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comparison libtommath/bn_mp_montgomery_setup.c @ 284:eed26cff980b

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propagate from branch 'au.asn.ucc.matt.ltm.dropbear' (head 6c790cad5a7fa866ad062cb3a0c279f7ba788583) to branch 'au.asn.ucc.matt.dropbear' (head fff0894a0399405a9410ea1c6d118f342cf2aa64)
author Matt Johnston <matt@ucc.asn.au>
date Wed, 08 Mar 2006 13:23:49 +0000
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children 5ff8218bcee9
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283:bd240aa12ba7 284:eed26cff980b
1 #include <tommath.h>
2 #ifdef BN_MP_MONTGOMERY_SETUP_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, [email protected], http://math.libtomcrypt.org
16 */
17
18 /* setups the montgomery reduction stuff */
19 int
20 mp_montgomery_setup (mp_int * n, mp_digit * rho)
21 {
22 mp_digit x, b;
23
24 /* fast inversion mod 2**k
25 *
26 * Based on the fact that
27 *
28 * XA = 1 (mod 2**n) => (X(2-XA)) A = 1 (mod 2**2n)
29 * => 2*X*A - X*X*A*A = 1
30 * => 2*(1) - (1) = 1
31 */
32 b = n->dp[0];
33
34 if ((b & 1) == 0) {
35 return MP_VAL;
36 }
37
38 x = (((b + 2) & 4) << 1) + b; /* here x*a==1 mod 2**4 */
39 x *= 2 - b * x; /* here x*a==1 mod 2**8 */
40 #if !defined(MP_8BIT)
41 x *= 2 - b * x; /* here x*a==1 mod 2**16 */
42 #endif
43 #if defined(MP_64BIT) || !(defined(MP_8BIT) || defined(MP_16BIT))
44 x *= 2 - b * x; /* here x*a==1 mod 2**32 */
45 #endif
46 #ifdef MP_64BIT
47 x *= 2 - b * x; /* here x*a==1 mod 2**64 */
48 #endif
49
50 /* rho = -1/m mod b */
51 *rho = (((mp_word)1 << ((mp_word) DIGIT_BIT)) - x) & MP_MASK;
52
53 return MP_OKAY;
54 }
55 #endif