Disclosed herein are methods, systems, and apparatus, including computer programs encoded on computer storage media, for retrieving data from external data sources for processing within a blockchain network. One of the methods includes receiving a request for data that includes encrypted data, the encrypted data including access data that is encrypted using a service public key of a key management node; selecting a relay system node from a plurality of relay system nodes that share a service private key of the key management node; transmitting the request to the relay system node; receiving a response provided from the relay system node, the response including result data and a digital signature, wherein the digital signature is generated based on the result data and the service private key of the key management node; and transmitting the response to a client.