導語

1. 數字簽名一般會使用RSA演算法，對於RSA演算法的理解，阮一峰也提供兩篇生動的文章來說明：

   • 上篇
   • 下篇

   如果看到一大段數學公式就不想往下看的同學，我這裡給個簡化版的：

   將兩個大質數相乘十分容易，但是想要對其乘積進行因式分解卻極其困難，文章中給出了的例子：
   將
   1230186684530117755130494958384962720772853569595334
   7921973224521517264005072636575187452021997864693899
   5647494277406384592519255732630345373154826850791702
   6122142913461670429214311602221240479274737794080665
   351419597459856902143413
   分解成兩個質數的乘積，因為現在的方法只能暴力破解，以目前電子計算機的運算速率來看，即使破解了，也會耗費相當長的時間，導致破解行為本身沒有什麼意義

一般網站的連結

google給的示例程式碼有些過於簡單，我添加了一些東西，複製貼上就可以執行：

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        new Thread() {
            @Override
 

            public void run() {
                try {
                    URL url = new URL("https://wikipedia.org");
                    URLConnection urlConnection = url.openConnection();
                    InputStream in = urlConnection.getInputStream();
                    printInputStream(in);
                } catch
 
 (MalformedURLException e) {
                    e.printStackTrace();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }.start();
    }

    private void printInputStream(InputStream is){
        BufferedReader reader = new BufferedReader(new InputStreamReader(is));
        StringBuffer sb = new StringBuffer();
        String line = null;
        try {
            while ((line = reader.readLine()) != null) {
                sb.append(line + "\n");
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                is.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        String rs = sb.toString();
        Log.e("inputSteam",rs);
    }
}

