Merge branch 'develop' into patch-1

Conflicts:
	system/libraries/Calendar.php

1 files changed, 504 insertions, 113 deletions

diff --git a/user_guide_src/source/libraries/encryption.rst b/user_guide_src/source/libraries/encryption.rst
index a38122203..cedc8d381 100644
--- a/user_guide_src/source/libraries/encryption.rst
+++ b/user_guide_src/source/libraries/encryption.rst
@@ -1,164 +1,555 @@
-################
-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
+##################
+Encryption Library
+##################
+
+The Encryption Library provides two-way data encryption. To do so in
+a cryptographically secure way, it utilizes PHP extensions that are
+unfortunately not always available on all systems.
+You must meet one of the following dependancies in order to use this
+library:
+
+- `OpenSSL <http://php.net/openssl>`_ (and PHP 5.3.3)
+- `MCrypt <http://php.net/mcrypt>`_ (and `MCRYPT_DEV_URANDOM` availability)
+
+If neither of the above dependancies is met, we simply cannot offer
+you a good enough implementation to meet the high standards required
+for proper cryptography.
+
+.. contents::
+  :local:
+
+.. raw:: html
+
+  <div class="custom-index container"></div>
+
+****************************
+Using the Encryption Library
+****************************
+
+Initializing the Class
+======================
+
+Like most other classes in CodeIgniter, the Encryption library is
+initialized in your controller using the ``$this->load->library()``
+method::
+
+	$this->load->library('encrypt');
+
+Once loaded, the Encryption library object will be available using::
+
+	$this->encrypt
+
+Default behavior
 ================
 
-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.
+By default, the Encryption Library will use the AES-128 cipher in CBC
+mode, using your configured *encryption_key* and SHA512 HMAC authentication.
+
+.. note:: AES-128 is chosen both because it is proven to be strong and
+	because of its wide availability across different cryptographic
+	software and programming languages' APIs.
+
+However, the *encryption_key* is not used as is.
+
+If you are somewhat familiar with cryptography, you should already know
+that a HMAC also requires a secret key and using the same key for both
+encryption and authentication is a bad practice.
+
+Because of that, two separate keys are derived from your already configured
+*encryption_key*: one for encryption and one for authentication. This is
+done via a technique called `HMAC-based Key Derivation Function
+<http://en.wikipedia.org/wiki/HKDF>`_ (HKDF).
+
+Setting your encryption_key
+===========================
+
+An *encryption key* is a piece of information that controls the
+cryptographic process and permits a plain-text string to be encrypted,
+and afterwards - decrypted. It is the secret "ingredient" in the whole
+process that allows you to be the only one who is able to decrypt data
+that you've decided to hide from the eyes of the public.
+After one key is used to encrypt data, that same key provides the **only**
+means to decrypt it, so not only must you chose one carefully, but you
+must not lose it or you will also use the encrypted data.
+
+It must be noted that to ensure maximum security, such key *should* not
+only be as strong as possible, but also often changed. Such behavior
+however is rarely practical or possible to implement, and that is why
+CodeIgniter gives you the ability to configure a single key that is to be
+used (almost) every time.
 
 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
+someone gain access to your key, the data will be easily decrypted. 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 encryption key should be as long as the encyption algorithm in use
+allows. For AES-128, that's 128 bits or 16 bytes (charcters) long. The
+key should be as random as possible and it should **not** be a simple
+text string.
+
+You will find a table below that shows the supported key lengths of
+different ciphers.
 
-Your key can be either stored in your **application/config/config.php**, or
+The 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.
+when encrypting/decrypting.
 
-To save your key to your **application/config/config.php**, open the file
+To save your key to your *application/config/config.php*, open the file
 and set::
 
