KeyAgreement (Java SE 26 & JDK 26)
Description: declaration: module: java.base, package: javax.crypto, class: KeyAgreement
Keywords: javax.crypto.KeyAgreement class

# 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` algorithms. For the "ECDH" algorithm, implementations must support the curves in parentheses.

- `DiffieHellman`
- `ECDH` (secp256r1, secp384r1)
- `X25519`

These algorithms are 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
External Specifications
  - Java Security Standard Algorithm Names
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 Object

  `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`
  Modifier and Type
  Method
  Description
  `protected Object`
  `clone()`
  Creates and returns a copy of this object.
  `boolean`
  `equals(Object obj)`
  Indicates whether some other object is "equal to" this one.
  `protected void`
  `finalize()`
  Deprecated, for removal: This API element is subject to removal in a future version.
  Finalization is deprecated and subject to removal in a future release.
  `final Class<?>`
  `getClass()`
  Returns the runtime class of this `Object`.
  `int`
  `hashCode()`
  Returns a hash code value for this object.
  `final void`
  `notify()`
  Wakes up a single thread that is waiting on this object's monitor.
  `final void`
  `notifyAll()`
  Wakes up all threads that are waiting on this object's monitor.
  `String`
  `toString()`
  Returns a string representation of the object.
  `final void`
  `wait()`
  Causes the current thread to wait until it is awakened, typically by being notified or interrupted.
  `final void`
  `wait(long timeoutMillis)`
  Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed.
  `final void`
  `wait(long timeoutMillis, int nanos)`
  Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed.

- ## 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 from 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`
    External Specifications
      - Java Security Standard Algorithm Names
    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`
    External Specifications
      - Java Security Standard Algorithm Names
    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 is returned. Note that the specified provider 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`
    External Specifications
      - Java Security Standard Algorithm Names
    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 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. 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 specified algorithm (e.g., the key material is too short)
    External Specifications
      - Java Security Standard Algorithm Names