Embodiments of the invention provide a decoder comprising at least one check node processing unit configured to receive at least three variable node messages from one or more variable node processing units and to determine one or more check node messages, wherein the at least one check node processing unit comprises at least two blocks of sub-check nodes, each block of sub-check node being configured to: determine a set of sub-check node syndromes from at least one variable node message among the at least three variable node messages; and determine at least one check node message from at least one syndrome.