Implementations of the present specification include storing a data item in a consortium blockchain; generating a first data digest based on the stored data item; sending the first data digest to verification nodes to cryptographically signs it and stores the signed first data digest in a public blockchain; receiving a request to retrieve the stored data item; retrieving the requested data item from the consortium blockchain; generating a second data digest based on the retrieved data item; sending the second data digest to verification nodes so that each verification node signs the second data digest; receiving the signed second data digests from the plurality of verification nodes; retrieving the signed first data digests from the public blockchain; determining that the signed first data digests match the signed second data digests; and sending a response indicating the stored data item is unchanged to the request to retrieve the stored data item.