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Tutorial
=========

Hello! If you're reading this file, it's because you want to add encryption to
one of your PHP projects. My job, as the person writing this documentation, is
to help you make sure you're doing the right thing and then show you how to use
this library to do it. To help me help you, please read the documentation
*carefully* and *deliberately*.

A Word of Caution
------------------

Encryption is not magic dust you can sprinkle on a system to make it more
secure. The way encryption is integrated into a system's design needs to be
carefully thought out. Sometimes, encryption is the wrong thing to use. Other
times, encryption needs to be used in a very specific way in order for it to
work as intended. Even if you are sure of what you are doing, we strongly
recommend seeking advice from an expert.

The first step is to think about your application's threat model. Ask yourself
the following questions. Who will want to attack my application, and what will
they get out of it? Are they trying to steal some information? Trying to alter
or destroy some information? Or just trying to make the system go down so people
can't access it? Then ask yourself how encryption can help combat those threats.
If you're going to add encryption to your application, you should have a very
clear idea of exactly which kinds of attacks it's helping to secure your
application against. Once you have your threat model, think about what kinds of
attacks it *does not* cover, and whether or not you should improve your threat
model to include those attacks.

**This isn't for storing user login passwords:** The most common use of
cryptography in web applications is to protect the users' login passwords. If
you're trying to use this library to "encrypt" your users' passwords, you're in
the wrong place. Passwords shouldn't be *encrypted*, they should be *hashed*
with a slow computation-heavy function that makes password guessing attacks more
expensive. See [How to Safely Store Your Users' Passwords in
2016](https://paragonie.com/blog/2016/02/how-safely-store-password-in-2016).

**This isn't for encrypting network communication:** Likewise, if you're trying
to encrypt messages sent between two parties over the Internet, you don't want
to be using this library. For that, set up a TLS connection between the two
points, or, if it's a chat app, use the [Signal
Protocol](https://whispersystems.org/blog/advanced-ratcheting/).

What this library provides is symmetric encryption for "data at rest." This
means it is not suitable for use in building protocols where "data is in motion"
(i.e. moving over a network) except in limited set of cases.

Please note that **encryption does not, and is not intended to, hide the
*length* of the data being encrypted.** For example, it is not safe to encrypt
a field in which only a small number of different-length values are possible
(e.g. "male" or "female") since it would be possible to tell what the plaintext
is by looking at the length of the ciphertext. In order to do this safely, it is
your responsibility to, before encrypting, pad the data out to the length of the
longest string that will ever be encrypted. This way, all plaintexts are the
same length, and no information about the plaintext can be gleaned from the
length of the ciphertext.

Getting the Code
-----------------

There are several different ways to obtain this library's code and to add it to
your project. Even if you've already cloned the code from GitHub, you should
take steps to verify the cryptographic signatures to make sure the code you got
was not intercepted and modified by an attacker.

Please head over to the [**Installing and
Verifying**](InstallingAndVerifying.md) documentation to get the code, and then
come back here to continue the tutorial.

Using the Library
------------------

I'm going to assume you know what symmetric encryption is, and the difference
between symmetric and asymmetric encryption. If you don't, I recommend taking
[Dan Boneh's Cryptography I course](https://www.coursera.org/learn/crypto/) on
Coursera.

To give you a quick introduction to the library, I'm going to explain how it
would be used in two sterotypical scenarios. Hopefully, one of these sterotypes
is close enough to what you want to do that you'll be able to figure out what
needs to be different on your own.

### Formal Documentation

While this tutorial should get you up and running fast, it's important to
understand how this library behaves. Please make sure to read the formal
documentation of all of the functions you're using, since there are some
important security warnings there.

The following classes are available for you to use:

- [Crypto](classes/Crypto.md): Encrypting and decrypting strings.
- [File](classes/File.md): Encrypting and decrypting files.
- [Key](classes/Key.md): Represents a secret encryption key.
- [KeyProtectedByPassword](classes/KeyProtectedByPassword.md): Represents
  a secret encryption key that needs to be "unlocked" by a password before it
  can be used.

### Scenario #1: Keep data secret from the database administrator

In this scenario, our threat model is as follows. Alice is a server
administrator responsible for managing a trusted web server. Eve is a database
administrator responsible for managing a database server. Dave is a web
developer working on code that will eventually run on the trusted web server.

Let's say Alice and Dave trust each other, and Alice is going to host Dave's
application on her server. But both Alice and Dave don't trust Eve. They know
Eve is a good database administrator, but she might have incentive to steal the
data from the database. They want to keep some of the web application's data
secret from Eve.

In order to do that, Alice will use the included `generate-defuse-key` script
which generates a random encryption key and prints it to standard output:

```sh
$ composer require defuse/php-encryption
$ vendor/bin/generate-defuse-key
```

Alice will run this script once and save the output to a configuration file, say
in `/etc/daveapp-secret-key.txt` and set the file permissions so that only the
user that the website PHP scripts run as can access it.