有數字簽名的網站

不做任何處理

官方給的處理

這裡同樣處理一下細節，複製可以直接執行

public class MainActivity extends AppCompatActivity {
    private final String load = "-----BEGIN CERTIFICATE-----\n" +
            "MIIEBzCCA3CgAwIBAgIBADANBgkqhkiG9w0BAQQFADCBlDELMAkGA1UEBhMCVVMxCzAJBgNVBAgT\n" +
            "AldBMSEwHwYDVQQKExhVbml2ZXJzaXR5IG9mIFdhc2hpbmd0b24xFDASBgNVBAsTC1VXIFNlcnZp\n" +
            "Y2VzMRcwFQYDVQQDEw5VVyBTZXJ2aWNlcyBDQTEmMCQGCSqGSIb3DQEJARYXaGVscEBjYWMud2Fz\n" +
            "aGluZ3Rvbi5lZHUwHhcNMDMwMjI1MTgyNTA5WhcNMzAwOTAzMTgyNTA5WjCBlDELMAkGA1UEBhMC\n" +
            "VVMxCzAJBgNVBAgTAldBMSEwHwYDVQQKExhVbml2ZXJzaXR5IG9mIFdhc2hpbmd0b24xFDASBgNV\n" +
            "BAsTC1VXIFNlcnZpY2VzMRcwFQYDVQQDEw5VVyBTZXJ2aWNlcyBDQTEmMCQGCSqGSIb3DQEJARYX\n" +
            "aGVscEBjYWMud2FzaGluZ3Rvbi5lZHUwgZ8wDQYJKoZIhvcNAQEBBQADgY0AMIGJAoGBALwCo6h4\n" +
            "T44m+7ve+BrnEqflqBISFaZTXyJTjIVQ39ZWhE0B3LafbbZYju0imlQLG+MEVAtNDdiYICcBcKsa\n" +
            "pr2dxOi31Nv0moCkOj7iQueMVU4E1TghYIR2I8hqixFCQIP/CMtSDail/POzFzzdVxI1pv2wRc5c\n" +
            "L6zNwV25gbn3AgMBAAGjggFlMIIBYTAdBgNVHQ4EFgQUVdfBM8b6k/gnPcsgS/VajliXfXQwgcEG\n" +
            "A1UdIwSBuTCBtoAUVdfBM8b6k/gnPcsgS/VajliXfXShgZqkgZcwgZQxCzAJBgNVBAYTAlVTMQsw\n" +
            "CQYDVQQIEwJXQTEhMB8GA1UEChMYVW5pdmVyc2l0eSBvZiBXYXNoaW5ndG9uMRQwEgYDVQQLEwtV\n" +
            "VyBTZXJ2aWNlczEXMBUGA1UEAxMOVVcgU2VydmljZXMgQ0ExJjAkBgkqhkiG9w0BCQEWF2hlbHBA\n" +
            "Y2FjLndhc2hpbmd0b24uZWR1ggEAMAwGA1UdEwQFMAMBAf8wKwYDVR0RBCQwIoYgaHR0cDovL2Nl\n" +
            "cnRzLmNhYy53YXNoaW5ndG9uLmVkdS8wQQYDVR0fBDowODA2oDSgMoYwaHR0cDovL2NlcnRzLmNh\n" +
            "Yy53YXNoaW5ndG9uLmVkdS9VV1NlcnZpY2VzQ0EuY3JsMA0GCSqGSIb3DQEBBAUAA4GBAIn0PNmI\n" +
            "JjT9bM5d++BtQ5UpccUBI9XVh1sCX/NdxPDZ0pPCw7HOOwILumpulT9hGZm9Rd+W4GnNDAMV40we\n" +
            "s8REptvOZObBBrjaaphDe1D/MwnrQythmoNKc33bFg9RotHrIfT4EskaIXSx0PywbyfIR1wWxMpr\n" +
            "8gbCjAEUHNF/\n" +
            "-----END CERTIFICATE-----";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        new Thread() {
            @Override
            public void run() {
                super.run();
                URL url = null;
                HttpsURLConnection urlConnection = null;
                InputStream in = null;
                try {
                    //1.生成證書:Certificate
                    CertificateFactory cf = CertificateFactory.getInstance("X.509");
                    InputStream caInput = new ByteArrayInputStream(load.getBytes());
                    Certificate ca = null;
                    try {
                        ca = cf.generateCertificate(caInput);
                        System.out.println("ca=" + ((X509Certificate) ca).getSubjectDN());
                    } catch (CertificateException e) {
                        e.printStackTrace();
                    } finally {
                        caInput.close();
                    }

                    //2.初始化公鑰:keyStore
                    String keyStoreType = KeyStore.getDefaultType();
                    KeyStore keyStore = KeyStore.getInstance(keyStoreType);
                    keyStore.load(null, null);
                    keyStore.setCertificateEntry("ca", ca);

                    //3.初始化TrustManagerFactory
                    String tmfAlgorithm = TrustManagerFactory.getDefaultAlgorithm();
                    TrustManagerFactory tmf = TrustManagerFactory.getInstance(tmfAlgorithm);
                    tmf.init(keyStore);

                    //4.初始化sslContext
                    SSLContext context = SSLContext.getInstance("TLS");
                    context.init(null, tmf.getTrustManagers(), null);

                    //5.建立Https連結
                    url = new URL("https://certs.cac.washington.edu/CAtest/");
                    //注意，這裡是HttpsURLConnection
                    urlConnection = (HttpsURLConnection) url.openConnection();
                    urlConnection.setSSLSocketFactory(context.getSocketFactory());
                    in = urlConnection.getInputStream();
                } catch (MalformedURLException e) {
                    e.printStackTrace();
                } catch (IOException e) {
                    e.printStackTrace();
                } catch (CertificateException e) {
                    e.printStackTrace();
                } catch (NoSuchAlgorithmException e) {
                    e.printStackTrace();
                } catch (KeyStoreException e) {
                    e.printStackTrace();
                } catch (KeyManagementException e) {
                    e.printStackTrace();
                }
                copyInputStreamToOutputStream(in);
            }
        }.start();
    }

    private void copyInputStreamToOutputStream(InputStream is) {
        if (is == null)
            return;
        BufferedReader reader = new BufferedReader(new InputStreamReader(is));
        StringBuffer sb = new StringBuffer();
        String line = null;
        try {
            while ((line = reader.readLine()) != null) {
                sb.append(line + "\n");
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                is.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        Log.e("inputstrem", sb.toString());
    }
}

