computer science

Rustls

Rustls is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Rustls rather than just read about it. In short: Rustls (pronounced "rustles") is an open-source implementation of the Transport Layer Security (TLS) cryptographic protocol written in the Rust programming language. TLS is essential to internet security, and Rustls aims to enable secure, fast TLS connections.

Key takeaways

Rustls belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Rustls to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Rustls from memory before moving on to harder problems.

Reference excerpt

Rustls (pronounced "rustles") is an open-source implementation of the Transport Layer Security (TLS) cryptographic protocol written in the Rust programming language. TLS is essential to internet security, and Rustls aims to enable secure, fast TLS connections. Rustls uses Rust's enforcement of memory safety to reduce the risk of security vulnerabilities. It is part of efforts to improve internet security by replacing memory-unsafe software libraries, such as OpenSSL, with memory-safe alternatives.

Team and funding Joe Birr-Pixton started Rustls in 2016 and remains the lead developer as of 2024. The Internet Security Research Group (ISRG), a nonprofit organization based in the United States, supported the project from 2021 to 2025 as part of its Prossimo initiative. As of 2025, Rustls is fiscally sponsored by the Rust Innovation Lab at the Rust Foundation. The shared goal is to make Rustls a viable alternative to OpenSSL, which is widely used by internet servers but difficult to use correctly and has had security bugs, such as Heartbleed, caused by memory-unsafe code. ISRG paid several programmers to work on Rustls, including Birr-Pixton, Daniel McCarney, and Dirkjan Ochtman, using money contributed by Google and other companies and organizations. In 2023, the Open Source Security Foundation's Alpha-Omega initiative gave ISRG $530,000 for development of the option to use different cryptographic backends and for the separate project Rust for Linux. That money came from Google, Amazon Web Services, and Microsoft. Amazon Web Services also gave ISRG $1 million in 2023 for memory-safety projects including Rustls. The Sovereign Tech Fund, supported by the German government, gave $1.5 million to ISRG in 2023 for work on Rustls and other projects that provide memory-safe versions of open source tools critical to internet security. Craig Newmark Philanthropies granted $100,000 to ISRG for memory safety projects in 2024. Additional funding has come from Fly.io, a cloud platform that uses Rustls. The United States Office of the National Cyber Director has encouraged work on memory-safe security software and complimented the Rustls team. Google awarded Open Source Peer Bonuses to Birr-Pixton and Ochtman for their work on Rustls.

Architecture and features Rustls is a low-level software library focused on TLS implementation. This means it does not support other internet protocols by itself, such as HTTPS, but software that implements other protocols may use Rustls as a component. By default Rustls uses cryptographic primitives from Amazon Web Services Libcrypto for Rust (aws-lc-rs), which supports Federal Information Processing Standards (FIPS). Rustls allows using alternative cryptographic libraries instead of aws-lc-rs, such as ring. The project has experimental support for post-quantum cryptography: a key exchange method with a special key encapsulation mechanism (Kyber). Rustls uses its own fork of the webpki library to verify public key infrastructure certificates, a step in the TLS handshake. Rustls supports Server Name Indication (SNI), which allows a web server to serve multiple HTTPS websites at the same IP address with different certificates. It also supports TLS certificates that contain IP addresses instead of domain names. C programs can use Rustls through a foreign function interface API, rustls-ffi. For example, cURL is a popular tool written in C, and it allows using Rustls through rustls-ffi. Rustls also has an OpenSSL compatibility layer that allows configuring the widely-used Nginx web server to use Rustls instead of OpenSSL. Rustls is available under multiple free software licenses: Apache 2.0, MIT, and ISC.

Evaluations In 2020, the Cloud Native Computing Foundation funded a security audit of Rustls and two Rust libraries it used, ring and webpki, with positive results. In 2019, benchmarks carried out by the Rustls developer showed better performance than OpenSSL. In 2024 the project conducted new performance comparisons with the latest version of OpenSSL, which showed some scenarios where Rustls was faster or more efficient and some where OpenSSL performed better.

Uses Like other TLS implementations, a computer user may use Rustls without being aware of it, as an underlying part of an application or website. A programmer can use Rustls directly or by configuring a higher-level library or tool to use it. In particular, Rustls is used by some projects that want to ensure they have a secure software supply chain. The US Cybersecurity and Infrastructure Security Agency has recommended using products in memory safe languages as part of its "Secure by Design" initiative. Some libraries support Rustls as one of several choices for TLS implementations. The reqwest HTTP client library offers the option to use Rustls for TLS instead of the system's default TLS library (for example, on Windows the default is the Security Support Provider Interface). In 2020 an ISRG software engineer enabled using Rustls as a TLS backend for cURL. s2n-quic, an implementation of the QUIC network protocol in Rust, supports both Rustls and s2n-tls for TLS. In 2021 Google funded the creation of mod_tls, a new TLS module for Apache HTTP Server using Rustls. The new module is intended to be a successor to the mod_ssl module that uses OpenSSL, as a more secure default. As of August 2024, mod_tls is available in the latest version of Apache but still marked as experimental. The Internet Society, a nonprofit that advocates for an open and secure internet, suggests that organizations use this module as a step toward increasing memory safety. Rustls is the default TLS implementation in some applications. The utility program cargo_audit, which checks Rust project dependencies for security vulnerabilities, uses Rustls. Linkerd, which "adds security, observability, and reliability to any Kubernetes cluster", includes a proxy server built with Rustls. Wolfi, a tool for making memory-safe Linux containers, uses Rustls. In 2024, ISRG announced plans to start replacing OpenSSL with Rustls in Let's Encrypt, their free certificate authority used by hundreds of millions of websites.

Illustrations

Worked examples

Example 1 — a first encounter with Rustls

Start with the simplest possible case. Write down what Rustls claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Rustls before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Rustls ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Rustls

In research: Rustls appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Rustls in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Rustls is common in secondary-school and first-year university syllabi. It links to neighbouring topics Free security software, Free software programmed in Rust, Software using the Apache license, so understanding it makes those chapters shorter.
In everyday life: Look for Rustls outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Rustls in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Rustls means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Rustls out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Rustls in simple terms?

Why does Rustls matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Rustls?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Rustls.