HTTPS
=====

OkHttp attempts to balance two competing concerns:

 * **Connectivity** to as many hosts as possible. That includes advanced hosts that run the latest versions of [boringssl](https://boringssl.googlesource.com/boringssl/) and less out of date hosts running older versions of [OpenSSL](https://www.openssl.org/).
 * **Security** of the connection. This includes verification of the remote webserver with certificates and the privacy of data exchanged with strong ciphers.

When negotiating a connection to an HTTPS server, OkHttp needs to know which [TLS versions](http://square.github.io/okhttp/4.x/okhttp/okhttp3/-tls-version/) and [cipher suites](http://square.github.io/okhttp/4.x/okhttp/okhttp3/-cipher-suite/) to offer. A client that wants to maximize connectivity would include obsolete TLS versions and weak-by-design cipher suites. A strict client that wants to maximize security would be limited to only the latest TLS version and strongest cipher suites.

Specific security vs. connectivity decisions are implemented by [ConnectionSpec](http://square.github.io/okhttp/4.x/okhttp/okhttp3/-connection-spec/). OkHttp includes four built-in connection specs:

 * `RESTRICTED_TLS` is a secure configuration, intended to meet stricter compliance requirements.
 * `MODERN_TLS` is a secure configuration that connects to modern HTTPS servers.
 * `COMPATIBLE_TLS` is a secure configuration that connects to secure–but not current–HTTPS servers.
 * `CLEARTEXT` is an insecure configuration that is used for `http://` URLs.

These loosely follow the model set in [Google Cloud Policies](https://cloud.google.com/load-balancing/docs/ssl-policies-concepts). We [track changes](tls_configuration_history.md) to this policy.

By default, OkHttp will attempt a `MODERN_TLS` connection.  However by configuring the client connectionSpecs you can allow a fall back to `COMPATIBLE_TLS` connection if the modern configuration fails.

```java
OkHttpClient client = new OkHttpClient.Builder()
    .connectionSpecs(Arrays.asList(ConnectionSpec.MODERN_TLS, ConnectionSpec.COMPATIBLE_TLS))
    .build();
```

The TLS versions and cipher suites in each spec can change with each release. For example, in OkHttp 2.2 we dropped support for SSL 3.0 in response to the [POODLE](http://googleonlinesecurity.blogspot.ca/2014/10/this-poodle-bites-exploiting-ssl-30.html) attack. And in OkHttp 2.3 we dropped support for [RC4](http://en.wikipedia.org/wiki/RC4#Security). As with your desktop web browser, staying up-to-date with OkHttp is the best way to stay secure.

You can build your own connection spec with a custom set of TLS versions and cipher suites. For example, this configuration is limited to three highly-regarded cipher suites. Its drawback is that it requires Android 5.0+ and a similarly current webserver.

```java
ConnectionSpec spec = new ConnectionSpec.Builder(ConnectionSpec.MODERN_TLS)
    .tlsVersions(TlsVersion.TLS_1_2)
    .cipherSuites(
          CipherSuite.TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256,
          CipherSuite.TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256,
          CipherSuite.TLS_DHE_RSA_WITH_AES_128_GCM_SHA256)
    .build();

OkHttpClient client = new OkHttpClient.Builder()
    .connectionSpecs(Collections.singletonList(spec))
    .build();
```

#### [Certificate Pinning](https://github.com/square/okhttp/blob/master/samples/guide/src/main/java/okhttp3/recipes/CertificatePinning.java)

By default, OkHttp trusts the certificate authorities of the host platform. This strategy maximizes connectivity, but it is subject to certificate authority attacks such as the [2011 DigiNotar attack](http://www.computerworld.com/article/2510951/cybercrime-hacking/hackers-spied-on-300-000-iranians-using-fake-google-certificate.html). It also assumes your HTTPS servers’ certificates are signed by a certificate authority.

Use [CertificatePinner](http://square.github.io/okhttp/4.x/okhttp/okhttp3/-certificate-pinner/) to restrict which certificates and certificate authorities are trusted. Certificate pinning increases security, but limits your server team’s abilities to update their TLS certificates. **Do not use certificate pinning without the blessing of your server’s TLS administrator!**

```java
  public CertificatePinning() {
    client = new OkHttpClient.Builder()
        .certificatePinner(new CertificatePinner.Builder()
            .add("publicobject.com", "sha256/afwiKY3RxoMmLkuRW1l7QsPZTJPwDS2pdDROQjXw8ig=")
            .build())
        .build();
  }

  public void run() throws Exception {
    Request request = new Request.Builder()
        .url("https://publicobject.com/robots.txt")
        .build();

    Response response = client.newCall(request).execute();
    if (!response.isSuccessful()) throw new IOException("Unexpected code " + response);

    for (Certificate certificate : response.handshake().peerCertificates()) {
      System.out.println(CertificatePinner.pin(certificate));
    }
  }
```

#### [Customizing Trusted Certificates](https://github.com/square/okhttp/blob/master/samples/guide/src/main/java/okhttp3/recipes/CustomTrust.java)

The full code sample shows how to replace the host platform’s certificate authorities with your own set. As above, **do not use custom certificates without the blessing of your server’s TLS administrator!**

```java
  private final OkHttpClient client;

  public CustomTrust() {
    SSLContext sslContext = sslContextForTrustedCertificates(trustedCertificatesInputStream());
    client = new OkHttpClient.Builder()
        .sslSocketFactory(sslContext.getSocketFactory())
        .build();
  }

  public void run() throws Exception {
    Request request = new Request.Builder()
        .url("https://publicobject.com/helloworld.txt")
        .build();

    Response response = client.newCall(request).execute();
    System.out.println(response.body().string());
  }

  private InputStream trustedCertificatesInputStream() {
    ... // Full source omitted. See sample.
  }

  public SSLContext sslContextForTrustedCertificates(InputStream in) {
    ... // Full source omitted. See sample.
  }
```