-	$config['encryption_key'] = "YOUR KEY";
+	$config['encryption_key'] = 'YOUR KEY';
+
+.. _ciphers-and-modes:
+
+Supported encryption ciphers and modes
+======================================
+
+.. note:: The terms 'cipher' and 'encryption algorithm' are interchangeable.
+
+Portable ciphers
+----------------
+
+Because MCrypt and OpenSSL (also called drivers throughout this document)
+each support different sets of encryption algorithms and often implement
+them in different ways, our Encryption library is designed to use them in
+a portable fashion, or in other words - it enables you to use them
+interchangeably, at least for the ciphers supported by both drivers.
+
+It is also implemented in a way that aims to match the standard
+implementations in other programming languages and libraries.
+
+Here's a list of the so called "portable" ciphers, where
+"CodeIgniter name" is the string value that you'd have to pass to the
+Encryption library to use that cipher:
+
+======================== ================== ============================ ===============================
+Cipher name              CodeIgniter name   Key lengths (bits / bytes)   Supported modes
+======================== ================== ============================ ===============================
+AES-128 / Rijndael-128   aes-128            128 / 16                     CBC, CTR, CFB, CFB8, OFB, ECB
+AES-192                  aes-192            192 / 24                     CBC, CTR, CFB, CFB8, OFB, ECB
+AES-256                  aes-256            256 / 32                     CBC, CTR, CFB, CFB8, OFB, ECB
+DES                      des                56 / 7                       CBC, CFB, CFB8, OFB, ECB
+TripleDES                tripledes          56 / 7, 112 / 14, 168 / 21   CBC, CFB, CFB8, OFB
+Blowfish                 blowfish           128-448 / 16-56              CBC, CFB, OFB, ECB
+CAST5 / CAST-128         cast5              40-128 / 5-16                CBC, CFB, OFB, ECB
+RC4 / ARCFour            rc4                40-2048 / 5-256              Stream
+======================== ================== ============================ ===============================
+
+.. important:: Because of how MCrypt works, if you fail to provide a key
+	with the appropriate length, you might end up using a different
+	algorithm than the one configured, so be really careful with that!
+
+.. note:: In case it isn't clear from the above table, Blowfish, CAST5
+	and RC4 support variable length keys. That is, any number in the
+	shown ranges is valid, although in bit terms that only happens
+	in 8-bit increments.
+
+.. note:: Even though CAST5 supports key lengths lower than 128 bits
+	(16 bytes), in fact they will just be zero-padded to the
+	maximum length, as specified in `RFC 2144
+	<http://tools.ietf.org/rfc/rfc2144.txt>`_.
+
+.. note:: Blowfish supports key lengths as small as 32 bits (4 bytes), but
+	our tests have shown that only lengths of 128 bits (16 bytes) or
+	higher are properly supported by both MCrypt and OpenSSL. It is
+	also a bad practice to use such low-length keys anyway.
+
+Driver-specific ciphers
+-----------------------
+
+As noted above, MCrypt and OpenSSL support different sets of encryption
+ciphers. For portability reasons and because we haven't tested them
+properly, we do not advise you to use the ones that are driver-specific,
+but regardless, here's a list of most of them:
+
+
+============== ========= ============================== =========================================
+Cipher name    Driver    Key lengths (bits / bytes)     Supported modes
+============== ========= ============================== =========================================
+AES-128        OpenSSL   128 / 16                       CBC, CTR, CFB, CFB8, OFB, ECB, GCM, XTS
+AES-192        OpenSSL   192 / 24                       CBC, CTR, CFB, CFB8, OFB, ECB, GCM, XTS
+AES-256        OpenSSL   256 / 32                       CBC, CTR, CFB, CFB8, OFB, ECB, GCM, XTS
+Rijndael-128   MCrypt    128 / 16, 192 / 24, 256 / 32   CBC, CTR, CFB, CFB8, OFB, OFB8, ECB
+Rijndael-192   MCrypt    128 / 16, 192 / 24, 256 / 32   CBC, CTR, CFB, CFB8, OFB, OFB8, ECB
+Rijndael-256   MCrypt    128 / 16, 192 / 24, 256 / 32   CBC, CTR, CFB, CFB8, OFB, OFB8, ECB
