diff --git a/lib/mpi/mpi-inv.c b/lib/mpi/mpi-inv.c
new file mode 100644
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);