/*
 * Nextcloud Android client application
 *
 * @author Tobias Kaminsky
 * Copyright (C) 2017 Tobias Kaminsky
 * Copyright (C) 2017 Nextcloud GmbH.
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Affero General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * at your option) any later version.
 *
 * This program 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 Affero General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * along with this program. If not, see <http://www.gnu.org/licenses/>.
 */

package com.owncloud.android.utils;

import android.accounts.Account;
import android.content.Context;
import android.os.Build;
import android.util.Base64;

import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;
import com.owncloud.android.datamodel.ArbitraryDataProvider;
import com.owncloud.android.datamodel.DecryptedFolderMetadata;
import com.owncloud.android.datamodel.EncryptedFolderMetadata;
import com.owncloud.android.datamodel.OCFile;
import com.owncloud.android.lib.common.OwnCloudClient;
import com.owncloud.android.lib.common.operations.RemoteOperationResult;
import com.owncloud.android.lib.common.utils.Log_OC;
import com.owncloud.android.lib.resources.e2ee.GetMetadataRemoteOperation;

import org.apache.commons.codec.binary.Hex;

import java.io.BufferedReader;
import java.io.ByteArrayInputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.RandomAccessFile;
import java.nio.charset.StandardCharsets;
import java.security.InvalidAlgorithmParameterException;
import java.security.InvalidKeyException;
import java.security.Key;
import java.security.KeyFactory;
import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.security.PrivateKey;
import java.security.PublicKey;
import java.security.SecureRandom;
import java.security.cert.CertificateException;
import java.security.cert.CertificateFactory;
import java.security.cert.X509Certificate;
import java.security.spec.InvalidKeySpecException;
import java.security.spec.KeySpec;
import java.security.spec.PKCS8EncodedKeySpec;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import javax.crypto.BadPaddingException;
import javax.crypto.Cipher;
import javax.crypto.IllegalBlockSizeException;
import javax.crypto.KeyGenerator;
import javax.crypto.NoSuchPaddingException;
import javax.crypto.SecretKey;
import javax.crypto.SecretKeyFactory;
import javax.crypto.spec.GCMParameterSpec;
import javax.crypto.spec.IvParameterSpec;
import javax.crypto.spec.PBEKeySpec;
import javax.crypto.spec.SecretKeySpec;

import androidx.annotation.Nullable;
import androidx.annotation.RequiresApi;

/**
 * Utils for encryption
 */
public final class EncryptionUtils {
    private static String TAG = EncryptionUtils.class.getSimpleName();

    public static final String PUBLIC_KEY = "PUBLIC_KEY";
    public static final String PRIVATE_KEY = "PRIVATE_KEY";
    public static final String MNEMONIC = "MNEMONIC";
    public static final int ivLength = 16;
    public static final int saltLength = 40;

    private static final String HASH_DELIMITER = "$";
    private static final String ivDelimiter = "fA=="; // "|" base64 encoded
    private static final int iterationCount = 1024;
    private static final int keyStrength = 256;
    private static final String AES_CIPHER = "AES/GCM/NoPadding";
    private static final String AES = "AES";
    private static final String RSA_CIPHER = "RSA/ECB/OAEPWithSHA-256AndMGF1Padding";
    private static final String RSA = "RSA";

    private EncryptionUtils() {
        // utility class -> private constructor
    }

    /*
    JSON
     */

    public static <T> T deserializeJSON(String json, TypeToken<T> type) {
        return new Gson().fromJson(json, type.getType());
    }

    public static String serializeJSON(Object data) {
        return new Gson().toJson(data);
    }

    /*
    METADATA
     */

    /**
     * Encrypt folder metaData
     *
     * @param decryptedFolderMetadata folder metaData to encrypt
     * @return EncryptedFolderMetadata encrypted folder metadata
     */
    @RequiresApi(api = Build.VERSION_CODES.KITKAT)
    public static EncryptedFolderMetadata encryptFolderMetadata(DecryptedFolderMetadata decryptedFolderMetadata,
                                                                String privateKey)
            throws NoSuchAlgorithmException, InvalidKeyException,
            InvalidAlgorithmParameterException, NoSuchPaddingException, BadPaddingException,
            IllegalBlockSizeException, InvalidKeySpecException {

