/*+
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** 2001 September 15+
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**+
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** The author disclaims copyright to this source code.  In place of+
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** a legal notice, here is a blessing:+
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**+
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**    May you do good and not evil.+
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**    May you find forgiveness for yourself and forgive others.+
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**    May you share freely, never taking more than you give.+
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**+
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*************************************************************************+
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** This file contains code to implement a pseudo-random number+
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** generator (PRNG) for SQLite.+
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**+
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** Random numbers are used by some of the database backends in order+
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** to generate random integer keys for tables or random filenames.+
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**+
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** $Id: random.cpp 1282 2008-11-13 09:31:33Z LarsPson $+
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*/+
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#include "sqliteInt.h"+
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+
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+
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/*+
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** Get a single 8-bit random value from the RC4 PRNG.  The Mutex+
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** must be held while executing this routine.+
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**+
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** Why not just use a library random generator like lrand48() for this?+
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** Because the OP_NewRowid opcode in the VDBE depends on having a very+
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** good source of random numbers.  The lrand48() library function may+
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** well be good enough.  But maybe not.  Or maybe lrand48() has some+
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** subtle problems on some systems that could cause problems.  It is hard+
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** to know.  To minimize the risk of problems due to bad lrand48()+
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** implementations, SQLite uses this random number generator based+
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** on RC4, which we know works very well.+
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**+
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** (Later):  Actually, OP_NewRowid does not depend on a good source of+
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** randomness any more.  But we will leave this code in all the same.+
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*/+
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static int randomByte(void){+
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  unsigned char t;+
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+
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  /* All threads share a single random number generator.+
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  ** This structure is the current state of the generator.+
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  */+
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  static struct {+
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    unsigned char isInit;          /* True if initialized */+
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    unsigned char i, j;            /* State variables */+
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    unsigned char s[256];          /* State variables */+
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  } prng;+
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+
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  /* Initialize the state of the random number generator once,+
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  ** the first time this routine is called.  The seed value does+
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  ** not need to contain a lot of randomness since we are not+
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  ** trying to do secure encryption or anything like that...+
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  **+
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  ** Nothing in this file or anywhere else in SQLite does any kind of+
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  ** encryption.  The RC4 algorithm is being used as a PRNG (pseudo-random+
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  ** number generator) not as an encryption device.+
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  */+
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  if( !prng.isInit ){+
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    int i;+
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    char k[256];+
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    prng.j = 0;+
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    prng.i = 0;+
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    sqlite3OsRandomness(sqlite3_vfs_find(0), 256, k);+
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    for(i=0; i<256; i++){+
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      prng.s[i] = i;+
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    }+
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    for(i=0; i<256; i++){+
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      prng.j += prng.s[i] + k[i];+
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      t = prng.s[prng.j];+
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      prng.s[prng.j] = prng.s[i];+
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      prng.s[i] = t;+
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    }+
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    prng.isInit = 1;+
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  }+
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+
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  /* Generate and return single random byte+
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  */+
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  prng.i++;+
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  t = prng.s[prng.i];+
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  prng.j += t;+
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  prng.s[prng.i] = prng.s[prng.j];+
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  prng.s[prng.j] = t;+
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  t += prng.s[prng.i];+
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  return prng.s[t];+
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}+
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+
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/*+
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** Return N random bytes.+
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*/+
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void sqlite3Randomness(int N, void *pBuf){+
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  unsigned char *zBuf = (unsigned char*)pBuf;+
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  static sqlite3_mutex *mutex = 0;+
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  if( mutex==0 ){+
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    mutex = sqlite3_mutex_alloc(SQLITE_MUTEX_STATIC_PRNG);+
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  }+
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  sqlite3_mutex_enter(mutex);+
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  while( N-- ){+
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    *(zBuf++) = randomByte();+
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  }+
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  sqlite3_mutex_leave(mutex);+
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}+
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