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1/*
2** 2001 September 15
3**
4** The author disclaims copyright to this source code. In place of
5** a legal notice, here is a blessing:
6**
7** May you do good and not evil.
8** May you find forgiveness for yourself and forgive others.
9** May you share freely, never taking more than you give.
10**
11*************************************************************************
12** This file contains code to implement a pseudo-random number
13** generator (PRNG) for SQLite.
14**
15** Random numbers are used by some of the database backends in order
16** to generate random integer keys for tables or random filenames.
17**
18** $Id: random.c,v 1.16 2007/01/05 14:38:56 drh Exp $
19*/
20#include "sqliteInt.h"
21#include "os.h"
22
23
24/*
25** Get a single 8-bit random value from the RC4 PRNG. The Mutex
26** must be held while executing this routine.
27**
28** Why not just use a library random generator like lrand48() for this?
29** Because the OP_NewRowid opcode in the VDBE depends on having a very
30** good source of random numbers. The lrand48() library function may
31** well be good enough. But maybe not. Or maybe lrand48() has some
32** subtle problems on some systems that could cause problems. It is hard
33** to know. To minimize the risk of problems due to bad lrand48()
34** implementations, SQLite uses this random number generator based
35** on RC4, which we know works very well.
36**
37** (Later): Actually, OP_NewRowid does not depend on a good source of
38** randomness any more. But we will leave this code in all the same.
39*/
40static int randomByte(void){
41 unsigned char t;
42
43 /* All threads share a single random number generator.
44 ** This structure is the current state of the generator.
45 */
46 static struct {
47 unsigned char isInit; /* True if initialized */
48 unsigned char i, j; /* State variables */
49 unsigned char s[256]; /* State variables */
50 } prng;
51
52 /* Initialize the state of the random number generator once,
53 ** the first time this routine is called. The seed value does
54 ** not need to contain a lot of randomness since we are not
55 ** trying to do secure encryption or anything like that...
56 **
57 ** Nothing in this file or anywhere else in SQLite does any kind of
58 ** encryption. The RC4 algorithm is being used as a PRNG (pseudo-random
59 ** number generator) not as an encryption device.
60 */
61 if( !prng.isInit ){
62 int i;
63 char k[256];
64 prng.j = 0;
65 prng.i = 0;
66 sqlite3OsRandomSeed(k);
67 for(i=0; i<256; i++){
68 prng.s[i] = i;
69 }
70 for(i=0; i<256; i++){
71 prng.j += prng.s[i] + k[i];
72 t = prng.s[prng.j];
73 prng.s[prng.j] = prng.s[i];
74 prng.s[i] = t;
75 }
76 prng.isInit = 1;
77 }
78
79 /* Generate and return single random byte
80 */
81 prng.i++;
82 t = prng.s[prng.i];
83 prng.j += t;
84 prng.s[prng.i] = prng.s[prng.j];
85 prng.s[prng.j] = t;
86 t += prng.s[prng.i];
87 return prng.s[t];
88}
89
90/*
91** Return N random bytes.
92*/
93void sqlite3Randomness(int N, void *pBuf){
94 unsigned char *zBuf = pBuf;
95 sqlite3OsEnterMutex();
96 while( N-- ){
97 *(zBuf++) = randomByte();
98 }
99 sqlite3OsLeaveMutex();
100}

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