lib/mpi: Extend the MPI library

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

diff --git a/lib/mpi/mpi-inv.c b/lib/mpi/mpi-inv.c
new file mode 100644 (file)
index 0000000..61e37d1
--- /dev/null
+++ b/lib/mpi/mpi-inv.c
@@ -0,0 +1,143 @@
+/* mpi-inv.c  -  MPI functions
+ *     Copyright (C) 1998, 2001, 2002, 2003 Free Software Foundation, Inc.
+ *
+ * This file is part of Libgcrypt.
+ *
+ * Libgcrypt is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as
+ * published by the Free Software Foundation; either version 2.1 of
+ * the License, or (at your option) any later version.
+ *
+ * Libgcrypt 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 Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this program; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "mpi-internal.h"
+
+/****************
+ * Calculate the multiplicative inverse X of A mod N
+ * That is: Find the solution x for
+ *             1 = (a*x) mod n
+ */
+int mpi_invm(MPI x, MPI a, MPI n)
+{
+       /* Extended Euclid's algorithm (See TAOCP Vol II, 4.5.2, Alg X)
+        * modified according to Michael Penk's solution for Exercise 35
+        * with further enhancement
+        */
+       MPI u, v, u1, u2 = NULL, u3, v1, v2 = NULL, v3, t1, t2 = NULL, t3;
+       unsigned int k;
+       int sign;
+       int odd;
+
+       if (!mpi_cmp_ui(a, 0))
+               return 0; /* Inverse does not exists.  */
+       if (!mpi_cmp_ui(n, 1))
+               return 0; /* Inverse does not exists.  */
+
+       u = mpi_copy(a);
+       v = mpi_copy(n);
+
+       for (k = 0; !mpi_test_bit(u, 0) && !mpi_test_bit(v, 0); k++) {
+               mpi_rshift(u, u, 1);
+               mpi_rshift(v, v, 1);
+       }
+       odd = mpi_test_bit(v, 0);
+
+       u1 = mpi_alloc_set_ui(1);
+       if (!odd)
+               u2 = mpi_alloc_set_ui(0);
+       u3 = mpi_copy(u);
+       v1 = mpi_copy(v);
+       if (!odd) {
+               v2 = mpi_alloc(mpi_get_nlimbs(u));
+               mpi_sub(v2, u1, u); /* U is used as const 1 */
+       }
+       v3 = mpi_copy(v);
+       if (mpi_test_bit(u, 0)) { /* u is odd */
+               t1 = mpi_alloc_set_ui(0);
+               if (!odd) {
+                       t2 = mpi_alloc_set_ui(1);
+                       t2->sign = 1;
+               }
+               t3 = mpi_copy(v);
+               t3->sign = !t3->sign;
+               goto Y4;
+       } else {
+               t1 = mpi_alloc_set_ui(1);
+               if (!odd)
+                       t2 = mpi_alloc_set_ui(0);
+               t3 = mpi_copy(u);
+       }
+
+       do {
+               do {
+                       if (!odd) {
+                               if (mpi_test_bit(t1, 0) || mpi_test_bit(t2, 0)) {
+                                       /* one is odd */
+                                       mpi_add(t1, t1, v);
+                                       mpi_sub(t2, t2, u);
+                               }
+                               mpi_rshift(t1, t1, 1);
+                               mpi_rshift(t2, t2, 1);
+                               mpi_rshift(t3, t3, 1);
+                       } else {
+                               if (mpi_test_bit(t1, 0))
+                                       mpi_add(t1, t1, v);
+                               mpi_rshift(t1, t1, 1);
+                               mpi_rshift(t3, t3, 1);
+                       }
+Y4:
+                       ;
+               } while (!mpi_test_bit(t3, 0)); /* while t3 is even */
+
+               if (!t3->sign) {
+                       mpi_set(u1, t1);
+                       if (!odd)
+                               mpi_set(u2, t2);
+                       mpi_set(u3, t3);
+               } else {
+                       mpi_sub(v1, v, t1);
+                       sign = u->sign; u->sign = !u->sign;
+                       if (!odd)
+                               mpi_sub(v2, u, t2);
+                       u->sign = sign;
+                       sign = t3->sign; t3->sign = !t3->sign;
+                       mpi_set(v3, t3);
+                       t3->sign = sign;
+               }
+               mpi_sub(t1, u1, v1);
+               if (!odd)
+                       mpi_sub(t2, u2, v2);
+               mpi_sub(t3, u3, v3);
+               if (t1->sign) {
+                       mpi_add(t1, t1, v);
+                       if (!odd)
+                               mpi_sub(t2, t2, u);
+               }
+       } while (mpi_cmp_ui(t3, 0)); /* while t3 != 0 */
+       /* mpi_lshift( u3, k ); */
+       mpi_set(x, u1);
+
+       mpi_free(u1);
+       mpi_free(v1);
+       mpi_free(t1);
+       if (!odd) {
+               mpi_free(u2);
+               mpi_free(v2);
+               mpi_free(t2);
+       }
+       mpi_free(u3);
+       mpi_free(v3);
+       mpi_free(t3);
+
+       mpi_free(u);
+       mpi_free(v);
+       return 1;
+}
+EXPORT_SYMBOL_GPL(mpi_invm);