|
1
|
1 /* LibTomMath, multiple-precision integer library -- Tom St Denis |
|
|
2 * |
|
|
3 * LibTomMath is a library that provides multiple-precision |
|
|
4 * integer arithmetic as well as number theoretic functionality. |
|
|
5 * |
|
|
6 * The library was designed directly after the MPI library by |
|
|
7 * Michael Fromberger but has been written from scratch with |
|
|
8 * additional optimizations in place. |
|
|
9 * |
|
|
10 * The library is free for all purposes without any express |
|
|
11 * guarantee it works. |
|
|
12 * |
|
|
13 * Tom St Denis, [email protected], http://math.libtomcrypt.org |
|
|
14 */ |
|
|
15 #include <tommath.h> |
|
|
16 |
|
|
17 /* calculate c = a**b using a square-multiply algorithm */ |
|
|
18 int mp_expt_d (mp_int * a, mp_digit b, mp_int * c) |
|
|
19 { |
|
|
20 int res, x; |
|
|
21 mp_int g; |
|
|
22 |
|
|
23 if ((res = mp_init_copy (&g, a)) != MP_OKAY) { |
|
|
24 return res; |
|
|
25 } |
|
|
26 |
|
|
27 /* set initial result */ |
|
|
28 mp_set (c, 1); |
|
|
29 |
|
|
30 for (x = 0; x < (int) DIGIT_BIT; x++) { |
|
|
31 /* square */ |
|
|
32 if ((res = mp_sqr (c, c)) != MP_OKAY) { |
|
|
33 mp_clear (&g); |
|
|
34 return res; |
|
|
35 } |
|
|
36 |
|
|
37 /* if the bit is set multiply */ |
|
|
38 if ((b & (mp_digit) (((mp_digit)1) << (DIGIT_BIT - 1))) != 0) { |
|
|
39 if ((res = mp_mul (c, &g, c)) != MP_OKAY) { |
|
|
40 mp_clear (&g); |
|
|
41 return res; |
|
|
42 } |
|
|
43 } |
|
|
44 |
|
|
45 /* shift to next bit */ |
|
|
46 b <<= 1; |
|
|
47 } |
|
|
48 |
|
|
49 mp_clear (&g); |
|
|
50 return MP_OKAY; |
|
|
51 } |
