Does OkHttp support accepting self-signed SSL certs?

Yes, It does.

Retrofit allows you to set your custom HTTP client, that is configured to your needs.

As for self-signed SSL certs there is a discussion here. The link contains code samples to add self-signed SSL to Android’s DefaultHttpClient and to load this client to Retrofit.

If you need OkHttpClient to accept self signed SSL, you need to pass it custom javax.net.ssl.SSLSocketFactory instance via setSslSocketFactory(SSLSocketFactory sslSocketFactory) method.

The easiest method to get a socket factory is to get one from javax.net.ssl.SSLContext as discussed here.

Here is a sample for configuring OkHttpClient:

OkHttpClient client = new OkHttpClient();
KeyStore keyStore = readKeyStore(); //your method to obtain KeyStore
SSLContext sslContext = SSLContext.getInstance("SSL");
TrustManagerFactory trustManagerFactory = TrustManagerFactory.getInstance(TrustManagerFactory.getDefaultAlgorithm());
trustManagerFactory.init(keyStore);
KeyManagerFactory keyManagerFactory = KeyManagerFactory.getInstance(KeyManagerFactory.getDefaultAlgorithm());
keyManagerFactory.init(keyStore, "keystore_pass".toCharArray());
sslContext.init(keyManagerFactory.getKeyManagers(),trustManagerFactory.getTrustManagers(), new SecureRandom());
client.setSslSocketFactory(sslContext.getSocketFactory());

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Updated code for okhttp3 (using builder):

    OkHttpClient client = new OkHttpClient.Builder()
            .sslSocketFactory(sslContext.getSocketFactory())
            .build();

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the client here is now configured to use certificates from your KeyStore. However it will only trust the certificates in your KeyStore and will not trust anything else, even if your system trust them by default. (If you have only self signed certs in your KeyStore and try to connect to Google main page via HTTPS you will get SSLHandshakeException).

You can obtain KeyStore instance from file as seen in docs:

KeyStore readKeyStore() {
    KeyStore ks = KeyStore.getInstance(KeyStore.getDefaultType());

    // get user password and file input stream
    char[] password = getPassword();

    java.io.FileInputStream fis = null;
    try {
        fis = new java.io.FileInputStream("keyStoreName");
        ks.load(fis, password);
    } finally {
        if (fis != null) {
            fis.close();
        }
    }
    return ks;
}

If you are on android you can put it in res/raw folder and get it from a Context instance using

fis = context.getResources().openRawResource(R.raw.your_keystore_filename);

There are several discussions on how to create your keystore. For example here