SSL TrustManager setup on lower Android APIs

I have an app communicating with a HTTPS RPC.
The HTTP server is using a CAcert signed certificate.

I'm using a custom TrustManager for validating the certificate.

Because I can not be sure, CAcert is included in all devices' trusted key store.
Because I want to allow only CAcert to sign a certificate for this connection.

However, I'm following Google's best practices. The only thing I changed is:

Load the CAcert root certificate from a static byte[] instead a file
Replace the last part, where the example code loads a file, with HttpsURLConnection.setDefaultSSLSocketFactory(sslContext.getSocketFactory());. There is a JSONRPC2 API on top of the UrlConnection.

Devices tested:

working on Nexus 4 / mako running API18 / CM10.2
working on API18 emulator
working on API17 emulator
working on API14 emulator
not working on a HTC G2 running API10 / CM7.*
not working on API8 emulator

On low API devices it fails verifying the certificate during SSL handshake.
When trying to load https://google.com with this TrustManager on API18, it fails as expected because no trust anchor could be found.
So basically, this code should work and all of the methods are API1...
I know, that UrlConnection was broken on some lower APIs.

How do I fix this?

Code:

/**
 * Trust only CAcert's CA. CA cert is injected as byte[]. Following best practices from
 * https://developer.android.com/training/articles/security-ssl.html#UnknownCa
 */
private static void trustCAcert() {
    try {
        // Load CAs from an InputStream
        CertificateFactory cf = CertificateFactory.getInstance("X.509");

        ByteArrayInputStream is = new ByteArrayInputStream(CACERTROOTDER);

        Certificate ca;
        try {
            ca = cf.generateCertificate(is);
            Log.d(TAG, "ca=", ((X509Certificate) ca).getSubjectDN());
        } finally {
            is.close();
        }

        // Create a KeyStore containing our trusted CAs
        String keyStoreType = KeyStore.getDefaultType();
        KeyStore keyStore = KeyStore.getInstance(keyStoreType);
        keyStore.load(null, null);
        keyStore.setCertificateEntry("ca", ca);

        // Create a TrustManager that trusts the CAs in our KeyStore
        String tmfAlgorithm = TrustManagerFactory.getDefaultAlgorithm();
        TrustManagerFactory tmf = TrustManagerFactory.getInstance(tmfAlgorithm);
        tmf.init(keyStore);

        // Create an SSLContext that uses our TrustManager
        SSLContext sslContext = SSLContext.getInstance("TLS");
        sslContext.init(null, tmf.getTrustManagers(), null);

        HttpsURLConnection.setDefaultSSLSocketFactory(
            sslContext.getSocketFactory());

        // added for testing only
        URL u = new URL(
            "https://myremoteapiurlsignedwiththesamecert.com/v1/doc.html");
        HttpsURLConnection con = (HttpsURLConnection) u.openConnection();
        con.setSSLSocketFactory(sslContext.getSocketFactory());
        BufferedReader r = new BufferedReader(
            new InputStreamReader(
                con.getInputStream())); // the exception is thrown here
        // because verification fails
        String l;
        while ((l = r.readLine()) != null) {
            Log.d(TAG, "l: ", l);
        }
    } catch (IOException e) { // none of the exceptions is thrown during setup
        Log.e(TAG, "IOException", e);
    } catch (CertificateException e) {
        Log.e(TAG, "CertificateException", e);
    } catch (NoSuchAlgorithmException e) {
        Log.e(TAG, "NoSuchAlgorithmException", e);
    } catch (KeyStoreException e) {
        Log.e(TAG, "KeyStoreException", e);
    } catch (KeyManagementException e) {
        Log.e(TAG, "KeyManagementException", e);
    }
}

Logs:

