# Class KeyAgreementSpi

java.lang.Object
javax.crypto.KeyAgreementSpi

public abstract class KeyAgreementSpi extends Object
This class defines the Service Provider Interface (SPI) for the `KeyAgreement` class. All the abstract methods in this class must be implemented by each cryptographic service provider who wishes to supply the implementation of a particular key agreement algorithm.

The keys involved in establishing a shared secret are created by one of the key generators (`KeyPairGenerator` or `KeyGenerator`), a `KeyFactory`, or as a result from an intermediate phase of the key agreement protocol (`engineDoPhase`).

For each of the correspondents in the key exchange, `engineDoPhase` needs to be called. For example, if the key exchange is with one other party, `engineDoPhase` needs to be called once, with the `lastPhase` flag set to `true`. If the key exchange is with two other parties, `engineDoPhase` needs to be called twice, the first time setting the `lastPhase` flag to `false`, and the second time setting it to `true`. There may be any number of parties involved in a key exchange.

Since:
  1.4
See Also:
  - `KeyGenerator`
  - `SecretKey`

- ## Constructor Summary

  Constructors
  Constructor
  Description
  `KeyAgreementSpi()`
  Constructor for subclasses to call.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `protected abstract Key`
  `engineDoPhase(Key key, boolean lastPhase)`
  Executes the next phase of this key agreement with the given key that was received from one of the other parties involved in this key agreement.
  `protected abstract byte[]`
  `engineGenerateSecret()`
  Generates the shared secret and returns it in a new buffer.
  `protected abstract int`
  `engineGenerateSecret(byte[] sharedSecret, int offset)`
  Generates the shared secret, and places it into the buffer `sharedSecret`, beginning at `offset` inclusive.
  `protected abstract SecretKey`
  `engineGenerateSecret(String algorithm)`
  Creates the shared secret and returns it as a secret key object of the requested algorithm type.
  `protected abstract void`
  `engineInit(Key key, SecureRandom random)`
  Initializes this key agreement with the given key and source of randomness.
  `protected abstract void`
  `engineInit(Key key, AlgorithmParameterSpec params, SecureRandom random)`
  Initializes this key agreement with the given key, set of algorithm parameters, and source of randomness.

  ### Methods declared in class Object

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

- ## Constructor Details

  - ### KeyAgreementSpi

    public KeyAgreementSpi()
    Constructor for subclasses to call.

- ## Method Details

  - ### engineInit

    protected abstract void engineInit(Key key, SecureRandom random) throws InvalidKeyException
    Initializes this key agreement with the given key and source of randomness. The given key is required to contain all the algorithm parameters required for this key agreement.

    If the key agreement algorithm requires random bytes, it gets them from the given source of randomness, `random`. However, if the underlying algorithm implementation does not require any random bytes, `random` is ignored.

    Parameters:
      `key` - the party's private information. For example, in the case of the Diffie-Hellman key agreement, this would be the party's own Diffie-Hellman private key.
      `random` - the source of randomness
    Throws:
      `InvalidKeyException` - if the given key is inappropriate for this key agreement, e.g., is of the wrong type or has an incompatible algorithm type.

  - ### engineInit

    protected abstract void engineInit(Key key, AlgorithmParameterSpec params, SecureRandom random) throws InvalidKeyException, InvalidAlgorithmParameterException
    Initializes this key agreement with the given key, set of algorithm parameters, and source of randomness.
    Parameters:
      `key` - the party's private information. For example, in the case of the Diffie-Hellman key agreement, this would be the party's own Diffie-Hellman private key.
      `params` - the key agreement parameters
      `random` - the source of randomness
    Throws:
      `InvalidKeyException` - if the given key is inappropriate for this key agreement, e.g., is of the wrong type or has an incompatible algorithm type.
      `InvalidAlgorithmParameterException` - if the given parameters are inappropriate for this key agreement.

