Every t-shirt has a unique QR code that can be used to verify its authenticity.
The QR code points to a URL on this website that will display some details of the t-shirt, but in case this website ever goes down it’s still possible to perform the verification steps manually:
The URL is in the form https://btcpricetshirt.com/v/<id>/<size>/<amount>/<signature>
The signature is an hex-encoded bitcoin signature made from the following address:
19QLmknNw9gR5Rgemjmm57DcJLG1X4X7yTThe signature covers the ASCII data “<id>:<size>:<amount>“.
For instance, it can be verified manually with:
bitcoin-cli verifymessage 19QLmknNw9gR5Rgemjmm57DcJLG1X4X7yT "$(echo -n "<hex sig>" | xxd -r -p | base64 -w0)" "ABCDEF:M:50000"