79 lines
2.6 KiB
C
79 lines
2.6 KiB
C
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#include "tommath_private.h"
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#ifdef S_MP_SQR_C
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/* LibTomMath, multiple-precision integer library -- Tom St Denis */
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/* SPDX-License-Identifier: Unlicense */
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/* low level squaring, b = a*a, HAC pp.596-597, Algorithm 14.16 */
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mp_err s_mp_sqr(const mp_int *a, mp_int *b)
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{
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mp_int t;
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int ix, pa;
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mp_err err;
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pa = a->used;
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if ((err = mp_init_size(&t, (2 * pa) + 1)) != MP_OKAY) {
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return err;
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}
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/* default used is maximum possible size */
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t.used = (2 * pa) + 1;
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for (ix = 0; ix < pa; ix++) {
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mp_digit u;
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int iy;
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/* first calculate the digit at 2*ix */
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/* calculate double precision result */
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mp_word r = (mp_word)t.dp[2*ix] +
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((mp_word)a->dp[ix] * (mp_word)a->dp[ix]);
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/* store lower part in result */
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t.dp[ix+ix] = (mp_digit)(r & (mp_word)MP_MASK);
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/* get the carry */
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u = (mp_digit)(r >> (mp_word)MP_DIGIT_BIT);
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for (iy = ix + 1; iy < pa; iy++) {
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/* first calculate the product */
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r = (mp_word)a->dp[ix] * (mp_word)a->dp[iy];
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/* now calculate the double precision result, note we use
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* addition instead of *2 since it's easier to optimize.
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*/
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/* Some architectures and/or compilers seem to prefer a bit-shift nowadays */
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r = (mp_word)t.dp[ix + iy] + (r<<1) + (mp_word)u;
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/* store lower part */
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t.dp[ix + iy] = (mp_digit)(r & (mp_word)MP_MASK);
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/* get carry */
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u = (mp_digit)(r >> (mp_word)MP_DIGIT_BIT);
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}
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/* propagate upwards */
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while (u != 0uL) {
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mp_digit tmp;
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/*
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"u" can get bigger than MP_DIGIT_MAX and would need a bigger type
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for the sum (mp_word). That is costly if mp_word is not a native
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integer but a bigint from the compiler library. We do a manual
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multiword addition instead.
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*/
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/* t.dp[ix + iy] has been masked off by MP_MASK and is hence of the correct size
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and we can just add the lower part of "u". Carry is guaranteed to fit into
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the type used for mp_digit, too, so we can extract it later. */
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tmp = t.dp[ix + iy] + (u & MP_MASK);
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/* t.dp[ix + iy] is set to the result minus the carry, carry is still in "tmp" */
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t.dp[ix + iy] = tmp & MP_MASK;
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/* Add high part of "u" and the carry from "tmp" to get the next "u" */
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u = (u >> MP_DIGIT_BIT) + (tmp >> MP_DIGIT_BIT);
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++iy;
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}
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}
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mp_clamp(&t);
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mp_exch(&t, b);
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mp_clear(&t);
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return MP_OKAY;
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}
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#endif
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