initially checkin

1 files changed, 247 insertions, 0 deletions

diff --git a/mpi/mpi-pow.c b/mpi/mpi-pow.c
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--- /dev/null
+++ b/mpi/mpi-pow.c
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+/* mpi-pow.c  -  MPI functions
+ *	Copyright (c) 1997 by Werner Koch (dd9jn)
+ *
+ * This file is part of G10.
+ *
+ * G10 is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * G10 is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
+ */
+
+#include <config.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include "mpi-internal.h"
+#include "longlong.h"
+#include <assert.h>
+
+
+/****************
+ * RES = BASE ^ EXP mod MOD
+ */
+void
+mpi_powm( MPI res, MPI base, MPI exp, MPI mod)
+{
+    mpi_ptr_t  rp, ep, mp, bp;
+    mpi_size_t esize, msize, bsize, rsize;
+    int        esign, msign, bsign, rsign;
+    mpi_size_t size;
+    int mod_shift_cnt;
+    int negative_result;
+    mpi_ptr_t mp_marker=NULL, bp_marker=NULL, ep_marker=NULL;
+    mpi_ptr_t xp_marker=NULL;
+    int assign_rp=0;
+
+    esize = exp->nlimbs;
+    msize = mod->nlimbs;
+    size = 2 * msize;
+    esign = exp->sign;
+    msign = mod->sign;
+
+    rp = res->d;
+    ep = exp->d;
+
+    if( !msize )
+	msize = 1 / msize;	    /* provoke a signal */
+
+    if( !esize ) {
+	/* Exponent is zero, result is 1 mod MOD, i.e., 1 or 0
+	 * depending on if MOD equals 1.  */
+	rp[0] = 1;
+	res->nlimbs = (msize == 1 && mod->d[0] == 1) ? 0 : 1;
+	res->sign = 0;
+	goto leave;
+    }
+
+    /* Normalize MOD (i.e. make its most significant bit set) as required by
+     * mpn_divrem.  This will make the intermediate values in the calculation
+     * slightly larger, but the correct result is obtained after a final
+     * reduction using the original MOD value.	*/
+    mp = mp_marker = mpi_alloc_limb_space(msize);
+    count_leading_zeros( mod_shift_cnt, mod->d[msize-1] );
+    if( mod_shift_cnt )
+	mpihelp_lshift( mp, mod->d, msize, mod_shift_cnt );
+    else
+	MPN_COPY( mp, mod->d, msize );
+
+    bsize = base->nlimbs;
+    bsign = base->sign;
+    if( bsize > msize ) { /* The base is larger than the module. Reduce it. */
+	/* Allocate (BSIZE + 1) with space for remainder and quotient.
+	 * (The quotient is (bsize - msize + 1) limbs.)  */
+	bp = bp_marker = mpi_alloc_limb_space( bsize + 1);
+	MPN_COPY( bp, base->d, bsize );
+	/* We don't care about the quotient, store it above the remainder,
+	 * at BP + MSIZE.  */
+	mpihelp_divrem( bp + msize, 0, bp, bsize, mp, msize );
+	bsize = msize;
+	/* Canonicalize the base, since we are going to multiply with it
+	 * quite a few times.  */
+	MPN_NORMALIZE( bp, bsize );
+    }
+    else
+	bp = base->d;
+
+    if( !bsize ) {
+	res->nlimbs = 0;
+	res->sign = 0;
+	goto leave;
+    }
+
+    if( res->alloced < size ) {
+	/* We have to allocate more space for RES.  If any of the input
+	 * parameters are identical to RES, defer deallocation of the old
+	 * space.  */
+	if( rp == ep || rp == mp || rp == bp ) {
+	    rp = mpi_alloc_limb_space( size );
+	    assign_rp = 1;
+	}
+	else {
+	    mpi_resize( res, size );
+	    rp = res->d;
+	}
+    }
+    else { /* Make BASE, EXP and MOD not overlap with RES.  */
+	if( rp == bp ) {
+	    /* RES and BASE are identical.  Allocate temp. space for BASE.  */
+	    assert( !bp_marker );
+	    bp = bp_marker = mpi_alloc_limb_space( bsize );
+	    MPN_COPY(bp, rp, bsize);
+	}
+	if( rp == ep ) {
+	    /* RES and EXP are identical.  Allocate temp. space for EXP.  */
+	    ep = ep_marker = mpi_alloc_limb_space( esize );
+	    MPN_COPY(ep, rp, esize);
+	}
+	if( rp == mp ) {
+	    /* RES and MOD are identical.  Allocate temporary space for MOD.*/
+	    assert( !mp_marker );
+	    mp = mp_marker = mpi_alloc_limb_space( msize );
+	    MPN_COPY(mp, rp, msize);
+	}
+    }
+
+    MPN_COPY( rp, bp, bsize );
+    rsize = bsize;
+    rsign = bsign;
+
+    {
+	mpi_size_t i;
+	mpi_ptr_t xp = xp_marker = mpi_alloc_limb_space( 2 * (msize + 1) );
+	int c;
+	mpi_limb_t e;
+	mpi_limb_t carry_limb;
+
+	negative_result = (ep[0] & 1) && base->sign;
+
+	i = esize - 1;
+	e = ep[i];
+	count_leading_zeros (c, e);
+	e = (e << c) << 1;     /* shift the exp bits to the left, lose msb */
+	c = BITS_PER_MPI_LIMB - 1 - c;
+
+	/* Main loop.