+GOST           MCrypt    256 / 32                       CBC, CTR, CFB, CFB8, OFB, OFB8, ECB
+Twofish        MCrypt    128 / 16, 192 / 24, 256 / 32   CBC, CTR, CFB, CFB8, OFB, OFB8, ECB
+CAST-256       MCrypt    128 / 16, 192 / 24, 256 / 32   CBC, CTR, CFB, CFB8, OFB, OFB8, ECB
+Loki97         MCrypt    128 / 16, 192 / 24, 256 / 32   CBC, CTR, CFB, CFB8, OFB, OFB8, ECB
+SaferPlus      MCrypt    128 / 16, 192 / 24, 256 / 32   CBC, CTR, CFB, CFB8, OFB, OFB8, ECB
+Serpent        MCrypt    128 / 16, 192 / 24, 256 / 32   CBC, CTR, CFB, CFB8, OFB, OFB8, ECB
+XTEA           MCrypt    128 / 16                       CBC, CTR, CFB, CFB8, OFB, OFB8, ECB
+RC2            MCrypt    8-1024 / 1-128                 CBC, CTR, CFB, CFB8, OFB, OFB8, ECB
+RC2            OpenSSL   8-1024 / 1-128                 CBC, CFB, OFB, ECB
+Camellia-128   OpenSSL   128 / 16                       CBC, CFB, CFB8, OFB, ECB
+Camellia-192   OpenSSL   192 / 24                       CBC, CFB, CFB8, OFB, ECB
+Camellia-256   OpenSSL   256 / 32                       CBC, CFB, CFB8, OFB, ECB
+Seed           OpenSSL   128 / 16                       CBC, CFB, OFB, ECB
+============== ========= ============================== =========================================
+
+.. note:: If you wish to use one of those ciphers, you'd have to pass
+	its name in lower-case to the Encryption library.
+
+.. note:: You've probably noticed that all AES cipers (and Rijndael-128)
+	are also listed in the portable ciphers list. This is because
+	drivers support different modes for these ciphers. Also, it is
+	important to note that AES-128 and Rijndael-128 are actually
+	the same cipher, but **only** when used with a 128-bit key.
+
+.. note:: RC2 is listed as supported by both MCrypt and OpenSSL.
+	However, both drivers implement them differently and they
+	are not portable. It is probably worth noting that we only
+	found one obscure source confirming that it is MCrypt that
+	is not properly implementing it.
+
+.. _encryption-modes:
+
+Encryption modes
+----------------
+
+Different modes of encryption have different characteristics and serve
+for different purposes. Some are stronger than others, some are faster
+and some offer extra features.
+We are not going in depth into that here, we'll leave that to the
+cryptography experts. The table below is to provide brief informational
+reference to our more experienced users. If you are a beginner, just
+stick to the CBC mode - it is widely accepted as strong and secure for
+general purposes.
+
+=========== ================== ================= ===================================================================================================================================================
+Mode name   CodeIgniter name   Driver support    Additional info
+=========== ================== ================= ===================================================================================================================================================
+CBC         cbc                MCrypt, OpenSSL   A safe default choice
+CTR         ctr                MCrypt, OpenSSL   Considered as theoretically better than CBC, but not as widely available
+CFB         cfb                MCrypt, OpenSSL   N/A
+CFB8        cfb8               MCrypt, OpenSSL   Same as CFB, but operates in 8-bit mode (not recommended).
+OFB         ofb                MCrypt, OpenSSL   N/A
+OFB8        ofb8               MCrypt            Same as OFB, but operates in 8-bit mode (not recommended).
+ECB         ecb                MCrypt, OpenSSL   Ignores IV (not recommended).
+GCM         gcm                OpenSSL           Provides authentication and therefore doesn't need a HMAC.
+XTS         xts                OpenSSL           Usually used for encrypting random access data such as RAM or hard-disk storage.
+Stream      stream             MCrypt, OpenSSL   This is not actually a mode, it just says that a stream cipher is being used. Required because of the general cipher+mode initialization process.
+=========== ================== ================= ===================================================================================================================================================
 
