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The Post-Quantum Signal Protocol : Secure Chat in a Quantum World

Ines Duits

Published Jan 1, 2019• 2 citations
OpenAlex

Abstract

The Signal Protocol provides end-to-end encryption, forward secrecy, backward secrecy, authentication and deniability for chat applications like WhatsApp. The Signal Protocol does this by using the ECDH Curve25519 key exchanges and SHA-512 key derivation. However, the ECDH key exchange is not quantum-safe. A post-quantum Signal Protocol requires a substitute for the ECDH key exchanges. Therefore, we look at post-quantum cryptography, which is secure against a quantum computer. In this thesis we analyse the Signal Protocol and explain our research. We compared 11 different post-quantum algorithms: SIDH and 10 different key encapsulation mechanism, which are in the NIST post-quantum standardisation process: BigQuake, BIKE, Frodo, Crystal-Kyber, Leda, Lima, New Hope, Saber, SIKE and Titanium. We conclude that it is feasible to have different post-quantum Signal Protocols considering the state of 2018 for an average user.

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The Signal Protocol provides end-to-end encryption, forward secrecy, backward secrecy, authentication and deniability for chat applications like WhatsApp. The Signal Protocol does this by using the ECDH Curve25519 key exchanges and SHA-512 key derivation.
However, the ECDH key exchange is not quantum-safe. A post-quantum Signal Protocol requires a substitute for the ECDH key exchanges. Therefore, we look at post-quantum cryptography, which is secure against a quantum computer.
In this thesis we analyse the Signal Protocol and explain our research. We compared 11 different post-quantum algorithms: SIDH and 10 different key encapsulation mechanism, which are in the NIST post-quantum standardisation process: BigQuake, BIKE, Frodo, Crystal-Kyber, Leda, Lima, New Hope, Saber, SIKE and Titanium.
We conclude that it is feasible to have different post-quantum Signal Protocols considering the state of 2018 for an average user.
Because of Shor’s method, it is expected that quantum computers will have the capability to decipher current public key encryption methods. Consequently, there is a need to replace existing public key cryptography with quantum-resistant cryptography, also known as post-quantum cryptography (PQC) algorithms. In the last twenty years, significant progress has been made in the field of PQC research, leading to the creation of various algorithms that are proven to be resistant to quantum attacks. Several organizations responsible for standardization have already selected and are in the process of standardizing specific PQC algorithms.In popular instant messaging and voice-over IP applications like Signal, Facebook Messenger, and WhatsApp, the Signal Protocol provides end-to-end encryption (E2EE) and additional security features such as forward secrecy, future secrecy, authentication, asynchronous communication, and deniability. The protocol utilizes the ECDH Curve25519 key exchange and hash-based key derivation algorithms to accomplish this. However, the ECDH key exchange is vulnerable to attacks from quantum computers, which can compromise the security of contemporary cryptographic techniques by obtaining the corresponding public keys. Therefore, there is a need to replace the ECDH key exchange with a quantum-safe key exchange scheme that is resistant to attacks from quantum computers.In this context, the focus is on investigating post-quantum cryptography (PQC) that can provide robust security against quantum computing threats and proposing an end-to-end encryption scheme.
Quantum computing poses a serious threat to traditional cryptography, with algorithms like RSA and ECC being vulnerable to attacks such as Shor’s algorithm. To address this, we propose a Post-Quantum Secure Chat Application using lattice-based cryptography, specifically CRYSTALS-Kyber for key exchange and CRYSTALS-Dilithium for digital signatures. The system provides end-to-end encryption with forward secrecy, ensuring message confidentiality, authenticity, and integrity. Mathematical formulations of key encapsulation, encryption, and signature verification are presented, alongside performance benchmarks. Experimental results show that the application achieves practical latency and throughput on real devices while resisting quantum attacks. This work demonstrates that integrating post-quantum algorithms into messaging platforms is both feasible and essential for secure future communication.
