/* -*-c-*-
*
- * $Id: mprand.c,v 1.1 1999/12/10 23:23:05 mdw Exp $
+ * $Id: mprand.c,v 1.3 2000/06/17 11:45:09 mdw Exp $
*
* Generate a random multiprecision integer
*
/*----- Revision history --------------------------------------------------*
*
* $Log: mprand.c,v $
+ * Revision 1.3 2000/06/17 11:45:09 mdw
+ * Major memory management overhaul. Added arena support. Use the secure
+ * arena for secret integers. Replace and improve the MP management macros
+ * (e.g., replace MP_MODIFY by MP_DEST).
+ *
+ * Revision 1.2 1999/12/22 15:55:33 mdw
+ * Modify `mprand' slightly. Add `mprand_range'.
+ *
* Revision 1.1 1999/12/10 23:23:05 mdw
* Support for generating random large integers.
*
mp *mprand(mp *d, unsigned b, grand *r, mpw or)
{
- size_t sz = (b + 7) / 8;
- octet *v = xmalloc(sz);
+ size_t sz = (b + 7) >> 3;
+ arena *a = (d && (d->f & MP_BURN)) ? arena_secure : arena_global;
+ octet *v = x_alloc(a, sz);
unsigned m;
/* --- Fill buffer with random data --- */
* This is slightly tricky. Oh, well.
*/
- b = (b - 1) % 8;
+ b = (b - 1) & 7;
m = (1 << b);
v[0] = (v[0] & (m - 1)) | m;
d = mp_loadb(d, v, sz);
d->v[0] |= or;
- free(v);
+ memset(v, 0, sz);
+ x_free(a, v);
+ return (d);
+}
+
+/* --- @mprand_range@ --- *
+ *
+ * Arguments: @mp *d@ = destination integer
+ * @mp *l@ = limit for random number
+ * @grand *r@ = random number source
+ * @mpw or@ = mask for low-order bits
+ *
+ * Returns: A pseudorandom integer, unformly distributed over the
+ * interval %$[0, l)$%.
+ *
+ * Use: Generates a uniformly-distributed pseudorandom number in the
+ * appropriate range.
+ */
+
+mp *mprand_range(mp *d, mp *l, grand *r, mpw or)
+{
+ size_t b = mp_bits(l);
+ size_t sz = (b + 7) >> 3;
+ arena *a = (d && (d->f & MP_BURN)) ? arena_secure : arena_global;
+ octet *v = x_alloc(a, sz);
+ unsigned m;
+
+ /* --- The algorithm --- *
+ *
+ * Rather simpler than most. Find the number of bits in the number %$l$%
+ * (i.e., the integer %$b$% such that %$2^{b - 1} \le l < 2^b$%), and
+ * generate pseudorandom integers with %$n$% bits (but not, unlike in the
+ * function above, with the top bit forced to 1). If the integer is
+ * greater than or equal to %$l$%, try again.
+ *
+ * This is similar to the algorithms used in @lcrand_range@ and friends,
+ * except that I've forced the `raw' range of the random numbers such that
+ * %$l$% itself is the largest multiple of %$l$% in the range (since, by
+ * the inequality above, %$2^b \le 2l$%). This removes the need for costly
+ * division and remainder operations.
+ *
+ * As usual, the number of iterations expected is two.
+ */
+
+ b = (b - 1) & 7;
+ m = (1 << b) - 1;
+ do {
+ r->ops->fill(r, v, sz);
+ v[0] &= m;
+ d = mp_loadb(d, v, sz);
+ d->v[0] |= or;
+ } while (MP_CMP(d, >=, l));
+
+ /* --- Done --- */
+
+ memset(v, 0, sz);
+ x_free(a, v);
return (d);
}