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annotate tomsfastmath/src/mont/fp_montgomery_setup.c @ 647:939cd3e22c87 dropbear-tfm

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- Fix constraints so we don't get warned about uninitialised variable (it isn't used as input by the asm)
author Matt Johnston <matt@ucc.asn.au>
date Wed, 30 Nov 2011 23:15:21 +0800
parents a362b62d38b2
children
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1 /* TomsFastMath, a fast ISO C bignum library.
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2 *
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3 * This project is meant to fill in where LibTomMath
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4 * falls short. That is speed ;-)
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5 *
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6 * This project is public domain and free for all purposes.
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7 *
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8 * Tom St Denis, [email protected]
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9 */
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10 #include <tfm.h>
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12 /* setups the montgomery reduction */
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13 int fp_montgomery_setup(fp_int *a, fp_digit *rho)
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14 {
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15 fp_digit x, b;
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17 /* fast inversion mod 2**k
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18 *
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19 * Based on the fact that
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20 *
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21 * XA = 1 (mod 2**n) => (X(2-XA)) A = 1 (mod 2**2n)
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22 * => 2*X*A - X*X*A*A = 1
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23 * => 2*(1) - (1) = 1
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24 */
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25 b = a->dp[0];
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27 if ((b & 1) == 0) {
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28 return FP_VAL;
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29 }
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31 x = (((b + 2) & 4) << 1) + b; /* here x*a==1 mod 2**4 */
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32 x *= 2 - b * x; /* here x*a==1 mod 2**8 */
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33 x *= 2 - b * x; /* here x*a==1 mod 2**16 */
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34 x *= 2 - b * x; /* here x*a==1 mod 2**32 */
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35 #ifdef FP_64BIT
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36 x *= 2 - b * x; /* here x*a==1 mod 2**64 */
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37 #endif
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39 /* rho = -1/m mod b */
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40 *rho = (((fp_word) 1 << ((fp_word) DIGIT_BIT)) - ((fp_word)x));
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42 return FP_OKAY;
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43 }
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46 /* $Source$ */
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47 /* $Revision$ */
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48 /* $Date$ */