draft of security section

docs/guide/security.md

+Security
+========
+
+Hashing and verifyig passwords
+------------------------------
+
+It is important not to store passwords in plain text but, contrary to popular belief, just using `md5` or `sha1` to
+compute and verify hashes isn't a good way either. Modern hardware allows to brute force these very fast.
+
+In order to truly secure user passwords even in case your database is leaked you need to use a function that is resistant
+to brute-force such as bcrypt. In PHP it can be achieved by using [crypt function](http://php.net/manual/en/function.crypt.php)
+but since usage isn't trivial and one can easily misuse it, Yii provides two helper functions for generating hash from
+password and verifying existing hash.
+
+When user sets his password we're taking password string from POST and then getting a hash:
+
+```php
+$hash = \yii\helpers\Security::generatePasswordHash($password);
+```
+
+The hash we've got is persisted to database to be used later.
+
+Then when user is trying to log in we're verifying the password he entered against a hash that we've previously persisted:
+
+```php
+if(Security::validatePassword($password, $hash)) {
+	// all good, logging user in
+}
+else {
+	// wrong password
+}
+```
+
+
+Random data
+-----------
+
+Random data is useful in many cases. For example, when resetting a password via email you need to generate a token,
+save it to database and send it via email to end user so he's able to prove that email belongs to him. It is very
+important for this token to be truly unique else there will be a possibility to predict a value and reset another user's
+password.
+
+Yii security helper makes it as simple as:
+
+```php
+$key = \yii\helpers\Security::generateRandomKey();
+```
+
+Encryption and decryption
+-------------------------
+
+In order to encrypt data so only person knowing a secret passphrase or having a secret key will be able to decrypt it.
+For example, we need to store some information in our database but we need to make sure only user knowing a secret code
+can view it (even if database is leaked):
+
+
+```php
+// $data and $secretWord are from the form
+$encryptedData = \yii\helpers\Security::encrypt($data, $secretWord);
+// store $encryptedData to database
+```
+
+Then when user want to read it:
+
+```php
+// $secretWord is from the form, $encryptedData is from database
+$data = \yii\helpers\Security::decrypt($encryptedData, $secretWord);
+```
+
+Making sure data wasn't modified
+--------------------------------
+
+hashData()
+validateData()
+
+
+Securing Cookies
+----------------
+
+- validation
+- httpOnly
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