The advantage of using a blockchain is that you don't have to rely on hundreds of 3rd party centralised systems (one per product type you want to track). You'r

The advantage of using a blockchain is that you don't have to rely on hundreds of 3rd party centralised systems (one per product type you want to track). You're not just trusting these 3rd parties to not rewrite history but trusting that they maintain their infrastructure indefinitely. With a popular blockchain there are distributed financial incentives to keep the whole system running (and honest) longer than any individual actor on the network. As for how much data the phone needs to store, I was imagining running something equivalent to an SPV client: https://bitcoin.stackexchange.com/questions/4649/what-is-an-spv-client https://bitcoin.stackexchange.com/questions/4649/what-is-an-... https://bitcoin.stackexchange.com/questions/48420/what-hardware-requirements-does-a-spv-client-have https://bitcoin.stackexchange.com/questions/48420/what-hardw... Alternatively the app can request the digitally signed proofs from the blockchain via a semi-trusted 3rd party gateway, with the advantage that you can choose your gateway at any time, and the gateway can't forge the cryptographic proofs that your client is checking for you. For example, if you buy a product from Manufacturer 1 whose web address is manufacturer1.example.com, you could visit that site and find a page with a QR code containing their public key (wallet address), which the app could scan. Once you've marked that public key as trusted, the app could check that the corresponding secret key was used to sign the data on the QR code you received on your certificate of authenticity. The tricky part is sending a transaction to the network which says "I have received the product with the following serial number...", but the QR code on the certificate could contain the secret key for a wallet which has been pre-loaded with just enough funds to send a microtransaction to the network.