        HashMap<String, EncryptedFolderMetadata.EncryptedFile> files = new HashMap<>();
        EncryptedFolderMetadata encryptedFolderMetadata = new EncryptedFolderMetadata(decryptedFolderMetadata
                .getMetadata(), files);

        // Encrypt each file in "files"
        for (Map.Entry<String, DecryptedFolderMetadata.DecryptedFile> entry : decryptedFolderMetadata
                .getFiles().entrySet()) {
            String key = entry.getKey();
            DecryptedFolderMetadata.DecryptedFile decryptedFile = entry.getValue();

            EncryptedFolderMetadata.EncryptedFile encryptedFile = new EncryptedFolderMetadata.EncryptedFile();
            encryptedFile.setInitializationVector(decryptedFile.getInitializationVector());
            encryptedFile.setMetadataKey(decryptedFile.getMetadataKey());
            encryptedFile.setAuthenticationTag(decryptedFile.getAuthenticationTag());

            byte[] decryptedMetadataKey = EncryptionUtils.decodeStringToBase64Bytes(EncryptionUtils.decryptStringAsymmetric(
                    decryptedFolderMetadata.getMetadata().getMetadataKeys().get(encryptedFile.getMetadataKey()),
                    privateKey));

            // encrypt
            String dataJson = EncryptionUtils.serializeJSON(decryptedFile.getEncrypted());
            encryptedFile.setEncrypted(EncryptionUtils.encryptStringSymmetric(dataJson, decryptedMetadataKey));

            files.put(key, encryptedFile);
        }

        return encryptedFolderMetadata;
    }

    /*
     * decrypt folder metaData with private key
     */
    @RequiresApi(api = Build.VERSION_CODES.KITKAT)
    public static DecryptedFolderMetadata decryptFolderMetaData(EncryptedFolderMetadata encryptedFolderMetadata,
                                                                String privateKey)
            throws NoSuchAlgorithmException, InvalidKeyException,
            InvalidAlgorithmParameterException, NoSuchPaddingException, BadPaddingException,
            IllegalBlockSizeException, InvalidKeySpecException {

        HashMap<String, DecryptedFolderMetadata.DecryptedFile> files = new HashMap<>();
        DecryptedFolderMetadata decryptedFolderMetadata = new DecryptedFolderMetadata(
                encryptedFolderMetadata.getMetadata(), files);

        for (Map.Entry<String, EncryptedFolderMetadata.EncryptedFile> entry : encryptedFolderMetadata
                .getFiles().entrySet()) {
            String key = entry.getKey();
            EncryptedFolderMetadata.EncryptedFile encryptedFile = entry.getValue();

            DecryptedFolderMetadata.DecryptedFile decryptedFile = new DecryptedFolderMetadata.DecryptedFile();
            decryptedFile.setInitializationVector(encryptedFile.getInitializationVector());
            decryptedFile.setMetadataKey(encryptedFile.getMetadataKey());
            decryptedFile.setAuthenticationTag(encryptedFile.getAuthenticationTag());

            byte[] decryptedMetadataKey = EncryptionUtils.decodeStringToBase64Bytes(
                    EncryptionUtils.decryptStringAsymmetric(decryptedFolderMetadata.getMetadata()
                            .getMetadataKeys().get(encryptedFile.getMetadataKey()), privateKey));

