CacheWord

CacheWord is an Android library project for passphrase caching and management. It helps app developers securely generate, store, and access secrets derived from a user's passphrase.

CacheWord is still under development. Proceed with caution�

Broadly speaking this library assists developers with two related problems:

1. Secrets Management: how the secret key material for your app is generated, stored, and accessed
2. Passphrase Caching: store the passphrase in memory to avoid constantly prompting the user

CacheWord manages key derivation, verification, persistence, passphrase resetting, and caching secret key material in memory.

Features:

• Strong key derivation (PBKDF2)
• Secure secret storage (AES-256 GCM)
• Persistent notification: informs the user the app data is unlocked
• Configurable timeout: after a specified time of inactivity the app locks itself
• Manual clearing: the user can forcibly lock the application
• Uses Android's Keystore on 4.x if available - Not Yet Implemented

CacheWord requires at least SDK version 2.3.3

Usage

Setup

(Eclipse) Import into your workspace

Before we begin, download CacheWord and import it into your Eclipse workspace. Then add it as a library project to your project in eclipse.

Project Properties > Android > (Library) Add

(Ant) Add the library project to your project.properties

You need to run android update project -p . inside the CacheWord/cachewordlib folder, then add the path to your project.properties file. For more information please see the Android developer guide for referencing library projects.

Edit your AndroidManifest.xml

Add the following to between the <application>....</application> tags

<service android:name="info.guardianproject.cacheword.CacheWordService" android:enabled="true" android:exported="false" />

Dependencies

• Android support library v4 (android-support-v4.jar; included)
• SQLCipher for Android >= v3.x (included)

CacheWord provides a support class for SQLCipher for Android. You probably want to use this.

Download the SQLCipher for Android v3.x release and copy the libs/ and assets/ dir into your Android project dir.

Integration

A CacheWordSubscriber is any component in your application interested in the secrets managed by CacheWord. Such components may be:

• Initialization Activity
• Login Activity
• Any Activity that handles sensitive data
• Encryption/Decryption wrappers
• SQLCipher Database Helper
• IOCipher Virtual File System
• etc.

For each of these interested components you must implement two things

1. Implement the ICacheWordSubscriber interface
2. Instantiate a CacheWordHandler to assist the component

TIP: Activities should implement CacheWordActivityHandler and propagate the lifecycle methods onPause and onResume instead of calling [dis]connect().

1. Implementing ICacheWordSubscriber

The ICacheWordSubscriber interface consists of three event methods.

These event methods are similar to the Android lifecycle methods onResume, onPause, etc. The appropriate event is guaranteed to be called for every CacheWord enhanced Activity during the onResume method when then lifecycle change is propagated correctly (see below). This ensures your Activities will always be aware of the current status of CacheWord.

In most applications, your state and view handling code that usually goes in onResume and onPause should instead go in one of the CacheWord event methods.

1. onCacheWordUninitialized

   This event is called when CacheWord determines there is no saved state. Usually this occurs the first time the user runs your app. At this point there should be no sensitive data stored, because there is no secret to encrypt it with.

   In this event you should prompt the user to create a password and the pass the new password to CacheWord with setCachedSecrets().

   This event could also be triggered after the Application's data is cleared or reset.

2. onCacheWordLocked

   This event signifies the secrets are unavailable or have become unavailable. It is triggered when the secrets expiration timeout is reached, or the user manually locks CacheWord.

   You should clear all UI components and data structures containing sensitive information and perhaps show a dedicated lock screen.

   At this stage your app should prompt the user for the passphrase and give it to CacheWord with setCachedSecrets()

3. onCacheWordOpened

   This event is triggered when CacheWord has received valid credentials via the setCachedSecrets() method.

   At this stage in your app you may call getCachedSecrets() to retrieve the unencrypted secrets from CacheWord.

Example:

public class MyActivity extends Activity implements ICacheWordSubscriber
{

    ...

    @Override
    public void onCacheWordUninitialized() {
        startCreatePassphraseActivity();
    }


    @Override
    public void onCacheWordLocked() {
        clearUi();
        startPassphrasePromptActivity();
    }

    @Override
    public void onCacheWordOpened() {
        decryptDataAndPopulateUi(mCacheWord.getEncryptionKey());
    }

    ...
}

2. Instantiate CacheWordHandler and propagate lifecycle Changes

CacheWordHandler is the class instantiated by any object interested in receiving CacheWord events. It does the heavy lifting of starting, connecting, and communicating with the CacheWordService. Each object that wants to be aware of CacheWord events should instantiate its own CacheWordHandler.

It is your object's responsibility to call CacheWordHandler.connectToService() and CacheWordHandler.disconnect() when your object wants to register and unregister from event notifications.

Disconnecting is important, because CacheWord maintains a list of connected clients and will not initiate the automatic timeout until all the connected clients have disconnected.

class YourClass implements ICacheWordSubscriber
{
        ...
        private CacheWordHandler mCacheWord;
        ...
        public YourClass() {
            ...
            mCacheWord = new CacheWordHandler(mContext, this);
            mCacheWord.connectToService()
            ...
        }

        // Called when your object no longer wants to be notified of CacheWord events
        public goodbyeYourClass() {
            mCacheWord.disconnect()
        }
        ...
}

Activity's and CacheWordActivityHandler

Most of the time it is Activity classes that need to instantiate CacheWordHandler, so for this use case there is a convenient class called CacheWordActivityHandler. Instead of calling connectToService and disconnect(), you simply need to propagate the Android lifecycle changes onPause() and onResume().

class YourActivity extends Activity implements ICacheWordSubscriber
{
        ...
        private CacheWordActivityHandler mCacheWord;
        ...
        @Override
        public void onCreate(Bundle savedInstanceState) {
            ...
            mCacheWord = new CacheWordActivityHandler(this);
            ...
        }

        @Override
        protected void onResume() {
            super.onStart();
            mCacheWord.onResume();
        }

        @Override
        protected void onPause() {
            super.onPause();
            mCacheWord.onPause();
        }
        ...
}

Configuration

Configuration is entirely optional as sane defaults are provided for every option.

For compile time configuration options, define the resources in XML (see res/values/cacheword.xml for the available options).

To configure the XML resources, simply copy cacheword.xml into your own res/values directory and edit the settings as you like.

Runtime configuration options can be set via the CacheWordSettings object.

Configurable options are:

• Whether to show a persistent unlocked notification
• Display options for the notification (title, message, icon, etc)
• Intent executed when the notification is clicked
• Timeout length after your app leaves the foreground
• Vibration on successful unlock

SQLCipher & IOCipher Support

If you use SQLCipher for encrypted database storage you should use CacheWord's SQLCipherOpenHelper. See the NoteCipher application for an example of how to use it.

Likewise if you use IOCipher for encrypted file storage you should use CacheWord's IOCipherHelper.

TODO: make example of IOCipherHelper

What Are These Cached Secrets?

The sensitive data that is cached by CacheWord can be specified by the user as a class implementing ICachedSecrets.

The default implementation of this class (PassphraseSecrets) attempts to provide for most use cases. It generates a 256 bit encryption key that can be used by other libraries like SQLCipher or IOCipher

In this case the user's password is used to encrypt (after being hashed of course) the generated encryption key, and is never written to disk.

Issues & Support

Bug? Please report any issues at our project issue tracker, no account required!

Question? Find us on IRC or our mailing list.

• IRC: #guardianproject @ freenode
• Mailing List: guardian-dev

Library Development

See HACKING.md

Security Design Notes

See SECURITY.md