Programming language: Rust
License: GNU General Public License v3.0 or later
Tags: Cryptography    

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Rustls is a modern TLS library written in Rust. It uses ring for cryptography and webpki for certificate verification.


Rustls is ready for use. There are no major breaking interface changes envisioned after the set included in the 0.20 release.

If you'd like to help out, please see [CONTRIBUTING.md](CONTRIBUTING.md).

Build Status Coverage Status (codecov.io) Documentation Chat

Release history

  • Next release
    • Planned: removal of unused signature verification schemes at link-time.
  • 0.20.8 (TBD)
    • Yield an error from ConnectionCommon::read_tls() if buffers are full. Both a full deframer buffer and a full incoming plaintext buffer will now cause an error to be returned. Callers should call process_new_packets() and read out the writer() after each successful call to read_tls().
  • 0.20.7 (2022-10-18)
    • Expose secret extraction API under the secret_extraction cargo feature. This is designed to enable switching from rustls to kTLS (kernel TLS offload) after a successful TLS 1.2/1.3 handshake, for example.
    • Move filtering of signature schemes after config selection, avoiding the need for linking in encryption/decryption code for all cipher suites at the cost of exposing more signature schemes in the ClientHello emitted by the Acceptor.
    • Expose AlertDescription, ContentType, and HandshakeType, SignatureAlgorithm, and NamedGroup as part of the stable API. Previously they were part of the unstable internals API, but were referenced by parts of the stable API.
    • We now have a Discord channel for community discussions.
    • The minimum supported Rust version is now 1.56.0 due to several dependencies requiring it.
  • 0.20.6 (2022-05-18)
    • 0.20.5 included a change to track more context for the Error::CorruptMessage which made API-incompatible changes to the Error type. We yanked 0.20.5 and have reverted that change as part of 0.20.6.
  • 0.20.5 (2022-05-14)
    • Correct compatbility with servers which return no TLS extensions and take advantage of a special case encoding.
    • Remove spurious warn-level logging introduced in 0.20.3.
    • Expose cipher suites in ClientHello type.
    • Allow verification of IP addresses with dangerous_config enabled.
    • Retry I/O operations in ConnectionCommon::complete_io() when interrupted.
    • Fix server::ResolvesServerCertUsingSni case sensitivity.
  • 0.20.4 (2022-02-19)
    • Correct regression in QUIC 0-RTT support.
  • 0.20.3 (2022-02-13)
    • Support loading ECDSA keys in SEC1 format.
    • Support receipt of 0-RTT "early data" in TLS1.3 servers. It is not enabled by default; opt in by setting ServerConfig::max_early_data_size to a non-zero value.
    • Support sending of data with the first server flight. This is also not enabled by default either: opt in by setting ServerConfig::send_half_rtt_data.
    • Support read_buf interface when compiled with nightly. This means data can be safely read out of a rustls connection into a buffer without the buffer requiring initialisation first. Set the read_buf feature to use this.
    • Improve efficiency when writing vectors of TLS types.
    • Reduce copying and improve efficiency in TLS1.2 handshake.
  • 0.20.2 (2021-11-21)
    • Fix CipherSuite::as_str() value (as introduced in 0.20.1).
  • 0.20.1 (2021-11-14)
    • Allow cipher suite enum items to be stringified.
    • Improve documentation of configuration builder types.
    • Ensure unused cipher suites can be removed at link-time.
    • Ensure single-use error types implement std::error::Error, and are public.

See [RELEASE_NOTES.md](RELEASE_NOTES.md) for further change history.


Lives here: https://docs.rs/rustls/


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 fragment size 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.
  • TLS1.3 0-RTT data for servers.
  • 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.


For reasons explained in the manual, rustls does not and will 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.

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.

Rustls requires Rust 1.56 or later.

Example code

There are two example programs which use mio to do asynchronous IO.

Client example program

The client example program is named tlsclient-mio. The interface looks like:

Connects to the TLS server at hostname:PORT.  The default PORT
is 443.  By default, this reads a request from stdin (to EOF)
before making the connection.  --http replaces this with a
basic HTTP GET request for /.

