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 1.25 2008/06/19 01:03:18 drh Exp $

    19 */

    20 #include "sqliteInt.h"

    21 

    22 

    23 /* All threads share a single random number generator.

    24 ** This structure is the current state of the generator.

    25 */

    26 static struct sqlite3PrngType {

    27   unsigned char isInit;          /* True if initialized */

    28   unsigned char i, j;            /* State variables */

    29   unsigned char s[256];          /* State variables */

    30 } sqlite3Prng;

    31 

    32 /*

    33 ** Get a single 8-bit random value from the RC4 PRNG.  The Mutex

    34 ** must be held while executing this routine.

    35 **

    36 ** Why not just use a library random generator like lrand48() for this?

    37 ** Because the OP_NewRowid opcode in the VDBE depends on having a very

    38 ** good source of random numbers.  The lrand48() library function may

    39 ** well be good enough.  But maybe not.  Or maybe lrand48() has some

    40 ** subtle problems on some systems that could cause problems.  It is hard

    41 ** to know.  To minimize the risk of problems due to bad lrand48()

    42 ** implementations, SQLite uses this random number generator based

    43 ** on RC4, which we know works very well.

    44 **

    45 ** (Later):  Actually, OP_NewRowid does not depend on a good source of

    46 ** randomness any more.  But we will leave this code in all the same.

    47 */

    48 static int randomByte(void){

    49   unsigned char t;

    50 

    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( !sqlite3Prng.isInit ){

    62     int i;

    63     char k[256];

    64     sqlite3Prng.j = 0;

    65     sqlite3Prng.i = 0;

    66     sqlite3OsRandomness(sqlite3_vfs_find(0), 256, k);

    67     for(i=0; i<256; i++){

    68       sqlite3Prng.s[i] = i;

    69     }

    70     for(i=0; i<256; i++){

    71       sqlite3Prng.j += sqlite3Prng.s[i] + k[i];

    72       t = sqlite3Prng.s[sqlite3Prng.j];

    73       sqlite3Prng.s[sqlite3Prng.j] = sqlite3Prng.s[i];

    74       sqlite3Prng.s[i] = t;

    75     }

    76     sqlite3Prng.isInit = 1;

    77   }

    78 

    79   /* Generate and return single random byte

    80   */

    81   sqlite3Prng.i++;

    82   t = sqlite3Prng.s[sqlite3Prng.i];

    83   sqlite3Prng.j += t;

    84   sqlite3Prng.s[sqlite3Prng.i] = sqlite3Prng.s[sqlite3Prng.j];

    85   sqlite3Prng.s[sqlite3Prng.j] = t;

    86   t += sqlite3Prng.s[sqlite3Prng.i];

    87   return sqlite3Prng.s[t];

    88 }

    89 

    90 /*

    91 ** Return N random bytes.

    92 */

    93 void sqlite3_randomness(int N, void *pBuf){

    94   unsigned char *zBuf = pBuf;

    95 #ifndef SQLITE_MUTEX_NOOP

    96   sqlite3_mutex *mutex = sqlite3MutexAlloc(SQLITE_MUTEX_STATIC_PRNG);

    97 #endif

    98   sqlite3_mutex_enter(mutex);

    99   while( N-- ){

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

   101   }

   102   sqlite3_mutex_leave(mutex);

   103 }

   104 

   105 #ifndef SQLITE_OMIT_BUILTIN_TEST

   106 /*

   107 ** For testing purposes, we sometimes want to preserve the state of

   108 ** PRNG and restore the PRNG to its saved state at a later time.

   109 ** The sqlite3_test_control() interface calls these routines to

   110 ** control the PRNG.

   111 */

   112 static struct sqlite3PrngType sqlite3SavedPrng;

   113 void sqlite3PrngSaveState(void){

   114   memcpy(&sqlite3SavedPrng, &sqlite3Prng, sizeof(sqlite3Prng));

   115 }

   116 void sqlite3PrngRestoreState(void){

   117   memcpy(&sqlite3Prng, &sqlite3SavedPrng, sizeof(sqlite3Prng));

   118 }

   119 void sqlite3PrngResetState(void){

   120   sqlite3Prng.isInit = 0;

   121 }

   122 #endif /* SQLITE_OMIT_BUILTIN_TEST */