Mercurial > dropbear
annotate libtommath/bn_mp_reduce_2k.c @ 1734:73646de50f13 DROPBEAR_2020.80
version 2020.80
author | Matt Johnston <matt@ucc.asn.au> |
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date | Fri, 26 Jun 2020 21:45:59 +0800 |
parents | 1051e4eea25a |
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1 #include "tommath_private.h" |
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2 #ifdef BN_MP_REDUCE_2K_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 /* reduces a modulo n where n is of the form 2**p - d */ |
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7 mp_err mp_reduce_2k(mp_int *a, const mp_int *n, mp_digit d) |
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8 { |
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9 mp_int q; |
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10 mp_err err; |
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11 int p; |
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13 if ((err = mp_init(&q)) != MP_OKAY) { |
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14 return err; |
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15 } |
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17 p = mp_count_bits(n); | |
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18 top: |
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19 /* q = a/2**p, a = a mod 2**p */ |
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20 if ((err = mp_div_2d(a, p, &q, a)) != MP_OKAY) { |
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21 goto LBL_ERR; |
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22 } |
1436 | 23 |
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24 if (d != 1u) { |
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25 /* q = q * d */ |
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26 if ((err = mp_mul_d(&q, d, &q)) != MP_OKAY) { |
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27 goto LBL_ERR; |
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28 } |
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29 } |
1436 | 30 |
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31 /* a = a + q */ |
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32 if ((err = s_mp_add(a, &q, a)) != MP_OKAY) { |
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33 goto LBL_ERR; |
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34 } |
1436 | 35 |
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36 if (mp_cmp_mag(a, n) != MP_LT) { |
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37 if ((err = s_mp_sub(a, n, a)) != MP_OKAY) { |
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38 goto LBL_ERR; |
1436 | 39 } |
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40 goto top; |
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41 } |
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43 LBL_ERR: |
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44 mp_clear(&q); |
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45 return err; |
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46 } |
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47 |
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48 #endif |