+	 *
+	 * Make the result be pointed to alternately by XP and RP.  This
+	 * helps us avoid block copying, which would otherwise be necessary
+	 * with the overlap restrictions of mpihelp_divmod. With 50% probability
+	 * the result after this loop will be in the area originally pointed
+	 * by RP (==RES->d), and with 50% probability in the area originally
+	 * pointed to by XP.
+	 */
+	for(;;) {
+	    while( c ) {
+		mpi_ptr_t tp;
+		mpi_size_t xsize;
+
+		mpihelp_mul_n(xp, rp, rp, rsize);
+		xsize = 2 * rsize;
+		if( xsize > msize ) {
+		    mpihelp_divrem(xp + msize, 0, xp, xsize, mp, msize);
+		    xsize = msize;
+		}
+
+		tp = rp; rp = xp; xp = tp;
+		rsize = xsize;
+
+		if( (mpi_limb_signed_t)e < 0 ) {
+		    mpihelp_mul( xp, rp, rsize, bp, bsize );
+		    xsize = rsize + bsize;
+		    if( xsize > msize ) {
+			mpihelp_divrem(xp + msize, 0, xp, xsize, mp, msize);
+			xsize = msize;
+		    }
+
+		    tp = rp; rp = xp; xp = tp;
+		    rsize = xsize;
+		}
+		e <<= 1;
+		c--;
+	    }
+
+	    i--;
+	    if( i < 0 )
+		break;
+	    e = ep[i];
+	    c = BITS_PER_MPI_LIMB;
+	}
+
+	/* We shifted MOD, the modulo reduction argument, left MOD_SHIFT_CNT
+	 * steps.  Adjust the result by reducing it with the original MOD.
+	 *
+	 * Also make sure the result is put in RES->d (where it already
+	 * might be, see above).
+	 */
+	if( mod_shift_cnt ) {
+	    carry_limb = mpihelp_lshift( res->d, rp, rsize, mod_shift_cnt);
+	    rp = res->d;
+	    if( carry_limb ) {
+		rp[rsize] = carry_limb;
+		rsize++;
+	    }
+	}
+	else {
+	    MPN_COPY( res->d, rp, rsize);
+	    rp = res->d;
+	}
+
+	if( rsize >= msize ) {
+	    mpihelp_divrem(rp + msize, 0, rp, rsize, mp, msize);
+	    rsize = msize;
+	}
+
+	/* Remove any leading zero words from the result.  */
+	if( mod_shift_cnt )
+	    mpihelp_rshift( rp, rp, rsize, mod_shift_cnt);
+	MPN_NORMALIZE (rp, rsize);
+    }
+
+    if( negative_result && rsize ) {
+	if( mod_shift_cnt )
+	    mpihelp_rshift( mp, mp, msize, mod_shift_cnt);
+	mpihelp_sub( rp, mp, msize, rp, rsize);
+	rsize = msize;
+	rsign = msign;
+	MPN_NORMALIZE(rp, rsize);
+    }
+    res->nlimbs = rsize;
+    res->sign = rsign;
+
+  leave:
+    if( assign_rp ) mpi_assign_limb_space( res, rp, size );
+    if( mp_marker ) mpi_free_limb_space( mp_marker );
+    if( bp_marker ) mpi_free_limb_space( bp_marker );
+    if( ep_marker ) mpi_free_limb_space( ep_marker );
+    if( xp_marker ) mpi_free_limb_space( xp_marker );
+}
+