java.security

## Class KeyFactory

- java.lang.Object
- - java.security.KeyFactory

-     public class KeyFactory
      extends Object

  Key factories are used to convert keys (opaque cryptographic keys of type `Key`) into key specifications (transparent representations of the underlying key material), and vice versa.

  Key factories are bi-directional. That is, they allow you to build an opaque key object from a given key specification (key material), or to retrieve the underlying key material of a key object in a suitable format.

  Multiple compatible key specifications may exist for the same key. For example, a DSA public key may be specified using `DSAPublicKeySpec` or `X509EncodedKeySpec`. A key factory can be used to translate between compatible key specifications.

  The following is an example of how to use a key factory in order to instantiate a DSA public key from its encoding. Assume Alice has received a digital signature from Bob. Bob also sent her his public key (in encoded format) to verify his signature. Alice then performs the following actions:

       X509EncodedKeySpec bobPubKeySpec = new X509EncodedKeySpec(bobEncodedPubKey);
       KeyFactory keyFactory = KeyFactory.getInstance("DSA");
       PublicKey bobPubKey = keyFactory.generatePublic(bobPubKeySpec);
       Signature sig = Signature.getInstance("DSA");
       sig.initVerify(bobPubKey);
       sig.update(data);
       sig.verify(signature);

  Every implementation of the Java platform is required to support the following standard `KeyFactory` algorithms:

  - `DiffieHellman`
  - `DSA`
  - `RSA`
  These algorithms are described in the KeyFactory section of the Java Cryptography Architecture Standard Algorithm Name Documentation. Consult the release documentation for your implementation to see if any other algorithms are supported.
  Since:
    1.2
  See Also:
    `Key`, `PublicKey`, `PrivateKey`, `KeySpec`, `DSAPublicKeySpec`, `X509EncodedKeySpec`

- - ### Constructor Summary

    Constructors
    Modifier | Constructor and Description
    --- | ---
    `protected` | `KeyFactory(KeyFactorySpi keyFacSpi, Provider provider, String algorithm)` Creates a KeyFactory object.

  - ### Method Summary

    Methods
    Modifier and Type | Method and Description
    --- | ---
    `PrivateKey` | `generatePrivate(KeySpec keySpec)` Generates a private key object from the provided key specification (key material).
    `PublicKey` | `generatePublic(KeySpec keySpec)` Generates a public key object from the provided key specification (key material).
    `String` | `getAlgorithm()` Gets the name of the algorithm associated with this `KeyFactory`.
    `static KeyFactory` | `getInstance(String algorithm)` Returns a KeyFactory object that converts public/private keys of the specified algorithm.
    `static KeyFactory` | `getInstance(String algorithm, Provider provider)` Returns a KeyFactory object that converts public/private keys of the specified algorithm.
    `static KeyFactory` | `getInstance(String algorithm, String provider)` Returns a KeyFactory object that converts public/private keys of the specified algorithm.
    `<T extends KeySpec> T` | `getKeySpec(Key key, Class<T> keySpec)` Returns a specification (key material) of the given key object.
    `Provider` | `getProvider()` Returns the provider of this key factory object.
    `Key` | `translateKey(Key key)` Translates a key object, whose provider may be unknown or potentially untrusted, into a corresponding key object of this key factory.

    - ### Methods inherited from class java.lang.Object

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

- - ### Constructor Detail

    - #### KeyFactory

          protected KeyFactory(KeyFactorySpi keyFacSpi,
                    Provider provider,
                    String algorithm)

      Creates a KeyFactory object.
      Parameters:
        `keyFacSpi` - the delegate
        `provider` - the provider
        `algorithm` - the name of the algorithm to associate with this `KeyFactory`

  - ### Method Detail

    - #### getInstance

          public static KeyFactory getInstance(String algorithm)
                                        throws NoSuchAlgorithmException

      Returns a KeyFactory object that converts public/private keys of the specified algorithm.

