func URLSession(session: NSURLSession,  didReceiveChallenge challenge: NSURLAuthenticationChallenge, completionHandler (NSURLSessionAuthChallengeDisposition, NSURLCredential?) -> Void) {
    let serverTrust = challenge.protectionSpace.serverTrust
    let certificate = SecTrustGetCertificateAtIndex(serverTrust!, 0)

    // Set SSL policies for domain name check
    let policies = NSMutableArray();
    policies.addObject(SecPolicyCreateSSL(true, (challenge.protectionSpace.host)))
    SecTrustSetPolicies(serverTrust!, policies);

    // Evaluate server certificate
    var result: SecTrustResultType = 0
    SecTrustEvaluate(serverTrust!, &result)
    let isServerTrusted:Bool = (Int(result) == kSecTrustResultUnspecified || Int(result) == kSecTrustResultProceed)

    // Get local and remote cert data
    let remoteCertificateData:NSData = SecCertificateCopyData(certificate!)
    let pathToCert = NSBundle.mainBundle().pathForResource(githubCert, ofType: "cer")
    let localCertificate:NSData = NSData(contentsOfFile: pathToCert!)!

    if (isServerTrusted && remoteCertificateData.isEqualToData(localCertificate)) {
        let credential:NSURLCredential = NSURLCredential(forTrust: serverTrust!)
        completionHandler(.UseCredential, credential)
    } else {
        completionHandler(.CancelAuthenticationChallenge, nil)
    }
}