            // decrypt
            String dataJson = EncryptionUtils.decryptStringSymmetric(encryptedFile.getEncrypted(), decryptedMetadataKey);
            decryptedFile.setEncrypted(EncryptionUtils.deserializeJSON(dataJson,
                    new TypeToken<DecryptedFolderMetadata.Data>() {
                    }));

            files.put(key, decryptedFile);
        }

        return decryptedFolderMetadata;
    }

    /**
     * Download metadata for folder and decrypt it
     *
     * @return decrypted metadata or null
     */
    @RequiresApi(api = Build.VERSION_CODES.KITKAT)
    public static @Nullable
    DecryptedFolderMetadata downloadFolderMetadata(OCFile folder, OwnCloudClient client,
                                                   Context context, Account account) {
        RemoteOperationResult getMetadataOperationResult = new GetMetadataRemoteOperation(folder.getLocalId())
            .execute(client, true);

        if (!getMetadataOperationResult.isSuccess()) {
            return null;
        }

        // decrypt metadata
        ArbitraryDataProvider arbitraryDataProvider = new ArbitraryDataProvider(context.getContentResolver());
        String serializedEncryptedMetadata = (String) getMetadataOperationResult.getData().get(0);
        String privateKey = arbitraryDataProvider.getValue(account.name, EncryptionUtils.PRIVATE_KEY);

        EncryptedFolderMetadata encryptedFolderMetadata = EncryptionUtils.deserializeJSON(
                serializedEncryptedMetadata, new TypeToken<EncryptedFolderMetadata>() {
                });

        try {
            return EncryptionUtils.decryptFolderMetaData(encryptedFolderMetadata, privateKey);
        } catch (Exception e) {
            Log_OC.e(TAG, e.getMessage());
            return null;
        }
    }

    /*
    BASE 64
     */

    public static byte[] encodeStringToBase64Bytes(String string) {
        try {
            return Base64.encode(string.getBytes(), Base64.NO_WRAP);
        } catch (Exception e) {
            return new byte[0];
        }
    }

    public static String decodeBase64BytesToString(byte[] bytes) {
        try {
            return new String(Base64.decode(bytes, Base64.NO_WRAP));
        } catch (Exception e) {
            return "";
        }
    }

    public static String encodeBytesToBase64String(byte[] bytes) {
        return Base64.encodeToString(bytes, Base64.NO_WRAP);
    }

    public static byte[] decodeStringToBase64Bytes(String string) {
        return Base64.decode(string, Base64.NO_WRAP);
    }

    /*
    ENCRYPTION
     */

    /**
     * @param ocFile             file do crypt
     * @param encryptionKeyBytes key, either from metadata or {@link EncryptionUtils#generateKey()}
     * @param iv                 initialization vector, either from metadata or {@link EncryptionUtils#randomBytes(int)}
     * @return encryptedFile with encryptedBytes and authenticationTag
     */
    @RequiresApi(api = Build.VERSION_CODES.KITKAT)
    public static EncryptedFile encryptFile(OCFile ocFile, byte[] encryptionKeyBytes, byte[] iv)
            throws NoSuchAlgorithmException,
            InvalidAlgorithmParameterException, NoSuchPaddingException, InvalidKeyException,
            BadPaddingException, IllegalBlockSizeException, IOException {
        File file = new File(ocFile.getStoragePath());

        return encryptFile(file, encryptionKeyBytes, iv);
    }

    /**
     * @param file               file do crypt
     * @param encryptionKeyBytes key, either from metadata or {@link EncryptionUtils#generateKey()}
     * @param iv                 initialization vector, either from metadata or {@link EncryptionUtils#randomBytes(int)}
     * @return encryptedFile with encryptedBytes and authenticationTag
     */
    @RequiresApi(api = Build.VERSION_CODES.KITKAT)
    public static EncryptedFile encryptFile(File file, byte[] encryptionKeyBytes, byte[] iv)
            throws NoSuchAlgorithmException,
            InvalidAlgorithmParameterException, NoSuchPaddingException, InvalidKeyException,
            BadPaddingException, IllegalBlockSizeException, IOException {

        Cipher cipher = Cipher.getInstance(AES_CIPHER);

        Key key = new SecretKeySpec(encryptionKeyBytes, AES);

        GCMParameterSpec spec = new GCMParameterSpec(128, iv);
        cipher.init(Cipher.ENCRYPT_MODE, key, spec);