第二行的load的由來：

1. 下載load-der.crt
2. 在cmd中輸入命令：keytool -printcert -rfc -file 檔案地址

實現的流程也很簡單：

Volley的示例

在實際開發中，我們使用網路框架來進行網路中的互動，所以上面的程式碼肯定是用不到的；現在通用的網路請求框架，都給HTTPS請求留出了空間來實現，下面以Volley為例：

public class MainActivity extends Activity {
    private final String load = "-----BEGIN CERTIFICATE-----\n" +
            "MIIEBzCCA3CgAwIBAgIBADANBgkqhkiG9w0BAQQFADCBlDELMAkGA1UEBhMCVVMxCzAJBgNVBAgT\n" +
            "AldBMSEwHwYDVQQKExhVbml2ZXJzaXR5IG9mIFdhc2hpbmd0b24xFDASBgNVBAsTC1VXIFNlcnZp\n" +
            "Y2VzMRcwFQYDVQQDEw5VVyBTZXJ2aWNlcyBDQTEmMCQGCSqGSIb3DQEJARYXaGVscEBjYWMud2Fz\n" +
            "aGluZ3Rvbi5lZHUwHhcNMDMwMjI1MTgyNTA5WhcNMzAwOTAzMTgyNTA5WjCBlDELMAkGA1UEBhMC\n" +
            "VVMxCzAJBgNVBAgTAldBMSEwHwYDVQQKExhVbml2ZXJzaXR5IG9mIFdhc2hpbmd0b24xFDASBgNV\n" +
            "BAsTC1VXIFNlcnZpY2VzMRcwFQYDVQQDEw5VVyBTZXJ2aWNlcyBDQTEmMCQGCSqGSIb3DQEJARYX\n" +
            "aGVscEBjYWMud2FzaGluZ3Rvbi5lZHUwgZ8wDQYJKoZIhvcNAQEBBQADgY0AMIGJAoGBALwCo6h4\n" +
            "T44m+7ve+BrnEqflqBISFaZTXyJTjIVQ39ZWhE0B3LafbbZYju0imlQLG+MEVAtNDdiYICcBcKsa\n" +
            "pr2dxOi31Nv0moCkOj7iQueMVU4E1TghYIR2I8hqixFCQIP/CMtSDail/POzFzzdVxI1pv2wRc5c\n" +
            "L6zNwV25gbn3AgMBAAGjggFlMIIBYTAdBgNVHQ4EFgQUVdfBM8b6k/gnPcsgS/VajliXfXQwgcEG\n" +
            "A1UdIwSBuTCBtoAUVdfBM8b6k/gnPcsgS/VajliXfXShgZqkgZcwgZQxCzAJBgNVBAYTAlVTMQsw\n" +
            "CQYDVQQIEwJXQTEhMB8GA1UEChMYVW5pdmVyc2l0eSBvZiBXYXNoaW5ndG9uMRQwEgYDVQQLEwtV\n" +
            "VyBTZXJ2aWNlczEXMBUGA1UEAxMOVVcgU2VydmljZXMgQ0ExJjAkBgkqhkiG9w0BCQEWF2hlbHBA\n" +
            "Y2FjLndhc2hpbmd0b24uZWR1ggEAMAwGA1UdEwQFMAMBAf8wKwYDVR0RBCQwIoYgaHR0cDovL2Nl\n" +
            "cnRzLmNhYy53YXNoaW5ndG9uLmVkdS8wQQYDVR0fBDowODA2oDSgMoYwaHR0cDovL2NlcnRzLmNh\n" +
            "Yy53YXNoaW5ndG9uLmVkdS9VV1NlcnZpY2VzQ0EuY3JsMA0GCSqGSIb3DQEBBAUAA4GBAIn0PNmI\n" +
            "JjT9bM5d++BtQ5UpccUBI9XVh1sCX/NdxPDZ0pPCw7HOOwILumpulT9hGZm9Rd+W4GnNDAMV40we\n" +
            "s8REptvOZObBBrjaaphDe1D/MwnrQythmoNKc33bFg9RotHrIfT4EskaIXSx0PywbyfIR1wWxMpr\n" +
            "8gbCjAEUHNF/\n" +
            "-----END CERTIFICATE-----";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        //生成SSLSocketFactory
        SSLSocketFactory sslSocketFactory = initSSLSocketFactory();

