/* Copyright (c) 2012 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.alorma.github.inapp;

import android.text.TextUtils;
import android.util.Log;
import com.alorma.github.Base64;
import com.alorma.github.Base64DecoderException;
import java.security.InvalidKeyException;
import java.security.KeyFactory;
import java.security.NoSuchAlgorithmException;
import java.security.PublicKey;
import java.security.Signature;
import java.security.SignatureException;
import java.security.spec.InvalidKeySpecException;
import java.security.spec.X509EncodedKeySpec;

/**
 * Security-related methods. For a secure implementation, all of this code
 * should be implemented on a server that communicates with the
 * application on the device. For the sake of simplicity and clarity of this
 * example, this code is included here and is executed on the device. If you
 * must verify the purchases on the phone, you should obfuscate this code to
 * make it harder for an attacker to replace the code with stubs that treat all
 * purchases as verified.
 */
public class Security {
  private static final String TAG = "IABUtil/Security";

  private static final String KEY_FACTORY_ALGORITHM = "RSA";
  private static final String SIGNATURE_ALGORITHM = "SHA1withRSA";

  /**
   * Verifies that the data was signed with the given signature, and returns
   * the verified purchase. The data is in JSON format and signed
   * with a private key. The data also contains the
   * and product ID of the purchase.
   *
   * @param base64PublicKey the base64-encoded public key to use for verifying.
   * @param signedData the signed JSON string (signed, not encrypted)
   * @param signature the signature for the data, signed with the private key
   */
  public static boolean verifyPurchase(String base64PublicKey, String signedData,
      String signature) {
    if (TextUtils.isEmpty(signedData) || TextUtils.isEmpty(base64PublicKey) ||
        TextUtils.isEmpty(signature)) {
      Log.e(TAG, "Purchase verification failed: missing data.");
      return false;
    }

    PublicKey key = Security.generatePublicKey(base64PublicKey);
    return Security.verify(key, signedData, signature);
  }

  /**
   * Generates a PublicKey instance from a string containing the
   * Base64-encoded public key.
   *
   * @param encodedPublicKey Base64-encoded public key
   * @throws IllegalArgumentException if encodedPublicKey is invalid
   */
  public static PublicKey generatePublicKey(String encodedPublicKey) {
    try {
      byte[] decodedKey = Base64.decode(encodedPublicKey);
      KeyFactory keyFactory = KeyFactory.getInstance(KEY_FACTORY_ALGORITHM);
      return keyFactory.generatePublic(new X509EncodedKeySpec(decodedKey));
    } catch (NoSuchAlgorithmException e) {
      throw new RuntimeException(e);
    } catch (InvalidKeySpecException e) {
      Log.e(TAG, "Invalid key specification.");
      throw new IllegalArgumentException(e);
    } catch (Base64DecoderException e) {
      Log.e(TAG, "Base64 decoding failed.");
      throw new IllegalArgumentException(e);
    }
  }

  /**
   * Verifies that the signature from the server matches the computed
   * signature on the data.  Returns true if the data is correctly signed.
   *
   * @param publicKey public key associated with the developer account
   * @param signedData signed data from server
   * @param signature server signature
   * @return true if the data and signature match
   */
  public static boolean verify(PublicKey publicKey, String signedData, String signature) {
    Signature sig;
    try {
      sig = Signature.getInstance(SIGNATURE_ALGORITHM);
      sig.initVerify(publicKey);
      sig.update(signedData.getBytes());
      if (!sig.verify(Base64.decode(signature))) {
        Log.e(TAG, "Signature verification failed.");
        return false;
      }
      return true;
    } catch (NoSuchAlgorithmException e) {
      Log.e(TAG, "NoSuchAlgorithmException.");
    } catch (InvalidKeyException e) {
      Log.e(TAG, "Invalid key specification.");
    } catch (SignatureException e) {
      Log.e(TAG, "Signature exception.");
    } catch (Base64DecoderException e) {
      Log.e(TAG, "Base64 decoding failed.");
    }
    return false;
  }
}