Swoosh drives the merchant API behind Swish, the Swedish mobile payment service: create a payment request, poll it, cancel it, and verify the callback Swish posts when it settles. It resolves the mutual-TLS certificate your bank issues and verifies Swish in return, and carries the m-commerce token that app-switch URLs and QR codes are built from. Rails applications get a railtie and a controller concern; plain Ruby, Sinatra and Hanami use the same API without them. No runtime dependencies.