X-codes: error control with unknowable inputs

This paper studies a class of codes used for digital system test compaction technique. An X-code produces a hash from an input vector in a way that guarantees that errors in the inputs change the resulting hash despite the presence of unknowable inputs. The value of an unknowable input cannot be inferred. Some classes of X-codes overlap with deterministic combinatorial group testing (CGT) solutions provides constructive bounds.