KeyPairGenerator

AI-generated Key Takeaways

• The KeyPairGenerator class in Java is used to create public and private key pairs for various cryptographic algorithms such as RSA, DSA, and EC.

• It offers two initialization approaches: algorithm-independent using key size and randomness source, and algorithm-specific using AlgorithmParameterSpec.

• Key pairs are generated using the generateKeyPair() method after the KeyPairGenerator is initialized with desired parameters.

• Developers can obtain a KeyPairGenerator instance for a specific algorithm using the getInstance() method.

• Android provides support for DH, DSA, RSA (API level 1+) and EC (API level 11+).

public abstract class KeyPairGenerator extends KeyPairGeneratorSpi

The KeyPairGenerator class is used to generate pairs of public and private keys. Key pair generators are constructed using the getInstance factory methods (static methods that return instances of a given class).

A Key pair generator for a particular algorithm creates a public/private key pair that can be used with this algorithm. It also associates algorithm-specific parameters with each of the generated keys.

There are two ways to generate a key pair: in an algorithm-independent manner, and in an algorithm-specific manner. The only difference between the two is the initialization of the object:

• Algorithm-Independent Initialization

  All key pair generators share the concepts of a keysize and a source of randomness. The keysize is interpreted differently for different algorithms (e.g., in the case of the DSA algorithm, the keysize corresponds to the length of the modulus). There is an initialize method in this KeyPairGenerator class that takes these two universally shared types of arguments. There is also one that takes just a keysize argument, and uses the SecureRandom implementation of the highest-priority installed provider as the source of randomness. (If none of the installed providers supply an implementation of SecureRandom, a system-provided source of randomness is used.)

  Since no other parameters are specified when you call the above algorithm-independent initialize methods, it is up to the provider what to do about the algorithm-specific parameters (if any) to be associated with each of the keys.

  If the algorithm is the DSA algorithm, and the keysize (modulus size) is 512, 768, or 1024, then the Sun provider uses a set of precomputed values for the p, q, and g parameters. If the modulus size is not one of the above values, the Sun provider creates a new set of parameters. Other providers might have precomputed parameter sets for more than just the three modulus sizes mentioned above. Still others might not have a list of precomputed parameters at all and instead always create new parameter sets.

• Algorithm-Specific Initialization

  For situations where a set of algorithm-specific parameters already exists (e.g., so-called community parameters in DSA), there are two initialize methods that have an AlgorithmParameterSpec argument. One also has a SecureRandom argument, while the the other uses the SecureRandom implementation of the highest-priority installed provider as the source of randomness. (If none of the installed providers supply an implementation of SecureRandom, a system-provided source of randomness is used.)

In case the client does not explicitly initialize the KeyPairGenerator (via a call to an initialize method), each provider must supply (and document) a default initialization. For example, the Sun provider uses a default modulus size (keysize) of 1024 bits.

Note that this class is abstract and extends from KeyPairGeneratorSpi for historical reasons. Application developers should only take notice of the methods defined in this KeyPairGenerator class; all the methods in the superclass are intended for cryptographic service providers who wish to supply their own implementations of key pair generators.

Android provides the following KeyPairGenerator algorithms:

Algorithm	Supported API Levels
DH	1+
DSA	1+
EC	11+
RSA	1+

These algorithms are described in the KeyPairGenerator section of the Java Cryptography Architecture Standard Algorithm Name Documentation.

See Also

• AlgorithmParameterSpec

Protected Constructor Summary

KeyPairGenerator(String algorithm) Creates a KeyPairGenerator object for the specified algorithm.