        //HurlStack兩個引數預設都是null,如果傳入SSLSocketFactory，那麼會以Https的方式來請求網路
        HurlStack stack = new HurlStack(null, sslSocketFactory);

        //通常，我們呼叫的是Volley.newRequestQueue(context),HurlStack為預設的，也就是不處理Https的情況
        //現在傳入處理Https的HurlStack，Volley就會去處理相應的請求
        RequestQueue requestQueue = Volley.newRequestQueue(this, stack);

        //去訪問網路
        StringRequest request = new StringRequest("https://certs.cac.washington.edu/CAtest/",
                new Response.Listener<String>() {
                    @Override
                    public void onResponse(String response) {
                        Log.e("onResponse", response);
                    }
                }, new Response.ErrorListener() {
            @Override
            public void onErrorResponse(VolleyError error) {
                Log.e("error", error.getMessage());
            }
        });
        requestQueue.add(request);
    }

    /**
     * 生成SSLSocketFactory
     * 這裡的程式碼與之前相比，沒有什麼不同
     *
     * @return
     */
    private SSLSocketFactory initSSLSocketFactory() {
        //生成證書:Certificate
        CertificateFactory cf = null;
        SSLSocketFactory factory = null;
        try {
            cf = CertificateFactory.getInstance("X.509");
            InputStream caInput = new ByteArrayInputStream(load.getBytes());
            Certificate ca = null;
            try {
                ca = cf.generateCertificate(caInput);
            } finally {
                try {
                    caInput.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }

            //初始化公鑰:keyStore
            String keyType = KeyStore.getDefaultType();
            KeyStore keyStore = KeyStore.getInstance(keyType);
            keyStore.load(null, null);
            keyStore.setCertificateEntry("ca", ca);

            //初始化TrustManagerFactory
            String algorithm = TrustManagerFactory.getDefaultAlgorithm();
            TrustManagerFactory managerFactory = TrustManagerFactory.getInstance(algorithm);
            managerFactory.init(keyStore);

            //初始化sslContext
            SSLContext sslContext = SSLContext.getInstance("TLS");
            sslContext.init(null, managerFactory.getTrustManagers(), null);
            factory = sslContext.getSocketFactory();

        } catch (CertificateException e) {
            e.printStackTrace();
        } catch (NoSuchAlgorithmException e) {
            e.printStackTrace();
        } catch (KeyStoreException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } catch (KeyManagementException e) {
            e.printStackTrace();
        }

        return factory;
    }
}

示例程式碼很簡單，如果有需要，在專案裡稍微寫寫程式碼，就可以整合進去。
成熟網路請求框架都給使用者留出了傳遞“SSLSocketFactory ”的入口，仔細閱讀下網路請求框架初始化的程式碼，就可以找對應的地方。

結語

HTTPS認證貌似要花不少的銀子，一般各位也不會用到，如果要使用HTTPS，可以參考下上面的示例程式碼。另外，開頭推薦阮一峰的幾篇文章，希望各位看官查閱。