        RandomAccessFile randomAccessFile = new RandomAccessFile(file, "r");
        byte[] fileBytes = new byte[(int) randomAccessFile.length()];
        randomAccessFile.readFully(fileBytes);

        byte[] cryptedBytes = cipher.doFinal(fileBytes);
        String authenticationTag = encodeBytesToBase64String(Arrays.copyOfRange(cryptedBytes,
                cryptedBytes.length - (128 / 8), cryptedBytes.length));

        return new EncryptedFile(cryptedBytes, authenticationTag);
    }

    /**
     * @param file               encrypted file
     * @param encryptionKeyBytes key from metadata
     * @param iv                 initialization vector from metadata
     * @param authenticationTag  authenticationTag from metadata
     * @return decrypted byte[]
     */
    @RequiresApi(api = Build.VERSION_CODES.KITKAT)
    public static byte[] decryptFile(File file, byte[] encryptionKeyBytes, byte[] iv, byte[] authenticationTag)
            throws NoSuchAlgorithmException,
            InvalidAlgorithmParameterException, NoSuchPaddingException, InvalidKeyException,
            BadPaddingException, IllegalBlockSizeException, IOException {


        Cipher cipher = Cipher.getInstance(AES_CIPHER);
        Key key = new SecretKeySpec(encryptionKeyBytes, AES);
        GCMParameterSpec spec = new GCMParameterSpec(128, iv);
        cipher.init(Cipher.DECRYPT_MODE, key, spec);

        RandomAccessFile randomAccessFile = new RandomAccessFile(file, "r");
        byte[] fileBytes = new byte[(int) randomAccessFile.length()];
        randomAccessFile.readFully(fileBytes);

        // check authentication tag
        byte[] extractedAuthenticationTag = Arrays.copyOfRange(fileBytes,
                fileBytes.length - (128 / 8), fileBytes.length);

        if (!Arrays.equals(extractedAuthenticationTag, authenticationTag)) {
            throw new SecurityException("Tag not correct");
        }

        return cipher.doFinal(fileBytes);
    }

    public static class EncryptedFile {
        public byte[] encryptedBytes;
        public String authenticationTag;

        public EncryptedFile(byte[] encryptedBytes, String authenticationTag) {
            this.encryptedBytes = encryptedBytes;
            this.authenticationTag = authenticationTag;
        }
    }

    /**
     * Encrypt string with RSA algorithm, ECB mode, OAEPWithSHA-256AndMGF1 padding
     * Asymmetric encryption, with private and public key
     *
     * @param string String to encrypt
     * @param cert   contains public key in it
     * @return encrypted string
     */
    @RequiresApi(api = Build.VERSION_CODES.KITKAT)
    public static String encryptStringAsymmetric(String string, String cert)
            throws NoSuchAlgorithmException,
            NoSuchPaddingException, InvalidKeyException,
        BadPaddingException, IllegalBlockSizeException,
        CertificateException {

        Cipher cipher = Cipher.getInstance(RSA_CIPHER);

        String trimmedCert = cert.replace("-----BEGIN CERTIFICATE-----\n", "")
                .replace("-----END CERTIFICATE-----\n", "");
        byte[] encodedCert = trimmedCert.getBytes(StandardCharsets.UTF_8);
        byte[] decodedCert = org.apache.commons.codec.binary.Base64.decodeBase64(encodedCert);

        CertificateFactory certFactory = CertificateFactory.getInstance("X.509");
        InputStream in = new ByteArrayInputStream(decodedCert);
        X509Certificate certificate = (X509Certificate) certFactory.generateCertificate(in);
        PublicKey realPublicKey = certificate.getPublicKey();

        cipher.init(Cipher.ENCRYPT_MODE, realPublicKey);

        byte[] bytes = encodeStringToBase64Bytes(string);
        byte[] cryptedBytes = cipher.doFinal(bytes);

        return encodeBytesToBase64String(cryptedBytes);
    }


    /**
     * Decrypt string with RSA algorithm, ECB mode, OAEPWithSHA-256AndMGF1 padding
     * Asymmetric encryption, with private and public key
     *
     * @param string           string to decrypt
     * @param privateKeyString private key
     * @return decrypted string
     */
    @RequiresApi(api = Build.VERSION_CODES.KITKAT)
    public static String decryptStringAsymmetric(String string, String privateKeyString)
            throws NoSuchAlgorithmException,
            NoSuchPaddingException, InvalidKeyException,
            BadPaddingException, IllegalBlockSizeException,
            InvalidKeySpecException {

