random.c
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- /*
- ** 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 1.23 2008/03/21 16:45:47 drh Exp $
- */
- #include "sqliteInt.h"
- /* All threads share a single random number generator.
- ** This structure is the current state of the generator.
- */
- static struct sqlite3PrngType {
- unsigned char isInit; /* True if initialized */
- unsigned char i, j; /* State variables */
- unsigned char s[256]; /* State variables */
- } sqlite3Prng;
- /*
- ** 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;
- /* 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( !sqlite3Prng.isInit ){
- int i;
- char k[256];
- sqlite3Prng.j = 0;
- sqlite3Prng.i = 0;
- sqlite3OsRandomness(sqlite3_vfs_find(0), 256, k);
- for(i=0; i<256; i++){
- sqlite3Prng.s[i] = i;
- }
- for(i=0; i<256; i++){
- sqlite3Prng.j += sqlite3Prng.s[i] + k[i];
- t = sqlite3Prng.s[sqlite3Prng.j];
- sqlite3Prng.s[sqlite3Prng.j] = sqlite3Prng.s[i];
- sqlite3Prng.s[i] = t;
- }
- sqlite3Prng.isInit = 1;
- }
- /* Generate and return single random byte
- */
- sqlite3Prng.i++;
- t = sqlite3Prng.s[sqlite3Prng.i];
- sqlite3Prng.j += t;
- sqlite3Prng.s[sqlite3Prng.i] = sqlite3Prng.s[sqlite3Prng.j];
- sqlite3Prng.s[sqlite3Prng.j] = t;
- t += sqlite3Prng.s[sqlite3Prng.i];
- return sqlite3Prng.s[t];
- }
- /*
- ** Return N random bytes.
- */
- void sqlite3_randomness(int N, void *pBuf){
- unsigned char *zBuf = pBuf;
- static sqlite3_mutex *mutex = 0;
- if( mutex==0 ){
- mutex = sqlite3_mutex_alloc(SQLITE_MUTEX_STATIC_PRNG);
- }
- sqlite3_mutex_enter(mutex);
- while( N-- ){
- *(zBuf++) = randomByte();
- }
- sqlite3_mutex_leave(mutex);
- }
- #ifndef SQLITE_OMIT_BUILTIN_TEST
- /*
- ** For testing purposes, we sometimes want to preserve the state of
- ** PRNG and restore the PRNG to its saved state at a later time.
- ** The sqlite3_test_control() interface calls these routines to
- ** control the PRNG.
- */
- static struct sqlite3PrngType sqlite3SavedPrng;
- void sqlite3PrngSaveState(void){
- memcpy(&sqlite3SavedPrng, &sqlite3Prng, sizeof(sqlite3Prng));
- }
- void sqlite3PrngRestoreState(void){
- memcpy(&sqlite3Prng, &sqlite3SavedPrng, sizeof(sqlite3Prng));
- }
- void sqlite3PrngResetState(void){
- sqlite3Prng.isInit = 0;
- }
- #endif /* SQLITE_OMIT_BUILTIN_TEST */