Cleaves the sulfated polysaccharide porphyran at the (1->4) linkages between beta-D-galactopyranose and alpha-L-galactopyranose-6-sulfate, forming mostly the disaccharide alpha-L-galactopyranose-6-sulfate-(1->3)-beta-D-galactose. Some longer oligosaccharides of even number of residues are also observed. Inactive on the non-sulfated agarose portion of the porphyran backbone. Displays a strict requirement for C6-sulfate in the -2 and +1-binding subsites. Hydrolysis of beta-D-galactopyranose-(1->4)-alpha-L-galactopyranose-6-sulfate linkages in porphyran. Belongs to the glycosyl hydrolase 16 family.