How do I implement a 'Remember me' function in an Android Activity?

Question

I have a username, password, and checkbox (next to the text \'remember me\').

How do I to implement a remember me function to keep username and password data??

Answer 1

It should be noted that although the accepted answer successfully stores and retrieves the credentials, it stores them as plain-text.

This means that the password will be readily visible on a rooted phone. If you choose to store sensitive information such as a password in your application using SharedPreferences, then as Reto Meier (Technical Lead on Android Development Relations) states: You should at the very least encrypt the password before writing it to disk.

Here's an implementation that encrypts the user information before storing it in SharedPreferences:

/*
Copyright (C) 2012 Sveinung Kval Bakken, sveinung.bakken@gmail.com

Permission is hereby granted, free of charge, to any person obtaining
a copy of this software and associated documentation files (the
"Software"), to deal in the Software without restriction, including
without limitation the rights to use, copy, modify, merge, publish,
distribute, sublicense, and/or sell copies of the Software, and to
permit persons to whom the Software is furnished to do so, subject to
the following conditions:

The above copyright notice and this permission notice shall be
included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

 */

import java.io.UnsupportedEncodingException;
import java.security.GeneralSecurityException;
import java.security.InvalidAlgorithmParameterException;
import java.security.InvalidKeyException;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;

import javax.crypto.Cipher;
import javax.crypto.spec.IvParameterSpec;
import javax.crypto.spec.SecretKeySpec;

import android.content.Context;
import android.content.SharedPreferences;
import android.util.Base64;


public class SecurePreferences {

    public static class SecurePreferencesException extends RuntimeException {

        public SecurePreferencesException(Throwable e) {
            super(e);
        }

    }

    private static final String TRANSFORMATION = "AES/CBC/PKCS5Padding";
    private static final String KEY_TRANSFORMATION = "AES/ECB/PKCS5Padding";
    private static final String SECRET_KEY_HASH_TRANSFORMATION = "SHA-256";
    private static final String CHARSET = "UTF-8";

    private final boolean encryptKeys;
    private final Cipher writer;
    private final Cipher reader;
    private final Cipher keyWriter;
    private final SharedPreferences preferences;

    /**
     * This will initialize an instance of the SecurePreferences class
     * @param context your current context.
     * @param preferenceName name of preferences file (preferenceName.xml)
     * @param secureKey the key used for encryption, finding a good key scheme is hard. 
     * Hardcoding your key in the application is bad, but better than plaintext preferences. Having the user enter the key upon application launch is a safe(r) alternative, but annoying to the user.
     * @param encryptKeys settings this to false will only encrypt the values, 
     * true will encrypt both values and keys. Keys can contain a lot of information about 
     * the plaintext value of the value which can be used to decipher the value.
     * @throws SecurePreferencesException
     */
    public SecurePreferences(Context context, String preferenceName, String secureKey, boolean encryptKeys) throws SecurePreferencesException {
        try {
            this.writer = Cipher.getInstance(TRANSFORMATION);
            this.reader = Cipher.getInstance(TRANSFORMATION);
            this.keyWriter = Cipher.getInstance(KEY_TRANSFORMATION);

            initCiphers(secureKey);

            this.preferences = context.getSharedPreferences(preferenceName, Context.MODE_PRIVATE);

            this.encryptKeys = encryptKeys;
        }
        catch (GeneralSecurityException e) {
            throw new SecurePreferencesException(e);
        }
        catch (UnsupportedEncodingException e) {
            throw new SecurePreferencesException(e);
        }
    }

    protected void initCiphers(String secureKey) throws UnsupportedEncodingException, NoSuchAlgorithmException, InvalidKeyException,
            InvalidAlgorithmParameterException {
        IvParameterSpec ivSpec = getIv();
        SecretKeySpec secretKey = getSecretKey(secureKey);

        writer.init(Cipher.ENCRYPT_MODE, secretKey, ivSpec);
        reader.init(Cipher.DECRYPT_MODE, secretKey, ivSpec);
        keyWriter.init(Cipher.ENCRYPT_MODE, secretKey);
    }

    protected IvParameterSpec getIv() {
        byte[] iv = new byte[writer.getBlockSize()];
        System.arraycopy("fldsjfodasjifudslfjdsaofshaufihadsf".getBytes(), 0, iv, 0, writer.getBlockSize());
        return new IvParameterSpec(iv);
    }

