A (transparent) specification of the key material that constitutes a cryptographic key.
If the key is stored on a hardware device, its specification may contain information that helps identify the key on the device.
A key may be specified in an algorithm-specific way, or in an
algorithm-independent encoding format (such as ASN.1).
For example, a DSA private key may be specified by its components
DSAPrivateKeySpec), or it may be
specified using its DER encoding
This interface contains no methods or constants. Its only purpose is to group (and provide type safety for) all key specifications. All key specifications must implement this interface.