Software Integrity

 

Proper use of Java SecureRandom

Java SecureRandom updates as of April 2016

There have been several changes to Java’s SecureRandom API since creating this post back in 2009. According to Oracle, the following interesting changes have been made:

  • For UNIX-like platforms, two new implementations have been introduced that provide blocking and non-blocking behavior: NativePRNGBlocking and NativePRNGNonBlocking.
  • The getInstanceStrong() method was introduced in JDK 8. This method returns an instance of the strongest SecureRandom implementation available on each platform.

Let’s take a look at each of these changes and see what they mean for your applications. Note that these details only apply to the Oracle JDK/JRE that is distributed with the SUN and SunMSCAPI Java Cryptography Extension providers.

New Implementations: NativePRNGBlocking/NativePRNGNonBlocking

The previous behavior of NativePRNG on Solaris/Linux/MacOS was that sun.security.provider.NativePRNG.generateSeed() could block because it used /dev/random, but getting outputs using nextBytes(), nextLong(), etc. would never block because they used /dev/urandom (this was true for default environment settings). NativePRNG still behaves like this, but there are now two additional implementations:

  • NativePRNGBlocking uses /dev/random for all of the following operations:
    • Initial seeding: This initializes an internal SHA1PRNG instance using 20 bytes from /dev/random
    • Calls to nextBytes(), nextInt(), etc.: This provides the XOR of the output from the internal SHA1PRNG instance (see above) and data read from /dev/random
    • Calls to getSeed(): This provides data read from /dev/random
  • Similarly, NativePRNGNonBlocking uses /dev/urandom for all of the following operations:
    • Initial seeding: This initializes an internal SHA1PRNG instance using 20 bytes from /dev/urandom
    • Calls to nextBytes(), nextInt(), etc.: This provides the XOR of the output from the internal SHA1PRNG instance (see above) and data read from /dev/urandom
    • Calls to getSeed(): This provides data read from /dev/urandom

NativePRNGBlocking will be problematic in any applications where you do not want your application to block while the system gathers more entropy. Getting any type of output from it could cause your application’s thread to hang. It is fine for use in a desktop application for generating a local cryptographic key (for example), but will almost never be okay to use in a web application.

Note that NativePRNGBlocking can be somewhat wasteful in its entropy usage. For performance reasons, it will read in 32 bytes of data at a time from /dev/random for nextBytes(), nextInt(), etc. Any data that doesn’t get used within 100 milliseconds will be discarded. As a result, each call to nextBoolean() will give you one bit of output, but the implementation may actually use up (and mostly discard) 256 bits of entropy.

SecureRandom.getInstanceStrong() Method

This method uses the securerandom.strongAlgorithms property in the java.security file to select a SecureRandom implementation.

If you simply use the old mechanism to get the highest priority SecureRandom implementation (i.e. new java.security.SecureRandom()), then you will get the following implementations depending on your platform:

[table "2" not found /]

If you use SecureRandom.getInstanceStrong(), then the following defaults are used instead:

[table "3" not found /]

Because of this default behavior, you should avoid using SecureRandom.getInstanceStrong() in any server-side code running on Solaris/Linux/MacOS where availability is important.

Original Post

When generating random numbers in Java for cryptographic purposes, many developers often use the java.security.SecureRandom class. And while the java.security.SecureRandom class is designed to generate cryptographically secure random numbers, there are a few subtleties in the API, and if it is used improperly the output can become predictable. At Cigital we have witnessed a number of cases where this is true. The following is a guide to the proper use of Java’s java.security.SecureRandom class.