If --cafile is not supplied, a built-in set of CA certificates
are used from the webpki-roots crate.

  tlsclient-mio [options] [--suite SUITE ...] [--proto PROTO ...] [--protover PROTOVER ...] <hostname>
  tlsclient-mio (--version | -v)
  tlsclient-mio (--help | -h)

    -p, --port PORT     Connect to PORT [default: 443].
    --http              Send a basic HTTP GET request for /.
    --cafile CAFILE     Read root certificates from CAFILE.
    --auth-key KEY      Read client authentication key from KEY.
    --auth-certs CERTS  Read client authentication certificates from CERTS.
                        CERTS must match up with KEY.
    --protover VERSION  Disable default TLS version list, and use
                        VERSION instead.  May be used multiple times.
    --suite SUITE       Disable default cipher suite list, and use
                        SUITE instead.  May be used multiple times.
    --proto PROTOCOL    Send ALPN extension containing PROTOCOL.
                        May be used multiple times to offer several protocols.
    --cache CACHE       Save session cache to file CACHE.
    --no-tickets        Disable session ticket support.
    --no-sni            Disable server name indication support.
    --insecure          Disable certificate verification.
    --verbose           Emit log output.
    --max-frag-size M   Limit outgoing messages to M bytes.
    --version, -v       Show tool version.
    --help, -h          Show this screen.

Some sample runs:

$ cargo run --bin tlsclient-mio -- --http mozilla-modern.badssl.com
HTTP/1.1 200 OK
Server: nginx/1.6.2 (Ubuntu)
Date: Wed, 01 Jun 2016 18:44:00 GMT
Content-Type: text/html
Content-Length: 644


$ cargo run --bin tlsclient-mio -- --http expired.badssl.com
TLS error: WebPkiError(CertExpired, ValidateServerCert)
Connection closed

Server example program

The server example program is named tlsserver-mio. The interface looks like:

Runs a TLS server on :PORT.  The default PORT is 443.

`echo' mode means the server echoes received data on each connection.

`http' mode means the server blindly sends a HTTP response on each

`forward' means the server forwards plaintext to a connection made to

`--certs' names the full certificate chain, `--key' provides the
RSA private key.

  tlsserver-mio --certs CERTFILE --key KEYFILE [--suite SUITE ...] [--proto PROTO ...] [--protover PROTOVER ...] [options] echo
  tlsserver-mio --certs CERTFILE --key KEYFILE [--suite SUITE ...] [--proto PROTO ...] [--protover PROTOVER ...] [options] http
  tlsserver-mio --certs CERTFILE --key KEYFILE [--suite SUITE ...] [--proto PROTO ...] [--protover PROTOVER ...] [options] forward <fport>
  tlsserver-mio (--version | -v)
  tlsserver-mio (--help | -h)

    -p, --port PORT     Listen on PORT [default: 443].
    --certs CERTFILE    Read server certificates from CERTFILE.
                        This should contain PEM-format certificates
                        in the right order (the first certificate should
                        certify KEYFILE, the last should be a root CA).
    --key KEYFILE       Read private key from KEYFILE.  This should be a RSA
                        private key or PKCS8-encoded private key, in PEM format.
    --ocsp OCSPFILE     Read DER-encoded OCSP response from OCSPFILE and staple
                        to certificate.  Optional.
    --auth CERTFILE     Enable client authentication, and accept certificates
                        signed by those roots provided in CERTFILE.
    --require-auth      Send a fatal alert if the client does not complete client
    --resumption        Support session resumption.
    --tickets           Support tickets.
    --protover VERSION  Disable default TLS version list, and use
                        VERSION instead.  May be used multiple times.
    --suite SUITE       Disable default cipher suite list, and use
                        SUITE instead.  May be used multiple times.
    --proto PROTOCOL    Negotiate PROTOCOL using ALPN.
                        May be used multiple times.
    --verbose           Emit log output.
    --version, -v       Show tool version.
    --help, -h          Show this screen.

Here's a sample run; we start a TLS echo server, then connect to it with openssl and tlsclient-mio:

$ cargo run --bin tlsserver-mio -- --certs test-ca/rsa/end.fullchain --key test-ca/rsa/end.rsa -p 8443 echo &
$ echo hello world | openssl s_client -ign_eof -quiet -connect localhost:8443
depth=2 CN = ponytown RSA CA
verify error:num=19:self signed certificate in certificate chain
hello world
$ echo hello world | cargo run --bin tlsclient-mio -- --cafile test-ca/rsa/ca.cert -p 8443 localhost
hello world


Rustls is distributed under the following three licenses:

  • Apache License version 2.0.
  • MIT license.
  • ISC license.

These are included as LICENSE-APACHE, LICENSE-MIT and LICENSE-ISC respectively. You may use this software under the terms of any of these licenses, at your option.

Code of conduct

This project adopts the Rust Code of Conduct. Please email [email protected] to report any instance of misconduct, or if you have any comments or questions on the Code of Conduct.

*Note that all licence references and agreements mentioned in the rustls README section above are relevant to that project's source code only.