mirror of https://gitee.com/openkylin/linux.git
156 lines
3.3 KiB
C
156 lines
3.3 KiB
C
/* mpi-mod.c - Modular reduction
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* Copyright (C) 1998, 1999, 2001, 2002, 2003,
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* 2007 Free Software Foundation, Inc.
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*
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* This file is part of Libgcrypt.
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*/
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#include "mpi-internal.h"
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#include "longlong.h"
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/* Context used with Barrett reduction. */
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struct barrett_ctx_s {
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MPI m; /* The modulus - may not be modified. */
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int m_copied; /* If true, M needs to be released. */
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int k;
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MPI y;
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MPI r1; /* Helper MPI. */
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MPI r2; /* Helper MPI. */
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MPI r3; /* Helper MPI allocated on demand. */
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};
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void mpi_mod(MPI rem, MPI dividend, MPI divisor)
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{
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mpi_fdiv_r(rem, dividend, divisor);
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}
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/* This function returns a new context for Barrett based operations on
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* the modulus M. This context needs to be released using
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* _gcry_mpi_barrett_free. If COPY is true M will be transferred to
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* the context and the user may change M. If COPY is false, M may not
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* be changed until gcry_mpi_barrett_free has been called.
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*/
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mpi_barrett_t mpi_barrett_init(MPI m, int copy)
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{
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mpi_barrett_t ctx;
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MPI tmp;
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mpi_normalize(m);
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ctx = kcalloc(1, sizeof(*ctx), GFP_KERNEL);
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if (copy) {
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ctx->m = mpi_copy(m);
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ctx->m_copied = 1;
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} else
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ctx->m = m;
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ctx->k = mpi_get_nlimbs(m);
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tmp = mpi_alloc(ctx->k + 1);
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/* Barrett precalculation: y = floor(b^(2k) / m). */
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mpi_set_ui(tmp, 1);
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mpi_lshift_limbs(tmp, 2 * ctx->k);
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mpi_fdiv_q(tmp, tmp, m);
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ctx->y = tmp;
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ctx->r1 = mpi_alloc(2 * ctx->k + 1);
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ctx->r2 = mpi_alloc(2 * ctx->k + 1);
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return ctx;
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}
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void mpi_barrett_free(mpi_barrett_t ctx)
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{
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if (ctx) {
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mpi_free(ctx->y);
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mpi_free(ctx->r1);
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mpi_free(ctx->r2);
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if (ctx->r3)
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mpi_free(ctx->r3);
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if (ctx->m_copied)
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mpi_free(ctx->m);
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kfree(ctx);
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}
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}
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/* R = X mod M
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*
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* Using Barrett reduction. Before using this function
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* _gcry_mpi_barrett_init must have been called to do the
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* precalculations. CTX is the context created by this precalculation
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* and also conveys M. If the Barret reduction could no be done a
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* straightforward reduction method is used.
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*
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* We assume that these conditions are met:
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* Input: x =(x_2k-1 ...x_0)_b
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* m =(m_k-1 ....m_0)_b with m_k-1 != 0
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* Output: r = x mod m
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*/
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void mpi_mod_barrett(MPI r, MPI x, mpi_barrett_t ctx)
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{
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MPI m = ctx->m;
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int k = ctx->k;
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MPI y = ctx->y;
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MPI r1 = ctx->r1;
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MPI r2 = ctx->r2;
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int sign;
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mpi_normalize(x);
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if (mpi_get_nlimbs(x) > 2*k) {
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mpi_mod(r, x, m);
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return;
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}
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sign = x->sign;
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x->sign = 0;
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/* 1. q1 = floor( x / b^k-1)
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* q2 = q1 * y
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* q3 = floor( q2 / b^k+1 )
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* Actually, we don't need qx, we can work direct on r2
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*/
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mpi_set(r2, x);
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mpi_rshift_limbs(r2, k-1);
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mpi_mul(r2, r2, y);
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mpi_rshift_limbs(r2, k+1);
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/* 2. r1 = x mod b^k+1
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* r2 = q3 * m mod b^k+1
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* r = r1 - r2
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* 3. if r < 0 then r = r + b^k+1
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*/
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mpi_set(r1, x);
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if (r1->nlimbs > k+1) /* Quick modulo operation. */
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r1->nlimbs = k+1;
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mpi_mul(r2, r2, m);
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if (r2->nlimbs > k+1) /* Quick modulo operation. */
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r2->nlimbs = k+1;
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mpi_sub(r, r1, r2);
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if (mpi_has_sign(r)) {
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if (!ctx->r3) {
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ctx->r3 = mpi_alloc(k + 2);
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mpi_set_ui(ctx->r3, 1);
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mpi_lshift_limbs(ctx->r3, k + 1);
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}
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mpi_add(r, r, ctx->r3);
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}
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/* 4. while r >= m do r = r - m */
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while (mpi_cmp(r, m) >= 0)
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mpi_sub(r, r, m);
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x->sign = sign;
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}
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void mpi_mul_barrett(MPI w, MPI u, MPI v, mpi_barrett_t ctx)
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{
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mpi_mul(w, u, v);
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mpi_mod_barrett(w, w, ctx);
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}
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