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      1 /*
      2  * Copyright (C) 2013 The Android Open Source Project
      3  *
      4  * Licensed under the Apache License, Version 2.0 (the "License");
      5  * you may not use this file except in compliance with the License.
      6  * You may obtain a copy of the License at
      7  *
      8  *      http://www.apache.org/licenses/LICENSE-2.0
      9  *
     10  * Unless required by applicable law or agreed to in writing, software
     11  * distributed under the License is distributed on an "AS IS" BASIS,
     12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     13  * See the License for the specific language governing permissions and
     14  * limitations under the License.
     15  */
     16 
     17 package com.example.android.vault;
     18 
     19 import android.content.Context;
     20 import android.security.KeyPairGeneratorSpec;
     21 
     22 import java.io.IOException;
     23 import java.math.BigInteger;
     24 import java.security.GeneralSecurityException;
     25 import java.security.KeyPair;
     26 import java.security.KeyPairGenerator;
     27 import java.security.KeyStore;
     28 import java.util.Calendar;
     29 import java.util.GregorianCalendar;
     30 
     31 import javax.crypto.Cipher;
     32 import javax.crypto.SecretKey;
     33 import javax.security.auth.x500.X500Principal;
     34 
     35 /**
     36  * Wraps {@link SecretKey} instances using a public/private key pair stored in
     37  * the platform {@link KeyStore}. This allows us to protect symmetric keys with
     38  * hardware-backed crypto, if provided by the device.
     39  * <p>
     40  * See <a href="http://en.wikipedia.org/wiki/Key_Wrap">key wrapping</a> for more
     41  * details.
     42  * <p>
     43  * Not inherently thread safe.
     44  */
     45 public class SecretKeyWrapper {
     46     private final Cipher mCipher;
     47     private final KeyPair mPair;
     48 
     49     /**
     50      * Create a wrapper using the public/private key pair with the given alias.
     51      * If no pair with that alias exists, it will be generated.
     52      */
     53     public SecretKeyWrapper(Context context, String alias)
     54             throws GeneralSecurityException, IOException {
     55         mCipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
     56 
     57         final KeyStore keyStore = KeyStore.getInstance("AndroidKeyStore");
     58         keyStore.load(null);
     59 
     60         if (!keyStore.containsAlias(alias)) {
     61             generateKeyPair(context, alias);
     62         }
     63 
     64         // Even if we just generated the key, always read it back to ensure we
     65         // can read it successfully.
     66         final KeyStore.PrivateKeyEntry entry = (KeyStore.PrivateKeyEntry) keyStore.getEntry(
     67                 alias, null);
     68         mPair = new KeyPair(entry.getCertificate().getPublicKey(), entry.getPrivateKey());
     69     }
     70 
     71     private static void generateKeyPair(Context context, String alias)
     72             throws GeneralSecurityException {
     73         final Calendar start = new GregorianCalendar();
     74         final Calendar end = new GregorianCalendar();
     75         end.add(Calendar.YEAR, 100);
     76 
     77         final KeyPairGeneratorSpec spec = new KeyPairGeneratorSpec.Builder(context)
     78                 .setAlias(alias)
     79                 .setSubject(new X500Principal("CN=" + alias))
     80                 .setSerialNumber(BigInteger.ONE)
     81                 .setStartDate(start.getTime())
     82                 .setEndDate(end.getTime())
     83                 .build();
     84 
     85         final KeyPairGenerator gen = KeyPairGenerator.getInstance("RSA", "AndroidKeyStore");
     86         gen.initialize(spec);
     87         gen.generateKeyPair();
     88     }
     89 
     90     /**
     91      * Wrap a {@link SecretKey} using the public key assigned to this wrapper.
     92      * Use {@link #unwrap(byte[])} to later recover the original
     93      * {@link SecretKey}.
     94      *
     95      * @return a wrapped version of the given {@link SecretKey} that can be
     96      *         safely stored on untrusted storage.
     97      */
     98     public byte[] wrap(SecretKey key) throws GeneralSecurityException {
     99         mCipher.init(Cipher.WRAP_MODE, mPair.getPublic());
    100         return mCipher.wrap(key);
    101     }
    102 
    103     /**
    104      * Unwrap a {@link SecretKey} using the private key assigned to this
    105      * wrapper.
    106      *
    107      * @param blob a wrapped {@link SecretKey} as previously returned by
    108      *            {@link #wrap(SecretKey)}.
    109      */
    110     public SecretKey unwrap(byte[] blob) throws GeneralSecurityException {
    111         mCipher.init(Cipher.UNWRAP_MODE, mPair.getPrivate());
    112         return (SecretKey) mCipher.unwrap(blob, "AES", Cipher.SECRET_KEY);
    113     }
    114 }
    115