A method of transferring transaction data in a payment processing system, the payment processing system comprising a mobile telephone arranged to send transaction data to a plurality of issuing bank computing systems (12A,12B) via a payment network using a first protocol, wherein the payment network comprises i) a plurality of payment service providers (41A,41B), ii) a plurality of acquiring bank data computing systems (42A,42B), and a card system data processing system, and wherein the first protocol defining a plurality of fields and allowing the transfer of data of a first type, and wherein at least one of the issuing bank computing systems (12A,12B) further requires data of a second type, not included in the data sent according to the first protocol, the method comprising: transferring first data to a plurality of the issuing bank computing systems (12A,12B) using the first protocol, the first data being of the first type and comprising data corresponding to at least one of the fields defined by the first protocol, wherein the first data is transferred to the plurality of issuing bank computing systems (12A,12B) via one of the plurality of payment service providers (41A,41B) and one of the plurality of acquiring bank data processing systems (42A,42B) supporting the first protocol to the card system data processing system and from the card system data processing system to the plurality of issuing banks (12A,12B), wherein the transferring comprises providing the first data to a merchant using near-field communications; storing, by the mobile telephone, second data in a memory, the second data being of the second type and comprising at least some data which does not correspond to the fields defined by the first protocol; and responsive to a request from an issuing bank computing system (12A,12B), providing, by the mobile telephone, the second data to the issuing bank computing system (12A,12B) requiring data of the second type via a mobile communications network, wherein at least one of the acquiring bank data computing systems (42A,42B) does not support a protocol capable of transferring data of the second type, wherein the first data comprises a plurality of first data items, and the second data comprises a plurality of second data items associated with corresponding first data items, the method comprising generating at least a part of a second data item using at least a part of a corresponding first data item as an input to a data processing algorithm which computes a hash function of the corresponding data item and digitally signs the output of the hash function. transferring first data to a plurality of the issuing bank computing systems (12A,12B) using the first protocol, the first data being of the first type and comprising data corresponding to at least one of the fields defined by the first protocol, wherein the first data is transferred to the plurality of issuing bank computing systems (12A,12B) via one of the plurality of payment service providers (41A,41B) and one of the plurality of acquiring bank data processing systems (42A,42B) supporting the first protocol to the card system data processing system and from the card system data processing system to the plurality of issuing banks (12A,12B), wherein the transferring comprises providing the first data to a merchant using near-field communications; storing, by the mobile telephone, second data in a memory, the second data being of the second type and comprising at least some data which does not correspond to the fields defined by the first protocol; and responsive to a request from an issuing bank computing system (12A,12B), providing, by the mobile telephone, the second data to the issuing bank computing system (12A,12B) requiring data of the second type via a mobile communications network, wherein at least one of the acquiring bank data computing systems (42A,42B) does not support a protocol capable of transferring data of the second type, wherein the first data comprises a plurality of first data items, and the second data comprises a plurality of second data items associated with corresponding first data items, the method comprising generating at least a part of a second data item using at least a part of a corresponding first data item as an input to a data processing algorithm which computes a hash function of the corresponding data item and digitally signs the output of the hash function.