k-9/src/com/fsck/k9/PRNGFixes.java

/*
 * 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 com.fsck.k9;

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}.
 */
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.
     */
    private static class LinuxPRNGSecureRandomProvider extends Provider {
        private static final long serialVersionUID = 6538669771360998378L;

        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}).
     */
    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 long serialVersionUID = 628140755730642770L;

        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;

        @Override
        protected void engineSetSeed(byte[] bytes) {
            // We use /dev/urandom so we don't need to worry about seeding
        }

        @Override
        protected void engineNextBytes(byte[] bytes) {
            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;
            }
        }
    }

    /**
     * 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");
        }
    }
}