Fig. 2.

The splitting mechanism for privacy preservation. Vertical-SecureBoost guarantees the privacy and security in the process when multiple parties jointly build the tree model. When the guest party figures out the best split feature, it will notify the party that holds the feature, denoted as host. Then, the host will search for its threshold value, split the local model, and get the left children and right children. After splitting the local model, the host will transfer its party id and the sample space in the left children node to the guest party, since the sample space in the right children can be inferred from the left children. The guest party then records the party id in the current node and splits the local tree model. Then, the guest party will send party id and the sample space in the left children node to the remaining party. In this way, although all the parties share the same tree model, the recorded information of each node of each party’s tree model may be different. Each party can only have the authority to see its own data information.