A substrate having a coding pattern disposed on a surface thereof. The coding pattern comprises a plurality of contiguous tags. Each tag comprises: a plurality of data symbols, including a plurality of first data symbols; a plurality of data elements; and one or more flags. The flags indicated either that: the first data symbols define first data contained in the tag; or the first data symbols define a fragment of second data. The second data is embedded in a block of said tags and is typically a public-key digital signature. The first data is typically a secret-key digital signature.