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Rustls is a modern TLS library written in Rust. 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

Build Status Coverage Status ( Documentation

Release history:

  • Next release:
    • Planned: removal of unused signature verification schemes at link-time.
    • Breaking change: internal buffers are now limited to 64 kB by default. Use Connection::set_buffer_limit to change the buffer limits to suit your application.
    • Breaking API change: PEM parsing now lives in the rustls-pemfile crate. This means rustls::internals::pemfile and rustls::RootCertStore::add_pem_file no longer exist.
    • Breaking API change: ServerCertVerifier::verify_server_cert and ClientCertVerifier::verify_client_cert pass the end-entity and intermediate certificates separately. This means rustls deals with the case where the certificate chain is empty, rather than leaving that to ServerCertVerifier/ClientCertVerifier implementation.
    • There are now 40% fewer unreachable unwraps in the core crate thanks to large refactoring efforts.
    • Breaking API change: the WebPkiError variant of rustls::Error now includes which operation failed.
    • Breaking API changes: These public API items have been renamed to meet naming guidelines:
      • rustls::TLSError to rustls::Error.
      • rustls::ResolvesServerCertUsingSNI to rustls::ResolvesServerCertUsingSni.
      • rustls::WebPKIVerifier to rustls::WebPkiVerifier.
      • rustls::ciphersuites to rustls::cipher_suites.
      • rustls::ClientHello::sigschemes to rustls::ClientHello::signature_schemes.
      • rustls::RootCertStore::get_subjects to rustls::RootCertStore::subjects.
      • rustls::ServerSession to rustls::ServerConnection.
      • rustls::ClientSession to rustls::ClientConnection.
      • rustls::ServerSession::get_sni_hostname to rustls::ServerConnection::sni_hostname.
      • rustls::ClientConfig::ciphersuites to rustls::ClientConfig::cipher_suites.
      • rustls::ServerConfig::ciphersuites to rustls::ServerConfig::cipher_suites.
      • rustls::ProducesTickets::get_lifetime to rustls::ProducesTickets::lifetime.
      • rustls::Session: get_peer_certificates to peer_certificates, get_alpn_protocol to alpn_protocol, get_protocol_version to protocol_version, get_negotiated_ciphersuite to negotiated_cipher_suite.
    • Breaking API change: ResolvesServerCert::resolve and ResolvesClientCert::resolve now return Option<Arc<CertifiedKey>> instead of Option<CertifiedKey>. CertifiedKey is now an immutable type.
    • Breaking API change: peer_certificates returns a borrow rather than a copy on the internally stored certificate chain.
    • Breaking API change: ClientConnection's DNS name parameter is now a new enum, ServerName, to allow future support for ECH and servers named by IP address.
  • 0.19.1 (2021-04-17):
    • Backport: fix security issue: there was a reachable panic in servers if a client sent an invalid ClientECDiffieHellmanPublic encoding, due to an errant unwrap() when parsing the encoding.
  • 0.19.0 (2020-11-22):
    • Ensured that get_peer_certificates is both better documented, and works uniformly for both full-handshake and resumed sessions.
    • Fix bug: fully qualified hostnames should have had their trailing dot stripped when quoted in the SNI extension.
  • 0.18.1 (2020-08-16):
    • Fix DoS vulnerability in TLS1.3 "Middlebox Compatibility Mode" CCS handling. This is thought to be quite minor -- see this commit message for a full discussion.
  • 0.18.0 (2020-07-04):
    • Allow custom certificate validation implementations to also handle handshake signature computation. This allows uses in non-web contexts, where webpki is not likely to process the certificates in use. Thanks to @DemiMarie-parity.
    • Performance improvements. Thanks to @nviennot.
    • Fixed client authentication being unduly rejected by client when server uses the superseded certificate_types field of CertificateRequest.
    • Breaking API change: The writev_tls API has been removed, in favour of using vectored IO support now offered by std::io::Write.
    • Added ed25519 support for authentication; thanks to @potatosalad.
    • Support removal of unused ciphersuites at link-time. To use this, call ClientConfig::with_ciphersuites instead of ClientConfig::new.

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, 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.
  • 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.

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:

$ cargo run --example 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


$ cargo run --example tlsclient -- --http
TLS error: WebPkiError(CertExpired, ValidateServerCert)
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:

$ cargo run --example 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 | cargo run --example 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.

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