An improved data system permits power efficient mask key write operations. A mask key selector implements criteria-based selection of mask keys for mask key write operations on blocks data. In one embodiment, a first set of mask keys is compared to data bytes of a data block that will be written to memory. The comparison culls keys from the list of candidates that match unmasked data bytes, that is, values that will be written to memory as “changed” data. A mask key is selected from the resulting set of candidates so a memory write operation consumes less power (relative to selection of other keys), or so that the operation minimizes switching noise. The selected mask key is then substituted by a controller into masked data values, and a modified data block is transmitted to memory, with the memory detecting masked data by identifying mask keys in the modified data block.