Security.java

Security.java

package com.holaxapps.chomdoi.support;

import javax.crypto.*;
import java.io.UnsupportedEncodingException;
import java.security.*;
import java.security.spec.InvalidKeySpecException;
import java.security.spec.PKCS8EncodedKeySpec;
import java.security.spec.X509EncodedKeySpec;
import java.util.Base64;

/**
 * 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 int DEFAULT_KEY_SIZE = 2048;

    private static final String KEY_FACTORY_ALGORITHM = "RSA";

    private static final String SIGNATURE_ALGORITHM = "SHA256withRSA";

    private static final String CHARSET = "UTF-8";

    /**
     * Generate key pair that public and private key by RSA
     * @return key pair
     */
    public static KeyPair generateKeyPair() throws NoSuchAlgorithmException {
        KeyPairGenerator generator = KeyPairGenerator.getInstance(KEY_FACTORY_ALGORITHM);
        generator.initialize(DEFAULT_KEY_SIZE, new SecureRandom());
        KeyPair pair = generator.generateKeyPair();
        return pair;
    }

    /**
     * Encrypt plain text by RSA algorithm
     * @param plainText
     * @param encodedPublicKey
     * @return cipher text
     * @throws NoSuchAlgorithmException
     */
    public static String encrypt(String plainText, byte[] encodedPublicKey) throws NoSuchAlgorithmException {
        PublicKey publicKey = Security.generatePublicKey(encodedPublicKey);
        try {
            Cipher cipher = Cipher.getInstance(KEY_FACTORY_ALGORITHM);
            cipher.init(Cipher.ENCRYPT_MODE, publicKey);
            byte[] bytes = cipher.doFinal(plainText.getBytes(CHARSET));
            return Base64.getEncoder().encodeToString(bytes);
        } catch (NoSuchPaddingException | InvalidKeyException | UnsupportedEncodingException | IllegalBlockSizeException | BadPaddingException e) {
            throw new RuntimeException(e);
        }
    }

    /**
     * Decrypt cipher text by RSA algorithm
     * @param cipherText
     * @param encodedPrivateKey
     * @return plain text
     * @throws NoSuchAlgorithmException
     */
    public static String decrypt(String cipherText, byte[] encodedPrivateKey) throws NoSuchAlgorithmException {
        PrivateKey privateKey = Security.generatePrivateKey(encodedPrivateKey);
        try {
            byte[] bytes = Base64.getDecoder().decode(cipherText);
            Cipher cipher = Cipher.getInstance(KEY_FACTORY_ALGORITHM);
            cipher.init(Cipher.DECRYPT_MODE, privateKey);
            return new String(cipher.doFinal(bytes), CHARSET);
        } catch (NoSuchPaddingException | InvalidKeyException | UnsupportedEncodingException | IllegalBlockSizeException | BadPaddingException e) {
            throw new RuntimeException(e);
        }
    }

    /**
     * Create signature by signing
     * @param plainText the signed JSON string (signed, not encrypted)
     * @param encodedPrivateKey the base64-encoded private key to use for signing.
     * @return signature text
     */
    public static String sign(String plainText, byte[] encodedPrivateKey) {
        try {
            Signature privateSignature = Signature.getInstance(SIGNATURE_ALGORITHM);
            privateSignature.initSign(Security.generatePrivateKey(encodedPrivateKey));
            privateSignature.update(plainText.getBytes(CHARSET));
            byte[] signature = privateSignature.sign();
            return Base64.getEncoder().encodeToString(signature);
        } catch (NoSuchAlgorithmException | InvalidKeyException | UnsupportedEncodingException | SignatureException e) {
            throw new RuntimeException(e);
        }
    }

    /**
     * Verifies that the data was signed with the given signature, and returns
     * the verified purchase. The data is in JSON format and signed
     * and product ID of the purchase.
     * @param plainText the signed JSON string (signed, not encrypted)
     * @param signature the signature for the data, signed with the private key
     * @param encodedPublicKey the base64-encoded public key to use for verifying.
     * @return result for verification
     */
    public static boolean verify(String plainText, String signature, byte[] encodedPublicKey) {
        PublicKey publicKey = Security.generatePublicKey(encodedPublicKey);
        return Security.verifySignarue(plainText, signature, publicKey);
    }

    private static PublicKey generatePublicKey(byte[] encodedPublicKey) {
        try {
            KeyFactory keyFactory = KeyFactory.getInstance(KEY_FACTORY_ALGORITHM);
            return keyFactory.generatePublic(new X509EncodedKeySpec(encodedPublicKey));
        } catch (NoSuchAlgorithmException e) {
            throw new RuntimeException(e);
        } catch (InvalidKeySpecException e) {
            throw new IllegalArgumentException(e);
        }
    }

    private static PrivateKey generatePrivateKey(byte[] encodedPrivateKey) {
        try {
            KeyFactory keyFactory = KeyFactory.getInstance(KEY_FACTORY_ALGORITHM);
            return keyFactory.generatePrivate(new PKCS8EncodedKeySpec(encodedPrivateKey));
        } catch (NoSuchAlgorithmException e) {
            throw new RuntimeException(e);
        } catch (InvalidKeySpecException e) {
            throw new IllegalArgumentException(e);
        }
    }

    private static boolean verifySignarue(String plainText, String signature, PublicKey publicKey) {
        Signature sig;
        try {
            sig = Signature.getInstance(SIGNATURE_ALGORITHM);
            sig.initVerify(publicKey);
            sig.update(plainText.getBytes());
            if (!sig.verify(Base64.getDecoder().decode(signature)))
                throw new InvalidSignatureException("It was awesome! Signature hasn't be invalid");
        } catch (NoSuchAlgorithmException | InvalidKeyException | SignatureException e) {
            throw new RuntimeException(e);
        }
        return true;
    }
}

class InvalidSignatureException extends RuntimeException {

    InvalidSignatureException(String message) {
        super(message);
    }
}