Public Method Summary

final KeyPair	genKeyPair() Generates a key pair.
KeyPair	generateKeyPair() Generates a key pair.
String	getAlgorithm() Returns the standard name of the algorithm for this key pair generator.
static KeyPairGenerator	getInstance(String algorithm) Returns a KeyPairGenerator object that generates public/private key pairs for the specified algorithm.
static KeyPairGenerator	getInstance(String algorithm, String provider) Returns a KeyPairGenerator object that generates public/private key pairs for the specified algorithm.
static KeyPairGenerator	getInstance(String algorithm, Provider provider) Returns a KeyPairGenerator object that generates public/private key pairs for the specified algorithm.
final Provider	getProvider() Returns the provider of this key pair generator object.
void	initialize(AlgorithmParameterSpec params, SecureRandom random) Initializes the key pair generator with the given parameter set and source of randomness.
void	initialize(AlgorithmParameterSpec params) Initializes the key pair generator using the specified parameter set and the SecureRandom implementation of the highest-priority installed provider as the source of randomness.
void	initialize(int keysize, SecureRandom random) Initializes the key pair generator for a certain keysize with the given source of randomness (and a default parameter set).
void	initialize(int keysize) Initializes the key pair generator for a certain keysize using a default parameter set and the SecureRandom implementation of the highest-priority installed provider as the source of randomness.

Inherited Method Summary

From class java.security.KeyPairGeneratorSpi

abstract KeyPair	generateKeyPair() Generates a key pair.
void	initialize(AlgorithmParameterSpec params, SecureRandom random) Initializes the key pair generator using the specified parameter set and user-provided source of randomness.
abstract void	initialize(int keysize, SecureRandom random) Initializes the key pair generator for a certain keysize, using the default parameter set.

From class java.lang.Object

Object	clone() Creates and returns a copy of this Object.
boolean	equals(Object obj) Compares this instance with the specified object and indicates if they are equal.
void	finalize() Invoked when the garbage collector has detected that this instance is no longer reachable.
final Class<?>	getClass() Returns the unique instance of Class that represents this object's class.
int	hashCode() Returns an integer hash code for this object.
final void	notify() Causes a thread which is waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
final void	notifyAll() Causes all threads which are waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
String	toString() Returns a string containing a concise, human-readable description of this object.
final void	wait(long timeout, int nanos) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait(long timeout) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait() Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object.

Protected Constructors

protected KeyPairGenerator (String algorithm)

Creates a KeyPairGenerator object for the specified algorithm.

Parameters

algorithm	the standard string name of the algorithm. See the KeyPairGenerator section in the Java Cryptography Architecture Standard Algorithm Name Documentation for information about standard algorithm names.

Public Methods

public final KeyPair genKeyPair ()

Generates a key pair.

If this KeyPairGenerator has not been initialized explicitly, provider-specific defaults will be used for the size and other (algorithm-specific) values of the generated keys.

This will generate a new key pair every time it is called.

This method is functionally equivalent to generateKeyPair.

Returns

• the generated key pair

public KeyPair generateKeyPair ()

Generates a key pair.

If this KeyPairGenerator has not been initialized explicitly, provider-specific defaults will be used for the size and other (algorithm-specific) values of the generated keys.

This will generate a new key pair every time it is called.

This method is functionally equivalent to genKeyPair.

Returns

• the generated key pair

public String getAlgorithm ()

Returns the standard name of the algorithm for this key pair generator. See the KeyPairGenerator section in the Java Cryptography Architecture Standard Algorithm Name Documentation for information about standard algorithm names.

Returns

• the standard string name of the algorithm.

public static KeyPairGenerator getInstance (String algorithm)

Returns a KeyPairGenerator object that generates public/private key pairs for the specified algorithm.

This method traverses the list of registered security Providers, starting with the most preferred Provider. A new KeyPairGenerator object encapsulating the KeyPairGeneratorSpi implementation from the first Provider that supports the specified algorithm is returned.

Note that the list of registered providers may be retrieved via the Security.getProviders() method.

Parameters

algorithm	the standard string name of the algorithm. See the KeyPairGenerator section in the Java Cryptography Architecture Standard Algorithm Name Documentation for information about standard algorithm names.

Returns

• the new KeyPairGenerator object.

Throws

NoSuchAlgorithmException	if no Provider supports a KeyPairGeneratorSpi implementation for the specified algorithm.

