Mercurial > dropbear

annotate libtommath/bn_mp_montgomery_setup.c @ 994:5c5ade336926

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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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1 #include <tommath.h>
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2 #ifdef BN_MP_MONTGOMERY_SETUP_C
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3 /* LibTomMath, multiple-precision integer library -- Tom St Denis
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4 *
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5 * LibTomMath is a library that provides multiple-precision
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6 * integer arithmetic as well as number theoretic functionality.
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7 *
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8 * The library was designed directly after the MPI library by
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9 * Michael Fromberger but has been written from scratch with
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10 * additional optimizations in place.
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11 *
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12 * The library is free for all purposes without any express
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13 * guarantee it works.
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14 *
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15 * Tom St Denis, [email protected], http://math.libtomcrypt.com
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16 */
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18 /* setups the montgomery reduction stuff */
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19 int
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20 mp_montgomery_setup (mp_int * n, mp_digit * rho)
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21 {
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22 mp_digit x, b;
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23
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24 /* fast inversion mod 2**k
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25 *
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26 * Based on the fact that
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27 *
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28 * XA = 1 (mod 2**n) => (X(2-XA)) A = 1 (mod 2**2n)
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29 * => 2*X*A - X*X*A*A = 1
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30 * => 2*(1) - (1) = 1
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31 */
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32 b = n->dp[0];
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34 if ((b & 1) == 0) {
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35 return MP_VAL;
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36 }
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38 x = (((b + 2) & 4) << 1) + b; /* here x*a==1 mod 2**4 */
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39 x *= 2 - b * x; /* here x*a==1 mod 2**8 */
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40 #if !defined(MP_8BIT)
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41 x *= 2 - b * x; /* here x*a==1 mod 2**16 */
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42 #endif
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43 #if defined(MP_64BIT) || !(defined(MP_8BIT) || defined(MP_16BIT))
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44 x *= 2 - b * x; /* here x*a==1 mod 2**32 */
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45 #endif
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46 #ifdef MP_64BIT
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47 x *= 2 - b * x; /* here x*a==1 mod 2**64 */
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48 #endif
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50 /* rho = -1/m mod b */
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51 *rho = (unsigned long)(((mp_word)1 << ((mp_word) DIGIT_BIT)) - x) & MP_MASK;
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53 return MP_OKAY;
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54 }
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55 #endif
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57 /* $Source: /cvs/libtom/libtommath/bn_mp_montgomery_setup.c,v $ */
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58 /* $Revision: 1.4 $ */
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59 /* $Date: 2006/12/04 21:34:03 $ */