rustls 0.18.1

Rustls is a modern TLS library written in Rust.

Rustls - a modern TLS library

Rustls is a TLS library that aims to provide a good level of cryptographic security, requires no configuration to achieve that security, and provides no unsafe features or obsolete cryptography.

Current features

  • TLS1.2 and TLS1.3.
  • ECDSA, Ed25519 or RSA server authentication by clients.
  • ECDSA, Ed25519 or RSA server authentication by servers.
  • Forward secrecy using ECDHE; with curve25519, nistp256 or nistp384 curves.
  • AES128-GCM and AES256-GCM bulk encryption, with safe nonces.
  • ChaCha20-Poly1305 bulk encryption (RFC7905).
  • ALPN support.
  • SNI support.
  • Tunable MTU to make TLS messages match size of underlying transport.
  • Optional use of vectored IO to minimise system calls.
  • TLS1.2 session resumption.
  • TLS1.2 resumption via tickets (RFC5077).
  • TLS1.3 resumption via tickets or session storage.
  • TLS1.3 0-RTT data for clients.
  • Client authentication by clients.
  • Client authentication by servers.
  • Extended master secret support (RFC7627).
  • Exporters (RFC5705).
  • OCSP stapling by servers.
  • SCT stapling by servers.
  • SCT verification by clients.

Possible future features

  • PSK support.
  • OCSP verification by clients.
  • Certificate pinning.


The following things are broken, obsolete, badly designed, underspecified, dangerous and/or insane. Rustls does not support:

  • SSL1, SSL2, SSL3, TLS1 or TLS1.1.
  • RC4.
  • DES or triple DES.
  • EXPORT ciphersuites.
  • MAC-then-encrypt ciphersuites.
  • Ciphersuites without forward secrecy.
  • Renegotiation.
  • Kerberos.
  • Compression.
  • Discrete-log Diffie-Hellman.
  • Automatic protocol version downgrade.
  • AES-GCM with unsafe nonces.

There are plenty of other libraries that provide these features should you need them.

Platform support

Rustls uses ring for implementing the cryptography in TLS. As a result, rustls only runs on platforms supported by ring. At the time of writing this means x86, x86-64, armv7, and aarch64.

Design Overview

Rustls does not take care of network IO

It doesn't make or accept TCP connections, or do DNS, or read or write files.

There's example client and server code which uses mio to do all needed network IO.

Rustls provides encrypted pipes

These are the ServerSession and ClientSession types. You supply raw TLS traffic on the left (via the read_tls() and write_tls() methods) and then read/write the plaintext on the right:

TLS                                   Plaintext
===                                   =========
read_tls()      +-----------------------+      io::Read
|                       |
+--------->     ClientSession     +--------->
|          or           |
<---------+     ServerSession     <---------+
|                       |
write_tls()     +-----------------------+      io::Write

Rustls takes care of server certificate verification

You do not need to provide anything other than a set of root certificates to trust. Certificate verification cannot be turned off or disabled in the main API.

Getting started

This is the minimum you need to do to make a TLS client connection.

First, we make a ClientConfig. You're likely to make one of these per process, and use it for all connections made by that process.

let mut config = rustls::ClientConfig::new();

Next we load some root certificates. These are used to authenticate the server. The recommended way is to depend on the webpki_roots crate which contains the Mozilla set of root certificates.


Now we can make a session. You need to provide the server's hostname so we know what to expect to find in the server's certificate.

# use rustls;
# use webpki;
# use std::sync::Arc;
# let mut config = rustls::ClientConfig::new();
let rc_config = Arc::new(config);
let example_com = webpki::DNSNameRef::try_from_ascii_str("").unwrap();
let mut client = rustls::ClientSession::new(&rc_config, example_com);

Now you should do appropriate IO for the client object. If client.wants_read() yields true, you should call client.read_tls() when the underlying connection has data. Likewise, if client.wants_write() yields true, you should call client.write_tls() when the underlying connection is able to send data. You should continue doing this as long as the connection is valid.

The return types of read_tls() and write_tls() only tell you if the IO worked. No parsing or processing of the TLS messages is done. After each read_tls() you should therefore call client.process_new_packets() which parses and processes the messages. Any error returned from process_new_packets is fatal to the session, and will tell you why. For example, if the server's certificate is expired process_new_packets will return Err(WebPKIError(CertExpired)). From this point on, process_new_packets will not do any new work and will return that error continually.

You can extract newly received data by calling (via the io::Read trait). You can send data to the peer by calling client.write() (via the io::Write trait). Note that client.write() buffers data you send if the TLS session is not yet established: this is useful for writing (say) a HTTP request, but don't write huge amounts of data.

The following code uses a fictional socket IO API for illustration, and does not handle errors.

use std::io;

client.write(b"GET / HTTP/1.0\r\n\r\n").unwrap();
let mut socket = connect("", 443);
loop {
if client.wants_read() && socket.ready_for_read() {
client.read_tls(&mut socket).unwrap();

let mut plaintext = Vec::new();
client.read_to_end(&mut plaintext).unwrap();

if client.wants_write() && socket.ready_for_write() {
client.write_tls(&mut socket).unwrap();



tlsserver and tlsclient are full worked examples. These both use mio.

Crate features

Here's a list of what features are exposed by the rustls crate and what they mean.

  • logging: this makes the rustls crate depend on the log crate. rustls outputs interesting protocol-level messages at trace! and debug! level, and protocol-level errors at warn! and error! level. The log messages do not contain secret key data, and so are safe to archive without affecting session security. This feature is in the default set.

  • dangerous_configuration: this feature enables a dangerous() method on ClientConfig and ServerConfig that allows setting inadvisable options, such as replacing the certificate verification process. Applications requesting this feature should be reviewed carefully.

  • quic: this feature exposes additional constructors and functions for using rustls as a TLS library for QUIC. See the quic module for details of these. You will only need this if you're writing a QUIC implementation.