Key Interfaces

The Key interface is the top-level interface for all opaque keys. It defines the functionality shared by all opaque key objects.

An opaque key representation is one in which you have no direct access to the key material that constitutes a key. In other words, an opaque key allows you limited access to the key - just the three methods defined by the Key interface: getAlgorithm, getFormat, and getEncoded. This is in contrast to a transparent representation, in which you can access each key material value individually, through one of the get methods defined in the corresponding specification class.

All opaque keys have three characteristics:
An Algorithm
The key algorithm for that key. The key algorithm is usually an encryption or asymmetric operation algorithm (such as DSA or RSA), which will work with those algorithms and with related algorithms (such as MD5 with RSA, SHA-1 with RSA, and so on.) The name of the algorithm of a key is obtained using this method: 
String getAlgorithm()
An Encoded Form
The external encoded form for the key used when a standard representation of the key is needed outside the Java™ Virtual Machine, as when transmitting the key to some other party. The key is encoded according to a standard format (such as X.509 or PKCS #8), and is returned using the method: 
byte[] getEncoded()
A Format
The name of the format of the encoded key. It is returned by the method: 
String getFormat()
Keys are generally obtained through key generators, certificates, key specifications (using a KeyFactory), or a KeyStore implementation accessing a keystore database used to manage keys.

It is possible to parse encoded keys, in an algorithm-dependent manner, using a KeyFactory.

It is also possible to parse certificates, using a CertificateFactory.

The interfaces that extend the Key interface in the java.security and javax.crypto packages are:
The PublicKey and PrivateKey Interfaces

The PublicKey and PrivateKey interfaces (which both extend the Key interface) are methodless interfaces, used for type-safety and type-identification.