79 lines
2.1 KiB
C
79 lines
2.1 KiB
C
/* LibTomMath, multiple-precision integer library -- Tom St Denis
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*
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* LibTomMath is a library that provides multiple-precision
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* integer arithmetic as well as number theoretic functionality.
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*
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* The library was designed directly after the MPI library by
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* Michael Fromberger but has been written from scratch with
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* additional optimizations in place.
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*
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* The library is free for all purposes without any express
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* guarantee it works.
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*
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* Tom St Denis, tomstdenis@iahu.ca, http://math.libtomcrypt.org
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*/
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#include <tommath.h>
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/* this is a shell function that calls either the normal or Montgomery
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* exptmod functions. Originally the call to the montgomery code was
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* embedded in the normal function but that wasted alot of stack space
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* for nothing (since 99% of the time the Montgomery code would be called)
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*/
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int mp_exptmod (mp_int * G, mp_int * X, mp_int * P, mp_int * Y)
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{
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int dr;
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/* modulus P must be positive */
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if (P->sign == MP_NEG) {
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return MP_VAL;
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}
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/* if exponent X is negative we have to recurse */
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if (X->sign == MP_NEG) {
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mp_int tmpG, tmpX;
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int err;
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/* first compute 1/G mod P */
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if ((err = mp_init(&tmpG)) != MP_OKAY) {
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return err;
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}
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if ((err = mp_invmod(G, P, &tmpG)) != MP_OKAY) {
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mp_clear(&tmpG);
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return err;
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}
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/* now get |X| */
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if ((err = mp_init(&tmpX)) != MP_OKAY) {
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mp_clear(&tmpG);
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return err;
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}
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if ((err = mp_abs(X, &tmpX)) != MP_OKAY) {
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mp_clear_multi(&tmpG, &tmpX, NULL);
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return err;
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}
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/* and now compute (1/G)**|X| instead of G**X [X < 0] */
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err = mp_exptmod(&tmpG, &tmpX, P, Y);
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mp_clear_multi(&tmpG, &tmpX, NULL);
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return err;
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}
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/* is it a DR modulus? */
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dr = mp_dr_is_modulus(P);
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/* if not, is it a uDR modulus? */
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if (dr == 0) {
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dr = mp_reduce_is_2k(P) << 1;
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}
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/* if the modulus is odd or dr != 0 use the fast method */
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if (mp_isodd (P) == 1 || dr != 0) {
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return mp_exptmod_fast (G, X, P, Y, dr);
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} else {
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/* otherwise use the generic Barrett reduction technique */
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return s_mp_exptmod (G, X, P, Y);
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}
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}
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