lib/random.c

   1 /*
   2  * Copyright (c) 2008, 2009, 2010, 2011, 2012 Nicira, Inc.
   3  *
   4  * Licensed under the Apache License, Version 2.0 (the "License");
   5  * you may not use this file except in compliance with the License.
   6  * You may obtain a copy of the License at:
   7  *
   8  *     http://www.apache.org/licenses/LICENSE-2.0
   9  *
  10  * Unless required by applicable law or agreed to in writing, software
  11  * distributed under the License is distributed on an "AS IS" BASIS,
  12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  13  * See the License for the specific language governing permissions and
  14  * limitations under the License.
  15  */
  16
  17 #include <config.h>
  18 #include "random.h"
  19
  20 #include <assert.h>
  21 #include <errno.h>
  22 #include <stdlib.h>
  23 #include <sys/time.h>
  24
  25 #include "entropy.h"
  26 #include "timeval.h"
  27 #include "util.h"
  28
  29 /* This is the 32-bit PRNG recommended in G. Marsaglia, "Xorshift RNGs",
  30  * _Journal of Statistical Software_ 8:14 (July 2003).  According to the paper,
  31  * it has a period of 2**32 - 1 and passes almost all tests of randomness.
  32  *
  33  * We use this PRNG instead of libc's rand() because rand() varies in quality
  34  * and because its maximum value also varies between 32767 and INT_MAX, whereas
  35  * we often want random numbers in the full range of uint32_t.
  36  *
  37  * This random number generator is intended for purposes that do not require
  38  * cryptographic-quality randomness. */
  39
  40 /* Current random state. */
  41 static uint32_t seed;
  42
  43 static uint32_t random_next(void);
  44
  45 void
  46 random_init(void)
  47 {
  48     while (!seed) {
  49         struct timeval tv;
  50         uint32_t entropy;
  51
  52         xgettimeofday(&tv);
  53         get_entropy_or_die(&entropy, 4);
  54
  55         seed = tv.tv_sec ^ tv.tv_usec ^ entropy;
  56     }
  57 }
  58
  59 void
  60 random_set_seed(uint32_t seed_)
  61 {
  62     assert(seed_);
  63     seed = seed_;
  64 }
  65
  66 void
  67 random_bytes(void *p_, size_t n)
  68 {
  69     uint8_t *p = p_;
  70
  71     random_init();
  72
  73     for (; n > 4; p += 4, n -= 4) {
  74         uint32_t x = random_next();
  75         memcpy(p, &x, 4);
  76     }
  77
  78     if (n) {
  79         uint32_t x = random_next();
  80         memcpy(p, &x, n);
  81     }
  82 }
  83
  84 uint8_t
  85 random_uint8(void)
  86 {
  87     return random_uint32();
  88 }
  89
  90 uint16_t
  91 random_uint16(void)
  92 {
  93     return random_uint32();
  94 }
  95
  96 uint32_t
  97 random_uint32(void)
  98 {
  99     random_init();
 100     return random_next();
 101 }
 102
 103 uint64_t
 104 random_uint64(void)
 105 {
 106     uint64_t x;
 107
 108     random_init();
 109
 110     x = random_next();
 111     x |= (uint64_t) random_next() << 32;
 112     return x;
 113 }
 114
 115 int
 116 random_range(int max)
 117 {
 118     return random_uint32() % max;
 119 }
 120
 121 static uint32_t
 122 random_next(void)
 123 {
 124     seed ^= seed << 13;
 125     seed ^= seed >> 17;
 126     seed ^= seed << 5;
 127
 128     return seed;
 129 }