Integrate CI_Encryption into the framework

TODO: Add documentation in user_guide_src/source/libraries/encryption.rst

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+################
+Encryption Class
+################
+
+The Encryption Class provides two-way data encryption. It uses a scheme
+that either compiles the message using a randomly hashed bitwise XOR
+encoding scheme, or is encrypted using the Mcrypt library. If Mcrypt is
+not available on your server the encoded message will still provide a
+reasonable degree of security for encrypted sessions or other such
+"light" purposes. If Mcrypt is available, you'll be provided with a high
+degree of security appropriate for storage.
+
+Setting your Key
+================
+
+A *key* is a piece of information that controls the cryptographic
+process and permits an encrypted string to be decoded. In fact, the key
+you chose will provide the **only** means to decode data that was
+encrypted with that key, so not only must you choose the key carefully,
+you must never change it if you intend use it for persistent data.
+
+It goes without saying that you should guard your key carefully. Should
+someone gain access to your key, the data will be easily decoded. If
+your server is not totally under your control it's impossible to ensure
+key security so you may want to think carefully before using it for
+anything that requires high security, like storing credit card numbers.
+
+To take maximum advantage of the encryption algorithm, your key should
+be 32 characters in length (256 bits). The key should be as random a
+string as you can concoct, with numbers and uppercase and lowercase
+letters. Your key should **not** be a simple text string. In order to be
+cryptographically secure it needs to be as random as possible.
+
+Your key can be either stored in your **application/config/config.php**, or
+you can design your own storage mechanism and pass the key dynamically
+when encoding/decoding.
+
+To save your key to your **application/config/config.php**, open the file
+and set::
+
+	$config['encryption_key'] = "YOUR KEY";
+
+Message Length
+==============
+
+It's important for you to know that the encoded messages the encryption
+function generates will be approximately 2.6 times longer than the
+original message. For example, if you encrypt the string "my super
+secret data", which is 21 characters in length, you'll end up with an
+encoded string that is roughly 55 characters (we say "roughly" because
+the encoded string length increments in 64 bit clusters, so it's not
+exactly linear). Keep this information in mind when selecting your data
+storage mechanism. Cookies, for example, can only hold 4K of
+information.
+
+Initializing the Class
+======================
+
+Like most other classes in CodeIgniter, the Encryption class is
+initialized in your controller using the **$this->load->library** function::
+
+	$this->load->library('encrypt');
+
+Once loaded, the Encrypt library object will be available using:
+$this->encrypt
+
+$this->encrypt->encode()
+========================
+
+Performs the data encryption and returns it as a string. Example::
+
+	$msg = 'My secret message';
+
+	$encrypted_string = $this->encrypt->encode($msg);
+	
+
+You can optionally pass your encryption key via the second parameter if
+you don't want to use the one in your config file::
+
+	$msg = 'My secret message';
+	$key = 'super-secret-key';
+
+	$encrypted_string = $this->encrypt->encode($msg, $key);
+
+$this->encrypt->decode()
+========================
+
+Decrypts an encoded string. Example::
+
+	$encrypted_string = 'APANtByIGI1BpVXZTJgcsAG8GZl8pdwwa84';
+
+	$plaintext_string = $this->encrypt->decode($encrypted_string);
+
+You can optionally pass your encryption key via the second parameter if
+you don't want to use the one in your config file::
+
+	$msg = 'My secret message';
+	$key = 'super-secret-key';
+
+	$encrypted_string = $this->encrypt->decode($msg, $key);
+
+$this->encrypt->set_cipher();
+==============================
+
+Permits you to set an Mcrypt cipher. By default it uses
+**MCRYPT_RIJNDAEL_256**. Example::
+
+	$this->encrypt->set_cipher(MCRYPT_BLOWFISH);
+
+Please visit php.net for a list of `available
+ciphers <http://php.net/mcrypt>`_.
+
+If you'd like to manually test whether your server supports Mcrypt you
+can use::
+
+	echo ( ! function_exists('mcrypt_encrypt')) ? 'Nope' : 'Yup';
+
+$this->encrypt->set_mode();
+============================
+
+Permits you to set an Mcrypt mode. By default it uses **MCRYPT_MODE_CBC**.
+Example::
+
+	$this->encrypt->set_mode(MCRYPT_MODE_CFB);
+
+Please visit php.net for a list of `available
+modes <http://php.net/mcrypt>`_.
+
+$this->encrypt->encode_from_legacy($orig_data, $legacy_mode = MCRYPT_MODE_ECB, $key = '');
+==========================================================================================
+
+Enables you to re-encode data that was originally encrypted with
+CodeIgniter 1.x to be compatible with the Encryption library in
+CodeIgniter 2.x. It is only necessary to use this method if you have
+encrypted data stored permanently such as in a file or database and are
+on a server that supports Mcrypt. "Light" use encryption such as
+encrypted session data or transitory encrypted flashdata require no
+intervention on your part. However, existing encrypted Sessions will be
+destroyed since data encrypted prior to 2.x will not be decoded.
+
+.. important::
+	**Why only a method to re-encode the data instead of maintaining legacy
+	methods for both encoding and decoding?** The algorithms in the
+	Encryption library have improved in CodeIgniter 2.x both for performance
+	and security, and we do not wish to encourage continued use of the older
+	methods. You can of course extend the Encryption library if you wish and
+	replace the new methods with the old and retain seamless compatibility
+	with CodeIgniter 1.x encrypted data, but this a decision that a
+	developer should make cautiously and deliberately, if at all.
+
+::
+
+	$new_data = $this->encrypt->encode_from_legacy($old_encrypted_string);
+
+======================	===============	 =======================================================================
+Parameter		 Default	  Description
+======================	===============  =======================================================================
+**$orig_data**		n/a 		 The original encrypted data from CodeIgniter 1.x's Encryption library
+**$legacy_mode**	MCRYPT_MODE_ECB	 The Mcrypt mode that was used to generate the original encrypted data.
+					 CodeIgniter 1.x's default was MCRYPT_MODE_ECB, and it will assume that
+					 to be the case unless overridden by this parameter.
+**$key**		n/a 		 The encryption key. This it typically specified in your config file as
+					 outlined above.
+======================	===============	 =======================================================================
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