The Signal protocol provides end-to-end security even when service providers are only partially trusted; however, it still lacks deniable authentication and does not resist quantum attacks in the messaging phase. This paper therefore presents a post-quantum enhancement of Signal that is efficient in both communication cost and end-to-end message latency. In the key-agreement phase, a lattice-based Gandalf ring signature is combined with the Kyber-1024 KEM to construct a pre-key mechanism that provides both post-quantum security and deniable authentication. In the messaging phase, a pair of post-quantum ratchets is layered on top of the classical Double Ratchet to form a classical-post-quantum hybrid ratchet, while the Skye key-derivation function and erasure-code-based block encapsulation reduce communication overhead. Experimental and analytical results show that the scheme preserves Signal’s end-to- end, forward, and post-compromise security, achieves practical post-quantum security and deniable authentication, and keeps the additional cryptographic delay negligible compared with typical network latency.
Quantum computing is a potentially significant threat to classical cryptographic methods, commonly influenced for mobile communication security. In this paper, we propose one secure channel for private, personal mobile devices that are based not only on post-quantum cryptographically algorithm but also to secure communications in the quantum era. NIST-spec kemsKyber2 and Saber represent for irreplaceable session keys in communication against prospective quantum threats. The keys are periodically replaced in order to provide forward secrecy and to protect against an eventual compromise. In addition to the features of mobile platforms, the scheme employs a lightweight authentication protocol applied to lattice-based cryptography which has been shown to be secure and low resource overhead. Testing the proposed solution was practicable and wielded manageable results since it was performed on Android devices with an average key exchange latency of less than 250 milliseconds and a secure message delivery of below 500 milliseconds without compromising the AES-256 equivalence level of privacy. The engineers and software developers with the results from this study could safely claim that practically, it is indeed doable to transfer secure messages on mobile platforms in the post-quantum era and they’d have that same level of confidence with respect to the adoption of cipher suites based on post-quantum standards within any future communication systems.Keywords: Post-quantum cryptography, quantum-resistant encryption, key encapsulation mechanism (KEM), end-to-end encryption (E2EE), quantum key distribution (QKD), mobile communication security, lattice-based cryptography, publickey infrastructure (PKI), quantum computing threats.
Signal is a new security protocol and accompanying app that provides end-to-end encryption for instant messaging. The core protocol has recently been adopted by WhatsApp, Facebook Messenger, and Google Allo among many others, the first two of these have at least 1 billion active users. Signal includes several uncommon security properties (such as "future secrecy" or "post-compromise security"), enabled by a novel technique called ratcheting in which session keys are updated with every message sent. Despite its importance and novelty, there has been little to no academic analysis of the Signal protocol. We conduct the first security analysis of Signal's key agreement and double ratchet as a multi-stage key exchange protocol. We extract from the implementation a formal description of the abstract protocol, and define a security model which can capture the "ratcheting" key update structure. We then prove the security of Signal's core in our model, demonstrating several standard security properties. We have found no major flaws in the design, and hope that our presentation and results can serve as a starting point for other analyses of this widely adopted protocol.
The rapid development of quantum computers is emerging as a significant threat to end to end encryption protocols such as Signal. To address this issue, this paper enhances the Signal protocol by integrating Post Quantum Cryptography(PQC) algorithms. Specifically, this project integrates the PQXDH algorithm, which is the combination of X3DH and Kyber Key Encapsulation Mechanism to provide quantum resistant key exchange and encryption. A distinctive contribution of this work is the architectural shift from centralized client-server models to a fully decentralized peer-to-peer (P2P) communication model. This ensures greater resilience, fault tolerance, and preservation of privacy. To validate the feasibility of the proposed design, a real-world IoT use case is presented, highlighting the applicability of the system in resource-constrained environments where computational efficiency and low communication overhead are critical. The implementation is developed on Linux and leverages modern cryptographic libraries, ensuring compatibility with emerging PQC standards and integration into existing security infrastructures. Overall, this work demonstrates the combination of the E2E protocol Signal with quantum resistant Kyber to provide a quantum resistant cryptographic algorithm that can be used in decentralized architecture demonstrated using the IoT use case. The result is a robust, scalable, and future-proof framework for quantum-secure distributed communication.