created_at2021-07-08 16:52:31.847235
updated_at2024-03-17 12:12:28.660208
descriptionasync ACME client for tls-alpn-01 challenge



Project Status: Active – The project has reached a stable, usable state and is being actively developed. Released API docs MIT licensed

A generic async ACME crate.

The main goal is to allow binaries to choose what async runtime and TLS library is used.


You need to specify via features what crates are used in the actual work.

feature flag Meaning
use_tokio Use tokio as async runtime
use_async_std Use async_std as async runtime
use_rustls Use rustls for HTTPS and generate Certificates tailored to it
hyper_rustls use_rustls+use_tokio GitHub Workflow Status
async_std_rustls use_rustls+use_async_std GitHub Workflow Status

Without anything specified you will end up with no async backend selected or no crypto backend selected. If you use this crate for a library, please reexport the appropriate features.


Rust offers different async runtimes that - on a high level - offer the same thing: asynchronous functions for files, sockets and so on.

So if you write a lib and need some basic features (like an http client) you sometimes have to make choices that are not what your crate's users would have liked. For example: I wrote a webserver based on hyper and wanted to add ACME. A crate I found did what I needed but used async-h1 and async-std. While that worked, it did increase the binary size and number of crates I depend on by a good amount.

So I wrote this. You can specify which backend to use. In the Webserver case, using --features="hyper_rustls" (same dependencies) instead of --features="async_std_rustls" lead to 81 less crates and a 350kB smaller binary. Using:

lto = "fat"
codegen-units = 1


These query certs from Let's Encrypt's Staging endpoint. In order for them to work you need to change the email and domain from to your own.

  1. Hyper server with rustls: cargo run --example hyper_rustls --features="hyper_rustls"
  2. async-std server with rustls: cargo run --example async_rustls --features="async_std_rustls"


  1. Add native_tls
  2. Add openssl cert generation
Commit count: 38

cargo fmt