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1 #include <tommath.h>
2 #ifdef BN_MP_PRIME_RANDOM_EX_C
3 /* LibTomMath, multiple-precision integer library -- Tom St Denis
4  *
5  * LibTomMath is a library that provides multiple-precision
6  * integer arithmetic as well as number theoretic functionality.
7  *
8  * The library was designed directly after the MPI library by
9  * Michael Fromberger but has been written from scratch with
10  * additional optimizations in place.
11  *
12  * The library is free for all purposes without any express
13  * guarantee it works.
14  *
15  * Tom St Denis, tomstdenis@gmail.com, http://math.libtomcrypt.com
16  */
17 
18 /* makes a truly random prime of a given size (bits),
19  *
20  * Flags are as follows:
21  *
22  *   LTM_PRIME_BBS      - make prime congruent to 3 mod 4
23  *   LTM_PRIME_SAFE     - make sure (p-1)/2 is prime as well (implies LTM_PRIME_BBS)
24  *   LTM_PRIME_2MSB_OFF - make the 2nd highest bit zero
25  *   LTM_PRIME_2MSB_ON  - make the 2nd highest bit one
26  *
27  * You have to supply a callback which fills in a buffer with random bytes.  "dat" is a parameter you can
28  * have passed to the callback (e.g. a state or something).  This function doesn't use "dat" itself
29  * so it can be NULL
30  *
31  */
32 
33 /* This is possibly the mother of all prime generation functions, muahahahahaha! */
mp_prime_random_ex(mp_int * a,int t,int size,int flags,ltm_prime_callback cb,void * dat)34 int mp_prime_random_ex(mp_int *a, int t, int size, int flags, ltm_prime_callback cb, void *dat)
35 {
36    unsigned char *tmp, maskAND, maskOR_msb, maskOR_lsb;
37    int res, err, bsize, maskOR_msb_offset;
38 
39    /* sanity check the input */
40    if (size <= 1 || t <= 0) {
41       return MP_VAL;
42    }
43 
44    /* LTM_PRIME_SAFE implies LTM_PRIME_BBS */
45    if (flags & LTM_PRIME_SAFE) {
46       flags |= LTM_PRIME_BBS;
47    }
48 
49    /* calc the byte size */
50    bsize = (size>>3) + ((size&7)?1:0);
51 
52    /* we need a buffer of bsize bytes */
53    tmp = OPT_CAST(unsigned char) XMALLOC(bsize);
54    if (tmp == NULL) {
55       return MP_MEM;
56    }
57 
58    /* calc the maskAND value for the MSbyte*/
59    maskAND = ((size&7) == 0) ? 0xFF : (0xFF >> (8 - (size & 7)));
60 
61    /* calc the maskOR_msb */
62    maskOR_msb        = 0;
63    maskOR_msb_offset = ((size & 7) == 1) ? 1 : 0;
64    if (flags & LTM_PRIME_2MSB_ON) {
65       maskOR_msb       |= 0x80 >> ((9 - size) & 7);
66    }
67 
68    /* get the maskOR_lsb */
69    maskOR_lsb         = 1;
70    if (flags & LTM_PRIME_BBS) {
71       maskOR_lsb     |= 3;
72    }
73 
74    do {
75       /* read the bytes */
76       if (cb(tmp, bsize, dat) != bsize) {
77          err = MP_VAL;
78          goto error;
79       }
80 
81       /* work over the MSbyte */
82       tmp[0]    &= maskAND;
83       tmp[0]    |= 1 << ((size - 1) & 7);
84 
85       /* mix in the maskORs */
86       tmp[maskOR_msb_offset]   |= maskOR_msb;
87       tmp[bsize-1]             |= maskOR_lsb;
88 
89       /* read it in */
90       if ((err = mp_read_unsigned_bin(a, tmp, bsize)) != MP_OKAY)     { goto error; }
91 
92       /* is it prime? */
93       if ((err = mp_prime_is_prime(a, t, &res)) != MP_OKAY)           { goto error; }
94       if (res == MP_NO) {
95          continue;
96       }
97 
98       if (flags & LTM_PRIME_SAFE) {
99          /* see if (a-1)/2 is prime */
100          if ((err = mp_sub_d(a, 1, a)) != MP_OKAY)                    { goto error; }
101          if ((err = mp_div_2(a, a)) != MP_OKAY)                       { goto error; }
102 
103          /* is it prime? */
104          if ((err = mp_prime_is_prime(a, t, &res)) != MP_OKAY)        { goto error; }
105       }
106    } while (res == MP_NO);
107 
108    if (flags & LTM_PRIME_SAFE) {
109       /* restore a to the original value */
110       if ((err = mp_mul_2(a, a)) != MP_OKAY)                          { goto error; }
111       if ((err = mp_add_d(a, 1, a)) != MP_OKAY)                       { goto error; }
112    }
113 
114    err = MP_OKAY;
115 error:
116    XFREE(tmp);
117    return err;
118 }
119 
120 
121 #endif
122 
123 /* $Source: /cvs/libtom/libtommath/bn_mp_prime_random_ex.c,v $ */
124 /* $Revision: 1.4 $ */
125 /* $Date: 2006/03/31 14:18:44 $ */
126