APIUtils  D  ca=OID.1.2.840.113549.1.9.1=#1612737570706F7274406361636572742E6F7267, CN=CA Cert Signing Authority, OU=http://www.cacert.org, O=Root CA
          E  IOException
          E  javax.net.ssl.SSLException: Not trusted server certificate
          E         at org.apache.harmony.xnet.provider.jsse.OpenSSLSocketImpl.startHandshake(OpenSSLSocketImpl.java:371)
          E         at org.apache.harmony.luni.internal.net.www.protocol.http.HttpConnection.getSecureSocket(HttpConnection.java:168)
          E         at org.apache.harmony.luni.internal.net.www.protocol.https.HttpsURLConnectionImpl$HttpsEngine.connect(HttpsURLConnectionImpl.java:399)
          E         at org.apache.harmony.luni.internal.net.www.protocol.http.HttpURLConnectionImpl.getInputStream(HttpURLConnectionImpl.java:1152)
          E         at org.apache.harmony.luni.internal.net.www.protocol.https.HttpsURLConnectionImpl.getInputStream(HttpsURLConnectionImpl.java:253)
          E         at de.terminbuddy.android.network.APIUtils.trustCAcert(APIUtils.java:294)
          E         at de.terminbuddy.android.network.APIUtils.initRpcSession(APIUtils.java:243)
          E         at de.terminbuddy.android.network.APIUtils.runRPC(APIUtils.java:323)
          E         at de.terminbuddy.android.network.AsyncJSONRPCTask.doInBackground(AsyncJSONRPCTask.java:55)
          E         at de.terminbuddy.android.network.AsyncJSONRPCTask.doInBackground(AsyncJSONRPCTask.java:17)
          E         at android.os.AsyncTask$2.call(AsyncTask.java:185)
          E         at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:305)
          E         at java.util.concurrent.FutureTask.run(FutureTask.java:137)
          E         at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1068)
          E         at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:561)
          E         at java.lang.Thread.run(Thread.java:1096)
          E  Caused by: java.security.cert.CertificateException: java.security.cert.CertPathValidatorException: Could not validate certificate signature.
          E         at org.apache.harmony.xnet.provider.jsse.TrustManagerImpl.checkServerTrusted(TrustManagerImpl.java:168)
          E         at org.apache.harmony.xnet.provider.jsse.OpenSSLSocketImpl.startHandshake(OpenSSLSocketImpl.java:366)
          E         ... 15 more
          E  Caused by: java.security.cert.CertPathValidatorException: Could not validate certificate signature.
          E         at org.bouncycastle.jce.provider.PKIXCertPathValidatorSpi.engineValidate(PKIXCertPathValidatorSpi.java:342)
          E         at java.security.cert.CertPathValidator.validate(CertPathValidator.java:202)
          E         at org.apache.harmony.xnet.provider.jsse.TrustManagerImpl.checkServerTrusted(TrustManagerImpl.java:164)
          E         ... 16 more
          E  Caused by: java.security.SignatureException: Signature was not verified.
          E         at org.apache.harmony.security.provider.cert.X509CertImpl.fastVerify(X509CertImpl.java:601)
          E         at org.apache.harmony.security.provider.cert.X509CertImpl.verify(X509CertImpl.java:544)
          E         at org.bouncycastle.jce.provider.PKIXCertPathValidatorSpi.engineValidate(PKIXCertPathValidatorSpi.java:337)
          E         ... 18 more

I found a solution for this problem:

It looks like CRL verification is some kind of broken (API<14/11) for CAcert's certificates.
SNI is involved which is not supported in API<8.

Basically I run my own checks on API<14 with the implementation found here: Validate X509 certificates using Java APis

private static final byte[] CACERTROOTDER = new byte[]{
        48, -126, 7, 61, 48, -126, 5, 37, -96, 3, 2, 1, 2, 2, 1, 0,
        // ...
        };

/**
 * Read x509 certificated file from byte[].
 *
 * @param bytes certificate in der format
 * @return certificate
 */
private static X509Certificate getCertificate(final byte[] bytes)
        throws IOException, CertificateException {
    CertificateFactory cf = CertificateFactory.getInstance("X.509");
    X509Certificate ca;
    ByteArrayInputStream is = new ByteArrayInputStream(bytes);
    try {
        ca = (X509Certificate) cf.generateCertificate(is);
        Log.d(TAG, "ca=", ca.getSubjectDN());
    } finally {
        is.close();
    }
    return ca;
}

