|
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 /* reduces a modulo n where n is of the form 2**p - d */ |
|
|
18 int |
|
|
19 mp_reduce_2k(mp_int *a, mp_int *n, mp_digit d) |
|
|
20 { |
|
|
21 mp_int q; |
|
|
22 int p, res; |
|
|
23 |
|
|
24 if ((res = mp_init(&q)) != MP_OKAY) { |
|
|
25 return res; |
|
|
26 } |
|
|
27 |
|
|
28 p = mp_count_bits(n); |
|
|
29 top: |
|
|
30 /* q = a/2**p, a = a mod 2**p */ |
|
|
31 if ((res = mp_div_2d(a, p, &q, a)) != MP_OKAY) { |
|
|
32 goto ERR; |
|
|
33 } |
|
|
34 |
|
|
35 if (d != 1) { |
|
|
36 /* q = q * d */ |
|
|
37 if ((res = mp_mul_d(&q, d, &q)) != MP_OKAY) { |
|
|
38 goto ERR; |
|
|
39 } |
|
|
40 } |
|
|
41 |
|
|
42 /* a = a + q */ |
|
|
43 if ((res = s_mp_add(a, &q, a)) != MP_OKAY) { |
|
|
44 goto ERR; |
|
|
45 } |
|
|
46 |
|
|
47 if (mp_cmp_mag(a, n) != MP_LT) { |
|
|
48 s_mp_sub(a, n, a); |
|
|
49 goto top; |
|
|
50 } |
|
|
51 |
|
|
52 ERR: |
|
|
53 mp_clear(&q); |
|
|
54 return res; |
|
|
55 } |
|
|
56 |
