# SQRT: A Secure Querying Scheme of Routing Table Based on Oblivious Transfer
## 1 Introduction  
Anonymous networks refer to hiding the users' privacy information such as the network address of communication entities and the communication relationship between entities in the transmission traffic through certain methods, so that attackers cannot directly know or speculate on the communication relationship between the two parties or the identity information or location information of the communication entity [1, 2]. Therefore, the security of routing information is an important factor in ensuring anonymous network security. In the onion routing (TOR) networks, some secure and reliable servers are set as directory servers (DSs), which can provide anonymous routing information to describe the current state of routing nodes. The user equipment (UE) can request and download the address information of routing nodes through HTTPS [3]. In anonymous networks, the identity and address information of routing nodes need to be properly protected from being identified by attackers. As shown in Figure 0(a), the UE in TOR networks queries the DS for the routing node (RN) list [4], and the DS returns \(n\) qualified routing nodes to the UE. The UE randomly selects \(m\) routing nodes from the \(n\) nodes to build a multi-hop transmission path in which \(m\), \(n\in N^{+}\), and \(m\leq n\). As shown in Figure 0(b), in the invisible Internet Project (I2P) networks, the UE queriesthe network database (NetDb) for routing information. NetDb stores and searches the routing information in \(n\) floodfill nodes through the Kad algorithm [5]. NetDb returns the RouterInfo and LeaseSet to the UE, and the UE establishes an outbound tunnel for the user equipment to send data according to the routing information. The outbound tunnel is from the gateway to the endpoint, and the LeaseSet contains the gateway of the recipient's inbound tunnel [6]. After receiving the data, the endpoint of outbound tunnel forwards the data to the gateway of the recipient's inbound tunnel. This routing scheme is also called the garlic routing [7]. The problem with the above two query schemes of the routing table is that the DS or NetDb knows the range of routing nodes that can be selected by the UE; at the same time, the UE also knows the information of other routing nodes in the DS and NetDb other than the routing nodes selected by itself. Take TOR networks as an example: according to the RN request of the UE, the DS feeds back \(n\) routing nodes meeting the requirements of the UE. The UE accordingly selects \(m\) routing nodes from the \(n\) routing nodes, with the result that the address information of _n_-_m_ routing nodes in the DS are leaked to the UE. Oblivious transfer (OT) is an important branch of cryptography research. It helps data owners provide data retrieval and data calculation based on their own data, while not disclosing their own data information to ensure that the data are available and invisible [8; 9]. Figure 1: Existing routing querying schemes of anonymous networks. (**a**) Tor networks and (**b**) I2P networks. This paper proposes a secure querying scheme of the routing table (SQRT) based on oblivious transfer. This scheme makes sure that the user equipment only obtains the routing node information fed back to the user equipment in the directory server without knowing the information of other routing nodes from the routing table in the directory server. The directory server knows neither the specific requirements of the routing nodes submitted by the user equipment nor which part of the node information is fed back to the user equipment so as to achieve a good effect of routing table data privacy protection. At present, there are some oblivious transfer schemes that can be applied to the malicious model. The BLAZE scheme presented in [10] tries to combine the semi-honest model with a consistency test to check the consistency by comparing the hash value of the share in the input stage with that in the output stage, which might lead to high communication traffic of oblivious transfer expansion.