Mercurial > dropbear
view libtommath/bn_mp_reduce.c @ 994:5c5ade336926
Prefer stronger algorithms in algorithm negotiation.
Prefer diffie-hellman-group14-sha1 (2048 bit) over
diffie-hellman-group1-sha1 (1024 bit).
Due to meet-in-the-middle attacks the effective key length of
three key 3DES is 112 bits. AES is stronger and faster then 3DES.
Prefer to delay the start of compression until after authentication
has completed. This avoids exposing compression code to attacks
from unauthenticated users.
(github pull request #9)
author | Fedor Brunner <fedor.brunner@azet.sk> |
---|---|
date | Fri, 23 Jan 2015 23:00:25 +0800 |
parents | 5ff8218bcee9 |
children | 60fc6476e044 |
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#include <tommath.h> #ifdef BN_MP_REDUCE_C /* LibTomMath, multiple-precision integer library -- Tom St Denis * * LibTomMath is a library that provides multiple-precision * integer arithmetic as well as number theoretic functionality. * * The library was designed directly after the MPI library by * Michael Fromberger but has been written from scratch with * additional optimizations in place. * * The library is free for all purposes without any express * guarantee it works. * * Tom St Denis, [email protected], http://math.libtomcrypt.com */ /* reduces x mod m, assumes 0 < x < m**2, mu is * precomputed via mp_reduce_setup. * From HAC pp.604 Algorithm 14.42 */ int mp_reduce (mp_int * x, mp_int * m, mp_int * mu) { mp_int q; int res, um = m->used; /* q = x */ if ((res = mp_init_copy (&q, x)) != MP_OKAY) { return res; } /* q1 = x / b**(k-1) */ mp_rshd (&q, um - 1); /* according to HAC this optimization is ok */ if (((unsigned long) um) > (((mp_digit)1) << (DIGIT_BIT - 1))) { if ((res = mp_mul (&q, mu, &q)) != MP_OKAY) { goto CLEANUP; } } else { #ifdef BN_S_MP_MUL_HIGH_DIGS_C if ((res = s_mp_mul_high_digs (&q, mu, &q, um)) != MP_OKAY) { goto CLEANUP; } #elif defined(BN_FAST_S_MP_MUL_HIGH_DIGS_C) if ((res = fast_s_mp_mul_high_digs (&q, mu, &q, um)) != MP_OKAY) { goto CLEANUP; } #else { res = MP_VAL; goto CLEANUP; } #endif } /* q3 = q2 / b**(k+1) */ mp_rshd (&q, um + 1); /* x = x mod b**(k+1), quick (no division) */ if ((res = mp_mod_2d (x, DIGIT_BIT * (um + 1), x)) != MP_OKAY) { goto CLEANUP; } /* q = q * m mod b**(k+1), quick (no division) */ if ((res = s_mp_mul_digs (&q, m, &q, um + 1)) != MP_OKAY) { goto CLEANUP; } /* x = x - q */ if ((res = mp_sub (x, &q, x)) != MP_OKAY) { goto CLEANUP; } /* If x < 0, add b**(k+1) to it */ if (mp_cmp_d (x, 0) == MP_LT) { mp_set (&q, 1); if ((res = mp_lshd (&q, um + 1)) != MP_OKAY) goto CLEANUP; if ((res = mp_add (x, &q, x)) != MP_OKAY) goto CLEANUP; } /* Back off if it's too big */ while (mp_cmp (x, m) != MP_LT) { if ((res = s_mp_sub (x, m, x)) != MP_OKAY) { goto CLEANUP; } } CLEANUP: mp_clear (&q); return res; } #endif /* $Source: /cvs/libtom/libtommath/bn_mp_reduce.c,v $ */ /* $Revision: 1.3 $ */ /* $Date: 2006/03/31 14:18:44 $ */