This blog post explores the challenges of creating a robust test suite for Time-Based One-Time Password (TOTP) algorithms. The author highlights the difficulty in balancing the need for deterministic, repeatable tests with the time-sensitive nature of TOTP codes. They propose using a fixed timestamp and shared secret as a starting point, then exploring variations in time steps and time drift to ensure the algorithm handles edge cases correctly. The post concludes with a call for collaboration and shared test vectors to improve the overall security and reliability of TOTP implementations.
This blog post by "shkspr" delves into the complexities of creating a robust test suite for Time-based One-Time Password (TOTP) algorithms. The author begins by highlighting the seemingly straightforward nature of TOTP, which involves generating a one-time password based on a shared secret key and the current time. However, they quickly point out that subtle implementation differences can lead to interoperability issues, emphasizing the need for thorough testing.
The core challenge, as described by the author, lies in the variability introduced by time itself. Testing requires predictable outputs, which conflicts with the time-dependent nature of TOTP. To address this, the author explores several strategies. Initially, they consider mocking the time function, effectively freezing time for testing purposes. However, this approach is deemed insufficient as it doesn't fully exercise the time-based aspects of the algorithm.
The post then introduces the concept of using pre-generated test vectors. These vectors would consist of specific secret keys, timestamps, and expected OTP values, providing deterministic test cases. The author discusses obtaining these vectors from RFC 6238 (which defines TOTP) and other publicly available sources, as well as potentially generating them using a known-good implementation. The benefit of this approach is the ability to verify the algorithm's correctness against established standards and other implementations.
Furthermore, the post emphasizes the importance of testing edge cases. These include scenarios like time drift, counter resets, and different time step sizes (the standard 30-second intervals, but also potentially others). Testing these scenarios is crucial to ensure the TOTP implementation is resilient to real-world conditions and potential issues.
The author concludes by acknowledging that building a comprehensive test suite for TOTP is a non-trivial task, but stresses its significance for ensuring secure and reliable two-factor authentication. They suggest that a combination of mocking, pre-generated test vectors, and rigorous edge-case testing offers the best path towards a robust and reliable testing strategy. While the author doesn't present a complete, ready-to-use test suite, they provide valuable insights and a clear direction for developers seeking to thoroughly test their TOTP implementations.
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https://news.ycombinator.com/item?id=43230922
The Hacker News comments discuss the practicality and usefulness of the proposed TOTP test suite. Several commenters point out that existing libraries like oathtool already provide robust implementations and question the need for a new test suite, suggesting that focusing on testing against these established libraries would be more effective. Others highlight the potential value in testing edge cases and different implementations, particularly for less common languages or when implementing TOTP from scratch. The difficulty in obtaining a diverse and representative set of real-world TOTP secrets for testing is also mentioned. Finally, some commenters express concern about the security implications of publishing a comprehensive test suite, fearing it could be misused for malicious purposes.
The Hacker News post "Towards a test-suite for TOTP codes" has generated a moderate discussion with several insightful comments.
One commenter highlights the inherent difficulty in creating a comprehensive test suite for TOTP due to the time-based nature of the algorithm. They explain that because TOTP codes are generated based on the current time, a pre-generated list of valid codes would quickly become outdated. They suggest that a more practical approach would be a test suite that verifies the process of generating TOTP codes, rather than testing against specific code values. This could involve testing the underlying HMAC-SHA1 algorithm and ensuring the correct time window and secret key are used.
Another commenter points out a potential vulnerability related to clock drift. They explain how a small difference between the server's clock and the client's clock can lead to valid TOTP codes being rejected. They suggest testing for resilience against such clock drift by allowing a tolerance of one or two time steps in either direction. This reinforces the idea that a robust test suite should focus on the algorithm's behavior under various conditions, including imperfect time synchronization.
A further comment discusses the practical challenges of testing TOTP in real-world scenarios. They mention the difficulty of simulating time changes and the need to mock or control the system clock during testing. This highlights the complexity of thoroughly testing time-dependent systems.
Finally, one commenter mentions the existence of RFC 6238, which specifies the TOTP algorithm. They suggest that any test suite should adhere to the guidelines and test vectors provided in the RFC. This ensures compliance with the standard and provides a baseline for interoperability.
The overall sentiment in the comments is that while creating a comprehensive, pre-generated test suite for TOTP codes is impractical, a valuable test suite can focus on validating the algorithm's implementation and its resilience to factors like clock drift and edge cases. The comments underscore the importance of testing the process of generating TOTP codes, rather than the codes themselves, and adhering to the RFC specifications.