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101 lines
3.0 KiB
C
101 lines
3.0 KiB
C
/*
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** 2001 September 15
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**
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** The author disclaims copyright to this source code. In place of
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** a legal notice, here is a blessing:
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**
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** May you do good and not evil.
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** May you find forgiveness for yourself and forgive others.
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** May you share freely, never taking more than you give.
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**
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*************************************************************************
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** This file contains code to implement a pseudo-random number
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** generator (PRNG) for SQLite.
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**
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** Random numbers are used by some of the database backends in order
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** to generate random integer keys for tables or random filenames.
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**
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** $Id$
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*/
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#include "sqliteInt.h"
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#include "os.h"
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/*
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** Get a single 8-bit random value from the RC4 PRNG. The Mutex
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** must be held while executing this routine.
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**
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** Why not just use a library random generator like lrand48() for this?
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** Because the OP_NewRowid opcode in the VDBE depends on having a very
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** good source of random numbers. The lrand48() library function may
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** well be good enough. But maybe not. Or maybe lrand48() has some
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** subtle problems on some systems that could cause problems. It is hard
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** to know. To minimize the risk of problems due to bad lrand48()
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** implementations, SQLite uses this random number generator based
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** on RC4, which we know works very well.
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**
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** (Later): Actually, OP_NewRowid does not depend on a good source of
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** randomness any more. But we will leave this code in all the same.
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*/
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static int randomByte(){
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unsigned char t;
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/* All threads share a single random number generator.
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** This structure is the current state of the generator.
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*/
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static struct {
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unsigned char isInit; /* True if initialized */
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unsigned char i, j; /* State variables */
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unsigned char s[256]; /* State variables */
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} prng;
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/* Initialize the state of the random number generator once,
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** the first time this routine is called. The seed value does
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** not need to contain a lot of randomness since we are not
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** trying to do secure encryption or anything like that...
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**
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** Nothing in this file or anywhere else in SQLite does any kind of
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** encryption. The RC4 algorithm is being used as a PRNG (pseudo-random
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** number generator) not as an encryption device.
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*/
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if( !prng.isInit ){
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int i;
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char k[256];
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prng.j = 0;
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prng.i = 0;
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sqlite3OsRandomSeed(k);
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for(i=0; i<256; i++){
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prng.s[i] = i;
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}
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for(i=0; i<256; i++){
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prng.j += prng.s[i] + k[i];
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t = prng.s[prng.j];
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prng.s[prng.j] = prng.s[i];
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prng.s[i] = t;
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}
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prng.isInit = 1;
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}
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/* Generate and return single random byte
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*/
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prng.i++;
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t = prng.s[prng.i];
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prng.j += t;
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prng.s[prng.i] = prng.s[prng.j];
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prng.s[prng.j] = t;
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t += prng.s[prng.i];
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return prng.s[t];
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}
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/*
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** Return N random bytes.
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*/
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void sqlite3Randomness(int N, void *pBuf){
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unsigned char *zBuf = pBuf;
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sqlite3OsEnterMutex();
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while( N-- ){
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*(zBuf++) = randomByte();
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}
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sqlite3OsLeaveMutex();
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}
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