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 1.13 2005/06/12 21:35:52 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 } 101