/**
 * Trust only CAcert's CA. CA cert is injected as byte[]. Following best practices from
 * https://developer.android.com/training/articles/security-ssl.html#UnknownCa
 */
private static void trustCAcert()
        throws KeyStoreException, IOException,
        CertificateException, NoSuchAlgorithmException,
        KeyManagementException {
    // Create a KeyStore containing our trusted CAs
    String keyStoreType = KeyStore.getDefaultType();
    final KeyStore keyStore = KeyStore.getInstance(keyStoreType);
    keyStore.load(null, null);
    keyStore.setCertificateEntry("CAcert-root", getCertificate(CACERTROOTDER));
    // if your HTTPd is not sending the full chain, add class3 cert to the key store
    // keyStore.setCertificateEntry("CAcert-class3", getCertificate(CACERTCLASS3DER));

    // Create a TrustManager that trusts the CAs in our KeyStore
    final TrustManagerFactory tmf = TrustManagerFactory.getInstance(
            TrustManagerFactory.getDefaultAlgorithm());
    tmf.init(keyStore);

    // Create an SSLContext that uses our TrustManager
    SSLContext sslContext = SSLContext.getInstance("TLS");

    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.ICE_CREAM_SANDWICH) {
        // may work on HC+, but there is no AVD or device to test it
        sslContext.init(null, tmf.getTrustManagers(), null);
    } else {
        // looks like CLR is broken in lower APIs. implement out own checks here :x
        // see https://stackoverflow.com/q/18713966/2331953
        HttpsURLConnection.setDefaultHostnameVerifier(new HostnameVerifier() {
            public boolean verify(final String hostname, final SSLSession session) {
                try {
                    // check if hostname matches DN
                    String dn = session.getPeerCertificateChain()[0].getSubjectDN().toString();

                    Log.d(TAG, "DN=", dn);
                    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.GINGERBREAD) {
                        return dn.equals("CN=" + hostname);
                    } else {
                        // no SNI on API<9, but I know the first vhost's hostname
                        return dn.equals("CN=" + hostname)
                                || dn.equals("CN=" + hostname.replace("jsonrpc", "rest"));
                    }
                } catch (Exception e) {
                    Log.e(TAG, "unexpected exception", e);
                    return false;
                }
            }
        });

        // build our own trust manager
        X509TrustManager tm = new X509TrustManager() {
            public X509Certificate[] getAcceptedIssuers() {
                // nothing to do
                return new X509Certificate[0];
            }

            @Override
            public void checkClientTrusted(final X509Certificate[] chain,
                    final String authType)
                    throws CertificateException {
                // nothing to do
            }

            @Override
            public void checkServerTrusted(final X509Certificate[] chain,
                    final String authType) throws CertificateException {
                // nothing to do
                Log.d(TAG, "checkServerTrusted(", chain, ")");
                X509Certificate cert = chain[0];

                cert.checkValidity();

                CertificateFactory cf = CertificateFactory.getInstance("X.509");
                ArrayList<X509Certificate> list = new ArrayList<X509Certificate>();
                list.add(cert);
                CertPath cp = cf.generateCertPath(list);
                try {
                    PKIXParameters params = new PKIXParameters(keyStore);
                    params.setRevocationEnabled(false); // CLR is broken, remember?
                    CertPathValidator cpv = CertPathValidator
                            .getInstance(CertPathValidator.getDefaultType());
                    cpv.validate(cp, params);
                } catch (KeyStoreException e) {
                    Log.d(TAG, "invalid key store", e);
                    throw new CertificateException(e);
                } catch (InvalidAlgorithmParameterException e) {
                    Log.d(TAG, "invalid algorithm", e);
                    throw new CertificateException(e);
                } catch (NoSuchAlgorithmException e) {
                    Log.d(TAG, "no such algorithm", e);
                    throw new CertificateException(e);
                } catch (CertPathValidatorException e) {
                    Log.d(TAG, "verification failed");
                    throw new CertificateException(e);
                }
                Log.d(TAG, "verification successful");
            }
        };
        sslContext.init(null, new X509TrustManager[]{tm}, null);
    }

    HttpsURLConnection.setDefaultSSLSocketFactory(sslContext.getSocketFactory());
}