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 1.23 2008/03/21 16:45:47 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 static sqlite3_mutex *mutex = 0; 96 if( mutex==0 ){ 97 mutex = sqlite3_mutex_alloc(SQLITE_MUTEX_STATIC_PRNG); 98 } 99 sqlite3_mutex_enter(mutex); 100 while( N-- ){ 101 *(zBuf++) = randomByte(); 102 } 103 sqlite3_mutex_leave(mutex); 104 } 105 106 #ifndef SQLITE_OMIT_BUILTIN_TEST 107 /* 108 ** For testing purposes, we sometimes want to preserve the state of 109 ** PRNG and restore the PRNG to its saved state at a later time. 110 ** The sqlite3_test_control() interface calls these routines to 111 ** control the PRNG. 112 */ 113 static struct sqlite3PrngType sqlite3SavedPrng; 114 void sqlite3PrngSaveState(void){ 115 memcpy(&sqlite3SavedPrng, &sqlite3Prng, sizeof(sqlite3Prng)); 116 } 117 void sqlite3PrngRestoreState(void){ 118 memcpy(&sqlite3Prng, &sqlite3SavedPrng, sizeof(sqlite3Prng)); 119 } 120 void sqlite3PrngResetState(void){ 121 sqlite3Prng.isInit = 0; 122 } 123 #endif /* SQLITE_OMIT_BUILTIN_TEST */ 124