Security.java

/*
 * Copyright (C) 2014 Vlad Mihalachi
 *
 * This file is part of Turbo Editor.
 *
 * Turbo Editor is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * Turbo Editor is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

package eu.se_bastiaan.popcorntimeremote.iab.utils;

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

import java.nio.charset.StandardCharsets;
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 final class Security {
    private static final String TAG = "IABUtil/Security";

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

    private Security() throws InstantiationException {
        throw new InstantiationException("This class is not created for instantiation");
    }

    /**
     * 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 {@link PurchaseState}
     * 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(StandardCharsets.UTF_8));
            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;
    }
}