 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.
+It's probably important for you to know that an encrypted string is usually
+longer than the original, plain-text string (depending on the cipher).
 
-Initializing the Class
-======================
+This is influenced by the cipher algorithm itself, the IV prepended to the
+cipher-text and (unless you are using GCM mode) the HMAC authentication
+message that is also prepended. Furthermore, the encrypted message is also
+Base64-encoded so that it is safe for storage and transmission, regardless
+of a possible character set in use.
 
-Like most other classes in CodeIgniter, the Encryption class is
-initialized in your controller using the **$this->load->library** function::
+Keep this information in mind when selecting your data storage mechanism.
+Cookies, for example, can only hold 4K of information.
 
-	$this->load->library('encrypt');
+.. _configuration:
+
+Configuring the library
+=======================
 
-Once loaded, the Encrypt library object will be available using:
-$this->encrypt
+For usability, performance, but also historical reasons tied to our old
+:doc:`Encrypt Class <encrypt>`, the Encryption library is designed to
+use repeatedly the same driver, encryption cipher, mode and key.
 
-$this->encrypt->encode()
-========================
+As noted in the "Default behavior" section above, this means using an
+auto-detected driver (OpenSSL has a higher priority), the AES-128 ciper
+in CBC mode, and your ``$config['encryption_key']`` value.
 
-Performs the data encryption and returns it as a string. Example::
+If you wish to change that however, you need to use the ``initialize()``
+method. It accepts an associative array of parameters, all of which are
+optional:
 
-	$msg = 'My secret message';
+======== ===============================================
+Option   Possible values
+======== ===============================================
+driver   'mcrypt', 'openssl'
+cipher   Cipher name (see :ref:`ciphers-and-modes`)
+mode     Encryption mode (see :ref:`encryption-modes`)
+key      Encryption key 
+======== ===============================================
 
-	$encrypted_string = $this->encrypt->encode($msg);
-	
+For example, if you were to change the encryption algorithm and
+mode to AES-256 in CTR mode, this is what you should do::
 
-You can optionally pass your encryption key via the second parameter if
-you don't want to use the one in your config file::
+	$this->encryption->initialize(
+		array(
+			'cipher' => 'aes-256',
+			'mode' => 'ctr',
+			'key' => '<a 32-character random string>'
+		)
+	);
 
-	$msg = 'My secret message';
-	$key = 'super-secret-key';
+Note that we only mentioned that you want to change the ciper and mode,
+but we also included a key in the example. As previously noted, it is
+important that you choose a key with a proper size for the used algorithm.
 
-	$encrypted_string = $this->encrypt->encode($msg, $key);
+There's also the ability to change the driver, if for some reason you
+have both, but want to use MCrypt instead of OpenSSL::
 
-$this->encrypt->decode()
-========================
+	// Switch to the MCrypt driver
+	$this->encryption->initialize(array('driver' => 'mcrypt'));
 
-Decrypts an encoded string. Example::
+	// Switch back to the OpenSSL driver
+	$this->encryption->initialize(array('driver' => 'openssl'));
 
-	$encrypted_string = 'APANtByIGI1BpVXZTJgcsAG8GZl8pdwwa84';
+Encrypting and decrypting data
+==============================
 
-	$plaintext_string = $this->encrypt->decode($encrypted_string);
+Encrypting and decrypting data with the already configured library
+settings is simple. As simple as just passing the string to the
+``encrypt()`` and/or ``decrypt()`` methods::
 
-You can optionally pass your encryption key via the second parameter if
-you don't want to use the one in your config file::
+	$plain_text = 'This is a plain-text message!';
+	$ciphertext = $this->encryption->encrypt($plain_text);
 
-	$msg = 'My secret message';
-	$key = 'super-secret-key';
+	// Outputs: This is a plain-text message!
+	echo $this->encryption->decrypt($ciphertext);
 
-	$encrypted_string = $this->encrypt->decode($msg, $key);
+And that's it! The Encryption library will do everything necessary
+for the whole process to be cryptographically secure out-of-the-box.
+You don't need to worry about it.
 
