rustls-platform-verifier

A Rust library to verify the validity of TLS certificates based on the operating system's certificate facilities. On operating systems that don't have these, webpki and/or rustls-native-certs is used instead.

This crate is advantageous over rustls-native-certs on its own for a few reasons:

• Improved correctness and security, as the OSes CA constraints will be taken into account.
• Better integration with OS certificate stores and enterprise CA deployments.
• Revocation support via verifying validity via OCSP and CRLs.
• Less I/O and memory overhead because all the platform CAs don't need to be loaded and parsed.

This library supports the following platforms and flows:

OS	Certificate Store	Verification Method	Revocation Support
Windows	Windows platform certificate store	Windows API certificate verification	Yes
macOS (10.14+)	macOS platform roots and keychain certificate	macOS Security.framework	Yes
iOS	iOS platform roots and keychain certificates	iOS Security.framework	Yes
Android	Android System Trust Store	Android Trust Manager	Sometimes1
Linux	System CA bundle, or user-provided certs2	webpki	No3
WASM	webpki roots	webpki	No3

Deployment Considerations

When choosing to use rustls-platform-verifier or another trust store option, these differences are important to consider. They are primarily about root certificate availability:

Backend	Updates	Roots used	Supports system-local roots
rustls-platform-verifier (non-Linux/BSD)	Updated by OS	System store, with full (dis)trust decisions from every source available.	Yes
rustls-native-certs + webpki	Updated by OS	System store, with no (dis)trust decisions. All roots are treated equally regardless of their status.	Yes, with exceptions
webpki-roots + webpki	Static, manual updates required	Hardcoded Mozilla CA roots, limited support for constrained roots.	No

In general: It is the opinion of the rustls and rustls-platform-verifier teams that this is the best default available for client-side libraries and applications making connections to TLS servers when running on common operating systems. This is because it gets both live trust information (new roots, explicit markers, and auto-managed CRLs) and better matches the common expectation of apps running on that platform (to use proxies, for example). Otherwise, it becomes your maintenance burden to ship updates right away in order to handle increasing numbers of positive and negative trust events in the WebPKI/certificate ecosystem, or risk availability and security concerns.

Linux/BSD

As of the time of writing, rustls-platform-verifier on these OSes only loads the trust stores from the OS once upon startup. This is the same behavior as rustls-native-certs, but the abstraction allows better behavior on the other platforms without extra work for downstreams.

Other

Alternatively, there is a clear answer to use static webpki-roots in your application instead if you are deploying containerized applications frequently, where root store changes will make it to production faster and any possibly used trust root is static by definition.

Even though platform verifiers are sometimes implemented in memory-unsafe languages, it is very unlikely that Rust apps using this library will become a point of weakness. This is due to either using a smaller set of servers or just being less exposed then other critical functions of the operating system, default web browser, etc. But if your activity is identical or close to one of the following examples that process large amounts of untrusted input, a 100% Rust option like webpki is a more secure option:

• Seeing how many TLS servers rustls with a specific configuration can connect to.
• Harvesting data from various untrusted TLS endpoints exposed on the internet.
• Extracting info from a known-evil endpoint.
• Scanning all TLS certificates on the open internet.

rustls-platform-verifier is widely deployed by several applications that use the rustls stack, such as 1Password, Bitwarden, Signal, and rustup, on a wide set of OSes. This means that it has received lots of exposure to edge cases and has real-world experience/expertise invested into it to ensure optimal compatibility and security.

Installation and setup

On most platforms, no setup should be required beyond adding the dependency via cargo:

rustls-platform-verifier = "0.3"

To get a rustls ClientConfig configured to use the platform verifier use:

let config = rustls_platform_verifier::tls_config();

This crate will use the rustls process-default crypto provider. To construct a ClientConfig with a different CryptoProvider, use:

let arc_crypto_provider = std::sync::Arc::new(rustls::crypto::ring::default_provider());
let config = rustls_platform_verifier::tls_config_with_provider(arc_crypto_provider);

If you want to adapt the configuration, you can build the ClientConfig like this:

use std::sync::Arc;
use rustls::ClientConfig;
use rustls_platform_verifier::Verifier;

let mut config = ClientConfig::builder()
    .dangerous() // The `Verifier` we're using is actually safe
    .with_custom_certificate_verifier(Arc::new(Verifier::new()))
    .with_no_client_auth();

Android

Some manual setup is required, outside of cargo, to use this crate on Android. In order to use Android's certificate verifier, the crate needs to call into the JVM. A small Kotlin component must be included in your app's build to support rustls-platform-verifier.