        Cipher cipher = Cipher.getInstance(RSA_CIPHER);

        byte[] privateKeyBytes = decodeStringToBase64Bytes(privateKeyString);
        PKCS8EncodedKeySpec keySpec = new PKCS8EncodedKeySpec(privateKeyBytes);
        KeyFactory kf = KeyFactory.getInstance(RSA);
        PrivateKey privateKey = kf.generatePrivate(keySpec);

        cipher.init(Cipher.DECRYPT_MODE, privateKey);

        byte[] bytes = decodeStringToBase64Bytes(string);
        byte[] encodedBytes = cipher.doFinal(bytes);

        return decodeBase64BytesToString(encodedBytes);
    }


    /**
     * Encrypt string with RSA algorithm, ECB mode, OAEPWithSHA-256AndMGF1 padding
     * Asymmetric encryption, with private and public key
     *
     * @param string             String to encrypt
     * @param encryptionKeyBytes key, either from metadata or {@link EncryptionUtils#generateKey()}
     * @return encrypted string
     */
    @RequiresApi(api = Build.VERSION_CODES.KITKAT)
    public static String encryptStringSymmetric(String string, byte[] encryptionKeyBytes)
            throws NoSuchAlgorithmException,
            InvalidAlgorithmParameterException, NoSuchPaddingException, InvalidKeyException,
            BadPaddingException, IllegalBlockSizeException {

        Cipher cipher = Cipher.getInstance(AES_CIPHER);
        byte[] iv = randomBytes(ivLength);

        Key key = new SecretKeySpec(encryptionKeyBytes, AES);
        GCMParameterSpec spec = new GCMParameterSpec(128, iv);
        cipher.init(Cipher.ENCRYPT_MODE, key, spec);

        byte[] bytes = encodeStringToBase64Bytes(string);
        byte[] cryptedBytes = cipher.doFinal(bytes);

        String encodedCryptedBytes = encodeBytesToBase64String(cryptedBytes);
        String encodedIV = encodeBytesToBase64String(iv);

        return encodedCryptedBytes + ivDelimiter + encodedIV;
    }


    /**
     * Decrypt string with RSA algorithm, ECB mode, OAEPWithSHA-256AndMGF1 padding
     * Asymmetric encryption, with private and public key
     *
     * @param string             string to decrypt
     * @param encryptionKeyBytes key from metadata
     * @return decrypted string
     */
    @RequiresApi(api = Build.VERSION_CODES.KITKAT)
    public static String decryptStringSymmetric(String string, byte[] encryptionKeyBytes)
            throws NoSuchAlgorithmException,
            InvalidAlgorithmParameterException, NoSuchPaddingException, InvalidKeyException,
            BadPaddingException, IllegalBlockSizeException {

        Cipher cipher = Cipher.getInstance(AES_CIPHER);

        int delimiterPosition = string.lastIndexOf(ivDelimiter);
        String cipherString = string.substring(0, delimiterPosition);
        String ivString = string.substring(delimiterPosition + ivDelimiter.length());

        byte[] iv = new IvParameterSpec(decodeStringToBase64Bytes(ivString)).getIV();

        Key key = new SecretKeySpec(encryptionKeyBytes, AES);

