bpo-40286: Add randbytes() method to random.Random (GH-19527)

Add random.randbytes() function and random.Random.randbytes()
method to generate random bytes.

Modify secrets.token_bytes() to use SystemRandom.randbytes()
rather than calling directly os.urandom().

Rename also genrand_int32() to genrand_uint32(), since it returns an
unsigned 32-bit integer, not a signed integer.

The _random module is now built with Py_BUILD_CORE_MODULE defined.

3 files changed, 59 insertions, 2 deletions

diff --git a/Lib/random.py b/Lib/random.py
index e24737d450..82345fab92 100644
--- a/Lib/random.py
+++ b/Lib/random.py
@@ -739,6 +739,12 @@ class SystemRandom(Random):
         x = int.from_bytes(_urandom(numbytes), 'big')
         return x >> (numbytes * 8 - k)                # trim excess bits
 
+    def randbytes(self, n):
+        """Generate n random bytes."""
+        # os.urandom(n) fails with ValueError for n < 0
+        # and returns an empty bytes string for n == 0.
+        return _urandom(n)
+
     def seed(self, *args, **kwds):
         "Stub method.  Not used for a system random number generator."
         return None
@@ -819,6 +825,7 @@ weibullvariate = _inst.weibullvariate
 getstate = _inst.getstate
 setstate = _inst.setstate
 getrandbits = _inst.getrandbits
+randbytes = _inst.randbytes
 
 if hasattr(_os, "fork"):
     _os.register_at_fork(after_in_child=_inst.seed)
diff --git a/Lib/secrets.py b/Lib/secrets.py
index 130434229e..a546efbdd4 100644
--- a/Lib/secrets.py
+++ b/Lib/secrets.py
@@ -14,7 +14,6 @@ __all__ = ['choice', 'randbelow', 'randbits', 'SystemRandom',
 
 import base64
 import binascii
-import os
 
 from hmac import compare_digest
 from random import SystemRandom
@@ -44,7 +43,7 @@ def token_bytes(nbytes=None):
     """
     if nbytes is None:
         nbytes = DEFAULT_ENTROPY
-    return os.urandom(nbytes)
+    return _sysrand.randbytes(nbytes)
 
 def token_hex(nbytes=None):
     """Return a random text string, in hexadecimal.
diff --git a/Lib/test/test_random.py b/Lib/test/test_random.py
index 548af706db..f709e52ecb 100644
--- a/Lib/test/test_random.py
+++ b/Lib/test/test_random.py
@@ -291,6 +291,22 @@ class TestBasicOps:
         k = sum(randrange(6755399441055744) % 3 == 2 for i in range(n))
         self.assertTrue(0.30 < k/n < .37, (k/n))
 
+    def test_randbytes(self):
+        # Verify ranges
+        for n in range(1, 10):
+            data = self.gen.randbytes(n)
+            self.assertEqual(type(data), bytes)
+            self.assertEqual(len(data), n)
+
+        self.assertEqual(self.gen.randbytes(0), b'')
+
+        # Verify argument checking
+        self.assertRaises(TypeError, self.gen.randbytes)
+        self.assertRaises(TypeError, self.gen.randbytes, 1, 2)
+        self.assertRaises(ValueError, self.gen.randbytes, -1)
+        self.assertRaises(TypeError, self.gen.randbytes, 1.0)
+
+
 try:
     random.SystemRandom().random()
 except NotImplementedError:
@@ -747,6 +763,41 @@ class MersenneTwister_TestBasicOps(TestBasicOps, unittest.TestCase):
         c = self.gen.choices(population, cum_weights=cum_weights, k=10000)
         self.assertEqual(a, c)
 
+    def test_randbytes(self):
+        super().test_randbytes()
+
+        # Mersenne Twister randbytes() is deterministic
+        # and does not depend on the endian and bitness.
+        seed = 8675309
+        expected = b'f\xf9\xa836\xd0\xa4\xf4\x82\x9f\x8f\x19\xf0eo\x02'
+
+        self.gen.seed(seed)
+        self.assertEqual(self.gen.randbytes(16), expected)
+
+        # randbytes(0) must not consume any entropy
+        self.gen.seed(seed)
+        self.assertEqual(self.gen.randbytes(0), b'')
+        self.assertEqual(self.gen.randbytes(16), expected)
+
+        # Four randbytes(4) calls give the same output than randbytes(16)
+        self.gen.seed(seed)
+        self.assertEqual(b''.join([self.gen.randbytes(4) for _ in range(4)]),
+                         expected)
+
+        # Each randbytes(2) or randbytes(3) call consumes 4 bytes of entropy
+        self.gen.seed(seed)
+        expected2 = b''.join(expected[i:i + 2]
+                             for i in range(0, len(expected), 4))
+        self.assertEqual(b''.join(self.gen.randbytes(2) for _ in range(4)),
+                         expected2)
+
+        self.gen.seed(seed)
+        expected3 = b''.join(expected[i:i + 3]
+                             for i in range(0, len(expected), 4))
+        self.assertEqual(b''.join(self.gen.randbytes(3) for _ in range(4)),
+                         expected3)
+
+
 def gamma(z, sqrt2pi=(2.0*pi)**0.5):
     # Reflection to right half of complex plane
     if z < 0.5: