## Artifact b6a57e85288470b013ad584a8813e893b60e62fe:

- File src/random.c — part of check-in [abff526d] at 2001-09-19 13:22:39 on branch trunk — Trying to get the OS abstraction layer to work. (CVS 256) (user: drh size: 3631)

`/* ** 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 1.6 2001/09/19 13:22:40 drh Exp $ */ #include "sqliteInt.h" #include "os.h" /* ** Get a single 8-bit random value from the RC4 PRNG. */ int sqliteRandomByte(void){ int t; /* ** The following structure holds the current state of the RC4 algorithm. ** We use RC4 as a random number generator. Each call to RC4 gives ** a random 8-bit number. ** ** 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. */ static struct { int isInit; int i, j; int s[256]; } prng_state; /* 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... */ if( !prng_state.isInit ){ int i; static char seed[] = " sqlite random seed abcdefghijklmnop"; char k[256]; sqliteOsRandomSeed(seed); prng_state.j = 0; prng_state.i = 0; for(i=0; i<256; i++){ prng_state.s[i] = i; k[i] = seed[i%sizeof(seed)]; } for(i=0; i<256; i++){ int t; prng_state.j = (prng_state.j + prng_state.s[i] + k[i]) & 0xff; t = prng_state.s[prng_state.j]; prng_state.s[prng_state.j] = prng_state.s[i]; prng_state.s[i] = t; } prng_state.isInit = 1; } /* Generate and return single random byte */ prng_state.i = (prng_state.i + 1) & 0xff; prng_state.j = (prng_state.j + prng_state.s[prng_state.i]) & 0xff; t = prng_state.s[prng_state.i]; prng_state.s[prng_state.i] = prng_state.s[prng_state.j]; prng_state.s[prng_state.j] = t; t = prng_state.s[prng_state.i] + prng_state.s[prng_state.j]; return prng_state.s[t & 0xff]; } /* ** Return a random 32-bit integer. The integer is generated by making ** 4 calls to sqliteRandomByte(). */ int sqliteRandomInteger(void){ int r; int i; r = sqliteRandomByte(); for(i=1; i<4; i++){ r = (r<<8) + sqliteRandomByte(); } return r; } /* ** Return a random 16-bit unsigned integer. The integer is generated by ** making 2 calls to sqliteRandomByte(). */ int sqliteRandomShort(void){ int r; r = sqliteRandomByte(); r = (r<<8) + sqliteRandomByte(); return r; } /* ** Generate a random filename with the given prefix. The new filename ** is written into zBuf[]. The calling function must insure that ** zBuf[] is big enough to hold the prefix plus 20 or so extra ** characters. ** ** Very random names are chosen so that the chance of a ** collision with an existing filename is very very small. */ void sqliteRandomName(char *zBuf, char *zPrefix){ int i, j; static const char zRandomChars[] = "abcdefghijklmnopqrstuvwxyz0123456789"; strcpy(zBuf, zPrefix); j = strlen(zBuf); for(i=0; i<15; i++){ int c = sqliteRandomByte() % (sizeof(zRandomChars) - 1); zBuf[j++] = zRandomChars[c]; } zBuf[j] = 0; }`