diff --git a/lib/mpi/mpi-inv.c b/lib/mpi/mpi-inv.c
new file mode 100644
index 0000000..0951f98
--- /dev/null
+++ b/lib/mpi/mpi-inv.c
@@ -0,0 +1,187 @@
+/* mpi-inv.c  -  MPI functions
+ * Copyright (C) 1998, 1999, 2000, 2001 Free Software Foundation, Inc.
+ *
+ * This file is part of GnuPG.
+ *
+ * GnuPG is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * GnuPG 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 General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
+ */
+
+#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, const MPI a, const MPI n)
+{
+	/* Extended Euclid's algorithm (See TAOPC Vol II, 4.5.2, Alg X)
+	 * modified according to Michael Penk's solution for Exercice 35
+	 * with further enhancement */
+	MPI u = NULL, v = NULL;
+	MPI u1 = NULL, u2 = NULL, u3 = NULL;
+	MPI v1 = NULL, v2 = NULL, v3 = NULL;
+	MPI t1 = NULL, t2 = NULL, t3 = NULL;
+	unsigned k;
+	int sign;
+	int odd = 0;
+	int rc = -ENOMEM;
+
+	if (mpi_copy(&u, a) < 0)
+		goto cleanup;
+	if (mpi_copy(&v, n) < 0)
+		goto cleanup;
+
+	for (k = 0; !mpi_test_bit(u, 0) && !mpi_test_bit(v, 0); k++) {
+		if (mpi_rshift(u, u, 1) < 0)
+			goto cleanup;
+		if (mpi_rshift(v, v, 1) < 0)
+			goto cleanup;
+	}
+	odd = mpi_test_bit(v, 0);
+
+	u1 = mpi_alloc_set_ui(1);
+	if (!u1)
+		goto cleanup;
+	if (!odd) {
+		u2 = mpi_alloc_set_ui(0);
+		if (!u2)
+			goto cleanup;
+	}
+	if (mpi_copy(&u3, u) < 0)
+		goto cleanup;
+	if (mpi_copy(&v1, v) < 0)
+		goto cleanup;
+	if (!odd) {
+		v2 = mpi_alloc(mpi_get_nlimbs(u));
+		if (!v2)
+			goto cleanup;
+		if (mpi_sub(v2, u1, u) < 0)
+			goto cleanup;	/* U is used as const 1 */
+	}
+	if (mpi_copy(&v3, v) < 0)
+		goto cleanup;
+	if (mpi_test_bit(u, 0)) {	/* u is odd */
+		t1 = mpi_alloc_set_ui(0);
+		if (!t1)
+			goto cleanup;
+		if (!odd) {
+			t2 = mpi_alloc_set_ui(1);
+			if (!t2)
+				goto cleanup;
+			t2->sign = 1;
+		}
+		if (mpi_copy(&t3, v) < 0)
+			goto cleanup;
+		t3->sign = !t3->sign;
+		goto Y4;
+	} else {
+		t1 = mpi_alloc_set_ui(1);
+		if (!t1)
+			goto cleanup;
+		if (!odd) {
+			t2 = mpi_alloc_set_ui(0);
+			if (!t2)
+				goto cleanup;
+		}
+		if (mpi_copy(&t3, u) < 0)
+			goto cleanup;
+	}
+	do {
+		do {
+			if (!odd) {
+				if (mpi_test_bit(t1, 0) || mpi_test_bit(t2, 0)) {	/* one is odd */
+					if (mpi_add(t1, t1, v) < 0)
+						goto cleanup;
+					if (mpi_sub(t2, t2, u) < 0)
+						goto cleanup;
+				}
+				if (mpi_rshift(t1, t1, 1) < 0)
+					goto cleanup;
+				if (mpi_rshift(t2, t2, 1) < 0)
+					goto cleanup;
+				if (mpi_rshift(t3, t3, 1) < 0)
+					goto cleanup;
+			} else {
+				if (mpi_test_bit(t1, 0))
+					if (mpi_add(t1, t1, v) < 0)
+						goto cleanup;
+				if (mpi_rshift(t1, t1, 1) < 0)
+					goto cleanup;
+				if (mpi_rshift(t3, t3, 1) < 0)
+					goto cleanup;
+			}
+Y4:
+			;
+		} while (!mpi_test_bit(t3, 0));	/* while t3 is even */
+
+		if (!t3->sign) {
+			if (mpi_set(u1, t1) < 0)
+				goto cleanup;
+			if (!odd)
+				if (mpi_set(u2, t2) < 0)
+					goto cleanup;
+			if (mpi_set(u3, t3) < 0)
+				goto cleanup;
+		} else {
+			if (mpi_sub(v1, v, t1) < 0)
+				goto cleanup;
+			sign = u->sign;
+			u->sign = !u->sign;
+			if (!odd)
+				if (mpi_sub(v2, u, t2) < 0)
+					goto cleanup;
+			u->sign = sign;
+			sign = t3->sign;
+			t3->sign = !t3->sign;
+			if (mpi_set(v3, t3) < 0)
+				goto cleanup;
+			t3->sign = sign;
+		}
+		if (mpi_sub(t1, u1, v1) < 0)
+			goto cleanup;
+		if (!odd)
+			if (mpi_sub(t2, u2, v2) < 0)
+				goto cleanup;
+		if (mpi_sub(t3, u3, v3) < 0)
+			goto cleanup;
+		if (t1->sign) {
+			if (mpi_add(t1, t1, v) < 0)
+				goto cleanup;
+			if (!odd)
+				if (mpi_sub(t2, t2, u) < 0)
+					goto cleanup;
+		}
+	} while (mpi_cmp_ui(t3, 0));	/* while t3 != 0 */
+	/* mpi_lshift( u3, k ); */
+	rc = mpi_set(x, u1);
+
+cleanup:
+	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 rc;
+}