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 1.11 2004/02/11 09:46:33 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_NewRecno 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 */
randomByte()37 static int randomByte(){
38 unsigned char t;
39
40 /* All threads share a single random number generator.
41 ** This structure is the current state of the generator.
42 */
43 static struct {
44 unsigned char isInit; /* True if initialized */
45 unsigned char i, j; /* State variables */
46 unsigned char s[256]; /* State variables */
47 } prng;
48
49 /* Initialize the state of the random number generator once,
50 ** the first time this routine is called. The seed value does
51 ** not need to contain a lot of randomness since we are not
52 ** trying to do secure encryption or anything like that...
53 **
54 ** Nothing in this file or anywhere else in SQLite does any kind of
55 ** encryption. The RC4 algorithm is being used as a PRNG (pseudo-random
56 ** number generator) not as an encryption device.
57 */
58 if( !prng.isInit ){
59 int i;
60 char k[256];
61 prng.j = 0;
62 prng.i = 0;
63 sqliteOsRandomSeed(k);
64 for(i=0; i<256; i++){
65 prng.s[i] = i;
66 }
67 for(i=0; i<256; i++){
68 prng.j += prng.s[i] + k[i];
69 t = prng.s[prng.j];
70 prng.s[prng.j] = prng.s[i];
71 prng.s[i] = t;
72 }
73 prng.isInit = 1;
74 }
75
76 /* Generate and return single random byte
77 */
78 prng.i++;
79 t = prng.s[prng.i];
80 prng.j += t;
81 prng.s[prng.i] = prng.s[prng.j];
82 prng.s[prng.j] = t;
83 t += prng.s[prng.i];
84 return prng.s[t];
85 }
86
87 /*
88 ** Return N random bytes.
89 */
sqliteRandomness(int N,void * pBuf)90 void sqliteRandomness(int N, void *pBuf){
91 unsigned char *zBuf = pBuf;
92 sqliteOsEnterMutex();
93 while( N-- ){
94 *(zBuf++) = randomByte();
95 }
96 sqliteOsLeaveMutex();
97 }
98