      This method traverses the list of registered security Providers, starting with the most preferred Provider. A new KeyFactory object encapsulating the KeyFactorySpi 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 name of the requested key algorithm. See the KeyFactory section in the Java Cryptography Architecture Standard Algorithm Name Documentation for information about standard algorithm names.
      Returns:
        the new KeyFactory object.
      Throws:
        `NoSuchAlgorithmException` - if no Provider supports a KeyFactorySpi implementation for the specified algorithm.
      See Also:
        `Provider`

    - #### getInstance

          public static KeyFactory getInstance(String algorithm,
                               String provider)
                                        throws NoSuchAlgorithmException,
                                               NoSuchProviderException

      Returns a KeyFactory object that converts public/private keys of the specified algorithm.

      A new KeyFactory object encapsulating the KeyFactorySpi 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 name of the requested key algorithm. See the KeyFactory section in the Java Cryptography Architecture Standard Algorithm Name Documentation for information about standard algorithm names.
        `provider` - the name of the provider.
      Returns:
        the new KeyFactory object.
      Throws:
        `NoSuchAlgorithmException` - if a KeyFactorySpi 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`

    - #### getInstance

          public static KeyFactory getInstance(String algorithm,
                               Provider provider)
                                        throws NoSuchAlgorithmException

      Returns a KeyFactory object that converts public/private keys of the specified algorithm.

      A new KeyFactory object encapsulating the KeyFactorySpi 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 name of the requested key algorithm. See the KeyFactory section in the Java Cryptography Architecture Standard Algorithm Name Documentation for information about standard algorithm names.
        `provider` - the provider.
      Returns:
        the new KeyFactory object.
      Throws:
        `NoSuchAlgorithmException` - if a KeyFactorySpi implementation for the specified algorithm is not available from the specified Provider object.
        `IllegalArgumentException` - if the specified provider is null.
      Since:
        1.4
      See Also:
        `Provider`

    - #### getProvider

          public final Provider getProvider()

      Returns the provider of this key factory object.
      Returns:
        the provider of this key factory object

    - #### getAlgorithm

          public final String getAlgorithm()

      Gets the name of the algorithm associated with this `KeyFactory`.
      Returns:
        the name of the algorithm associated with this `KeyFactory`

    - #### generatePublic

          public final PublicKey generatePublic(KeySpec keySpec)
                                         throws InvalidKeySpecException

      Generates a public key object from the provided key specification (key material).
      Parameters:
        `keySpec` - the specification (key material) of the public key.
      Returns:
        the public key.
      Throws:
        `InvalidKeySpecException` - if the given key specification is inappropriate for this key factory to produce a public key.

    - #### generatePrivate

          public final PrivateKey generatePrivate(KeySpec keySpec)
                                           throws InvalidKeySpecException

      Generates a private key object from the provided key specification (key material).
      Parameters:
        `keySpec` - the specification (key material) of the private key.
      Returns:
        the private key.
      Throws:
        `InvalidKeySpecException` - if the given key specification is inappropriate for this key factory to produce a private key.

    - #### getKeySpec

          public final <T extends KeySpec> T getKeySpec(Key key,
                                         Class<T> keySpec)
                                             throws InvalidKeySpecException

      Returns a specification (key material) of the given key object. `keySpec` identifies the specification class in which the key material should be returned. It could, for example, be `DSAPublicKeySpec.class`, to indicate that the key material should be returned in an instance of the `DSAPublicKeySpec` class.
      Parameters:
        `key` - the key.
        `keySpec` - the specification class in which the key material should be returned.
      Returns:
        the underlying key specification (key material) in an instance of the requested specification class.
      Throws:
        `InvalidKeySpecException` - if the requested key specification is inappropriate for the given key, or the given key cannot be processed (e.g., the given key has an unrecognized algorithm or format).

    - #### translateKey

          public final Key translateKey(Key key)
                                 throws InvalidKeyException

      Translates a key object, whose provider may be unknown or potentially untrusted, into a corresponding key object of this key factory.
      Parameters:
        `key` - the key whose provider is unknown or untrusted.
      Returns:
        the translated key.
      Throws:
        `InvalidKeyException` - if the given key cannot be processed by this key factory.

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