/*** 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 1.15 2006/01/06 14:32:20 drh Exp $*/#include "sqliteInt.h"#include "os.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(){  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;    sqlite3OsRandomSeed(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;  sqlite3OsEnterMutex();  while( N-- ){    *(zBuf++) = randomByte();  }  sqlite3OsLeaveMutex();}