iaik.security.random

Class VarLengthSeedGenerator

java.lang.Object

iaik.security.random.SeedGenerator

iaik.security.random.VarLengthSeedGenerator

Direct Known Subclasses:

AutoSeedGenerator, HashObjectSeedGenerator, JDKSeedGenerator, MetaSeedGenerator

public abstract class VarLengthSeedGenerator
extends SeedGenerator

A SeedGenerator is a class that "computes" random bits that can be used as seed for pseudo-random number generators (PRNG). This is an abstract class, subclasses have to implement this functionality. The generated bits should be truly random and non-deterministically generated based on e.g. user input, specialized external devices, or possibly the non-deterministic behavior of a multi-tasking operating system. Ideally all random bits needed for cryptographic applications would be generated in such a truly non-deterministic, random way. However, this turns out to be quite difficult on our (mostly ;) deterministic computers, therefore, only a random seed value is generated and then used with a deterministic PRNG. However, you should not use a single seed to generate a extremely long pseudo random sequence. In those cases it is advisable that you create a new seed every now and then.

The important point is that an attacker should have a probability no larger than 2^-n of guessing the correct n bit seed, nor should he be able to manipulate the random bit generation in a way to increase his probability of guessing it. Note that the attacker could reside on (in order of increasing difficulty for us) on (a) some machine connected to us via the Internet or some other WAN (b) on the same LAN (c) running tasks on the same machine (d) have full control over the OS run on our machine and (e) be the user running the seed generator himself. Obviously applications will usually not be concerned all of these cases, but only the first two or three.

Also note that if a seed of e.g. 64 bit is fed to a PRNG which is later used to generate a 128 bit random encryption key, the effective strength of that key is only 64 bit because an attacker could recreate the deterministic steps used to blow up the seed from 64 to 128 bit. On the other hand it would be no problem at all to generate an arbitrary number of 56 bit keys from our 64 bit seed. In short, this means that the seed should always be at least as long as the longest independent random value that needs to be generated from it.

General API usage:

1. In contrast to ordinary seed generators (that are only based on the class SeedGenerator), classes that subclass VarLengthSeedGenerator are able to provide seeds of variable length. Use VarLengthSeedGenerator.setDefault() to set the default variable length seed generator and VarLengthSeedGenerator.getDefault() to obtain an instance of the according variable length seed generator. Contrary to ordinary seed generators this call allows the user to set the length of the seed returned by getSeed().
2. Note that classes that subclass VarLengthSeedGenerator have to check whether the seed length has changed via VarLengthSeedGenerator.hasSeedLengthChanged() inside getSeed().
3. Depending on the generator used, you might have to perform some actions (i.e. call methods) before the seed can be fetched via getSeed() or it might be available immediately. You can check whether the seed is ready by using a SeedGenListener, you can call seedAvailable(), etc.
4. Fetch the random bits by calling getSeed(). Calling getSeed() again will yield the same seed, if you want to generate more random bits you have to create a new SeedGenerator object.

See Also:

SeedGenerator

Field Summary

Fields inherited from class iaik.security.random.SeedGenerator

seedGenListener

Constructor Summary

Constructors

Modifier	Constructor and Description
protected	VarLengthSeedGenerator(int numBits) Constructs a new variable length seed generator.

Method Summary

Methods

Modifier and Type	Method and Description
static VarLengthSeedGenerator	getDefault(int numBits) Get a new instance of the default seed generator.
int	getSeedLength() Gets the bitlength of the seed.
protected boolean	hasSeedLengthChanged() Returns true, if the seed length has changed in the meantime.
void	reinit(int numBits) Sets the number of bits to generate.
static void	setDefault(java.lang.Class clazz) Set the default seed generator.
protected void	setSeedLength(int numBits) Sets the bitlength of the seed.

Methods inherited from class iaik.security.random.SeedGenerator

callSeedGenListener, getDefault, getDefaultSeedGenerator, getSeed, getStatus, seedAvailable, setDefaultSeedGenerator, setSeedGenListener

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

VarLengthSeedGenerator

protected VarLengthSeedGenerator(int numBits)

Constructs a new variable length seed generator.

Parameters:

numBits - the seed length in bits

Method Detail

reinit

public void reinit(int numBits)

Sets the number of bits to generate. The purpose of this method is to change the number of seed bits to be generated, after a seed generator instance has been created using the default constructor. This has only an effect, if this method is called before getSeed() gets called.

Parameters:

numBits - the number of bits to generate

setDefault

public static void setDefault(java.lang.Class clazz)

Set the default seed generator. Instances of the default seed generator should not require any special initialization procedures or user interaction. The given class object must represent a SeedGenerator that has a no-args constructor accessible to this class.

Parameters:

clazz - the class that implements the default seed generator to be used

Throws:

java.lang.IllegalArgumentException - if there was an error with the SeedGenerator

getDefault

public static VarLengthSeedGenerator getDefault(int numBits)

Get a new instance of the default seed generator. This instance returns a variable length seed of bitlength numBits.

Parameters:

numBits - the bitlength of the seed

Returns:

The SeedGenerator instance.

Throws:

IllegalSeedGeneratorException

hasSeedLengthChanged

protected boolean hasSeedLengthChanged()

Returns true, if the seed length has changed in the meantime.

Returns:

true iff the seed length has changed

getSeedLength

public int getSeedLength()

Gets the bitlength of the seed.

Returns:

the number of bits to generate

setSeedLength

protected void setSeedLength(int numBits)

Sets the bitlength of the seed.