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1 /* LibTomMath, multiple-precision integer library -- Tom St Denis |
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2 * |
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3 * LibTomMath is a library that provides multiple-precision |
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4 * integer arithmetic as well as number theoretic functionality. |
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5 * |
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6 * The library was designed directly after the MPI library by |
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7 * Michael Fromberger but has been written from scratch with |
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8 * additional optimizations in place. |
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9 * |
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10 * The library is free for all purposes without any express |
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11 * guarantee it works. |
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12 * |
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13 * Tom St Denis, [email protected], http://math.libtomcrypt.org |
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14 */ |
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15 #include <tommath.h> |
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16 |
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17 /* shift right a certain amount of digits */ |
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18 void mp_rshd (mp_int * a, int b) |
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19 { |
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20 int x; |
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21 |
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22 /* if b <= 0 then ignore it */ |
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23 if (b <= 0) { |
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24 return; |
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25 } |
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26 |
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27 /* if b > used then simply zero it and return */ |
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28 if (a->used <= b) { |
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29 mp_zero (a); |
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30 return; |
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31 } |
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32 |
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33 { |
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34 register mp_digit *bottom, *top; |
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35 |
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36 /* shift the digits down */ |
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37 |
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38 /* bottom */ |
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39 bottom = a->dp; |
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40 |
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41 /* top [offset into digits] */ |
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42 top = a->dp + b; |
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43 |
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44 /* this is implemented as a sliding window where |
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45 * the window is b-digits long and digits from |
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46 * the top of the window are copied to the bottom |
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47 * |
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48 * e.g. |
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49 |
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50 b-2 | b-1 | b0 | b1 | b2 | ... | bb | ----> |
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51 /\ | ----> |
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52 \-------------------/ ----> |
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53 */ |
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54 for (x = 0; x < (a->used - b); x++) { |
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55 *bottom++ = *top++; |
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56 } |
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57 |
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58 /* zero the top digits */ |
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59 for (; x < a->used; x++) { |
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60 *bottom++ = 0; |
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61 } |
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62 } |
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63 |
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64 /* remove excess digits */ |
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65 a->used -= b; |
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66 } |
