A decoder of LDPC codewords using the iterative belief propagation algorithm stores a posteriori information on variables. An updating device updates the a posteriori information on variables, and a first computation device computes variables to constrain messages from a posteriori information on variables and variable to constraint messages from previous iteration. A second computation device computes a constraint to variable messages from variable to constraint messages computed by the first computation device. A further computation device updates the a posteriori information on variables. A shuffle device transfers the a posteriori information on variables to the first computation device, and a further shuffle device transfers information from the second computation device to the storing device. The decoder further includes a device for compression-storage-decompression of the constraint to variable messages. The disclosure also relates to a corresponding method, computer program and system.