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

Exceptions:

InvalidKeyException		if the given key is inappropriate for this phase.
IllegalStateException		if this key agreement has not been initialized.