Put 4 litres (140 fl oz/16 cups) of water in a large saucepan and bring to the boil. Add the salt, ginger, rosemary and pork skin, reduce to a simmer and poach for 1–1½ hours until the skin is soft enough to be pierced easily with the tip of a knife. Remove the skin from the water and leave to rest for 10 minutes or until cool enough to handle.
Using a sharp knife, start to remove the fat from the loin. Then, using a spoon, scrape the remaining fat away to leave a clean skin. Rinse the skin under cold running water, pat dry with paper towel, then cut into 3 pieces. Lay the pork skin on a dehydrator tray in a food dehydrator and dehydrate at 55°C (130°F) for 36–48 hours, until completely dry.
Break the pork skin into small pieces. Pulse a little of the dried pork skin in a blender to make roughly even-sized pieces, then transfer into a container. Repeat with the remaining skin.
Line a sieve with filter paper and place another sieve on top. Place the sieves over a saucepan large enough to hold the oil.
Pour the vegetable oil into a deep heavy-based saucepan, no more than one-third full, then heat to 170°C (325°F). Carefully add about 20 g (¾ oz) of the pork skin and fry until the skin puffs up. Pour the oil through the sieves to strain out the puffs and then tip the puffs onto a tray lined with paper towel to absorb the excess oil. Season well with the nori salt. Repeat with the remaining pork. Keep warm until required.