Security.java

/*
 * 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.jdroid.android.google.inappbilling;

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;
import android.text.TextUtils;
import android.util.Log;

/**
 * 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.
	 * 
	 * @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
	 * @return Whether the verification was successful or not
	 */
	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
	 * @return The {@link PublicKey}
	 * @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;
	}
}