See Also

• Provider

public static KeyPairGenerator getInstance (String algorithm, String provider)

Returns a KeyPairGenerator object that generates public/private key pairs for the specified algorithm.

A new KeyPairGenerator object encapsulating the KeyPairGeneratorSpi implementation from the specified provider is returned. The specified provider must be registered in the security provider list.

Note that the list of registered providers may be retrieved via the Security.getProviders() method.

Parameters

algorithm	the standard string name of the algorithm. See the KeyPairGenerator section in the Java Cryptography Architecture Standard Algorithm Name Documentation for information about standard algorithm names.
provider	the string name of the provider.

Returns

• the new KeyPairGenerator object.

Throws

NoSuchAlgorithmException	if a KeyPairGeneratorSpi implementation for the specified algorithm is not available from the specified provider.
NoSuchProviderException	if the specified provider is not registered in the security provider list.
IllegalArgumentException	if the provider name is null or empty.

See Also

• Provider

public static KeyPairGenerator getInstance (String algorithm, Provider provider)

Returns a KeyPairGenerator object that generates public/private key pairs for the specified algorithm.

A new KeyPairGenerator object encapsulating the KeyPairGeneratorSpi implementation from the specified Provider object is returned. Note that the specified Provider object does not have to be registered in the provider list.

Parameters

algorithm	the standard string name of the algorithm. See the KeyPairGenerator section in the Java Cryptography Architecture Standard Algorithm Name Documentation for information about standard algorithm names.
provider	the provider.

Returns

• the new KeyPairGenerator object.

Throws

NoSuchAlgorithmException	if a KeyPairGeneratorSpi implementation for the specified algorithm is not available from the specified Provider object.
IllegalArgumentException	if the specified provider is null.

See Also

• Provider

public final Provider getProvider ()

Returns the provider of this key pair generator object.

Returns

• the provider of this key pair generator object

public void initialize (AlgorithmParameterSpec params, SecureRandom random)

Initializes the key pair generator with the given parameter set and source of randomness.

This concrete method has been added to this previously-defined abstract class. This method calls the KeyPairGeneratorSpi initialize method, passing it params and random. That initialize method always throws an UnsupportedOperationException if it is not overridden by the provider.

Parameters

params	the parameter set used to generate the keys.
random	the source of randomness.

Throws

InvalidAlgorithmParameterException	if the given parameters are inappropriate for this key pair generator.

public void initialize (AlgorithmParameterSpec params)

Initializes the key pair generator using the specified parameter set and the SecureRandom implementation of the highest-priority installed provider as the source of randomness. (If none of the installed providers supply an implementation of SecureRandom, a system-provided source of randomness is used.).

This concrete method has been added to this previously-defined abstract class. This method calls the KeyPairGeneratorSpi initialize method, passing it params and a source of randomness (obtained from the highest-priority installed provider or system-provided if none of the installed providers supply one). That initialize method always throws an UnsupportedOperationException if it is not overridden by the provider.

Parameters

params	the parameter set used to generate the keys.

Throws

InvalidAlgorithmParameterException	if the given parameters are inappropriate for this key pair generator.

public void initialize (int keysize, SecureRandom random)

Initializes the key pair generator for a certain keysize with the given source of randomness (and a default parameter set).

Parameters

keysize	the keysize. This is an algorithm-specific metric, such as modulus length, specified in number of bits.
random	the source of randomness.

Throws

InvalidParameterException	if the keysize is not supported by this KeyPairGenerator object.

public void initialize (int keysize)

Initializes the key pair generator for a certain keysize using a default parameter set and the SecureRandom implementation of the highest-priority installed provider as the source of randomness. (If none of the installed providers supply an implementation of SecureRandom, a system-provided source of randomness is used.)

Parameters

keysize	the keysize. This is an algorithm-specific metric, such as modulus length, specified in number of bits.

Throws

InvalidParameterException	if the keysize is not supported by this KeyPairGenerator object.