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 1.15 2006/01/06 14:32:20 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 */

    40 static int randomByte(){

    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 */

    93 void sqlite3Randomness(int N, void *pBuf){

    94   unsigned char *zBuf = pBuf;

    95   sqlite3OsEnterMutex();

    96   while( N-- ){

    97     *(zBuf++) = randomByte();

    98   }

    99   sqlite3OsLeaveMutex();

   100 }