Mercurial > dropbear
annotate libtommath/bn_mp_dr_reduce.c @ 1861:2b3a8026a6ce
Add re-exec for server
This allows ASLR to re-randomize the address
space for every connection, preventing some
vulnerabilities from being exploitable by
repeated probing.
Overhead (memory and time) is yet to be confirmed.
At present this is only enabled on Linux. Other BSD platforms
with fexecve() would probably also work though have not been tested.
| author | Matt Johnston <matt@ucc.asn.au> |
|---|---|
| date | Sun, 30 Jan 2022 10:14:56 +0800 |
| parents | 1051e4eea25a |
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| 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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26 |
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27 /* m = digits in modulus */ |
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28 m = n->used; |
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29 |
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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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44 |
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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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47 |
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48 /* set carry to zero */ |
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49 mu = 0; |
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50 |
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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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57 |
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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 |