/* ** 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(); }

Generated by Doxygen 1.6.0 Back to index