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annotate libtommath/bn_s_mp_mul_high_digs_fast.c @ 1788:1fc0012b9c38

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Fix handling of replies to global requests (#112) The current code assumes that all global requests want / need a reply. This isn't always true and the request itself indicates if it wants a reply or not. It causes a specific problem with [email protected] messages. These are sent by OpenSSH after authentication to inform the client of potential other host keys for the host. This can be used to add a new type of host key or to rotate host keys. The initial information message from the server is sent as a global request, but with want_reply set to false. This means that the server doesn't expect an answer to this message. Instead the client needs to send a prove request as a reply if it wants to receive proof of ownership for the host keys. The bug doesn't cause any current problems with due to how OpenSSH treats receiving the failure message. It instead treats it as a keepalive message and further ignores it. Arguably this is a protocol violation though of Dropbear and it is only accidental that it doesn't cause a problem with OpenSSH. The bug was found when adding host keys support to libssh, which is more strict protocol wise and treats the unexpected failure message an error, also see https://gitlab.com/libssh/libssh-mirror/-/merge_requests/145 for more information. The fix here is to honor the want_reply flag in the global request and to only send a reply if the other side expects a reply.
author Dirkjan Bussink <d.bussink@gmail.com>
date Thu, 10 Dec 2020 16:13:13 +0100
parents 1051e4eea25a
children
Ignore whitespace changes - Everywhere: Within whitespace: At end of lines:
rev   line source
1692
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1 #include "tommath_private.h"
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2 #ifdef BN_S_MP_MUL_HIGH_DIGS_FAST_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 /* this is a modified version of fast_s_mul_digs that only produces
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7 * output digits *above* digs. See the comments for fast_s_mul_digs
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8 * to see how it works.
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9 *
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10 * This is used in the Barrett reduction since for one of the multiplications
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11 * only the higher digits were needed. This essentially halves the work.
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12 *
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13 * Based on Algorithm 14.12 on pp.595 of HAC.
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14 */
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15 mp_err s_mp_mul_high_digs_fast(const mp_int *a, const mp_int *b, mp_int *c, int digs)
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16 {
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17 int olduse, pa, ix, iz;
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18 mp_err err;
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19 mp_digit W[MP_WARRAY];
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20 mp_word _W;
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21
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22 /* grow the destination as required */
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23 pa = a->used + b->used;
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24 if (c->alloc < pa) {
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25 if ((err = mp_grow(c, pa)) != MP_OKAY) {
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26 return err;
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27 }
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28 }
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29
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30 /* number of output digits to produce */
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31 pa = a->used + b->used;
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32 _W = 0;
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33 for (ix = digs; ix < pa; ix++) {
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34 int tx, ty, iy;
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35 mp_digit *tmpx, *tmpy;
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36
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37 /* get offsets into the two bignums */
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38 ty = MP_MIN(b->used-1, ix);
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39 tx = ix - ty;
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40
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41 /* setup temp aliases */
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42 tmpx = a->dp + tx;
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43 tmpy = b->dp + ty;
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44
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45 /* this is the number of times the loop will iterrate, essentially its
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46 while (tx++ < a->used && ty-- >= 0) { ... }
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47 */
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48 iy = MP_MIN(a->used-tx, ty+1);
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49
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50 /* execute loop */
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51 for (iz = 0; iz < iy; iz++) {
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52 _W += (mp_word)*tmpx++ * (mp_word)*tmpy--;
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53 }
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54
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55 /* store term */
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56 W[ix] = (mp_digit)_W & MP_MASK;
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57
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58 /* make next carry */
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59 _W = _W >> (mp_word)MP_DIGIT_BIT;
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60 }
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61
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62 /* setup dest */
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63 olduse = c->used;
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64 c->used = pa;
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65
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66 {
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67 mp_digit *tmpc;
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68
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69 tmpc = c->dp + digs;
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70 for (ix = digs; ix < pa; ix++) {
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71 /* now extract the previous digit [below the carry] */
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72 *tmpc++ = W[ix];
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73 }
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74
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75 /* clear unused digits [that existed in the old copy of c] */
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76 MP_ZERO_DIGITS(tmpc, olduse - ix);
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77 }
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78 mp_clamp(c);
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79 return MP_OKAY;
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80 }
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81 #endif