Add libmp.

diff --git a/src/libmp/port/mpinvert.c b/src/libmp/port/mpinvert.c
new file mode 100644
index 0000000..ee26307
--- /dev/null
+++ b/src/libmp/port/mpinvert.c

@@ -0,0 +1,21 @@
+#include "os.h"
+#include <mp.h>
+
+#define iseven(a)	(((a)->p[0] & 1) == 0)
+
+// use extended gcd to find the multiplicative inverse
+// res = b**-1 mod m
+void
+mpinvert(mpint *b, mpint *m, mpint *res)
+{
+	mpint *dc1, *dc2;	// don't care
+
+	dc1 = mpnew(0);
+	dc2 = mpnew(0);
+	mpextendedgcd(b, m, dc1, res, dc2);
+	if(mpcmp(dc1, mpone) != 0)
+		abort();
+	mpmod(res, m, res);
+	mpfree(dc1);
+	mpfree(dc2);
+}