KeyAgreementSpi

AI-generated Key Takeaways

• KeyAgreementSpi is a Service Provider Interface (SPI) for the KeyAgreement class, defining how key agreement algorithms are implemented by providers.

• It involves multiple phases, using engineDoPhase with keys from other parties, potentially needing multiple calls depending on the number of participants.

• Shared secrets are generated and returned using engineGenerateSecret, which can be returned as a byte array or a SecretKey object.

• Initialization is done using engineInit, requiring a private key and optionally algorithm parameters and a source of randomness.

• The class can be reused for further key agreements after generating a secret unless re-initialized.

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.

See Also

• KeyGenerator
• SecretKey

Public Constructor Summary

KeyAgreementSpi()

Protected Method Summary

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.
abstract byte[]	engineGenerateSecret() Generates the shared secret and returns it in a new buffer.
abstract SecretKey	engineGenerateSecret(String algorithm) Creates the shared secret and returns it as a secret key object of the requested algorithm type.
abstract int	engineGenerateSecret(byte[] sharedSecret, int offset) Generates the shared secret, and places it into the buffer sharedSecret, beginning at offset inclusive.
abstract void	engineInit(Key key, SecureRandom random) Initializes this key agreement with the given key and source of randomness.
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.

Inherited Method Summary

From class java.lang.Object

Object	clone() Creates and returns a copy of this Object.
boolean	equals(Object obj) Compares this instance with the specified object and indicates if they are equal.
void	finalize() Invoked when the garbage collector has detected that this instance is no longer reachable.
final Class<?>	getClass() Returns the unique instance of Class that represents this object's class.
int	hashCode() Returns an integer hash code for this object.
final void	notify() Causes a thread which is waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
final void	notifyAll() Causes all threads which are waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
String	toString() Returns a string containing a concise, human-readable description of this object.
final void	wait(long timeout, int nanos) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait(long timeout) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait() Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object.

Public Constructors

public KeyAgreementSpi ()

Protected Methods

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.

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.

protected abstract byte[] engineGenerateSecret ()

Generates the shared secret and returns it in a new buffer.

This method resets this KeyAgreementSpi object, so that it can be reused for further key agreements. Unless this key agreement is reinitialized with one of the engineInit methods, the same private information and algorithm parameters will be used for subsequent key agreements.

Returns

• the new buffer with the shared secret

Throws

IllegalStateException	if this key agreement has not been completed yet

protected abstract SecretKey engineGenerateSecret (String algorithm)

Creates the shared secret and returns it as a secret key object of the requested algorithm type.

This method resets this KeyAgreementSpi object, so that it can be reused for further key agreements. Unless this key agreement is reinitialized with one of the engineInit methods, the same private information and algorithm parameters will be used for subsequent key agreements.

Parameters

algorithm	the requested secret key algorithm

Returns

• the shared secret key

Throws

IllegalStateException	if this key agreement has not been completed yet
NoSuchAlgorithmException	if the requested secret key algorithm is not available
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)

protected abstract int engineGenerateSecret (byte[] sharedSecret, int offset)

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, so that it can be reused for further key agreements. Unless this key agreement is reinitialized with one of the engineInit methods, the same private information and algorithm parameters will be used for subsequent key agreements.

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 completed yet
ShortBufferException	if the given output buffer is too small to hold the secret

protected abstract void engineInit (Key key, SecureRandom random)

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.

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.

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.