Feature Flag For Ring Crypto Provider In Rustls Discussion And Benefits

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Introduction

This article discusses the possibility of adding a feature flag to allow users to switch to the Ring cryptographic library as the Rustls crypto provider. This suggestion stems from a previous experience where switching from Ring to aws-lc-rs resulted in a significant increase in binary size. While Ring was initially marked as unmaintained, this status has since been retracted, and the Rustls team has committed to its security maintenance. Therefore, providing users with the option to choose Ring could be beneficial for those prioritizing smaller binary sizes.

Background

In a previous attempt to optimize the Servo browser engine, a switch was made from the Ring cryptographic library to aws-lc-rs. This change, implemented in Pull Request #35106, unfortunately led to an unexpected consequence: a regression in binary size of approximately 1.5MB. This regression was observed and documented on the Bencher platform, as detailed in the provided link (https://bencher.dev/console/projects/servo/perf?lower_value=false&upper_value=false&lower_boundary=false&upper_boundary=false&x_axis=date_time&branches=52e1e9bb-959c-4171-a53d-e06bd694a6c1&testbeds=d742c702-3842-4108-9d0c-2db74e57599a&benchmarks=864410ba-a36c-4e5e-a822-64d828f89160%2C46c23cda-549d-4ffc-88ae-0c5298aab5b9&measures=678e4118-c8a5-494d-8799-08abc3021cd5&start_time=1738022400000&end_time=1738281600000&tab=plots&plots_search=9b77de8b-0330-44cb-805c-4cbf8743b115&key=true&reports_per_page=4&branches_per_page=8&testbeds_per_page=8&benchmarks_per_page=8&plots_per_page=8&reports_page=1&branches_page=1&testbeds_page=1&benchmarks_page=1&plots_page=1). At the time, the investigation into the root cause of this size increase was not pursued extensively, primarily because Ring was marked as unmaintained. This perceived lack of long-term support made aws-lc-rs the more appealing option, despite the larger binary size. The initial assessment of Ring's maintenance status played a crucial role in the decision-making process, highlighting the importance of considering the long-term viability of dependencies.

However, the situation has since evolved. The "unmaintained" designation for Ring has been retracted, and the Rustls team has been granted write access to the project. This significant development signals a renewed commitment to the security and maintenance of Ring. The updated advisory, available at https://rustsec.org/advisories/RUSTSEC-2025-0007.html, reflects this change in status. This retraction of the unmaintained status and the Rustls team's involvement drastically alter the landscape. With Ring now actively maintained and receiving security updates, it becomes a much more viable option for projects concerned about binary size. The initial concerns that led to its dismissal are now largely mitigated, paving the way for a re-evaluation of its suitability.

The regression in binary size caused by switching away from Ring highlights a common trade-off in software development: performance versus size. In this case, aws-lc-rs may offer performance advantages in certain scenarios, but it comes at the cost of a larger binary. For applications where size is a critical factor, such as embedded systems or web browsers targeting resource-constrained devices, this trade-off may not be acceptable. The ability to choose between different crypto providers allows developers to optimize for their specific needs and constraints. This flexibility is particularly important in the Rust ecosystem, where a strong emphasis is placed on performance, security, and resource efficiency. By offering Ring as an alternative, projects can potentially achieve a smaller footprint without sacrificing security, thanks to the renewed maintenance efforts.

Proposal: Feature Flag for Ring

Given the current circumstances, it is proposed that a feature flag be introduced to allow users to easily switch to Ring as the Rustls crypto provider. This feature flag would provide a simple mechanism for projects to opt-in to using Ring, enabling them to potentially reduce their binary size. The primary motivation behind this proposal is to offer users greater control over their build configuration and to enable them to make informed decisions about the trade-offs between binary size and other factors, such as performance or specific cryptographic algorithm support. A feature flag provides a clean and idiomatic way to achieve this in Rust, allowing users to selectively enable or disable Ring support during compilation.

Implementing a feature flag offers several benefits. First, it allows users who prioritize smaller binary sizes to use Ring without affecting other users who may prefer aws-lc-rs for other reasons. This ensures that the default configuration remains suitable for the majority of users while providing an escape hatch for those with specific requirements. Second, it simplifies the process of experimenting with different crypto providers. Users can easily switch between Ring and aws-lc-rs by toggling the feature flag, allowing them to benchmark and compare the performance and size characteristics of each provider in their specific use case. This experimentation can lead to valuable insights and help users optimize their applications for their target environments. Third, it provides a clear and explicit way to declare the dependency on Ring. By enabling the feature flag, users explicitly indicate that they want to use Ring, which helps to avoid potential conflicts or unexpected behavior. This explicit declaration also makes it easier to track and manage dependencies over time.

This approach aligns with the Rust philosophy of providing developers with fine-grained control over their dependencies and build configurations. Rust's feature flag system is a powerful tool for managing optional dependencies and conditional compilation, and it is well-suited for this scenario. By leveraging feature flags, we can provide a flexible and user-friendly mechanism for selecting the desired crypto provider. This approach also promotes code modularity and maintainability, as the Ring-specific code can be conditionally compiled based on the feature flag. This reduces the overall complexity of the codebase and makes it easier to maintain and update the crypto provider implementations.

Advantages of Using Ring

The primary advantage of using Ring in this context is the potential for a smaller binary size. As demonstrated by the previous experience with Servo, switching from Ring to aws-lc-rs resulted in a noticeable increase in binary size. For applications where size is a critical constraint, this difference can be significant. Smaller binaries lead to faster download times, reduced storage requirements, and improved performance on resource-constrained devices. In environments such as embedded systems, mobile applications, and web browsers, these benefits can be crucial. By offering Ring as an option, we empower users to optimize their applications for these scenarios.

