xref: /titanic_51/usr/src/lib/libsqlite/src/random.c (revision 53ed03b54d7e8d004eeb46c17e2cfd6b4616945f)
1 
2 #pragma ident	"%Z%%M%	%I%	%E% SMI"
3 
4 /*
5 ** 2001 September 15
6 **
7 ** The author disclaims copyright to this source code.  In place of
8 ** a legal notice, here is a blessing:
9 **
10 **    May you do good and not evil.
11 **    May you find forgiveness for yourself and forgive others.
12 **    May you share freely, never taking more than you give.
13 **
14 *************************************************************************
15 ** This file contains code to implement a pseudo-random number
16 ** generator (PRNG) for SQLite.
17 **
18 ** Random numbers are used by some of the database backends in order
19 ** to generate random integer keys for tables or random filenames.
20 **
21 ** $Id: random.c,v 1.11 2004/02/11 09:46:33 drh Exp $
22 */
23 #include "sqliteInt.h"
24 #include "os.h"
25 
26 
27 /*
28 ** Get a single 8-bit random value from the RC4 PRNG.  The Mutex
29 ** must be held while executing this routine.
30 **
31 ** Why not just use a library random generator like lrand48() for this?
32 ** Because the OP_NewRecno opcode in the VDBE depends on having a very
33 ** good source of random numbers.  The lrand48() library function may
34 ** well be good enough.  But maybe not.  Or maybe lrand48() has some
35 ** subtle problems on some systems that could cause problems.  It is hard
36 ** to know.  To minimize the risk of problems due to bad lrand48()
37 ** implementations, SQLite uses this random number generator based
38 ** on RC4, which we know works very well.
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     sqliteOsRandomSeed(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 sqliteRandomness(int N, void *pBuf){
94   unsigned char *zBuf = pBuf;
95   sqliteOsEnterMutex();
96   while( N-- ){
97     *(zBuf++) = randomByte();
98   }
99   sqliteOsLeaveMutex();
100 }
101