Mercurial > dropbear
annotate libtommath/bn_s_mp_sub.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> |
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| 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_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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5 |
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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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11 |
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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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15 |
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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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24 |
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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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28 |
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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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39 |
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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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46 |
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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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50 |
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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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55 |
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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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58 |
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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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62 |
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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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70 |
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71 #endif |