annotate libtommath/bn_s_mp_mul_digs.c @ 1790:42745af83b7d

Introduce extra delay before closing unauthenticated sessions To make it harder for attackers, introduce a delay to keep an unauthenticated session open a bit longer, thus blocking a connection slot until after the delay. Without this, while there is a limit on the amount of attempts an attacker can make at the same time (MAX_UNAUTH_PER_IP), the time taken by dropbear to handle one attempt is still short and thus for each of the allowed parallel attempts many attempts can be chained one after the other. The attempt rate is then: "MAX_UNAUTH_PER_IP / <process time of one attempt>". With the delay, this rate becomes: "MAX_UNAUTH_PER_IP / UNAUTH_CLOSE_DELAY".
author Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
date Wed, 15 Feb 2017 13:53:04 +0100
parents 1051e4eea25a
children
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1 #include "tommath_private.h"
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2 #ifdef BN_S_MP_MUL_DIGS_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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6 /* multiplies |a| * |b| and only computes upto digs digits of result
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7 * HAC pp. 595, Algorithm 14.12 Modified so you can control how
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8 * many digits of output are created.
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9 */
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10 mp_err s_mp_mul_digs(const mp_int *a, const mp_int *b, mp_int *c, int digs)
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11 {
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12 mp_int t;
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13 mp_err err;
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14 int pa, pb, ix, iy;
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15 mp_digit u;
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16 mp_word r;
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17 mp_digit tmpx, *tmpt, *tmpy;
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18
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19 /* can we use the fast multiplier? */
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20 if ((digs < MP_WARRAY) &&
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21 (MP_MIN(a->used, b->used) < MP_MAXFAST)) {
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22 return s_mp_mul_digs_fast(a, b, c, digs);
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23 }
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24
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25 if ((err = mp_init_size(&t, digs)) != MP_OKAY) {
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26 return err;
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27 }
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28 t.used = digs;
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29
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30 /* compute the digits of the product directly */
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31 pa = a->used;
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32 for (ix = 0; ix < pa; ix++) {
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33 /* set the carry to zero */
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34 u = 0;
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35
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36 /* limit ourselves to making digs digits of output */
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37 pb = MP_MIN(b->used, digs - ix);
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38
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39 /* setup some aliases */
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40 /* copy of the digit from a used within the nested loop */
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41 tmpx = a->dp[ix];
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42
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43 /* an alias for the destination shifted ix places */
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44 tmpt = t.dp + ix;
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46 /* an alias for the digits of b */
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47 tmpy = b->dp;
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48
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49 /* compute the columns of the output and propagate the carry */
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50 for (iy = 0; iy < pb; iy++) {
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51 /* compute the column as a mp_word */
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52 r = (mp_word)*tmpt +
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53 ((mp_word)tmpx * (mp_word)*tmpy++) +
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54 (mp_word)u;
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55
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56 /* the new column is the lower part of the result */
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57 *tmpt++ = (mp_digit)(r & (mp_word)MP_MASK);
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58
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59 /* get the carry word from the result */
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60 u = (mp_digit)(r >> (mp_word)MP_DIGIT_BIT);
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61 }
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62 /* set carry if it is placed below digs */
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63 if ((ix + iy) < digs) {
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64 *tmpt = u;
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65 }
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66 }
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67
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68 mp_clamp(&t);
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69 mp_exch(&t, c);
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70
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71 mp_clear(&t);
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72 return MP_OKAY;
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73 }
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74 #endif