, pseudo-random number generator.
+// It's purpose is to simplified interface for C++11's <random> library.
+// Some of the methods use a "device specific" random seed, if the
+// compiler/device does not support device specific randomizers, then the
+// actual value supplied may not be random.
+class CdmRandomGenerator {
+ public:
+  // The maximum number of bytes that can be generated at once for
+  // `RandomData()`.
+  static constexpr size_t kMaxRandomDataLength = 8192;  // 8 kB
+
+  // Initializes the pseudo-random generator with a value from a device
+  // specific random number generator.
+  CdmRandomGenerator();
+
+  // Initializes the pseudo-random generator with the specified seed value.
+  explicit CdmRandomGenerator(unsigned int seed) : generator_(seed) {}
+
+  // All of these methods are thread-safe.
+
+  // Seeds the pseudo-random generator with a value from a device specific
+  // random number generator.
+  void Seed();
+
+  // Seeds the pseudo-random generator with the specified seed value.
+  // This is somewhat similar to `srand()` from the C standard library;
+  // except that the sequence generated from successive calls to `Rand()`
+  // will not necessarily be the same as they would be from the
+  // standard library `rand()`.  This is due to the underlying pseudo-random
+  // generator that is used.
+  void Seed(unsigned int seed);
+
+  // Returns a pseudo-random integer.
+  // This is similar to `rand()` from the C standard library.  The integer
+  // returned is in the range of [0, RAND_MAX].
+  int Rand();
+
+  // Allows for RNG to be callable, this is to make it similar to the
+  // C++11 generator interfaces.
+  int operator()() { return Rand(); }
+
+  // Returns a pseudo-random integer within the provided inclusive range.
+  uint64_t RandomInRange(uint64_t lower, uint64_t upper);
+  uint64_t RandomInRange(uint64_t upper) { return RandomInRange(0, upper); }
+
+  // Returns a byte string containing randomized bytes of the specified
+  // length.
+  // If |length| is greater than |CdmRandomGenerator::kMaxRandomDataLength|,
+  // then an error is logged and an empty string is returned.
+  std::string RandomData(size_t length);
+
+  // Random true/false using Bernoulli distribution of equal probability.
+  bool RandomBool();
+
+ private:
+  // Mutex is used to lock the object, and allowing it to be used
+  // concurrently in different threads.
+  std::mutex generator_lock_;
+
+  // The `default_random_engine` depends on the compiler used and
+  // potentially its version.  This is important to know if you need to
+  // create reproducible tests between platforms.
+  std::default_random_engine generator_;
+};
+
+// Provides a static interface to a process-wide instance of
+// CdmRandomGenerator.
+class CdmRandom {
+ public:
+  static int Rand() { return GetInstance()->Rand(); }
+  static uint64_t RandomInRange(uint64_t lower, uint64_t upper) {
+    return GetInstance()->RandomInRange(lower, upper);
+  }
+  static uint64_t RandomInRange(uint64_t upper) {
+    return GetInstance()->RandomInRange(upper);
+  }
+
+  static std::string RandomData(size_t length) {
+    return GetInstance()->RandomData(length);
+  }
+
+  static bool RandomBool() { return GetInstance()->RandomBool(); }
+
+ private:
+  // These are intended to be used by tests if needed.
+  static void Seed(unsigned int seed) { GetInstance()->Seed(seed); }
+  static void Seed() { GetInstance()->Seed(); }
+
+  // Returns the process-wide instance of CdmRandomGenerator.
+  // It the global instance has not yet been created, then a new instance
+  // is created using a device-specific random seed.
+  static CdmRandomGenerator* GetInstance();
+};
+
+}  // namespace wvcdm
+
+#endif  // WVCDM_CORE_CDM_RANDOM_H_