-$this->encrypt->set_cipher();
-==============================
+.. important:: Both methods will return FALSE in case of an error.
+	While for ``encrypt()`` this can only mean incorrect
+	configuration, you should always check the return value
+	of ``decrypt()`` in production code.
+
+How it works
+------------
+
+If you must know how the process works, here's what happens under
+the hood:
+
+- ``$this->encryption->encrypt($plain_text)``
+
+  #. Derive an encryption key and a HMAC key from your configured
+     *encryption_key* via HKDF, using the SHA-512 digest algorithm.
+
+  #. Generate a random initialization vector (IV).
+
+  #. Encrypt the data via AES-128 in CBC mode (or another previously
+     configured cipher and mode), using the above-mentioned derived
+     encryption key and IV.
+
+  #. Prepend said IV to the resulting cipher-text.
+
+  #. Base64-encode the resulting string, so that it can be safely
+     stored or transferred without worrying about character sets.
+
+  #. Create a SHA-512 HMAC authentication message using the derived
+     HMAC key to ensure data integrity and prepend it to the Base64
+     string.
+
+- ``$this->encryption->decrypt($ciphertext)``
+
+  #. Derive an encryption key and a HMAC key from your configured
+     *encryption_key* via HKDF, using the SHA-512 digest algorithm.
+     Because your configured *encryption_key* is the same, this
+     will produce the same result as in the ``encrypt()`` method
+     above - otherwise you won't be able to decrypt it.
+
+  #. Check if the string is long enough, separate the HMAC out of
+     it and validate if it is correct (this is done in a way that
+     prevents timing attacks agains it). Return FALSE if either of
+     the checks fails.
+
+  #. Base64-decode the string.
+
+  #. Separate the IV out of the cipher-text and decrypt the said
+     cipher-text using that IV and the derived encryption key.
+
+.. _custom-parameters:
+
+Using custom parameters
+-----------------------
+
+Let's say you have to interact with another system that is out
+of your control and uses another method to encrypt data. A
+method that will most certainly not match the above-described
+sequence and probably not use all of the steps either.
+
+The Encryption library allows you to change how its encryption
+and decryption processes work, so that you can easily tailor a
+custom solution for such situations.
+
+.. note:: It is possible to use the library in this way, without
+	setting an *encryption_key* in your configuration file.
+
+All you have to do is to pass an associative array with a few
+parameters to either the ``encrypt()`` or ``decrypt()`` method.
+Here's an example::
+
+	// Assume that we have $ciphertext, $key and $hmac_key
+	// from on outside source
+
+	$message = $this->encryption->decrypt(
+		$ciphertext,
+		array(
+			'cipher' => 'blowfish',
+			'mode' => 'cbc',
+			'key' => $key,
+			'hmac_digest' => 'sha256',
+			'hmac_key' => $hmac_key
+		)
+	);
+
+In the above example, we are decrypting a message that was encrypted
+using the Blowfish cipher in CBC mode and authenticated via a SHA-256
+HMAC.
+
+.. important:: Note that both 'key' and 'hmac_key' are used in this
+	example. When using custom parameters, encryption and HMAC keys
+	are not derived like the default behavior of the library is.
+
+Below is a list of the available options.
+
+However, unless you really need to and you know what you are doing,
+we advise you to not change the encryption process as this could
+impact security, so please do so with caution.
+
+============= =============== ============================= ======================================================
+Option        Default value   Mandatory / Optional          Description
+============= =============== ============================= ======================================================
+cipher        N/A             Yes                           Encryption algorithm (see :ref:`ciphers-and-modes`).
+mode          N/A             Yes                           Encryption mode (see :ref:`encryption-modes`).
+key           N/A             Yes                           Encryption key.
+iv            N/A             No                            Initialization vector (IV).
+                                                            If not provided it will be automatically generated
+                                                            during encryption and looked for during decryption.
+hmac          TRUE            No                            Whether to use a HMAC.
+                                                            Boolean. If set to FALSE, then *hmac_digest* and
+                                                            *hmac_key* will be ignored.
+hmac_digest   sha512          No                            HMAC message digest algorithm (see :ref:`digests`).
+hmac_key      N/A             Yes, unless *hmac* is FALSE   HMAC key.
+raw_data      FALSE           No                            Whether the cipher-text should be raw.
+                                                            Boolean. If set to TRUE, then Base64 encoding and
+                                                            decoding will not be performed and HMAC will not
+                                                            be a hexadecimal string.
+============= =============== ============================= ======================================================
+
+.. important:: ``encrypt()`` and ``decrypt()`` will return FALSE if
+	a mandatory parameter is not provided or if a provided
+	value is incorrect. This includes *hmac_key*, unless *hmac*
+	is set to FALSE.
+
+.. note:: If GCM mode is used, *hmac* will always be FALSE. This is
+	because GCM mode itself provides authentication.
+
+.. _digests:
+
+Supported HMAC authentication algorithms
+----------------------------------------
+
+For HMAC message authentication, the Encryption library supports
+usage of the SHA-2 family of algorithms:
+
+=========== ==================== ============================
+Algorithm   Raw length (bytes)   Hex-encoded length (bytes)
+=========== ==================== ============================
+sha512      64                   128
+sha384      48                   96
+sha256      32                   64
+sha224      28                   56
+=========== ==================== ============================
+
+The reason for not including other popular algorithms, such as
+MD5 or SHA1 is that they are no longer considered secure enough
+and as such, we don't want to encourage their usage.
+If you absolutely need to use them, it is easy to do so via PHP's
+native `hash_hmac() <http://php.net/hash_hmac()>`_ function.
+
+Stronger algorithms of course will be added in the future as they
+appear and become widely available.
+
+***************
+Class Reference
+***************
+
+.. class:: CI_Encryption
+
+	.. method:: initialize($params)
+
+		:param	array	$params: Configuration parameters
+		:returns:	CI_Encryption instance (method chaining)
+		:rtype:	CI_Encryption
+
+		Initializes (configures) the library to use a different
+		driver, cipher, mode or key.
+
+		Example::
+
+			$this->encryption->initialize(
+				array('mode' => 'ctr')
+			);
+
+		Please refer to the :ref:`configuration` section for detailed info.
+
+	.. method:: encrypt($data[, $params = NULL])
+
+		:param	string	$data: Data to encrypt
+		:param	array	$params: Optional parameters
+		:returns:	Encrypted data or FALSE on failure
+		:rtype:	string
+
+		Encrypts the input data and returns its ciphertext.
+
+		Example::
+
+			$ciphertext = $this->encryption->encrypt('My secret message');
+
+		Please refer to the :ref:`custom-parameters` section for information
+		on the optional parameters.
 
