KeystoreHelper.java

/*
   AES-256-GCM key-wrapping helper backed by Android Keystore.
 
   A random 256-bit database key is generated once, wrapped with the
   Keystore-resident master key, and stored in SharedPreferences as
   Base64( IV[12] || ciphertext ).
*/
 
package com.freerdp.freerdpcore.security;
 
import android.content.Context;
import android.content.SharedPreferences;
import android.security.keystore.KeyGenParameterSpec;
import android.security.keystore.KeyProperties;
import android.util.Base64;
 
import java.security.KeyStore;
import java.security.SecureRandom;
 
import javax.crypto.Cipher;
import javax.crypto.KeyGenerator;
import javax.crypto.SecretKey;
import javax.crypto.spec.GCMParameterSpec;
 
public final class KeystoreHelper
{
  private static final String KEYSTORE_PROVIDER = "AndroidKeyStore";
  private static final String KEY_ALIAS = "freerdp_db_master_key";
  private static final String PREFS_NAME = "freerdp_security_prefs";
  private static final String PREF_ENCRYPTED_DB_KEY = "encrypted_db_key";
  private static final String CIPHER_TRANSFORMATION = "AES/GCM/NoPadding";
  private static final int GCM_IV_LENGTH = 12;
  private static final int GCM_TAG_LENGTH = 128; // bits
  private static final int DB_KEY_LENGTH = 32;   // bytes (256-bit)
 
  private static volatile KeystoreHelper instance;
 
  private final Context applicationContext;
 
  private KeystoreHelper(Context context)
  {
    this.applicationContext = context.getApplicationContext();
  }
 
  public static KeystoreHelper getInstance(Context context)
  {
    if (instance == null)
    {
      synchronized (KeystoreHelper.class)
      {
        if (instance == null)
        {
          instance = new KeystoreHelper(context);
        }
      }
    }
    return instance;
  }
 
  public byte[] getOrCreateDbKey() throws KeystoreException
  {
    SharedPreferences prefs =
        applicationContext.getSharedPreferences(PREFS_NAME, Context.MODE_PRIVATE);
    String encoded = prefs.getString(PREF_ENCRYPTED_DB_KEY, null);
 
    if (encoded != null)
    {
      return decryptDbKey(encoded);
    }
 
    byte[] rawKey = new byte[DB_KEY_LENGTH];
    new SecureRandom().nextBytes(rawKey);
    String encrypted = encryptDbKey(rawKey);
    prefs.edit().putString(PREF_ENCRYPTED_DB_KEY, encrypted).apply();
    return rawKey;
  }
 
  private String encryptDbKey(byte[] rawKey) throws KeystoreException
  {
    try
    {
      SecretKey masterKey = getOrCreateMasterKey();
      Cipher cipher = Cipher.getInstance(CIPHER_TRANSFORMATION);
      cipher.init(Cipher.ENCRYPT_MODE, masterKey);
      byte[] iv = cipher.getIV();
      byte[] ciphertext = cipher.doFinal(rawKey);
 
      // Serialise as Base64( IV[12] || ciphertext )
      byte[] packed = new byte[GCM_IV_LENGTH + ciphertext.length];
      System.arraycopy(iv, 0, packed, 0, GCM_IV_LENGTH);
      System.arraycopy(ciphertext, 0, packed, GCM_IV_LENGTH, ciphertext.length);
      return Base64.encodeToString(packed, Base64.NO_WRAP);
    }
    catch (Exception e)
    {
      throw new KeystoreException("Failed to encrypt database key", e);
    }
  }
 
  private byte[] decryptDbKey(String encoded) throws KeystoreException
  {
    try
    {
      byte[] packed = Base64.decode(encoded, Base64.NO_WRAP);
      byte[] iv = new byte[GCM_IV_LENGTH];
      byte[] ciphertext = new byte[packed.length - GCM_IV_LENGTH];
      System.arraycopy(packed, 0, iv, 0, GCM_IV_LENGTH);
      System.arraycopy(packed, GCM_IV_LENGTH, ciphertext, 0, ciphertext.length);
 
      SecretKey masterKey = getOrCreateMasterKey();
      Cipher cipher = Cipher.getInstance(CIPHER_TRANSFORMATION);
      cipher.init(Cipher.DECRYPT_MODE, masterKey, new GCMParameterSpec(GCM_TAG_LENGTH, iv));
      return cipher.doFinal(ciphertext);
    }
    catch (Exception e)
    {
      throw new KeystoreException("Failed to decrypt database key", e);
    }
  }
 
  private SecretKey getOrCreateMasterKey() throws Exception
  {
    KeyStore keyStore = KeyStore.getInstance(KEYSTORE_PROVIDER);
    keyStore.load(null);
 
    if (keyStore.containsAlias(KEY_ALIAS))
    {
      return ((KeyStore.SecretKeyEntry)keyStore.getEntry(KEY_ALIAS, null)).getSecretKey();
    }
 
    KeyGenerator keyGen =
        KeyGenerator.getInstance(KeyProperties.KEY_ALGORITHM_AES, KEYSTORE_PROVIDER);
    keyGen.init(
        new KeyGenParameterSpec
            .Builder(KEY_ALIAS, KeyProperties.PURPOSE_ENCRYPT | KeyProperties.PURPOSE_DECRYPT)
            .setKeySize(256)
            .setBlockModes(KeyProperties.BLOCK_MODE_GCM)
            .setEncryptionPaddings(KeyProperties.ENCRYPTION_PADDING_NONE)
            .build());
    return keyGen.generateKey();
  }
 
  public static final class KeystoreException extends Exception
  {
    public KeystoreException(String message, Throwable cause)
    {
      super(message, cause);
    }
  }
}