Mercurial > dropbear

annotate libtommath/bn_mp_dr_reduce.c @ 1715:3974f087d9c0

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Disallow leading lines before the ident for server (#102) Per RFC4253 4.2 clients must be able to process other lines of data before the version string, server behavior is not defined neither with MUST/SHOULD nor with MAY. If server process up to 50 lines too - it may cause too long hanging session with invalid/evil client that consume host resources and potentially may lead to DDoS on poor embedded boxes. Let's require first line from client to be version string and fail early if it's not - matches both RFC and real OpenSSH behavior.
author Vladislav Grishenko <themiron@users.noreply.github.com>
date Mon, 15 Jun 2020 18:22:18 +0500
parents 1051e4eea25a
children
Ignore whitespace changes - Everywhere: Within whitespace: At end of lines:
rev   line source
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1 #include "tommath_private.h"
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2 #ifdef BN_MP_DR_REDUCE_C
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3 /* LibTomMath, multiple-precision integer library -- Tom St Denis */
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4 /* SPDX-License-Identifier: Unlicense */
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5
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6 /* reduce "x" in place modulo "n" using the Diminished Radix algorithm.
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7 *
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8 * Based on algorithm from the paper
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9 *
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10 * "Generating Efficient Primes for Discrete Log Cryptosystems"
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11 * Chae Hoon Lim, Pil Joong Lee,
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12 * POSTECH Information Research Laboratories
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13 *
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14 * The modulus must be of a special format [see manual]
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15 *
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16 * Has been modified to use algorithm 7.10 from the LTM book instead
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17 *
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18 * Input x must be in the range 0 <= x <= (n-1)**2
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19 */
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20 mp_err mp_dr_reduce(mp_int *x, const mp_int *n, mp_digit k)
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21 {
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22 mp_err err;
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23 int i, m;
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24 mp_word r;
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25 mp_digit mu, *tmpx1, *tmpx2;
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27 /* m = digits in modulus */
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28 m = n->used;
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30 /* ensure that "x" has at least 2m digits */
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31 if (x->alloc < (m + m)) {
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32 if ((err = mp_grow(x, m + m)) != MP_OKAY) {
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33 return err;
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34 }
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35 }
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36
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37 /* top of loop, this is where the code resumes if
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38 * another reduction pass is required.
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39 */
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40 top:
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41 /* aliases for digits */
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42 /* alias for lower half of x */
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43 tmpx1 = x->dp;
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45 /* alias for upper half of x, or x/B**m */
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46 tmpx2 = x->dp + m;
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48 /* set carry to zero */
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49 mu = 0;
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51 /* compute (x mod B**m) + k * [x/B**m] inline and inplace */
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52 for (i = 0; i < m; i++) {
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53 r = ((mp_word)*tmpx2++ * (mp_word)k) + *tmpx1 + mu;
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54 *tmpx1++ = (mp_digit)(r & MP_MASK);
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55 mu = (mp_digit)(r >> ((mp_word)MP_DIGIT_BIT));
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56 }
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58 /* set final carry */
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59 *tmpx1++ = mu;
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60
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61 /* zero words above m */
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62 MP_ZERO_DIGITS(tmpx1, (x->used - m) - 1);
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63
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64 /* clamp, sub and return */
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65 mp_clamp(x);
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66
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67 /* if x >= n then subtract and reduce again
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68 * Each successive "recursion" makes the input smaller and smaller.
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69 */
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70 if (mp_cmp_mag(x, n) != MP_LT) {
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71 if ((err = s_mp_sub(x, n, x)) != MP_OKAY) {
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72 return err;
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73 }
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74 goto top;
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75 }
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76 return MP_OKAY;
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77 }
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78 #endif