-Permits you to set an Mcrypt cipher. By default it uses
-**MCRYPT_RIJNDAEL_256**. Example::
+	.. method:: decrypt($data[, $params = NULL])
 
-	$this->encrypt->set_cipher(MCRYPT_BLOWFISH);
+		:param	string	$data: Data to decrypt
+		:param	array	$params: Optional parameters
+		:returns:	Decrypted data or FALSE on failure
+		:rtype:	string
 
-Please visit php.net for a list of `available
-ciphers <http://php.net/mcrypt>`_.
+		Decrypts the input data and returns it in plain-text.
 
-If you'd like to manually test whether your server supports Mcrypt you
-can use::
+		Example::
 
-	echo ( ! function_exists('mcrypt_encrypt')) ? 'Nope' : 'Yup';
+			echo $this->encryption->decrypt($ciphertext);
 
-$this->encrypt->set_mode();
-============================
+		Please refer to the :ref:`custom-parameters` secrion for information
+		on the optional parameters.
 
-Permits you to set an Mcrypt mode. By default it uses **MCRYPT_MODE_CBC**.
-Example::
+	.. method:: hkdf($key[, $digest = 'sha512'[, $salt = NULL[, $length = NULL[, $info = '']]]])
 
-	$this->encrypt->set_mode(MCRYPT_MODE_CFB);
+		:param	string	$key: Input key material
+		:param	string	$digest: A SHA-2 family digest algorithm
+		:param	string	$salt: Optional salt
+		:param	int	$length: Optional output length
+		:param	string	$info: Optional context/application-specific info
+		:returns:	A pseudo-random key or FALSE on failure
 
-Please visit php.net for a list of `available
-modes <http://php.net/mcrypt>`_.
+		Derives a key from another, presumably weaker key.
 
-$this->encrypt->encode_from_legacy($orig_data, $legacy_mode = MCRYPT_MODE_ECB, $key = '');
-==========================================================================================
+		This method is used internally to derive an encryption and HMAC key
+		from your configured *encryption_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.
+		It is publicly available due to its otherwise general purpose. It is
+		described in `RFC 5869 <https://tools.ietf.org/rfc/rfc5869.txt>`_.
 
-.. 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.
+		However, as opposed to the description in RFC 5869, this implementation
+		doesn't support SHA1.
 
-::
+		Example::
 
-	$new_data = $this->encrypt->encode_from_legacy($old_encrypted_string);
+			$hmac_key = $this->encryption->hkdf(
+				$key,
+				'sha512',
+				NULL,
+				NULL,
+				'authentication'
+			);
 
-======================	===============	 =======================================================================
-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.
-======================	===============	 =======================================================================
\ No newline at end of file
+			// $hmac_key is a pseudo-random key with a length of 64 bytes
\ No newline at end of file