Methods, systems, and apparatus, including computer programs encoded on computer storage media, for retrieval of data from external data sources for processing within a blockchain network. One of the methods includes receiving a request for data from a data source, the request including plaintext data and encrypted data, the encrypted data including access data and a hash of the plaintext data; transmitting the request to one relay system component external to the blockchain network; receiving a result from the relay system component that is digitally signed using a private key of the relay system component; verifying an integrity of the result based on a public key of the relay system component; and transmitting the result to a user computing device in response to verifying the integrity of the result.