/*
 * This software is provided 'as-is', without any express or implied
 * warranty.  In no event will Google be held liable for any damages
 * arising from the use of this software.
 *
 * Permission is granted to anyone to use this software for any purpose,
 * including commercial applications, and to alter it and redistribute it
 * freely, as long as the origin is not misrepresented.
 */

package org.sufficientlysecure.keychain.util;

import android.os.Build;
import android.os.Process;

import java.io.ByteArrayOutputStream;
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.UnsupportedEncodingException;
import java.security.NoSuchAlgorithmException;
import java.security.Provider;
import java.security.SecureRandom;
import java.security.SecureRandomSpi;
import java.security.Security;

/**
 * Fixes for the output of the default PRNG having low entropy.
 * 
 * The fixes need to be applied via {@link #apply()} before any use of Java Cryptography
 * Architecture primitives. A good place to invoke them is in the application's {@code onCreate}.
 * 
 * copied from http://android-developers.blogspot.de/2013/08/some-securerandom-thoughts.html
 * 
 * 
 * More information on these Android bugs:
 * http://blog.k3170makan.com/2013/08/more-details-on-android-jca-prng-flaw.html
 * Paper: "Randomly failed! Weaknesses in Java Pseudo Random Number Generators (PRNGs)"
 * 
 * 
 * Sep 15, 2013:
 * On some devices /dev/urandom is non-writable!
 * No need to seed /dev/urandom. urandom should have enough seeds from the OS and kernel.
 * Only OpenSSL seeds are broken. See http://emboss.github.io/blog/2013/08/21/openssl-prng-is-not-really-fork-safe
 * 
 * see also:
 * https://github.com/k9mail/k-9/commit/dda8f64276d4d29c43f86237cd77819c28f22f21
 * In addition to a couple of custom ROMs linking /dev/urandom to a non-writable
 * random version, now Samsung's SELinux policy also prevents apps from opening
 * /dev/urandom for writing. Since we shouldn't need to write to /dev/urandom anyway
 * we now simply don't.
 */
public final class PRNGFixes {

    private static final int VERSION_CODE_JELLY_BEAN = 16;
    private static final int VERSION_CODE_JELLY_BEAN_MR2 = 18;
    private static final byte[] BUILD_FINGERPRINT_AND_DEVICE_SERIAL = getBuildFingerprintAndDeviceSerial();

    /** Hidden constructor to prevent instantiation. */
    private PRNGFixes() {
    }

    /**
     * Applies all fixes.
     * 
     * @throws SecurityException
     *             if a fix is needed but could not be applied.
     */
    public static void apply() {
        applyOpenSSLFix();
        installLinuxPRNGSecureRandom();
    }

    /**
     * Applies the fix for OpenSSL PRNG having low entropy. Does nothing if the fix is not needed.
     * 
     * @throws SecurityException
     *             if the fix is needed but could not be applied.
     */
    private static void applyOpenSSLFix() throws SecurityException {
        if ((Build.VERSION.SDK_INT < VERSION_CODE_JELLY_BEAN)
                || (Build.VERSION.SDK_INT > VERSION_CODE_JELLY_BEAN_MR2)) {
            // No need to apply the fix
            return;
        }

        try {
            // Mix in the device- and invocation-specific seed.
            Class.forName("org.apache.harmony.xnet.provider.jsse.NativeCrypto")
                    .getMethod("RAND_seed", byte[].class).invoke(null, generateSeed());

            // Mix output of Linux PRNG into OpenSSL's PRNG
            int bytesRead = (Integer) Class
                    .forName("org.apache.harmony.xnet.provider.jsse.NativeCrypto")
                    .getMethod("RAND_load_file", String.class, long.class)
                    .invoke(null, "/dev/urandom", 1024);
            if (bytesRead != 1024) {
                throw new IOException("Unexpected number of bytes read from Linux PRNG: "
                        + bytesRead);
            }
        } catch (Exception e) {
            throw new SecurityException("Failed to seed OpenSSL PRNG", e);
        }
    }

