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 1.11 2004/02/11 09:46:33 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_NewRecno 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 static int randomByte(){ 38 unsigned char t; 39 40 /* All threads share a single random number generator. 41 ** This structure is the current state of the generator. 42 */ 43 static struct { 44 unsigned char isInit; /* True if initialized */ 45 unsigned char i, j; /* State variables */ 46 unsigned char s[256]; /* State variables */ 47 } prng; 48 49 /* Initialize the state of the random number generator once, 50 ** the first time this routine is called. The seed value does 51 ** not need to contain a lot of randomness since we are not 52 ** trying to do secure encryption or anything like that... 53 ** 54 ** Nothing in this file or anywhere else in SQLite does any kind of 55 ** encryption. The RC4 algorithm is being used as a PRNG (pseudo-random 56 ** number generator) not as an encryption device. 57 */ 58 if( !prng.isInit ){ 59 int i; 60 char k[256]; 61 prng.j = 0; 62 prng.i = 0; 63 sqliteOsRandomSeed(k); 64 for(i=0; i<256; i++){ 65 prng.s[i] = i; 66 } 67 for(i=0; i<256; i++){ 68 prng.j += prng.s[i] + k[i]; 69 t = prng.s[prng.j]; 70 prng.s[prng.j] = prng.s[i]; 71 prng.s[i] = t; 72 } 73 prng.isInit = 1; 74 } 75 76 /* Generate and return single random byte 77 */ 78 prng.i++; 79 t = prng.s[prng.i]; 80 prng.j += t; 81 prng.s[prng.i] = prng.s[prng.j]; 82 prng.s[prng.j] = t; 83 t += prng.s[prng.i]; 84 return prng.s[t]; 85 } 86 87 /* 88 ** Return N random bytes. 89 */ 90 void sqliteRandomness(int N, void *pBuf){ 91 unsigned char *zBuf = pBuf; 92 sqliteOsEnterMutex(); 93 while( N-- ){ 94 *(zBuf++) = randomByte(); 95 } 96 sqliteOsLeaveMutex(); 97 } 98