Assertions to prevent us from doing anything we can't support

src/gadget.tcc

@@ -7,6 +7,7 @@ sodoku_encryption_key<FieldT>::sodoku_encryption_key(protoboard<FieldT> &pb,
 {
     assert(seed_key.size() == (256-8));
     unsigned int num_key_digests = div_ceil(dimension * dimension * 8, 256);
+    assert(num_key_digests < 256); // after this it will cycle
 
     padding_var.reset(new digest_variable<FieldT>(pb, 256, "padding"));
 
@@ -163,6 +164,8 @@ sodoku_gadget<FieldT>::sodoku_gadget(protoboard<FieldT> &pb, unsigned int n) :
 {
     dimension = n * n;
 
+    assert(dimension < 256); // any more will overflow the 8 bit storage
+
     const size_t input_size_in_bits = dimension * dimension * 8;
     {
         const size_t input_size_in_field_elements = div_ceil(input_size_in_bits, FieldT::capacity());