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bpo-40286: Makes simpler the relation between randbytes() and getrandbits() by serhiy-storchaka · Pull Request #19574 · python/cpython · GitHub

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bpo-40286: Makes simpler the relation between randbytes() and getrandbits() #19574

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serhiy-storchaka merged 1 commit into from
Apr 17, 2020
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bpo-40286: Makes simpler the relation between randbytes() and getrandbits() #19574

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24 changes: 20 additions & 4 deletions Lib/test/test_random.py
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Original file line number Diff line number Diff line change
Expand Up @@ -758,7 +758,7 @@ def test_randbytes(self):
# 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'
expected = b'3\xa8\xf9f\xf4\xa4\xd06\x19\x8f\x9f\x82\x02oe\xf0'

self.gen.seed(seed)
self.assertEqual(self.gen.randbytes(16), expected)
Expand All @@ -773,19 +773,35 @@ def test_randbytes(self):
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
# Each randbytes(1), randbytes(2) or randbytes(3) call consumes
# 4 bytes of entropy
self.gen.seed(seed)
expected2 = b''.join(expected[i:i + 2]
expected1 = expected[3::4]
self.assertEqual(b''.join(self.gen.randbytes(1) for _ in range(4)),
expected1)

self.gen.seed(seed)
expected2 = b''.join(expected[i + 2: i + 4]
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]
expected3 = b''.join(expected[i + 1: i + 4]
for i in range(0, len(expected), 4))
self.assertEqual(b''.join(self.gen.randbytes(3) for _ in range(4)),
expected3)

def test_randbytes_getrandbits(self):
# There is a simple relation between randbytes() and getrandbits()
seed = 2849427419
gen2 = random.Random()
self.gen.seed(seed)
gen2.seed(seed)
for n in range(9):
self.assertEqual(self.gen.randbytes(n),
gen2.getrandbits(n * 8).to_bytes(n, 'little'))


def gamma(z, sqrt2pi=(2.0*pi)**0.5):
# Reflection to right half of complex plane
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18 changes: 6 additions & 12 deletions Modules/_randommodule.c
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Expand Up @@ -534,31 +534,25 @@ _random_Random_randbytes_impl(RandomObject *self, Py_ssize_t n)
return NULL;
}

if (n == 0) {
/* Don't consume any entropy */
return PyBytes_FromStringAndSize(NULL, 0);
}

PyObject *bytes = PyBytes_FromStringAndSize(NULL, n);
if (bytes == NULL) {
return NULL;
}
uint8_t *ptr = (uint8_t *)PyBytes_AS_STRING(bytes);

do {
for (; n; ptr += 4, n -= 4) {
uint32_t word = genrand_uint32(self);
#if PY_LITTLE_ENDIAN
/* Convert to big endian */
#if PY_BIG_ENDIAN
/* Convert to little endian */
word = _Py_bswap32(word);
#endif
if (n < 4) {

pitrou Apr 17, 2020

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

Just a matter of taste, but personally I would move this epilog out of the loop and change the loop condition to n >= 4.

serhiy-storchaka Apr 17, 2020

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

Then you would need to repeat the following 5 lines:

        uint32_t word = genrand_uint32(self);
#if PY_BIG_ENDIAN
        /* Convert to little endian */
        word = _Py_bswap32(word);
#endif
memcpy(ptr, &word, n);
/* Drop least significant bits */
memcpy(ptr, (uint8_t *)&word + (4 - n), n);

pitrou Apr 17, 2020

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

It looks like this will give different results on big-endian and little-endian platforms, no?

serhiy-storchaka Apr 17, 2020

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

word was already converted to little endian.
break;
}
memcpy(ptr, &word, 4);
ptr += 4;
n -= 4;
} while (n);
}

return bytes;
}
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