Mercurial > dropbear

annotate libtommath/bn_s_mp_sub.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:
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f52919ffd3b1 update ltm to 1.1.0 and enable FIPS 186.4 compliant key-generation (#79)
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1 #include "tommath_private.h"
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2 #ifdef BN_S_MP_SUB_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 /* low level subtraction (assumes |a| > |b|), HAC pp.595 Algorithm 14.9 */
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7 mp_err s_mp_sub(const mp_int *a, const mp_int *b, mp_int *c)
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8 {
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9 int olduse, min, max;
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10 mp_err err;
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12 /* find sizes */
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13 min = b->used;
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14 max = a->used;
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16 /* init result */
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17 if (c->alloc < max) {
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18 if ((err = mp_grow(c, max)) != MP_OKAY) {
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19 return err;
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20 }
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21 }
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22 olduse = c->used;
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23 c->used = max;
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25 {
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26 mp_digit u, *tmpa, *tmpb, *tmpc;
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27 int i;
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29 /* alias for digit pointers */
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30 tmpa = a->dp;
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31 tmpb = b->dp;
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32 tmpc = c->dp;
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33
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34 /* set carry to zero */
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35 u = 0;
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36 for (i = 0; i < min; i++) {
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37 /* T[i] = A[i] - B[i] - U */
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38 *tmpc = (*tmpa++ - *tmpb++) - u;
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40 /* U = carry bit of T[i]
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41 * Note this saves performing an AND operation since
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42 * if a carry does occur it will propagate all the way to the
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43 * MSB. As a result a single shift is enough to get the carry
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44 */
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45 u = *tmpc >> (MP_SIZEOF_BITS(mp_digit) - 1u);
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47 /* Clear carry from T[i] */
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48 *tmpc++ &= MP_MASK;
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49 }
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51 /* now copy higher words if any, e.g. if A has more digits than B */
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52 for (; i < max; i++) {
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53 /* T[i] = A[i] - U */
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54 *tmpc = *tmpa++ - u;
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56 /* U = carry bit of T[i] */
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57 u = *tmpc >> (MP_SIZEOF_BITS(mp_digit) - 1u);
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59 /* Clear carry from T[i] */
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60 *tmpc++ &= MP_MASK;
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61 }
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63 /* clear digits above used (since we may not have grown result above) */
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64 MP_ZERO_DIGITS(tmpc, olduse - c->used);
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65 }
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66
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67 mp_clamp(c);
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68 return MP_OKAY;
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69 }
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71 #endif