First, let’s take a quick look at how the java.security.SecureRandom API works. The java.security.SecureRandom class does not actually implement a pseudorandom number generator (PRNG) itself. It uses PRNG implementations in other classes to generate random numbers. A number of actual PRNGs may actually be used when an instance of java.security.SecureRandom is created. The PRNGs are part of Java cryptographic service providers (CSPs). In Sun’s Java implementation, the SUN CSP is used by default. On Windows, the SUN CSP uses the SHA1PRNG implemented in sun.security.provider.SecureRandom by default. On Solaris and Linux, the SUN CSP default is to use sun.security.provider.NativePRNG which simply provides the output of the /dev/urandom PRNG provided by the operating system. If however, on Solaris/Linux, the java.security configuration file in the JRE is modified such that securerandom.source is set to something other than file:/dev/urandom, then the SHA1PRNG implemented in sun.security.provider.SecureRandom is used, as on Windows. Of course, an application can choose not to use the defaults, and can always specify a particular PRNG implemented by a particular cryptographic provider. In Cigital’s experience, most Java applications that use java.security.SecureRandom actually use the SHA1PRNG provided by the SUN CSP under the cover.

Now, an application will end up with an instance of SHA1PRNG implemented in sun.security.provider.SecureRandom using the following calls:

// The following will create SUN SHA1PRNG on Windows with 
// default configuration and Sun JRE, and on Solaris/Linux
// if securerandom.source is modified in java.security
SecureRandom sr1 = new SecureRandom();

// The following will create SUN SHA1PRNG if the highest 
// priority CSP is SUN
SecureRandom sr2 = SecureRandom.getInstance("SHA1PRNG");

// The following will always create SUN SHA1PRNG
SecureRandom sr3 = SecureRandom.getInstance("SHA1PRNG", "SUN");

Note that according to Sun’s documentation, the returned java.security.SecureRandom instance is not seeded by any of these calls. If after one of these calls, java.security.SecureRandom.nextBytes(byte[]) is called, then the PRNG is seeded using a secure mechanism provided by the underlying operating system (starting with JRE 1.4.1 in Windows and JRE 1.4.2 in Linux and Solaris). If java.security.SecureRandom.setSeed(long) or java.security.SecureRandom.setSeed(byte[]) is called before a call to java.security.SecureRandom.nextBytes(byte[]), then the internal seeding mechanism is bypassed, and only the provided seed is used to generate random numbers.

For those who are not familiar with the inner workings of cryptographic PRNGs, their job is to take a relatively small random seed and use it to produce deterministic output that seems random to anybody that does not know what the seed is. The PRNG tries to ensure that the output does not reveal any information about the seed, and that somebody observing the output cannot predict future outputs without knowing the seed.

By bypassing the internal secure seeding mechanism of the SHA1PRNG, you may compromise the security of your PRNG output. If you seed it with anything that an attacker can potentially predict (e.g. the time when the PRNG instance was created), then using java.security.SecureRandom may not provide the level of security that you need.

Finally, regardless of how well the PRNG is seeded, it should not be used indefinitely without reseeding. There are two approaches that can be used for longer-term security of PRNG output:

  • Periodically throw away the existing java.security.SecureRandom instance and create a new one. This will generate a new instance with a new seed.
  • Periodically add new random material to the PRNG seed by making a call to java.security.SecureRandom.setSeed(java.security.SecureRandom.generateSeed(int)).

In summary, keep the following in mind when using java.security.SecureRandom:

  • Always specify the exact PRNG and provider that you wish to use. If you just use the default PRNG, you may end up with different PRNGs on different installations of your application that may need to be called differently in order to work properly. Using the following code to get a PRNG instance is appropriate:
    SecureRandom sr = SecureRandom.getInstance("SHA1PRNG", "SUN");
  • When using the SHA1PRNG, always call java.security.SecureRandom.nextBytes(byte[]) immediately after creating a new instance of the PRNG. This will force the PRNG to seed itself securely. If for testing purposes, you need predictable output, ignoring this rule and seeding the PRNG with hard-coded/predictable values may be appropriate.
  • Use at least JRE 1.4.1 on Windows and at least JRE 1.4.2 on Solaris and Linux. Earlier versions do not seed the SHA1PRNG securely.
  • Periodically reseed your PRNG as observing a large amount of PRNG output generated using one seed may allow the attacker to determine the seed and thus predict all future outputs.

See other posts on SecureRandom

Issues to be aware of when using Java’s SecureRandom
SecureRandom Implementation (sun.security.provider.SecureRandom – SHA1PRNG)
SecureRandom Implementation (sun.security.provider.NativePRNG)