This class defines the Service Provider Interface (SPI) for the Cipher 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 cipher algorithm.

In order to create an instance of Cipher, which encapsulates an instance of this CipherSpi class, an application calls one of the getInstance factory methods of the Cipher engine class and specifies the requested transformation. Optionally, the application may also specify the name of a provider.

A transformation is a string that describes the operation (or set of operations) to be performed on the given input, to produce some output. A transformation always includes the name of a cryptographic algorithm (e.g., DES), and may be followed by a feedback mode and padding scheme.

A transformation is of the form:

"algorithm/mode/padding" or
"algorithm"

(in the latter case, provider-specific default values for the mode and padding scheme are used). For example, the following is a valid transformation:

     Cipher c = Cipher.getInstance("DES/CBC/PKCS5Padding");

A provider may supply a separate class for each combination of algorithm/mode/padding, or may decide to provide more generic classes representing sub-transformations corresponding to algorithm or algorithm/mode or algorithm//padding (note the double slashes), in which case the requested mode and/or padding are set automatically by the getInstance methods of Cipher, which invoke the engineSetMode and engineSetPadding methods of the provider's subclass of CipherSpi.

A Cipher property in a provider master class may have one of the following formats:

     // provider's subclass of "CipherSpi" implements "algName" with
     // pluggable mode and padding
     Cipher.algName

     // provider's subclass of "CipherSpi" implements "algName" in the
     // specified "mode", with pluggable padding
     Cipher.algName/mode

     // provider's subclass of "CipherSpi" implements "algName" with the
     // specified "padding", with pluggable mode
     Cipher.algName//padding

     // provider's subclass of "CipherSpi" implements "algName" with the
     // specified "mode" and "padding"
     Cipher.algName/mode/padding

For example, a provider may supply a subclass of CipherSpi that implements DES/ECB/PKCS5Padding, one that implements DES/CBC/PKCS5Padding, one that implements DES/CFB/PKCS5Padding, and yet another one that implements DES/OFB/PKCS5Padding. That provider would have the following Cipher properties in its master class:

```
     Cipher.DES/ECB/PKCS5Padding
 
```
```
     Cipher.DES/CBC/PKCS5Padding
 
```
```
     Cipher.DES/CFB/PKCS5Padding
 
```
```
     Cipher.DES/OFB/PKCS5Padding
 
```

Another provider may implement a class for each of the above modes (i.e., one class for ECB, one for CBC, one for CFB, and one for OFB), one class for PKCS5Padding, and a generic DES class that subclasses from CipherSpi. That provider would have the following Cipher properties in its master class:

```
     Cipher.DES
 
```

The getInstance factory method of the Cipher engine class follows these rules in order to instantiate a provider's implementation of CipherSpi for a transformation of the form "algorithm":

Check if the provider has registered a subclass of CipherSpi for the specified "algorithm".
If the answer is YES, instantiate this class, for whose mode and padding scheme default values (as supplied by the provider) are used.
If the answer is NO, throw a NoSuchAlgorithmException exception.

The getInstance factory method of the Cipher engine class follows these rules in order to instantiate a provider's implementation of CipherSpi for a transformation of the form "algorithm/mode/padding":

Check if the provider has registered a subclass of CipherSpi for the specified "algorithm/mode/padding" transformation.
If the answer is YES, instantiate it.
If the answer is NO, go to the next step.
Check if the provider has registered a subclass of CipherSpi for the sub-transformation "algorithm/mode".
If the answer is YES, instantiate it, and call engineSetPadding(padding) on the new instance.
If the answer is NO, go to the next step.
Check if the provider has registered a subclass of CipherSpi for the sub-transformation "algorithm//padding" (note the double slashes).
If the answer is YES, instantiate it, and call engineSetMode(mode) on the new instance.
If the answer is NO, go to the next step.
Check if the provider has registered a subclass of CipherSpi for the sub-transformation "algorithm".
If the answer is YES, instantiate it, and call engineSetMode(mode) and engineSetPadding(padding) on the new instance.
If the answer is NO, throw a NoSuchAlgorithmException exception.