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annotate libtommath/bn_mp_dr_reduce.c @ 1930:299f4f19ba19

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Add /usr/sbin and /sbin to default root PATH When dropbear is used in a very restricted environment (such as in a initrd), the default user shell is often also very restricted and doesn't take care of setting the PATH so the user ends up with the PATH set by dropbear. Unfortunately, dropbear always sets "/usr/bin:/bin" as default PATH even for the root user which should have /usr/sbin and /sbin too. For a concrete instance of this problem, see the "Remote Unlocking" section in this tutorial: https://paxswill.com/blog/2013/11/04/encrypted-raspberry-pi/ It speaks of a bug in the initramfs script because it's written "blkid" instead of "/sbin/blkid"... this is just because the scripts from the initramfs do not expect to have a PATH without the sbin directories and because dropbear is not setting the PATH appropriately for the root user. I'm thus suggesting to use the attached patch to fix this misbehaviour (I did not test it, but it's easy enough). It might seem anecdotic but multiple Kali users have been bitten by this. From https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=903403
author Raphael Hertzog <hertzog@debian.org>
date Mon, 09 Jul 2018 16:27:53 +0200
parents 1051e4eea25a
children
Ignore whitespace changes - Everywhere: Within whitespace: At end of lines:
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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