[Paper Review] Multiparty Routing: Secure Routing for Mixnets

Fatemeh Shirazi, Elena Andreeva+2 more|Lirias (KU Leuven)|Aug 10, 2017

Internet Traffic Analysis and Secure E-voting25 references3 citations

TL;DR

This paper proposes multiparty routing (MPR), a novel secure routing protocol for mixnets that decentralizes routing decisions using multiparty computation to prevent route capture attacks while maintaining scalability. By distributing routing decisions and verifying correctness via cryptographic commitments and verifiable shuffles, MPR ensures routing integrity, load balancing, and resistance to global active adversaries, offering a scalable and secure alternative to traditional source or hop-by-hop routing.

ABSTRACT

Anonymous communication networks are important building blocks for online privacy protection. One approach to achieve anonymity is to relay messages through multiple routers, where each router shuffles messages independently. To achieve anonymity, at least one router needs to be honest. In the presence of an adversary that is controlling a subset of the routers unbiased routing is important for guaranteeing anonymity. However, the routing strategy also influenced other factors such as the scalability and the performance of the system. One solution is to use a fixed route for relaying all messages with many routers. If the route is not fixed the routing decision can either be made by the communication initiator or the intermediate routers. However, the existing routing types each have limitations. For example, one faces scalability issues when increasing the throughput of systems with fixed routes. Moreover, when the routing decision is left to the initiator, the initiator needs to maintain an up-to-date view of the system at all times, which also does not scale. If the routing decision is left to intermediate routers the routing of the communication can be influenced by an adversary. In this work, we propose a novel multiparty routing approach for anonymous communication that addresses these shortcomings. We distribute the routing decision and verify the correctness of routing to achieve routing integrity. More concretely, we provide a mixnet design that uses our routing approach and that in addition, addresses load balancing. We show that our system is secure against a global active adversary.

Motivation & Objective

• To address the scalability and security limitations of existing anonymous communication routing models, including fixed cascades, source routing, and hop-by-hop routing.
• To eliminate the need for communication initiators to maintain a complete view of the network, improving system scalability.
• To prevent route capture attacks common in hop-by-hop routing by decentralizing routing decisions using cryptographic multiparty computation.
• To ensure routing integrity and correctness through verifiable commitments and cryptographic verification of shuffle outputs.
• To integrate load balancing into the routing protocol by assigning ciphertexts proportionally based on relay throughput.

Proposed method

• The protocol uses a joint random number $Rand$ generated via multiparty computation to deterministically permute ciphertext indices using a cryptographic hash function $h$, avoiding collisions through iterative rehashing.
• A mapping function $Map$ assigns permuted ciphertexts to relays in the next layer based on their relative throughput, ensuring load balancing across the network.
• Each relay commits to its input and output ciphertexts using cryptographic commitments, which are later opened and verified on a public bulletin board.
• The routing verification algorithm checks the correctness of $Rand$ commitments and ensures that each relay's output matches the expected ciphertexts based on the permuted indices and layer throughput.
• The system employs provable shuffles, threshold decryption, and re-randomizable encryption to ensure end-to-end anonymity and verifiability.
• Auditors verify routing integrity by confirming that the sum of individual $Rand$ values equals the global $Rand$, and that each relay correctly signed only the ciphertexts it was assigned.

Experimental results

Research questions

• RQ1 Can a routing protocol be designed that avoids the scalability bottleneck of source routing while preventing route capture attacks inherent in hop-by-hop routing?
• RQ2 How can routing decisions be decentralized such that no single entity controls the path, yet routing integrity is cryptographically verifiable?
• RQ3 Can load balancing be natively integrated into the routing protocol without compromising anonymity or security?
• RQ4 What cryptographic mechanisms are required to ensure that both shuffling and routing are correct and publicly verifiable?
• RQ5 How can the system maintain resilience and performance under adversarial conditions, including active attacks and relay failures?

Key findings

• The proposed multiparty routing protocol achieves routing integrity by cryptographically binding routing decisions to a jointly generated random value $Rand$, which is publicly verifiable.
• The system ensures load balancing by assigning ciphertexts to relays in proportion to their throughput, using a throughput-weighted mapping function.
• The protocol prevents route capture attacks by decentralizing routing decisions and requiring cryptographic verification of all routing assignments.
• Routing verification is performed by auditors using a Boolean function that checks commitment correctness and ensures that each relay only signs its assigned ciphertexts.
• The system is secure against a global active adversary, as proven through the use of verifiable shuffles, threshold decryption, and commitment schemes.
• The protocol reduces initiator overhead by requiring only a small number of known relays, significantly improving scalability compared to source routing.