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lib/mpi/mpi-pow.c
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// SPDX-License-Identifier: GPL-2.0-or-later |
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/* mpi-pow.c - MPI functions * Copyright (C) 1994, 1996, 1998, 2000 Free Software Foundation, Inc. * * This file is part of GnuPG. * |
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* Note: This code is heavily based on the GNU MP Library. * Actually it's the same code with only minor changes in the * way the data is stored; this is to support the abstraction * of an optional secure memory allocation which may be used * to avoid revealing of sensitive data due to paging etc. * The GNU MP Library itself is published under the LGPL; * however I decided to publish this code under the plain GPL. */ |
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#include <linux/sched.h> |
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#include <linux/string.h> #include "mpi-internal.h" #include "longlong.h" /**************** * RES = BASE ^ EXP mod MOD */ int mpi_powm(MPI res, MPI base, MPI exp, MPI mod) { mpi_ptr_t mp_marker = NULL, bp_marker = NULL, ep_marker = NULL; |
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struct karatsuba_ctx karactx = {}; |
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mpi_ptr_t xp_marker = NULL; mpi_ptr_t tspace = NULL; mpi_ptr_t rp, ep, mp, bp; mpi_size_t esize, msize, bsize, rsize; |
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int msign, bsign, rsign; |
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mpi_size_t size; int mod_shift_cnt; int negative_result; int assign_rp = 0; mpi_size_t tsize = 0; /* to avoid compiler warning */ /* fixme: we should check that the warning is void */ int rc = -ENOMEM; esize = exp->nlimbs; msize = mod->nlimbs; size = 2 * msize; |
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msign = mod->sign; rp = res->d; ep = exp->d; if (!msize) |
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return -EINVAL; |
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if (!esize) { /* Exponent is zero, result is 1 mod MOD, i.e., 1 or 0 * depending on if MOD equals 1. */ |
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res->nlimbs = (msize == 1 && mod->d[0] == 1) ? 0 : 1; |
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if (res->nlimbs) { if (mpi_resize(res, 1) < 0) goto enomem; rp = res->d; rp[0] = 1; } |
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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); if (!mp) goto enomem; |
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mod_shift_cnt = count_leading_zeros(mod->d[msize - 1]); |
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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); if (!bp) goto enomem; 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); if (!rp) goto enomem; assign_rp = 1; } else { if (mpi_resize(res, size) < 0) goto enomem; 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. */ BUG_ON(bp_marker); bp = bp_marker = mpi_alloc_limb_space(bsize); if (!bp) goto enomem; 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); if (!ep) goto enomem; MPN_COPY(ep, rp, esize); } if (rp == mp) { /* RES and MOD are identical. Allocate temporary space for MOD. */ BUG_ON(mp_marker); mp = mp_marker = mpi_alloc_limb_space(msize); if (!mp) goto enomem; MPN_COPY(mp, rp, msize); } } MPN_COPY(rp, bp, bsize); rsize = bsize; rsign = bsign; { mpi_size_t i; mpi_ptr_t xp; int c; mpi_limb_t e; mpi_limb_t carry_limb; |
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xp = xp_marker = mpi_alloc_limb_space(2 * (msize + 1)); if (!xp) goto enomem; |
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negative_result = (ep[0] & 1) && base->sign; i = esize - 1; e = ep[i]; |
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c = count_leading_zeros(e); |
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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; /*if (mpihelp_mul_n(xp, rp, rp, rsize) < 0) goto enomem */ if (rsize < KARATSUBA_THRESHOLD) mpih_sqr_n_basecase(xp, rp, rsize); else { if (!tspace) { tsize = 2 * rsize; tspace = mpi_alloc_limb_space(tsize); if (!tspace) goto enomem; } else if (tsize < (2 * rsize)) { mpi_free_limb_space(tspace); tsize = 2 * rsize; tspace = mpi_alloc_limb_space(tsize); if (!tspace) goto enomem; } mpih_sqr_n(xp, rp, rsize, tspace); } 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 ); */ if (bsize < KARATSUBA_THRESHOLD) { mpi_limb_t tmp; if (mpihelp_mul (xp, rp, rsize, bp, bsize, &tmp) < 0) goto enomem; } else { if (mpihelp_mul_karatsuba_case (xp, rp, rsize, bp, bsize, &karactx) < 0) goto enomem; } 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--; |
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cond_resched(); |
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} 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); |
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} 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: rc = 0; enomem: |
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mpihelp_release_karatsuba_ctx(&karactx); |
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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); if (tspace) mpi_free_limb_space(tspace); return rc; } EXPORT_SYMBOL_GPL(mpi_powm); |