    /**
     * Installs a Linux PRNG-backed {@code SecureRandom} implementation as the default. Does nothing
     * if the implementation is already the default or if there is not need to install the
     * implementation.
     * 
     * @throws SecurityException
     *             if the fix is needed but could not be applied.
     */
    private static void installLinuxPRNGSecureRandom() throws SecurityException {
        if (Build.VERSION.SDK_INT > VERSION_CODE_JELLY_BEAN_MR2) {
            // No need to apply the fix
            return;
        }

        // Install a Linux PRNG-based SecureRandom implementation as the
        // default, if not yet installed.
        Provider[] secureRandomProviders = Security.getProviders("SecureRandom.SHA1PRNG");
        if ((secureRandomProviders == null)
                || (secureRandomProviders.length < 1)
                || (!LinuxPRNGSecureRandomProvider.class
                        .equals(secureRandomProviders[0].getClass()))) {
            Security.insertProviderAt(new LinuxPRNGSecureRandomProvider(), 1);
        }

        // Assert that new SecureRandom() and
        // SecureRandom.getInstance("SHA1PRNG") return a SecureRandom backed
        // by the Linux PRNG-based SecureRandom implementation.
        SecureRandom rng1 = new SecureRandom();
        if (!LinuxPRNGSecureRandomProvider.class.equals(rng1.getProvider().getClass())) {
            throw new SecurityException("new SecureRandom() backed by wrong Provider: "
                    + rng1.getProvider().getClass());
        }

        SecureRandom rng2;
        try {
            rng2 = SecureRandom.getInstance("SHA1PRNG");
        } catch (NoSuchAlgorithmException e) {
            throw new SecurityException("SHA1PRNG not available", e);
        }
        if (!LinuxPRNGSecureRandomProvider.class.equals(rng2.getProvider().getClass())) {
            throw new SecurityException("SecureRandom.getInstance(\"SHA1PRNG\") backed by wrong"
                    + " Provider: " + rng2.getProvider().getClass());
        }
    }

    /**
     * {@code Provider} of {@code SecureRandom} engines which pass through all requests to the Linux
     * PRNG.
     */
    @SuppressWarnings("serial")
    private static class LinuxPRNGSecureRandomProvider extends Provider {

        public LinuxPRNGSecureRandomProvider() {
            super("LinuxPRNG", 1.0, "A Linux-specific random number provider that uses"
                    + " /dev/urandom");
            // Although /dev/urandom is not a SHA-1 PRNG, some apps
            // explicitly request a SHA1PRNG SecureRandom and we thus need to
            // prevent them from getting the default implementation whose output
            // may have low entropy.
            put("SecureRandom.SHA1PRNG", LinuxPRNGSecureRandom.class.getName());
            put("SecureRandom.SHA1PRNG ImplementedIn", "Software");
        }
    }

    /**
     * {@link SecureRandomSpi} which passes all requests to the Linux PRNG ({@code /dev/urandom}).
     */
    @SuppressWarnings("serial")
    public static class LinuxPRNGSecureRandom extends SecureRandomSpi {

        /*
         * IMPLEMENTATION NOTE: Requests to generate bytes and to mix in a seed are passed through
         * to the Linux PRNG (/dev/urandom). Instances of this class seed themselves by mixing in
         * the current time, PID, UID, build fingerprint, and hardware serial number (where
         * available) into Linux PRNG.
         * 
         * Concurrency: Read requests to the underlying Linux PRNG are serialized (on sLock) to
         * ensure that multiple threads do not get duplicated PRNG output.
         */

        private static final File URANDOM_FILE = new File("/dev/urandom");

        private static final Object sLock = new Object();

        /**
         * Input stream for reading from Linux PRNG or {@code null} if not yet opened.
         * 
         * @GuardedBy("sLock")
         */
        private static DataInputStream sUrandomIn;

//        /**
//         * Output stream for writing to Linux PRNG or {@code null} if not yet opened.
//         * 
//         * @GuardedBy("sLock")
//         */
//        private static OutputStream sUrandomOut;
//
//        /**
//         * Whether this engine instance has been seeded. This is needed because each instance needs
//         * to seed itself if the client does not explicitly seed it.
//         */
//        private boolean mSeeded;