Gradle Setup

rustls-platform-verifier bundles the required native components in the crate, but the project must be setup to locate them automatically and correctly. These steps assume you are using .gradle Groovy files because they're the most common, but everything is 100% applicable to Kotlin script (.gradle.kts) configurations too with a few replacements.

Inside of your project's build.gradle file, add the following code and Maven repository definition. If applicable, this should only be the one "app" sub-project that will actually be using this crate at runtime. With multiple projects running this, your Gradle configuration performance may degrade.

$PATH_TO_DEPENDENT_CRATE is the relative path to the Cargo manifest (Cargo.toml) of any crate in your workspace that depends on rustls-platform-verifier from the location of your build.gradle file:

import groovy.json.JsonSlurper

// ...Your own script code could be here...

repositories {
    // ... Your other repositories could be here...
    maven {
        url = findRustlsPlatformVerifierProject()
        metadataSources.artifact()
    }
}

String findRustlsPlatformVerifierProject() {
    def dependencyText = providers.exec {
        it.workingDir = new File("../")
        commandLine("cargo", "metadata", "--format-version", "1", "--manifest-path", "$PATH_TO_DEPENDENT_CRATE/Cargo.toml")
    }.standardOutput.asText.get()

    def dependencyJson = new JsonSlurper().parseText(dependencyText)
    def manifestPath = file(dependencyJson.packages.find { it.name == "rustls-platform-verifier-android" }.manifest_path)
    return new File(manifestPath.parentFile, "maven").path
}

Then, wherever you declare your dependencies, add the following:

implementation "rustls:rustls-platform-verifier:latest.release"

Cargo automatically handles finding the downloaded crate in the correct location for your project. It also handles updating the version when new releases of rustls-platform-verifier are published. If you only use published releases, no extra maintenance should be required.

These script snippets can be tweaked as best suits your project, but the cargo metadata invocation must be included so that the Android implementation part can be located on-disk.

Proguard

If your Android application makes use of Proguard for optimizations, its important to make sure that the Android verifier component isn't optimized out because it looks like dead code. Proguard is unable to see any JNI usage, so your rules must manually opt into keeping it. The following rule can do this for you:

-keep, includedescriptorclasses class org.rustls.platformverifier.** { *; }

Crate initialization

In order for the crate to call into the JVM, it needs handles from Android. These are provided either the init_external or init_hosted function. These give rustls-platform-verifier the resources it needs to make calls into the Android certificate verifier.

As an example, if your Rust Android component which the "native" Android part of your app calls at startup has an initialization, like this:

#[export_name = "Java_com_orgname_android_rust_init"]
extern "C" fn java_init(
    env: JNIEnv,
    _class: JClass,
    context: JObject,
) -> jboolean {
    // ... initialize your app's other parts here.
}

In the simplest case, you should to insert a call to rustls_platform_verifier::android::init_hosted() here, before any networking has a chance to run. This only needs to be called once and the verifier will be valid for the lifetime of your app's process.

extern "C" fn java_init(
    env: JNIEnv,
    _class: JClass,
    context: JObject,
) -> jboolean {
    // ... initialize your app's other parts here.

    // Then, initialize the certificate verifier for future use.
    rustls_platform_verifier::android::init_hosted(&env, context);
}

In more advanced cases, such as where your code already stores long-lived handles into the Android environment, you can alternatively use init_external. This function takes a &'static reference to something that implements the android::Runtime trait, which the crate then uses to obtain the access when required to the JVM.

Credits

Made with ❤️ by the 1Password and rustls teams. Portions of the Android and Windows implementation were adapted and referenced from Chromium's previous verifier implementations as well.

License

^{Licensed under either of Apache License, Version 2.0 or MIT license at your option.}
_{Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in this crate by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.}

Footnotes

1. On Android, revocation checking requires API version >= 24 (e.g. at least Android 7.0, August 2016). When available, revocation checking is only performed for the end-entity certificate. If a stapled OCSP response for the end-entity cert isn't provided, and the certificate omits both a OCSP responder URL and CRL distribution point to fetch revocation information from, revocation checking may fail. ↩

2. On Linux the rustls-native-certs and openssl-probe crates are used to try and discover the system CA bundle. Users may wish to augment these certificates with webpki-roots using Verifier::new_with_extra_roots in case a system CA bundle is unavailable. ↩

3. The fall-back webpki verifier configured for Linux/WASM does not support providing CRLs for revocation checking. If you require revocation checking on these platforms, prefer constructing your own WebPkiServerVerifier, providing necessary CRLs. See the Rustls ServerCertVerifierBuilder docs for more information. ↩ ↩²