Signal's cryptography is generally well-regarded, using established and vetted protocols like X3DH and Double Ratchet for secure messaging. The blog post author reviewed Signal's implementation and found it largely sound, praising the clarity of the documentation and the overall design. While some minor theoretical improvements were suggested, like using a more modern key derivation function (HKDF over SHA-256) and potentially exploring post-quantum cryptography for future-proofing, the author concludes that Signal's current cryptographic choices are robust and secure, offering strong confidentiality and integrity protections for users.
This blog post by Soatok, titled "Reviewing the Cryptography Used by Signal," provides an in-depth examination of the cryptographic choices and implementations within the Signal messaging protocol, focusing on both its strengths and potential areas for improvement. The author prefaces their analysis by acknowledging Signal's strong reputation for security and privacy, emphasizing that their goal is not to condemn Signal, but rather to offer constructive criticism and promote a broader understanding of cryptographic design principles.
The review begins with a dissection of Signal's "Double Ratchet" algorithm, which combines a Diffie-Hellman (DH) ratchet with a symmetric-key ratchet to provide forward secrecy and post-compromise security. Soatok explains how this mechanism ensures that the compromise of a single device's keys doesn't compromise past or future message exchanges. The author also details how the X3DH key agreement protocol is employed for initial key exchange, ensuring secure communication even when users are offline.
A significant portion of the post delves into the nuances of Signal's use of the Extended Triple Diffie-Hellman (X3DH) key agreement protocol, praising its robust security properties. The author meticulously describes how X3DH establishes shared secrets between users, utilizing both long-term identity keys and ephemeral prekeys to provide a balance between security and efficiency. The discussion extends to the management of these prekeys and the implications for message delivery when devices are offline.
Further, Soatok discusses Signal's implementation of the Double Ratchet algorithm and its integration with the X3DH protocol. They explain how these mechanisms work in concert to provide end-to-end encryption and forward secrecy, protecting messages from eavesdropping and ensuring that past messages remain confidential even if a device is compromised. The author also touches upon the concept of "future secrecy," which is partially achieved by the Double Ratchet but acknowledges the limitations in preventing access to future messages if a device is currently compromised.
Beyond the core cryptographic protocols, the post examines Signal's choice of cryptographic primitives, specifically the use of the XEdDSA and VXEdDSA signature schemes. The author explains the benefits of these schemes in terms of security and efficiency, and notes the distinction between XEdDSA, used for signing prekeys, and VXEdDSA, used in the Double Ratchet. They also highlight Signal's transition to more modern cryptographic primitives, illustrating their commitment to staying up-to-date with best practices.
While largely positive about Signal's cryptographic design, Soatok does raise some minor concerns. These include the potential complexity of implementing the Double Ratchet correctly and the need for careful consideration of key management practices. The author also suggests potential areas for future improvement, such as incorporating post-quantum cryptography to prepare for future advancements in computing.
Concluding the review, the author reiterates their respect for Signal's security efforts and emphasizes the importance of ongoing scrutiny and improvement in the field of cryptography. The post serves as a valuable resource for anyone seeking a deeper understanding of the cryptographic underpinnings of Signal and the broader principles of secure messaging.
Summary of Comments ( 132 )
https://news.ycombinator.com/item?id=43088785
Hacker News users discussed the Signal cryptography review, mostly agreeing with the author's points. Several highlighted the importance of Signal's Double Ratchet algorithm and the trade-offs involved in achieving strong security while maintaining usability. Some questioned the practicality of certain theoretical attacks, emphasizing the difficulty of exploiting them in the real world. Others discussed the value of formal verification efforts and the overall robustness of Signal's protocol design despite minor potential vulnerabilities. The conversation also touched upon the importance of accessible security audits and the challenges of maintaining privacy in messaging apps.
The Hacker News post titled "Reviewing the Cryptography Used by Signal" (linking to soatok.blog/2025/02/18/reviewing-the-cryptography-used-by-signal) generated a moderate discussion with a handful of comments focusing on specific aspects of Signal's cryptography and its review.
Several commenters highlight the rigor and trustworthiness of Signal's cryptographic choices. One notes that Signal's use of the Double Ratchet Algorithm provides strong forward secrecy and resilience against compromise. They emphasize the importance of this feature in protecting past messages even if a current key is compromised. Another commenter concurs, pointing out Signal's proactive approach to security and its history of public audits, which contributes significantly to trust in their implementation.
Some discussion revolves around the nuances of cryptographic implementations. One commenter mentions the subtle complexities involved in securely implementing the Double Ratchet, and how even minor deviations or errors can potentially introduce vulnerabilities. This highlights the importance of careful review and scrutiny even for well-established cryptographic primitives. Another thread focuses on the X3DH key agreement protocol, discussing its role in establishing secure communication channels and its resistance to certain types of attacks.
One commenter expresses skepticism, questioning the necessity of such a detailed review given Signal's established reputation and existing audit history. They argue that the effort might be better spent elsewhere, perhaps on less scrutinized projects. This prompts a counter-argument that ongoing reviews are essential, especially in a constantly evolving threat landscape, and that even widely trusted systems benefit from periodic reassessment.
The conversation also touches upon the broader importance of secure messaging and the role of Signal in that ecosystem. One commenter expresses appreciation for Signal's commitment to open-source development and transparent security practices, contrasting it with closed-source alternatives where independent verification is difficult or impossible.
Overall, the comments on Hacker News generally reflect a positive view of Signal's cryptography, acknowledging its strengths and robustness while also highlighting the ongoing need for review and scrutiny in the field of security. The discussion avoids deeply technical jargon, making it accessible to a broader audience while still touching on key cryptographic concepts.