package oak.util;
/*
* 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.
*
* http://android-developers.blogspot.com/2013/08/some-securerandom-thoughts.html
*/
import android.os.Build;
import android.os.Process;
import android.util.Log;
import java.io.ByteArrayOutputStream;
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.OutputStream;
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.
* <p/>
* 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 {
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 java.security.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 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();
} catch (IOException e) {
// On a small fraction of devices /dev/urandom is not writable.
// Log and ignore.
Log.w(PRNGFixes.class.getSimpleName(),
"Failed to mix seed into " + URANDOM_FILE);
} finally {
mSeeded = true;
}
}
@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() throws IOException {
synchronized (sLock) {
if (sUrandomOut == null) {
sUrandomOut = new FileOutputStream(URANDOM_FILE);
}
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");
}
}
}