lib/mpi: Extend the MPI library

[linux-2.6-block.git] / lib / mpi / mpi-mod.c

diff --git a/lib/mpi/mpi-mod.c b/lib/mpi/mpi-mod.c
new file mode 100644 (file)
index 0000000..47bc59e
--- /dev/null
+++ b/lib/mpi/mpi-mod.c
@@ -0,0 +1,155 @@
+/* mpi-mod.c -  Modular reduction
+ * Copyright (C) 1998, 1999, 2001, 2002, 2003,
+ *               2007  Free Software Foundation, Inc.
+ *
+ * This file is part of Libgcrypt.
+ */
+
+
+#include "mpi-internal.h"
+#include "longlong.h"
+
+/* Context used with Barrett reduction.  */
+struct barrett_ctx_s {
+       MPI m;   /* The modulus - may not be modified. */
+       int m_copied;   /* If true, M needs to be released.  */
+       int k;
+       MPI y;
+       MPI r1;  /* Helper MPI. */
+       MPI r2;  /* Helper MPI. */
+       MPI r3;  /* Helper MPI allocated on demand. */
+};
+
+
+
+void mpi_mod(MPI rem, MPI dividend, MPI divisor)
+{
+       mpi_fdiv_r(rem, dividend, divisor);
+}
+
+/* This function returns a new context for Barrett based operations on
+ * the modulus M.  This context needs to be released using
+ * _gcry_mpi_barrett_free.  If COPY is true M will be transferred to
+ * the context and the user may change M.  If COPY is false, M may not
+ * be changed until gcry_mpi_barrett_free has been called.
+ */
+mpi_barrett_t mpi_barrett_init(MPI m, int copy)
+{
+       mpi_barrett_t ctx;
+       MPI tmp;
+
+       mpi_normalize(m);
+       ctx = kcalloc(1, sizeof(*ctx), GFP_KERNEL);
+
+       if (copy) {
+               ctx->m = mpi_copy(m);
+               ctx->m_copied = 1;
+       } else
+               ctx->m = m;
+
+       ctx->k = mpi_get_nlimbs(m);
+       tmp = mpi_alloc(ctx->k + 1);
+
+       /* Barrett precalculation: y = floor(b^(2k) / m). */
+       mpi_set_ui(tmp, 1);
+       mpi_lshift_limbs(tmp, 2 * ctx->k);
+       mpi_fdiv_q(tmp, tmp, m);
+
+       ctx->y  = tmp;
+       ctx->r1 = mpi_alloc(2 * ctx->k + 1);
+       ctx->r2 = mpi_alloc(2 * ctx->k + 1);
+
+       return ctx;
+}
+
+void mpi_barrett_free(mpi_barrett_t ctx)
+{
+       if (ctx) {
+               mpi_free(ctx->y);
+               mpi_free(ctx->r1);
+               mpi_free(ctx->r2);
+               if (ctx->r3)
+                       mpi_free(ctx->r3);
+               if (ctx->m_copied)
+                       mpi_free(ctx->m);
+               kfree(ctx);
+       }
+}
+
+
+/* R = X mod M
+ *
+ * Using Barrett reduction.  Before using this function
+ * _gcry_mpi_barrett_init must have been called to do the
+ * precalculations.  CTX is the context created by this precalculation
+ * and also conveys M.  If the Barret reduction could no be done a
+ * straightforward reduction method is used.
+ *
+ * We assume that these conditions are met:
+ * Input:  x =(x_2k-1 ...x_0)_b
+ *     m =(m_k-1 ....m_0)_b      with m_k-1 != 0
+ * Output: r = x mod m
+ */
+void mpi_mod_barrett(MPI r, MPI x, mpi_barrett_t ctx)
+{
+       MPI m = ctx->m;
+       int k = ctx->k;
+       MPI y = ctx->y;
+       MPI r1 = ctx->r1;
+       MPI r2 = ctx->r2;
+       int sign;
+
+       mpi_normalize(x);
+       if (mpi_get_nlimbs(x) > 2*k) {
+               mpi_mod(r, x, m);
+               return;
+       }
+
+       sign = x->sign;
+       x->sign = 0;
+
+       /* 1. q1 = floor( x / b^k-1)
+        *    q2 = q1 * y
+        *    q3 = floor( q2 / b^k+1 )
+        * Actually, we don't need qx, we can work direct on r2
+        */
+       mpi_set(r2, x);
+       mpi_rshift_limbs(r2, k-1);
+       mpi_mul(r2, r2, y);
+       mpi_rshift_limbs(r2, k+1);
+
+       /* 2. r1 = x mod b^k+1
+        *      r2 = q3 * m mod b^k+1
+        *      r  = r1 - r2
+        * 3. if r < 0 then  r = r + b^k+1
+        */
+       mpi_set(r1, x);
+       if (r1->nlimbs > k+1) /* Quick modulo operation.  */
+               r1->nlimbs = k+1;
+       mpi_mul(r2, r2, m);
+       if (r2->nlimbs > k+1) /* Quick modulo operation. */
+               r2->nlimbs = k+1;
+       mpi_sub(r, r1, r2);
+
+       if (mpi_has_sign(r)) {
+               if (!ctx->r3) {
+                       ctx->r3 = mpi_alloc(k + 2);
+                       mpi_set_ui(ctx->r3, 1);
+                       mpi_lshift_limbs(ctx->r3, k + 1);
+               }
+               mpi_add(r, r, ctx->r3);
+       }
+
+       /* 4. while r >= m do r = r - m */
+       while (mpi_cmp(r, m) >= 0)
+               mpi_sub(r, r, m);
+
+       x->sign = sign;
+}
+
+
+void mpi_mul_barrett(MPI w, MPI u, MPI v, mpi_barrett_t ctx)
+{
+       mpi_mul(w, u, v);
+       mpi_mod_barrett(w, w, ctx);
+}