Beyond the size advantage, Ring also has a reputation for strong security. It is a widely used cryptographic library that has been rigorously audited and tested. Its focus on security makes it a reliable choice for applications that require strong cryptographic protection. The recent commitment from the Rustls team to maintain Ring further strengthens its security profile, ensuring that it will continue to receive updates and security patches in the future. This ongoing maintenance is crucial for maintaining the long-term security and stability of applications that rely on Ring.

Furthermore, Ring is known for its performance in certain cryptographic operations. While aws-lc-rs may offer better performance in some cases, Ring can be more efficient for specific tasks. The optimal choice between Ring and aws-lc-rs depends on the specific cryptographic needs of the application. By providing both options, we allow users to select the provider that best suits their performance requirements. This flexibility is particularly important in applications that perform a wide range of cryptographic operations, as different providers may excel in different areas.

Finally, supporting Ring provides a valuable alternative in case of issues or vulnerabilities in other crypto providers. Having multiple options available increases the resilience of the ecosystem and reduces the risk of being solely dependent on a single provider. If a vulnerability is discovered in aws-lc-rs, for example, users can quickly switch to Ring as a temporary mitigation. This diversity in crypto providers enhances the overall security and stability of the Rust ecosystem. By maintaining support for Ring, we ensure that users have a backup option in case of unforeseen circumstances.

Addressing Concerns About Ring

One of the main reasons for the initial switch away from Ring was its status as "unmaintained." This designation raised concerns about the long-term security and stability of the library. However, as mentioned earlier, this status has since been retracted, and the Rustls team has committed to maintaining Ring. This commitment addresses the primary concern that led to the previous decision. With the Rustls team actively involved in Ring's development and maintenance, the library is now a much more viable option for long-term use.

Another potential concern is the completeness of Ring's feature set compared to aws-lc-rs. While Ring provides a comprehensive set of cryptographic primitives, it may not support all of the advanced features or algorithms available in aws-lc-rs. This is a valid consideration, and it is important for users to evaluate whether Ring meets their specific cryptographic requirements. However, for many common use cases, Ring provides sufficient functionality. The feature flag approach allows users to make an informed decision based on their needs. If they require specific features that are only available in aws-lc-rs, they can simply disable the feature flag and continue using aws-lc-rs.

It is also worth noting that the Rustls project itself benefits from having multiple crypto provider implementations. This diversity makes the project more resilient to vulnerabilities and reduces the risk of being tied to a single provider. By supporting both Ring and aws-lc-rs, Rustls can offer a more robust and flexible cryptographic foundation. This benefits not only Rustls itself but also the wider ecosystem of applications that rely on it.

Implementation Details

Implementing the feature flag would involve modifying the Rustls build configuration to conditionally compile the Ring backend. This would likely involve adding a new feature to the Cargo.toml file and using conditional compilation attributes (#[cfg(feature = "ring")]) to include or exclude the Ring-specific code. The implementation should be designed to minimize code duplication and maximize code reuse between the different crypto provider implementations. This can be achieved by defining a common interface for cryptographic operations and implementing it for both Ring and aws-lc-rs.

The feature flag should be clearly documented, and users should be provided with guidance on how to choose the appropriate crypto provider for their needs. The documentation should explain the trade-offs between Ring and aws-lc-rs, highlighting the size advantages of Ring and the potential performance advantages of aws-lc-rs in certain scenarios. It should also emphasize the importance of considering security requirements and selecting a provider that meets those requirements.

Testing is crucial to ensure that the feature flag works correctly and that both Ring and aws-lc-rs implementations are functioning as expected. The test suite should include unit tests, integration tests, and benchmarks to verify the correctness, performance, and security of the different crypto provider implementations. The benchmarks should be designed to measure the performance of key cryptographic operations, such as encryption, decryption, signing, and verification. This will help users make informed decisions about which provider to use based on their performance requirements.

Community Discussion

This proposal is intended to initiate a discussion within the Servo and Rustls communities. Feedback and suggestions are welcome on the feasibility, design, and implementation of the feature flag. The goal is to arrive at a solution that is beneficial to the community and that addresses the needs of a wide range of users. Open communication and collaboration are essential to ensure that the feature flag is implemented in a way that is consistent with the goals and principles of the Rust ecosystem.

Specific areas for discussion include:

  • The naming of the feature flag (e.g., ring, rustls-ring, etc.)
  • The default behavior when the feature flag is not specified
  • The documentation and guidance provided to users
  • The testing strategy
  • The potential impact on the Rustls API and ABI

By engaging in a constructive dialogue, we can ensure that the feature flag is implemented in a way that is both effective and user-friendly. This will benefit the entire community by providing a valuable tool for optimizing binary size and improving the flexibility of Rustls.

Conclusion

Adding a feature flag for Ring as the Rustls crypto provider presents a valuable opportunity to empower users with greater control over their build configurations and optimize for smaller binary sizes. Given Ring's renewed maintenance and security commitments, it is a viable alternative to aws-lc-rs for projects where size is a critical factor. This proposal encourages further discussion and collaboration within the community to determine the best approach for implementing this feature. By providing this flexibility, we can enhance the Rustls ecosystem and cater to a wider range of use cases. This ultimately strengthens the Rust community's commitment to performance, security, and resource efficiency.

Next Steps

The next steps in this process involve:

  1. Gathering feedback from the community on this proposal.
  2. Addressing any concerns or questions raised by the community.
  3. Developing a detailed implementation plan.
  4. Implementing the feature flag in Rustls.
  5. Thoroughly testing the implementation.
  6. Documenting the feature flag and its usage.

By following these steps, we can ensure that the feature flag is implemented successfully and that it provides a valuable benefit to the Rustls community.