    protected SecretKeySpec getSecretKey(String key) throws UnsupportedEncodingException, NoSuchAlgorithmException {
        byte[] keyBytes = createKeyBytes(key);
        return new SecretKeySpec(keyBytes, TRANSFORMATION);
    }

    protected byte[] createKeyBytes(String key) throws UnsupportedEncodingException, NoSuchAlgorithmException {
        MessageDigest md = MessageDigest.getInstance(SECRET_KEY_HASH_TRANSFORMATION);
        md.reset();
        byte[] keyBytes = md.digest(key.getBytes(CHARSET));
        return keyBytes;
    }

    public void put(String key, String value) {
        if (value == null) {
            preferences.edit().remove(toKey(key)).commit();
        }
        else {
            putValue(toKey(key), value);
        }
    }

    public boolean containsKey(String key) {
        return preferences.contains(toKey(key));
    }

    public void removeValue(String key) {
        preferences.edit().remove(toKey(key)).commit();
    }

    public String getString(String key) throws SecurePreferencesException {
        if (preferences.contains(toKey(key))) {
            String securedEncodedValue = preferences.getString(toKey(key), "");
            return decrypt(securedEncodedValue);
        }
        return null;
    }

    public void clear() {
        preferences.edit().clear().commit();
    }

    private String toKey(String key) {
        if (encryptKeys)
            return encrypt(key, keyWriter);
        else return key;
    }

    private void putValue(String key, String value) throws SecurePreferencesException {
        String secureValueEncoded = encrypt(value, writer);

        preferences.edit().putString(key, secureValueEncoded).commit();
    }

    protected String encrypt(String value, Cipher writer) throws SecurePreferencesException {
        byte[] secureValue;
        try {
            secureValue = convert(writer, value.getBytes(CHARSET));
        }
        catch (UnsupportedEncodingException e) {
            throw new SecurePreferencesException(e);
        }
        String secureValueEncoded = Base64.encodeToString(secureValue, Base64.NO_WRAP);
        return secureValueEncoded;
    }

    protected String decrypt(String securedEncodedValue) {
        byte[] securedValue = Base64.decode(securedEncodedValue, Base64.NO_WRAP);
        byte[] value = convert(reader, securedValue);
        try {
            return new String(value, CHARSET);
        }
        catch (UnsupportedEncodingException e) {
            throw new SecurePreferencesException(e);
        }
    }

    private static byte[] convert(Cipher cipher, byte[] bs) throws SecurePreferencesException {
        try {
            return cipher.doFinal(bs);
        }
        catch (Exception e) {
            throw new SecurePreferencesException(e);
        }
    }
}

The above code is not my own, here is the GitHub Page where it is sourced.

Here's a sample usage of the code:

SecurePreferences preferences = new SecurePreferences(context, "user-info", 
                                                       "YourSecurityKey", true);
// Put (all puts are automatically committed)
preferences.put("username", "MyUser");
preferences.put("password", "MyPassword");
// Get
String username = preferences.getString("username");
String password = preferences.getString("password");

Disclaimer: If someone has physical access to the phone, they could potentially gain access to the preferences file and to the application's binary. Meaning they could decompile the application and reveal the key used in the encryption and then unencrypt the user information.

However, nothing is 100% secure. The most secure method to store any information is to not store it at all. A determined hacker can hack almost anything, but encrypting the information will make it more difficult.

Answer 2

You can save values associated with your application using Preferences.

Define some statics to store the preference file name and the keys you're going to use:

public static final String PREFS_NAME = "MyPrefsFile";
private static final String PREF_USERNAME = "username";
private static final String PREF_PASSWORD = "password";

You'd then save the username and password as follows:

getSharedPreferences(PREFS_NAME,MODE_PRIVATE)
        .edit()
        .putString(PREF_USERNAME, username)
        .putString(PREF_PASSWORD, password)
        .commit();

So you would retrieve them like this:

SharedPreferences pref = getSharedPreferences(PREFS_NAME,MODE_PRIVATE);   
String username = pref.getString(PREF_USERNAME, null);
String password = pref.getString(PREF_PASSWORD, null);

if (username == null || password == null) {
    //Prompt for username and password
}

Alternatively, if you don't want to name a preferences file you can just use the default:

SharedPreferences pref = PreferenceManager.getDefaultSharedPreferences(this);