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/* Statistical tests for arc4random-related functions.
Copyright (C) 2022-2025 Free Software Foundation, Inc.
This file is part of the GNU C Library.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; if not, see
<https://www.gnu.org/licenses/>. */
#include <array_length.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <support/check.h>
enum
{
arc4random_key_size = 32
};
struct key
{
unsigned char data[arc4random_key_size];
};
/* With 12,000 keys, the probability that a byte in a predetermined
position does not have a predetermined value in all generated keys
is about 4e-21. The probability that this happens with any of the
16 * 256 possible byte position/values is 1.6e-17. This results in
an acceptably low false-positive rate. */
enum { key_count = 12000 };
static struct key keys[key_count];
/* Used to perform the distribution check. */
static int byte_counts[arc4random_key_size][256];
/* Bail out after this many failures. */
enum { failure_limit = 100 };
static void
find_stuck_bytes (bool (*func) (unsigned char *key))
{
memset (&keys, 0xcc, sizeof (keys));
int failures = 0;
for (int key = 0; key < key_count; ++key)
{
while (true)
{
if (func (keys[key].data))
break;
++failures;
if (failures >= failure_limit)
{
printf ("warning: bailing out after %d failures\n", failures);
return;
}
}
}
printf ("info: key generation finished with %d failures\n", failures);
memset (&byte_counts, 0, sizeof (byte_counts));
for (int key = 0; key < key_count; ++key)
for (int pos = 0; pos < arc4random_key_size; ++pos)
++byte_counts[pos][keys[key].data[pos]];
for (int pos = 0; pos < arc4random_key_size; ++pos)
for (int byte = 0; byte < 256; ++byte)
if (byte_counts[pos][byte] == 0)
{
support_record_failure ();
printf ("error: byte %d never appeared at position %d\n", byte, pos);
}
}
/* Test adapter for arc4random. */
static bool
generate_arc4random (unsigned char *key)
{
uint32_t words[arc4random_key_size / 4];
_Static_assert (sizeof (words) == arc4random_key_size, "sizeof (words)");
for (int i = 0; i < array_length (words); ++i)
words[i] = arc4random ();
memcpy (key, &words, arc4random_key_size);
return true;
}
/* Test adapter for arc4random_buf. */
static bool
generate_arc4random_buf (unsigned char *key)
{
arc4random_buf (key, arc4random_key_size);
return true;
}
/* Test adapter for arc4random_uniform. */
static bool
generate_arc4random_uniform (unsigned char *key)
{
for (int i = 0; i < arc4random_key_size; ++i)
key[i] = arc4random_uniform (256);
return true;
}
/* Test adapter for arc4random_uniform with argument 257. This means
that byte 0 happens more often, but we do not perform such a
statistical check, so the test will still pass */
static bool
generate_arc4random_uniform_257 (unsigned char *key)
{
for (int i = 0; i < arc4random_key_size; ++i)
key[i] = arc4random_uniform (257);
return true;
}
static int
do_test (void)
{
puts ("info: arc4random implementation test");
find_stuck_bytes (generate_arc4random);
puts ("info: arc4random_buf implementation test");
find_stuck_bytes (generate_arc4random_buf);
puts ("info: arc4random_uniform implementation test");
find_stuck_bytes (generate_arc4random_uniform);
puts ("info: arc4random_uniform implementation test (257 variant)");
find_stuck_bytes (generate_arc4random_uniform_257);
return 0;
}
#include <support/test-driver.c>
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