加密Android

Question

I'm trying to understand a code that lists all supported encryption algorithms on an Android device. I'm just wondering why is it necessary to add the step with ('Alg.Alias') and remove these characters from the service name ? The application stops working without it and I don't understand why !

package com.example.lab_enc_dec;

import java.security.Provider;
import java.security.Security;
import java.util.Iterator;
import java.util.Set;
import java.util.TreeSet;

import com.example.lab_enc_dec.R;

import android.app.Activity;
import android.os.Bundle;
import android.widget.TextView;

public class ListCryptoAlgorithms extends Activity {
    static final String TAG = "ListCryptoAlgorithms";

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

        setContentView(R.layout.algorithm_list);

        ListSupportedAlgorithms();
    }

    public void ListSupportedAlgorithms() {
        String result = "";

        // get all the providers
        Provider[] providers = Security.getProviders();

        for (int p = 0; p < providers.length; p++) {
            // get all service types for a specific provider
            Set<Object> ks = providers[p].keySet();
            Set<String> servicetypes = new TreeSet<String>();
            for (Iterator<Object> it = ks.iterator(); it.hasNext();) {
                String k = it.next().toString();
                k = k.split(" ")[0];
                if (k.startsWith("Alg.Alias."))
                    k = k.substring(10);                

                servicetypes.add(k.substring(0, k.indexOf('.')));
            }

            // get all algorithms for a specific service type
            int s = 1;
            for (Iterator<String> its = servicetypes.iterator(); its.hasNext();) {
                String stype = its.next();
                Set<String> algorithms = new TreeSet<String>();
                for (Iterator<Object> it = ks.iterator(); it.hasNext();) {
                    String k = it.next().toString();
                    k = k.split(" ")[0];
                    if (k.startsWith(stype + "."))
                        algorithms.add(k.substring(stype.length() + 1));
                    else if (k.startsWith("Alg.Alias." + stype +".")) 
                        algorithms.add(k.substring(stype.length() + 11));
                }

                int a = 1;
                for (Iterator<String> ita = algorithms.iterator(); ita.hasNext();) {
                    result += ("[P#" + (p + 1) + ":" + providers[p].getName() + "]" +
                            "[S#" + s + ":" + stype + "]" +
                            "[A#" + a + ":" + ita.next() + "]\n");
                    a++;
                }

                s++;
            }
        }

        TextView tv = (TextView)findViewById(R.id.supp_alg_result);
        tv.setText(result);
    }

}

Answer 1

Aliases are aliases to existing algorithms that have been registered under a different name. Those names are prefixed with "Alg.Alias." in the list so they can be distinguished from the initial registration. However, you should not use "Alg.Alias" in the getInstance methods of the various JCE classes such as Cihper . Hence the list removes the "Alg.Alias." from the service name.