rustls 0.16.0

Rustls is a modern TLS library written in Rust.


Rustls is a modern TLS library written in Rust. It's pronounced 'rustles'. It uses ring for cryptography and libwebpki for certificate verification.


Rustls is ready for use. There are no major breaking interface changes expected. Here's what I'm working on now.

If you'd like to help out, please see

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Release history:

  • 0.16.0 (2019-08-10):
    • Optimisation of read path for polled non-blocking IO.
    • Correct an omission in TLS1.3 middlebox compatibility mode, causing handshake failures with servers behind buggy middleboxes.
    • Move to ring 0.16.
    • Assorted refactoring to reduce memory usage during and after handshake.
    • Update other dependencies.
  • 0.15.2 (2019-04-02):
    • Moved example code around for benefit of Fuchsia.
    • Example code fixes for Windows -- Windows is now a tested platform.
    • QUIC-specific bug fixes.
    • Update dependencies.
  • 0.15.1 (2019-01-29):
    • Fix incorrect offering of SHA1.
  • 0.15.0 (2019-01-20):
    • Update dependencies.
    • Breaking API change: ALPN protocols are now encoded as a Vec<u8>, not a String. This alters the type of:
      • ClientConfig::alpn_protocols
      • ClientConfig::set_protocols
      • ServerConfig::alpn_protocols
      • ServerConfig::set_protocols
      • Session::get_alpn_protocol
    • Emit a warning when receiving an invalid SNI extension, such as one including an IP address.
    • Extended QUIC support for later QUIC drafts.
    • Correct bug where we'd send more than one fatal alert for handshake failure cases.
    • Discontinue support for SHA1 signatures.
    • Move to Rust 2018 edition.
  • 0.14.0 (2018-09-30):
    • Introduce client-side support for 0-RTT data in TLS1.3.
    • Fix a bug in rustls::Stream for non-blocking transports.
    • Move TLS1.3 support from draft 23 to final RFC8446 version.
    • Don't offer (eg) TLS1.3 if no TLS1.3 suites are configured.
    • Support stateful resumption in TLS1.3. Stateless resumption was previously supported, but is not the default configuration.
    • Breaking API change: generate() removed from StoresServerSessions trait.
    • Breaking API change: take() added to StoresServerSessions trait.
  • 0.13.1 (2018-08-17):
    • Fix a bug in rustls::Stream for non-blocking transports (backport).
  • 0.13.0 (2018-07-15):
    • Move TLS1.3 support from draft 22 to 23.
    • Add support for SSLKEYLOGFILE; not enabled by default.
    • Add support for basic usage in QUIC.
    • ServerConfig::set_single_cert and company now report errors.
    • Add support for vectored IO: writev_tls can now be used to optimise system call usage.
    • Support ECDSA signing for server and client authentication.
    • Add type like rustls::Stream which owns its underlying TCP stream and rustls session.

See for further change history.


Lives here:


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 or RSA server authentication by clients.
  • ECDSA 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.
  • Chacha20Poly1305 bulk encryption.
  • 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.

Example code

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

Client example program

The client example program is named tlsclient. 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 [options] [--suite SUITE ...] [--proto PROTO ...] <hostname>
  tlsclient (--version | -v)
  tlsclient (--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.
    --mtu MTU           Limit outgoing messages to MTU bytes.
    --version, -v       Show tool version.
    --help, -h          Show this screen.

Some sample runs:

$ ./tlsclient --http
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


$ ./target/debug/examples/tlsclient --http
TLS error: WebPKIError(CertExpired)
Connection closed

Server example program

The server example program is named tlsserver. 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 --certs CERTFILE --key KEYFILE [--suite SUITE ...] [--proto PROTO ...] [options] echo
  tlsserver --certs CERTFILE --key KEYFILE [--suite SUITE ...] [--proto PROTO ...] [options] http
  tlsserver --certs CERTFILE --key KEYFILE [--suite SUITE ...] [--proto PROTO ...] [options] forward <fport>
  tlsserver (--version | -v)
  tlsserver (--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:

$ ./tlsserver --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 | ./tlsclient --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.