ECDSA signatures need a zero padding for the ASN.1 storage of the R and S values

ctaocrypt/src/integer.c

@@ -179,6 +179,28 @@ mp_count_bits (mp_int * a)
 }
 
 
+int mp_leading_bit (mp_int * a)
+{
+    int bit = 0;
+    mp_int t;
+
+    if (mp_init_copy(&t, a) != MP_OKAY)
+        return 0;
+
+    while (mp_iszero(&t) == 0) {
+#ifndef MP_8BIT
+        bit = (t.dp[0] & 0x80) != 0;
+#else
+        bit = (t.dp[0] | ((t.dp[1] & 0x01) << 7)) & 0x80 != 0;
+#endif
+        if (mp_div_2d (&t, 8, &t, NULL) != MP_OKAY)
+            break;
+    }
+    mp_clear(&t);
+    return bit;
+}
+
+
 /* store in unsigned [big endian] format */
 int mp_to_unsigned_bin (mp_int * a, unsigned char *b)
 {