DefaultSignatureValidator.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.github.jberkel.pay.me.validator;

import android.text.TextUtils;
import android.util.Base64;
import android.util.Log;

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;

/**
 * A signature validator using a Base64 encoded public key.
 * 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 DefaultSignatureValidator implements SignatureValidator {
    private static final String TAG = "IAB/DefaultSignatureValidator";

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

    private final PublicKey mPublicKey;

    public DefaultSignatureValidator(String base64EncodedKey) {
        mPublicKey = generatePublicKey(base64EncodedKey);
    }

    @Override
    public boolean validate(String signedData, String signature) {
        if (TextUtils.isEmpty(signedData) || TextUtils.isEmpty(signature)) {
            Log.e(TAG, "Purchase verification failed: missing data.");
            return false;
        } else if (!verify(signedData, signature)) {
            Log.w(TAG, "signature does not match data.");
            return false;
        }  else {
            return true;
        }
    }

    private boolean verify(String signedData, String signature) {
        final byte[] decodedSig;
        try {
            decodedSig = Base64.decode(signature, Base64.DEFAULT);
        } catch (IllegalArgumentException e) {
            Log.e(TAG, "Error decoding signature.");
            return false;
        }

        try {
            Signature sig = Signature.getInstance(SIGNATURE_ALGORITHM);
            sig.initVerify(mPublicKey);
            sig.update(signedData.getBytes());

            if (!sig.verify(decodedSig)) {
                Log.e(TAG, "Signature verification failed.");
                return false;
            } else {
                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.");
        }
        return false;
    }

    private static PublicKey generatePublicKey(String encodedPublicKey) {
        try {
            byte[] decodedKey = Base64.decode(encodedPublicKey, Base64.DEFAULT);
            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);
        }
    }
}