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## javax.crypto
Class ExemptionMechanism

    java.lang.Object
      javax.crypto.ExemptionMechanism

    public class ExemptionMechanism

extends Object

This class provides the functionality of an exemption mechanism, examples of which are key recovery, key weakening, and key escrow.

Applications or applets that use an exemption mechanism may be granted stronger encryption capabilities than those which don't.

Since:
  1.4

Constructor Summary
`protected` | `ExemptionMechanism(ExemptionMechanismSpi exmechSpi, Provider provider, String mechanism)` Creates a ExemptionMechanism object.

Method Summary
`protected void` | `finalize()` Ensures that the key stored away by this ExemptionMechanism object will be wiped out when there are no more references to it.
`byte[]` | `genExemptionBlob()` Generates the exemption mechanism key blob.
`int` | `genExemptionBlob(byte[] output)` Generates the exemption mechanism key blob, and stores the result in the `output` buffer.
`int` | `genExemptionBlob(byte[] output, int outputOffset)` Generates the exemption mechanism key blob, and stores the result in the `output` buffer, starting at `outputOffset` inclusive.
`static ExemptionMechanism` | `getInstance(String algorithm)` Returns an `ExemptionMechanism` object that implements the specified exemption mechanism algorithm.
`static ExemptionMechanism` | `getInstance(String algorithm, Provider provider)` Returns an `ExemptionMechanism` object that implements the specified exemption mechanism algorithm.
`static ExemptionMechanism` | `getInstance(String algorithm, String provider)` Returns an `ExemptionMechanism` object that implements the specified exemption mechanism algorithm.
`String` | `getName()` Returns the exemption mechanism name of this `ExemptionMechanism` object.
`int` | `getOutputSize(int inputLen)` Returns the length in bytes that an output buffer would need to be in order to hold the result of the next `genExemptionBlob` operation, given the input length `inputLen` (in bytes).
`Provider` | `getProvider()` Returns the provider of this `ExemptionMechanism` object.
`void` | `init(Key key)` Initializes this exemption mechanism with a key.
`void` | `init(Key key, AlgorithmParameters params)` Initializes this exemption mechanism with a key and a set of algorithm parameters.
`void` | `init(Key key, AlgorithmParameterSpec params)` Initializes this exemption mechanism with a key and a set of algorithm parameters.
`boolean` | `isCryptoAllowed(Key key)` Returns whether the result blob has been generated successfully by this exemption mechanism.

Methods inherited from class java.lang.Object
`clone, equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

Constructor Detail

### ExemptionMechanism

    protected ExemptionMechanism(ExemptionMechanismSpi exmechSpi,
                                 Provider provider,
                                 String mechanism)

  Creates a ExemptionMechanism object.

  Parameters:
    `exmechSpi` - the delegate
    `provider` - the provider
    `mechanism` - the exemption mechanism

Method Detail

### getName

    public final String getName()

  Returns the exemption mechanism name of this `ExemptionMechanism` object.

  This is the same name that was specified in one of the `getInstance` calls that created this `ExemptionMechanism` object.

  Returns:
    the exemption mechanism name of this `ExemptionMechanism` object.

### getInstance

    public static final ExemptionMechanism getInstance(String algorithm)
                                                throws NoSuchAlgorithmException

  Returns an `ExemptionMechanism` object that implements the specified exemption mechanism algorithm.

  This method traverses the list of registered security Providers, starting with the most preferred Provider. A new ExemptionMechanism object encapsulating the ExemptionMechanismSpi 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 name of the requested exemption mechanism. See Appendix A in the Java Cryptography Extension Reference Guide for information about standard exemption mechanism names.
  Returns:
    the new `ExemptionMechanism` object.
  Throws:
    `NullPointerException` - if `algorithm` is null.
    `NoSuchAlgorithmException` - if no Provider supports an ExemptionMechanismSpi implementation for the specified algorithm.
  See Also:
    `Provider`

### getInstance

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

  Returns an `ExemptionMechanism` object that implements the specified exemption mechanism algorithm.

  A new ExemptionMechanism object encapsulating the ExemptionMechanismSpi 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 name of the requested exemption mechanism. See Appendix A in the Java Cryptography Extension Reference Guide for information about standard exemption mechanism names.
    `provider` - the name of the provider
  Returns:
    the new `ExemptionMechanism` object.
  Throws:
    `NullPointerException` - if `algorithm` is null.
    `NoSuchAlgorithmException` - if an ExemptionMechanismSpi 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` is null or empty.
  See Also:
    `Provider`

### getInstance

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

  Returns an `ExemptionMechanism` object that implements the specified exemption mechanism algorithm.

  A new ExemptionMechanism object encapsulating the ExemptionMechanismSpi 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 name of the requested exemption mechanism. See Appendix A in the Java Cryptography Extension Reference Guide for information about standard exemption mechanism names.
    `provider` - the provider.
  Returns:
    the new `ExemptionMechanism` object.
  Throws:
    `NullPointerException` - if `algorithm` is null.
    `NoSuchAlgorithmException` - if an ExemptionMechanismSpi implementation for the specified algorithm is not available from the specified Provider object.
    `IllegalArgumentException` - if the `provider` is null.
  See Also:
    `Provider`

### getProvider

    public final Provider getProvider()

  Returns the provider of this `ExemptionMechanism` object.

  Returns:
    the provider of this `ExemptionMechanism` object.

### isCryptoAllowed

    public final boolean isCryptoAllowed(Key key)
                                  throws ExemptionMechanismException

  Returns whether the result blob has been generated successfully by this exemption mechanism.

  The method also makes sure that the key passed in is the same as the one this exemption mechanism used in initializing and generating phases.

