keyAgreement
Performs Key Agreement using this key and otherKey.
If the key needs unlocking before use (for example user authentication in any shape or form) and keyUnlockData isn't set or doesn't contain what's needed, KeyLockedException is thrown.
Return
the shared secret as a SecureByteString.
Parameters
otherKey
public key of the other party
Throws
if this key does not support key agreement (e.g. RSA).
if the key needs unlocking.
if the key is no longer usable.