        GCMParameterSpec spec = new GCMParameterSpec(128, iv);
        cipher.init(Cipher.DECRYPT_MODE, key, spec);

        byte[] bytes = decodeStringToBase64Bytes(cipherString);
        byte[] encodedBytes = cipher.doFinal(bytes);

        return decodeBase64BytesToString(encodedBytes);
    }

    /**
     * Encrypt private key with symmetric AES encryption, GCM mode mode and no padding
     *
     * @param privateKey byte64 encoded string representation of private key
     * @param keyPhrase  key used for encryption, e.g. 12 random words
     *                   {@link EncryptionUtils#getRandomWords(int, Context)}
     * @return encrypted string, bytes first encoded base64, IV separated with "|", then to string
     */
    public static String encryptPrivateKey(String privateKey, String keyPhrase) throws NoSuchPaddingException,
            NoSuchAlgorithmException, InvalidKeyException, BadPaddingException,
            IllegalBlockSizeException, InvalidKeySpecException {
        Cipher cipher = Cipher.getInstance(AES_CIPHER);

        SecretKeyFactory factory = SecretKeyFactory.getInstance("PBKDF2WithHmacSHA1");
        byte[] salt = randomBytes(saltLength);
        KeySpec spec = new PBEKeySpec(keyPhrase.toCharArray(), salt, iterationCount, keyStrength);
        SecretKey tmp = factory.generateSecret(spec);
        SecretKeySpec key = new SecretKeySpec(tmp.getEncoded(), AES);

        cipher.init(Cipher.ENCRYPT_MODE, key);
        byte[] bytes = encodeStringToBase64Bytes(privateKey);
        byte[] encrypted = cipher.doFinal(bytes);

        byte[] iv = cipher.getIV();
        String encodedIV = encodeBytesToBase64String(iv);
        String encodedSalt = encodeBytesToBase64String(salt);
        String encodedEncryptedBytes = encodeBytesToBase64String(encrypted);

        return encodedEncryptedBytes + ivDelimiter + encodedIV + ivDelimiter + encodedSalt;
    }

    /**
     * Decrypt private key with symmetric AES encryption, GCM mode mode and no padding
     *
     * @param privateKey byte64 encoded string representation of private key, IV separated with "|"
     * @param keyPhrase  key used for encryption, e.g. 12 random words
     *                   {@link EncryptionUtils#getRandomWords(int, Context)}
     * @return decrypted string
     */
    public static String decryptPrivateKey(String privateKey, String keyPhrase) throws NoSuchPaddingException,
            NoSuchAlgorithmException, InvalidKeyException, BadPaddingException,
            IllegalBlockSizeException, InvalidKeySpecException, InvalidAlgorithmParameterException {

        // split up iv, salt
        String[] strings = privateKey.split(ivDelimiter);
        String realPrivateKey = strings[0];
        byte[] iv = decodeStringToBase64Bytes(strings[1]);
        byte[] salt = decodeStringToBase64Bytes(strings[2]);

        Cipher cipher = Cipher.getInstance(AES_CIPHER);
        SecretKeyFactory factory = SecretKeyFactory.getInstance("PBKDF2WithHmacSHA1");
        KeySpec spec = new PBEKeySpec(keyPhrase.toCharArray(), salt, iterationCount, keyStrength);
        SecretKey tmp = factory.generateSecret(spec);
        SecretKeySpec key = new SecretKeySpec(tmp.getEncoded(), AES);

        cipher.init(Cipher.DECRYPT_MODE, key, new IvParameterSpec(iv));

        byte[] bytes = decodeStringToBase64Bytes(realPrivateKey);
        byte[] decrypted = cipher.doFinal(bytes);

        String pemKey = decodeBase64BytesToString(decrypted);

        return pemKey.replaceAll("\n", "").replace("-----BEGIN PRIVATE KEY-----", "")
                .replace("-----END PRIVATE KEY-----", "");
    }

    public static String privateKeyToPEM(PrivateKey privateKey) {
        String privateKeyString = encodeBytesToBase64String(privateKey.getEncoded());

        return "-----BEGIN PRIVATE KEY-----\n" + privateKeyString.replaceAll("(.{65})", "$1\n")
                + "\n-----END PRIVATE KEY-----";
    }

