Purchase transaction system with encrypted payment card data

Systems and methods are provided for securing payment card information. A user may present a payment card such as a credit card to point-of-sale equipment. The point-of-sale equipment may encrypt the payment card information. An encryption algorithm may be used that takes as inputs a first part of the payment card information, a tweak formed by a second part of the payment card information, and an encryption key. The encrypted payment card information may be conveyed to a gateway over a communications network. The gateway may identify which encryption algorithm was used in encrypting the payment card information and may re-encrypt the payment card information using a format preserving encryption algorithm. A network-based service may be used to remotely perform functions for the gateway.