Module java.base
Package javax.crypto

# Class KeyAgreement

java.lang.Object
javax.crypto.KeyAgreement

public class KeyAgreement extends Object
This class provides the functionality of a key agreement (or key exchange) protocol.

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.

For each of the correspondents in the key exchange, `doPhase` needs to be called. For example, if this key exchange is with one other party, `doPhase` needs to be called once, with the `lastPhase` flag set to `true`. If this key exchange is with two other parties, `doPhase` 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. However, support for key exchanges with more than two parties is implementation specific or as specified by the standard key agreement algorithm.

Every implementation of the Java platform is required to support the following standard `KeyAgreement` algorithm:

- `DiffieHellman`

This algorithm is described in the KeyAgreement section of the Java Security Standard Algorithm Names Specification. Consult the release documentation for your implementation to see if any other algorithms are supported.

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

## Constructor Summary

Constructors
Modifier
Constructor
Description
`protected`
`KeyAgreement(KeyAgreementSpi keyAgreeSpi, Provider provider, String algorithm)`
Creates a KeyAgreement object.

## Method Summary

Modifier and Type
Method
Description
`final Key`
`doPhase(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.
`final byte[]`
`generateSecret()`
Generates the shared secret and returns it in a new buffer.
`final int`
`generateSecret(byte[] sharedSecret, int offset)`
Generates the shared secret, and places it into the buffer `sharedSecret`, beginning at `offset` inclusive.
`final SecretKey`
`generateSecret(String algorithm)`
Creates the shared secret and returns it as a `SecretKey` object of the specified algorithm.
`final String`
`getAlgorithm()`
Returns the algorithm name of this `KeyAgreement` object.
`static final KeyAgreement`
`getInstance(String algorithm)`
Returns a `KeyAgreement` object that implements the specified key agreement algorithm.
`static final KeyAgreement`
`getInstance(String algorithm, String provider)`
Returns a `KeyAgreement` object that implements the specified key agreement algorithm.
`static final KeyAgreement`
`getInstance(String algorithm, Provider provider)`
Returns a `KeyAgreement` object that implements the specified key agreement algorithm.
`final Provider`
`getProvider()`
Returns the provider of this `KeyAgreement` object.
`final void`
`init(Key key)`
Initializes this key agreement with the given key, which is required to contain all the algorithm parameters required for this key agreement.
`final void`
`init(Key key, SecureRandom random)`
Initializes this key agreement with the given key and source of randomness.
`final void`
`init(Key key, AlgorithmParameterSpec params)`
Initializes this key agreement with the given key and set of algorithm parameters.
`final void`
`init(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 java.lang.Object

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

## Constructor Details

### KeyAgreement

protected KeyAgreement(KeyAgreementSpi keyAgreeSpi, Provider provider, String algorithm)
Creates a KeyAgreement object.

Parameters:
  `keyAgreeSpi` - the delegate
  `provider` - the provider
  `algorithm` - the algorithm

## Method Details

### getAlgorithm

public final String getAlgorithm()
Returns the algorithm name of this `KeyAgreement` object.

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

Returns:
  the algorithm name of this `KeyAgreement` object.

### getInstance

public static final KeyAgreement getInstance(String algorithm) throws NoSuchAlgorithmException
Returns a `KeyAgreement` object that implements the specified key agreement algorithm.

This method traverses the list of registered security Providers, starting with the most preferred Provider. A new KeyAgreement object encapsulating the KeyAgreementSpi 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.

Implementation Note:
  The JDK Reference Implementation additionally uses the `jdk.security.provider.preferred` `Security` property to determine the preferred provider order for the specified algorithm. This may be different than the order of providers returned by `Security.getProviders()`.
Parameters:
  `algorithm` - the standard name of the requested key agreement algorithm. See the KeyAgreement section in the Java Security Standard Algorithm Names Specification for information about standard algorithm names.
Returns:
  the new `KeyAgreement` object
Throws:
  `NoSuchAlgorithmException` - if no `Provider` supports a `KeyAgreementSpi` implementation for the specified algorithm
  `NullPointerException` - if `algorithm` is `null`
See Also:
  - `Provider`

### getInstance

public static final KeyAgreement getInstance(String algorithm, String provider) throws NoSuchAlgorithmException, NoSuchProviderException
Returns a `KeyAgreement` object that implements the specified key agreement algorithm.

A new KeyAgreement object encapsulating the KeyAgreementSpi 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 key agreement algorithm. See the KeyAgreement section in the Java Security Standard Algorithm Names Specification for information about standard algorithm names.
  `provider` - the name of the provider.
Returns:
  the new `KeyAgreement` object
Throws:
  `IllegalArgumentException` - if the `provider` is `null` or empty
  `NoSuchAlgorithmException` - if a `KeyAgreementSpi` 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
  `NullPointerException` - if `algorithm` is `null`
See Also:
  - `Provider`

### getInstance

public static final KeyAgreement getInstance(String algorithm, Provider provider) throws NoSuchAlgorithmException
Returns a `KeyAgreement` object that implements the specified key agreement algorithm.

A new KeyAgreement object encapsulating the KeyAgreementSpi 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 key agreement algorithm. See the KeyAgreement section in the Java Security Standard Algorithm Names Specification for information about standard algorithm names.
  `provider` - the provider.
Returns:
  the new `KeyAgreement` object
Throws:
  `IllegalArgumentException` - if the `provider` is `null`
  `NoSuchAlgorithmException` - if a `KeyAgreementSpi` implementation for the specified algorithm is not available from the specified Provider object
  `NullPointerException` - if `algorithm` is `null`
See Also:
  - `Provider`

### getProvider

public final Provider getProvider()
Returns the provider of this `KeyAgreement` object.

Returns:
  the provider of this `KeyAgreement` object

### init

public final void init(Key key) throws InvalidKeyException
Initializes this key agreement with the given key, which is required to contain all the algorithm parameters required for this key agreement.

If this key agreement requires any random bytes, it will get them using 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 will be used.)

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.
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.

### init

public final void init(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.

### init

public final void init(Key key, AlgorithmParameterSpec params) throws InvalidKeyException, InvalidAlgorithmParameterException
Initializes this key agreement with the given key and set of algorithm parameters.

If this key agreement requires any random bytes, it will get them using 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 will be used.)

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
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.

### init

public final void init(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.

### doPhase

public final Key doPhase(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 or not 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.

### generateSecret

public final byte[] generateSecret() throws IllegalStateException
Generates the shared secret and returns it in a new buffer.

This method resets this `KeyAgreement` 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

### generateSecret

public final int generateSecret(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 `KeyAgreement` 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

### generateSecret

public final SecretKey generateSecret(String algorithm) throws IllegalStateException, NoSuchAlgorithmException, InvalidKeyException
Creates the shared secret and returns it as a `SecretKey` object of the specified algorithm.

This method resets this `KeyAgreement` 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
Returns:
  the shared secret key
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 available
  `InvalidKeyException` - if the shared secret-key material cannot be used to generate a secret key of the specified algorithm (e.g., the key material is too short)