        @Override
        protected void engineSetSeed(byte[] bytes) {
//            try {
//                OutputStream out;
//                synchronized (sLock) {
//                    out = getUrandomOutputStream();
//                }
//                out.write(bytes);
//                out.flush();
//                mSeeded = true;
//            } catch (IOException e) {
//                throw new SecurityException("Failed to mix seed into " + URANDOM_FILE, e);
//            }
        }

        @Override
        protected void engineNextBytes(byte[] bytes) {
//            if (!mSeeded) {
//                // Mix in the device- and invocation-specific seed.
//                engineSetSeed(generateSeed());
//            }

            try {
                DataInputStream in;
                synchronized (sLock) {
                    in = getUrandomInputStream();
                }
                synchronized (in) {
                    in.readFully(bytes);
                }
            } catch (IOException e) {
                throw new SecurityException("Failed to read from " + URANDOM_FILE, e);
            }
        }

        @Override
        protected byte[] engineGenerateSeed(int size) {
            byte[] seed = new byte[size];
            engineNextBytes(seed);
            return seed;
        }

        private DataInputStream getUrandomInputStream() {
            synchronized (sLock) {
                if (sUrandomIn == null) {
                    // NOTE: Consider inserting a BufferedInputStream between
                    // DataInputStream and FileInputStream if you need higher
                    // PRNG output performance and can live with future PRNG
                    // output being pulled into this process prematurely.
                    try {
                        sUrandomIn = new DataInputStream(new FileInputStream(URANDOM_FILE));
                    } catch (IOException e) {
                        throw new SecurityException("Failed to open " + URANDOM_FILE
                                + " for reading", e);
                    }
                }
                return sUrandomIn;
            }
        }

//        private OutputStream getUrandomOutputStream() {
//            synchronized (sLock) {
//                if (sUrandomOut == null) {
//                    try {
//                        sUrandomOut = new FileOutputStream(URANDOM_FILE);
//                    } catch (IOException e) {
//                        throw new SecurityException("Failed to open " + URANDOM_FILE
//                                + " for writing", e);
//                    }
//                }
//                return sUrandomOut;
//            }
//        }
    }

    /**
     * Generates a device- and invocation-specific seed to be mixed into the Linux PRNG.
     */
    private static byte[] generateSeed() {
        try {
            ByteArrayOutputStream seedBuffer = new ByteArrayOutputStream();
            DataOutputStream seedBufferOut = new DataOutputStream(seedBuffer);
            seedBufferOut.writeLong(System.currentTimeMillis());
            seedBufferOut.writeLong(System.nanoTime());
            seedBufferOut.writeInt(Process.myPid());
            seedBufferOut.writeInt(Process.myUid());
            seedBufferOut.write(BUILD_FINGERPRINT_AND_DEVICE_SERIAL);
            seedBufferOut.close();
            return seedBuffer.toByteArray();
        } catch (IOException e) {
            throw new SecurityException("Failed to generate seed", e);
        }
    }

    /**
     * Gets the hardware serial number of this device.
     * 
     * @return serial number or {@code null} if not available.
     */
    private static String getDeviceSerialNumber() {
        // We're using the Reflection API because Build.SERIAL is only available
        // since API Level 9 (Gingerbread, Android 2.3).
        try {
            return (String) Build.class.getField("SERIAL").get(null);
        } catch (Exception ignored) {
            return null;
        }
    }

    private static byte[] getBuildFingerprintAndDeviceSerial() {
        StringBuilder result = new StringBuilder();
        String fingerprint = Build.FINGERPRINT;
        if (fingerprint != null) {
            result.append(fingerprint);
        }
        String serial = getDeviceSerialNumber();
        if (serial != null) {
            result.append(serial);
        }
        try {
            return result.toString().getBytes("UTF-8");
        } catch (UnsupportedEncodingException e) {
            throw new RuntimeException("UTF-8 encoding not supported");
        }
    }
}