  - ### engineDoPhase

    protected abstract Key engineDoPhase(Key key, boolean lastPhase) throws InvalidKeyException, IllegalStateException
    Executes the next phase of this key agreement with the given key that was received from one of the other parties involved in this key agreement.
    Parameters:
      `key` - the key for this phase. For example, in the case of Diffie-Hellman between 2 parties, this would be the other party's Diffie-Hellman public key.
      `lastPhase` - flag which indicates whether this is the last phase of this key agreement.
    Returns:
      the (intermediate) key resulting from this phase, or `null` if this phase does not yield a key
    Throws:
      `InvalidKeyException` - if the given key is inappropriate for this phase.
      `IllegalStateException` - if this key agreement has not been initialized.

  - ### engineGenerateSecret

    protected abstract byte[] engineGenerateSecret() throws IllegalStateException
    Generates the shared secret and returns it in a new buffer.

    This method resets this `KeyAgreementSpi` object to the state that it was in after the most recent call to one of the `init` methods. After a call to `generateSecret`, the object can be reused for further key agreement operations by calling `doPhase` to supply new keys, and then calling `generateSecret` to produce a new secret. In this case, the private information and algorithm parameters supplied to `init` will be used for multiple key agreement operations. The `init` method can be called after `generateSecret` to change the private information used in subsequent operations.

    Returns:
      the new buffer with the shared secret
    Throws:
      `IllegalStateException` - if this key agreement has not been initialized or if `doPhase` has not been called to supply the keys for all parties in the agreement

  - ### engineGenerateSecret

    protected abstract int engineGenerateSecret(byte[] sharedSecret, int offset) throws IllegalStateException, ShortBufferException
    Generates the shared secret, and places it into the buffer `sharedSecret`, beginning at `offset` inclusive.

    If the `sharedSecret` buffer is too small to hold the result, a `ShortBufferException` is thrown. In this case, this call should be repeated with a larger output buffer.

    This method resets this `KeyAgreementSpi` object to the state that it was in after the most recent call to one of the `init` methods. After a call to `generateSecret`, the object can be reused for further key agreement operations by calling `doPhase` to supply new keys, and then calling `generateSecret` to produce a new secret. In this case, the private information and algorithm parameters supplied to `init` will be used for multiple key agreement operations. The `init` method can be called after `generateSecret` to change the private information used in subsequent operations.

    Parameters:
      `sharedSecret` - the buffer for the shared secret
      `offset` - the offset in `sharedSecret` where the shared secret will be stored
    Returns:
      the number of bytes placed into `sharedSecret`
    Throws:
      `IllegalStateException` - if this key agreement has not been initialized or if `doPhase` has not been called to supply the keys for all parties in the agreement
      `ShortBufferException` - if the given output buffer is too small to hold the secret

  - ### engineGenerateSecret

    protected abstract SecretKey engineGenerateSecret(String algorithm) throws IllegalStateException, NoSuchAlgorithmException, InvalidKeyException
    Creates the shared secret and returns it as a secret key object of the requested algorithm type.

    This method resets this `KeyAgreementSpi` object to the state that it was in after the most recent call to one of the `init` methods. After a call to `generateSecret`, the object can be reused for further key agreement operations by calling `doPhase` to supply new keys, and then calling `generateSecret` to produce a new secret. In this case, the private information and algorithm parameters supplied to `init` will be used for multiple key agreement operations. The `init` method can be called after `generateSecret` to change the private information used in subsequent operations.

    Parameters:
      `algorithm` - the requested secret key algorithm. This is different from the `KeyAgreement` algorithm provided to the `getInstance` method. See the SecretKey Algorithms section in the Java Security Standard Algorithm Names Specification for information about standard secret key algorithm names. Specify "Generic" if the output will be used as the input keying material of a key derivation function (KDF).
    Returns:
      the shared secret key. The length of the key material may be adjusted to be compatible with the specified algorithm, regardless of whether the key is extractable. If `algorithm` is specified as "Generic" and it is supported by the implementation, the full shared secret is returned.
    Throws:
      `IllegalStateException` - if this key agreement has not been initialized or if `doPhase` has not been called to supply the keys for all parties in the agreement
      `NoSuchAlgorithmException` - if the specified secret key algorithm is not supported
      `InvalidKeyException` - if the shared secret key material cannot be used to generate a secret key of the requested algorithm type (e.g., the key material is too short)
    External Specifications
      - Java Security Standard Algorithm Names