    /*
    Helper
     */

    public static String getMD5Sum(File file) {
        FileInputStream fileInputStream = null;
        try {
            fileInputStream = new FileInputStream(file);
            MessageDigest md5 = MessageDigest.getInstance("MD5");
            byte[] bytes = new byte[2048];
            int readBytes;

            while ((readBytes = fileInputStream.read(bytes)) != -1) {
                md5.update(bytes, 0, readBytes);
            }

            return new String(Hex.encodeHex(md5.digest()));

        } catch (Exception e) {
            Log_OC.e(TAG, e.getMessage());
        } finally {
            if (fileInputStream != null) {
                try {
                    fileInputStream.close();
                } catch (IOException e) {
                    Log_OC.e(TAG, "Error getting MD5 checksum for file", e);
                }
            }
        }

        return "";
    }

    public static List<String> getRandomWords(int count, Context context) throws IOException {
        InputStream ins = context.getResources().openRawResource(context.getResources()
                .getIdentifier("encryption_key_words", "raw", context.getPackageName()));

        InputStreamReader inputStreamReader = new InputStreamReader(ins);

        BufferedReader bufferedReader = new BufferedReader(inputStreamReader);

        List<String> lines = new ArrayList<>();
        String line;
        while ((line = bufferedReader.readLine()) != null) {
            lines.add(line);
        }

        SecureRandom random = new SecureRandom();

        List<String> outputLines = new ArrayList<>();
        for (int i = 0; i < count; i++) {
            int randomLine = random.nextInt(lines.size());
            outputLines.add(lines.get(randomLine));
        }

        return outputLines;
    }

    public static KeyPair generateKeyPair() throws NoSuchAlgorithmException {
        KeyPairGenerator keyGen = KeyPairGenerator.getInstance(RSA);
        keyGen.initialize(2048, new SecureRandom());
        return keyGen.generateKeyPair();
    }

    public static byte[] generateKey() {
        KeyGenerator keyGenerator;
        try {
            keyGenerator = KeyGenerator.getInstance(AES);
            keyGenerator.init(128);

            return keyGenerator.generateKey().getEncoded();
        } catch (NoSuchAlgorithmException e) {
            Log_OC.e(TAG, e.getMessage());
        }

        return null;
    }

    public static byte[] randomBytes(int size) {
        SecureRandom random = new SecureRandom();
        final byte[] iv = new byte[size];
        random.nextBytes(iv);

        return iv;
    }

    /**
     * Generate a SHA512 with appended salt
     *
     * @param token token to be hashed
     * @return SHA512 with appended salt, delimiter HASH_DELIMITER
     */
    public static String generateSHA512(String token) {
        String salt = EncryptionUtils.encodeBytesToBase64String(EncryptionUtils.randomBytes(EncryptionUtils.saltLength));

        return generateSHA512(token, salt);
    }

    /**
     * Generate a SHA512 with appended salt
     *
     * @param token token to be hashed
     * @return SHA512 with appended salt, delimiter HASH_DELIMITER
     */
    public static String generateSHA512(String token, String salt) {
        MessageDigest digest;
        String hashedToken = "";
        byte[] hash;
        try {
            digest = MessageDigest.getInstance("SHA-512");
            digest.update(salt.getBytes());
            hash = digest.digest(token.getBytes());

            StringBuilder stringBuilder = new StringBuilder();
            for (byte hashByte : hash) {
                stringBuilder.append(Integer.toString((hashByte & 0xff) + 0x100, 16).substring(1));
            }

            stringBuilder.append(HASH_DELIMITER).append(salt);

            hashedToken = stringBuilder.toString();

        } catch (NoSuchAlgorithmException e) {
            Log_OC.e(TAG, "Generating SHA512 failed", e);
        }

        return hashedToken;
    }

    public static boolean verifySHA512(String hashWithSalt, String compareToken) {
        String salt = hashWithSalt.split("\\" + HASH_DELIMITER)[1];

        String newHash = generateSHA512(compareToken, salt);

        return hashWithSalt.equals(newHash);
    }
}
