Add libmp.
diff --git a/src/libmp/port/mptobe.c b/src/libmp/port/mptobe.c
new file mode 100644
index 0000000..e08ae56
--- /dev/null
+++ b/src/libmp/port/mptobe.c
@@ -0,0 +1,57 @@
+#include "os.h"
+#include <mp.h>
+#include "dat.h"
+
+// convert an mpint into a big endian byte array (most significant byte first)
+// return number of bytes converted
+// if p == nil, allocate and result array
+int
+mptobe(mpint *b, uchar *p, uint n, uchar **pp)
+{
+ int i, j, suppress;
+ mpdigit x;
+ uchar *e, *s, c;
+
+ if(p == nil){
+ n = (b->top+1)*Dbytes;
+ p = malloc(n);
+ }
+ if(p == nil)
+ return -1;
+ if(pp != nil)
+ *pp = p;
+ memset(p, 0, n);
+
+ // special case 0
+ if(b->top == 0){
+ if(n < 1)
+ return -1;
+ else
+ return 1;
+ }
+
+ s = p;
+ e = s+n;
+ suppress = 1;
+ for(i = b->top-1; i >= 0; i--){
+ x = b->p[i];
+ for(j = Dbits-8; j >= 0; j -= 8){
+ c = x>>j;
+ if(c == 0 && suppress)
+ continue;
+ if(p >= e)
+ return -1;
+ *p++ = c;
+ suppress = 0;
+ }
+ }
+
+ // guarantee at least one byte
+ if(s == p){
+ if(p >= e)
+ return -1;
+ *p++ = 0;
+ }
+
+ return p - s;
+}