  Parameters:
    `key` - the key the crypto is going to use.
  Returns:
    whether the result blob of the same key has been generated successfully by this exemption mechanism; false if `key` is null.
  Throws:
    `ExemptionMechanismException` - if problem(s) encountered while determining whether the result blob has been generated successfully by this exemption mechanism object.

### getOutputSize

    public final int getOutputSize(int inputLen)
                            throws IllegalStateException

  Returns the length in bytes that an output buffer would need to be in order to hold the result of the next `genExemptionBlob` operation, given the input length `inputLen` (in bytes).

  The actual output length of the next `genExemptionBlob` call may be smaller than the length returned by this method.

  Parameters:
    `inputLen` - the input length (in bytes)
  Returns:
    the required output buffer size (in bytes)
  Throws:
    `IllegalStateException` - if this exemption mechanism is in a wrong state (e.g., has not yet been initialized)

### init

    public final void init(Key key)
                    throws InvalidKeyException,
                           ExemptionMechanismException

  Initializes this exemption mechanism with a key.

  If this exemption mechanism requires any algorithm parameters that cannot be derived from the given `key`, the underlying exemption mechanism implementation is supposed to generate the required parameters itself (using provider-specific default values); in the case that algorithm parameters must be specified by the caller, an `InvalidKeyException` is raised.

  Parameters:
    `key` - the key for this exemption mechanism
  Throws:
    `InvalidKeyException` - if the given key is inappropriate for this exemption mechanism.
    `ExemptionMechanismException` - if problem(s) encountered in the process of initializing.

### init

    public final void init(Key key,
                           AlgorithmParameterSpec params)
                    throws InvalidKeyException,
                           InvalidAlgorithmParameterException,
                           ExemptionMechanismException

  Initializes this exemption mechanism with a key and a set of algorithm parameters.

  If this exemption mechanism requires any algorithm parameters and `params` is null, the underlying exemption mechanism implementation is supposed to generate the required parameters itself (using provider-specific default values); in the case that algorithm parameters must be specified by the caller, an `InvalidAlgorithmParameterException` is raised.

  Parameters:
    `key` - the key for this exemption mechanism
    `params` - the algorithm parameters
  Throws:
    `InvalidKeyException` - if the given key is inappropriate for this exemption mechanism.
    `InvalidAlgorithmParameterException` - if the given algorithm parameters are inappropriate for this exemption mechanism.
    `ExemptionMechanismException` - if problem(s) encountered in the process of initializing.

### init

    public final void init(Key key,
                           AlgorithmParameters params)
                    throws InvalidKeyException,
                           InvalidAlgorithmParameterException,
                           ExemptionMechanismException

  Initializes this exemption mechanism with a key and a set of algorithm parameters.

  If this exemption mechanism requires any algorithm parameters and `params` is null, the underlying exemption mechanism implementation is supposed to generate the required parameters itself (using provider-specific default values); in the case that algorithm parameters must be specified by the caller, an `InvalidAlgorithmParameterException` is raised.

  Parameters:
    `key` - the key for this exemption mechanism
    `params` - the algorithm parameters
  Throws:
    `InvalidKeyException` - if the given key is inappropriate for this exemption mechanism.
    `InvalidAlgorithmParameterException` - if the given algorithm parameters are inappropriate for this exemption mechanism.
    `ExemptionMechanismException` - if problem(s) encountered in the process of initializing.

### genExemptionBlob

    public final byte[] genExemptionBlob()
                                  throws IllegalStateException,
                                         ExemptionMechanismException

  Generates the exemption mechanism key blob.

  Returns:
    the new buffer with the result key blob.
  Throws:
    `IllegalStateException` - if this exemption mechanism is in a wrong state (e.g., has not been initialized).
    `ExemptionMechanismException` - if problem(s) encountered in the process of generating.

### genExemptionBlob

    public final int genExemptionBlob(byte[] output)
                               throws IllegalStateException,
                                      ShortBufferException,
                                      ExemptionMechanismException

  Generates the exemption mechanism key blob, and stores the result in the `output` buffer.

  If the `output` buffer is too small to hold the result, a `ShortBufferException` is thrown. In this case, repeat this call with a larger output buffer. Use `getOutputSize` to determine how big the output buffer should be.

  Parameters:
    `output` - the buffer for the result
  Returns:
    the number of bytes stored in `output`
  Throws:
    `IllegalStateException` - if this exemption mechanism is in a wrong state (e.g., has not been initialized).
    `ShortBufferException` - if the given output buffer is too small to hold the result.
    `ExemptionMechanismException` - if problem(s) encountered in the process of generating.

### genExemptionBlob

    public final int genExemptionBlob(byte[] output,
                                      int outputOffset)
                               throws IllegalStateException,
                                      ShortBufferException,
                                      ExemptionMechanismException

  Generates the exemption mechanism key blob, and stores the result in the `output` buffer, starting at `outputOffset` inclusive.

  If the `output` buffer is too small to hold the result, a `ShortBufferException` is thrown. In this case, repeat this call with a larger output buffer. Use `getOutputSize` to determine how big the output buffer should be.

  Parameters:
    `output` - the buffer for the result
    `outputOffset` - the offset in `output` where the result is stored
  Returns:
    the number of bytes stored in `output`
  Throws:
    `IllegalStateException` - if this exemption mechanism is in a wrong state (e.g., has not been initialized).
    `ShortBufferException` - if the given output buffer is too small to hold the result.
    `ExemptionMechanismException` - if problem(s) encountered in the process of generating.

### finalize

    protected void finalize()

  Ensures that the key stored away by this ExemptionMechanism object will be wiped out when there are no more references to it.

  Overrides:
    `finalize` in class `Object`

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