/*
** 2001 September 15
**
** The author disclaims copyright to this source code. In place of
** a legal notice, here is a blessing:
**
** May you do good and not evil.
** May you find forgiveness for yourself and forgive others.
** May you share freely, never taking more than you give.
**
*************************************************************************
** This file contains code to implement a pseudo-random number
** generator (PRNG) for SQLite.
**
** Random numbers are used by some of the database backends in order
** to generate random integer keys for tables or random filenames.
**
** $Id: random.c,v 1.19 2007/08/21 10:44:16 drh Exp $
*/
#include "sqliteInt.h"
/*
** Get a single 8-bit random value from the RC4 PRNG. The Mutex
** must be held while executing this routine.
**
** Why not just use a library random generator like lrand48() for this?
** Because the OP_NewRowid opcode in the VDBE depends on having a very
** good source of random numbers. The lrand48() library function may
** well be good enough. But maybe not. Or maybe lrand48() has some
** subtle problems on some systems that could cause problems. It is hard
** to know. To minimize the risk of problems due to bad lrand48()
** implementations, SQLite uses this random number generator based
** on RC4, which we know works very well.
**
** (Later): Actually, OP_NewRowid does not depend on a good source of
** randomness any more. But we will leave this code in all the same.
*/
static int randomByte(void){
unsigned char t;
/* All threads share a single random number generator.
** This structure is the current state of the generator.
*/
static struct {
unsigned char isInit; /* True if initialized */
unsigned char i, j; /* State variables */
unsigned char s[256]; /* State variables */
} prng;
/* Initialize the state of the random number generator once,
** the first time this routine is called. The seed value does
** not need to contain a lot of randomness since we are not
** trying to do secure encryption or anything like that...
**
** Nothing in this file or anywhere else in SQLite does any kind of
** encryption. The RC4 algorithm is being used as a PRNG (pseudo-random
** number generator) not as an encryption device.
*/
if( !prng.isInit ){
int i;
char k[256];
prng.j = 0;
prng.i = 0;
sqlite3OsRandomness(sqlite3_vfs_find(0), 256, k);
for(i=0; i<256; i++){
prng.s[i] = i;
}
for(i=0; i<256; i++){
prng.j += prng.s[i] + k[i];
t = prng.s[prng.j];
prng.s[prng.j] = prng.s[i];
prng.s[i] = t;
}
prng.isInit = 1;
}
/* Generate and return single random byte
*/
prng.i++;
t = prng.s[prng.i];
prng.j += t;
prng.s[prng.i] = prng.s[prng.j];
prng.s[prng.j] = t;
t += prng.s[prng.i];
return prng.s[t];
}
/*
** Return N random bytes.
*/
void sqlite3Randomness(int N, void *pBuf){
unsigned char *zBuf = pBuf;
sqlite3_mutex_enter(sqlite3_mutex_alloc(SQLITE_MUTEX_PRNG));
while( N-- ){
*(zBuf++) = randomByte();
}
sqlite3_mutex_leave(sqlite3_mutex_alloc(SQLITE_MUTEX_PRNG));
}