Dave will write his code to load the key from the configuration file:

```php
<?php
use Defuse\Crypto\Key;

function loadEncryptionKeyFromConfig()
{
    $keyAscii = // ... load the contents of /etc/daveapp-secret-key.txt
    return Key::loadFromAsciiSafeString($keyAscii);
}
```

Then, whenever Dave wants to save a secret value to the database, he will first
encrypt it:

```php
<?php
use Defuse\Crypto\Crypto;

// ...
$key = loadEncryptionKeyFromConfig();
// ...
$ciphertext = Crypto::encrypt($secret_data, $key);
// ... save $ciphertext into the database ...
```

Whenever Dave wants to get the value back from the database, he must decrypt it
using the same key:

```php
<?php
use Defuse\Crypto\Crypto;

// ...
$key = loadEncryptionKeyFromConfig();
// ...
$ciphertext = // ... load $ciphertext from the database
try {
    $secret_data = Crypto::decrypt($ciphertext, $key);
} catch (\Defuse\Crypto\Exception\WrongKeyOrModifiedCiphertextException $ex) {
    // An attack! Either the wrong key was loaded, or the ciphertext has
    // changed since it was created -- either corrupted in the database or
    // intentionally modified by Eve trying to carry out an attack.

// ... handle this case in a way that's suitable to your application ...
}
```

Note that if anyone ever steals the key from Alice's server, they can decrypt
all of the ciphertexts that are stored in the database. As part of our threat
model, we are assuming Alice's server administration skills and Dave's secure
coding skills are good enough to stop Eve from being able to steal the key.
Under those assumptions, this solution will prevent Eve from seeing data that's
stored in the database.

However, notice that our threat model says nothing about what could happen if
Eve wants to *modify* the data. With this solution, Eve will not be able to
alter any individual ciphertext (because each ciphertext has its own
cryptographic integrity check), but Eve *will* be able to swap ciphertexts for
one another, and revert ciphertexts to what they used to be at previous times.
If we needed to defend against such attacks, we would have to re-design our
threat model and come up with a different solution.

### Scenario #2: Encrypting account data with the user's login password

This scenario is like Scenario 1, but subtly different. The threat model is as
follows. We have Alice, a server administrator, and Dave, the developer. Alice
and Dave trust each other, and Alice wants to host Dave's web application,
including its database, on her server. Alice is worried about her server getting
hacked. The application will store the users' credit card numbers, and Alice
wants to protect them in case the server gets hacked.

We can model the situation like this: after the server gets hacked, the attacker
will have read and write access to all data on it until the attack is detected
and Alice rebuilds the server. We'll call the time the attacker has access to
the server the *exposure window.* One idea to minimize loss is to encrypt the
users' credit card numbers using a key made from their login password. Then, as
long as the users all have strong passwords, and they are never logged in during
the exposure window, their credit cards will be protected from the attacker.

To implement this, Dave will use the `KeyProtectedByPassword` class. When a new
user account is created, Dave will save a new key to their account, one that's
protected by their login password:

```php
<?php
use Defuse\Crypto\KeyProtectedByPassword;

function CreateUserAccount($username, $password)
{
    // ... other user account creation stuff, including password hashing

$protected_key = KeyProtectedByPassword::createRandomPasswordProtectedKey($password);
    $protected_key_encoded = $protected_key->saveToAsciiSafeString();
    // ... save $protected_key_encoded into the user's account record
}
```

**WARNING:** Because of the way `KeyProtectedByPassword` is implemented, knowing
`SHA256($password)` is enough to decrypt a `KeyProtectedByPassword`. To be
secure, your application MUST NOT EVER compute `SHA256($password)` and use or
store it for any reason. You must also make sure that other libraries your
application is using don't compute it either.

Then, when the user logs in, Dave's code will load the protected key from the
user's account record, unlock it to get a `Key` object, and save the `Key`
object somewhere safe (like temporary memory-backed session storage or
a cookie). Note that wherever Dave's code saves the key, it must be destroyed
once the user logs out, or else the attacker might be able to find users' keys
even if they were never logged in during the attack.

```php
<?php
use Defuse\Crypto\KeyProtectedByPassword;

// ... authenticate the user using a good password hashing scheme
// keep the user's password in $password

$protected_key_encoded = // ... load it from the user's account record
$protected_key = KeyProtectedByPassword::loadFromAsciiSafeString($protected_key_encoded);
$user_key = $protected_key->unlockKey($password);
$user_key_encoded = $user_key->saveToAsciiSafeString();
// ... save $user_key_encoded in a cookie
```

```php
<?php
// ... when the user is logging out ...
// ... securely wipe the saved $user_key_encoded from the system ...
```

When a user adds their credit card number, Dave's code will get the key from the
memory-backed session or cookie and use it to encrypt the credit card number:

```php
<?php
use Defuse\Crypto\Crypto;
use Defuse\Crypto\Key;

// ...

$user_key_encoded = // ... get it out of the cookie ...
$user_key = Key::loadFromAsciiSafeString($user_key_encoded);

// ...

$credit_card_number = // ... get credit card number from the user
$encrypted_card_number = Crypto::encrypt($credit_card_number, $user_key);
// ... save $encrypted_card_number in the database
```

When the application needs to use the credit card number, it will decrypt it:

```php
<?php
use Defuse\Crypto\Crypto;
use Defuse\Crypto\Key;

// ...

$user_key_encoded = // ... get it out of the cookie
$user_key = Key::loadFromAsciiSafeString($user_key_encoded);

// ...

$encrypted_card_number = // ... load it from the database ...
try {
    $credit_card_number = Crypto::decrypt($encrypted_card_number, $user_key);
} catch (Defuse\Crypto\Exception\WrongKeyOrModifiedCiphertextException $ex) {
    // Either there's a bug in our code, we're trying to decrypt with the
    // wrong key, or the encrypted credit card number was corrupted in the
    // database.

// ... handle this case ...
}
```

With all caveats carefully heeded, this solution limits credit card number
exposure in the case where Alice's server gets hacked for a short amount of
time. Remember to think about the attacks that *aren't* included in our threat
model. The attacker is still free to do all sorts of harmful things like
modifying the server's data which may go undetected if Alice doesn't have secure
backups to compare against.

Getting Help
-------------

If you're having difficulty using the library, see if your problem is already
solved by an answer in the [FAQ](FAQ.md).

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