Implement Encrypt and Decrypt in KeyStore [+more KeyStore support]

[EduardDurech]

Feature description
Implement encrypt and decrypt in the termux-keystore (currently it only supports signing and verifying) via Cipher

• KeyStore Supported Ciphers

This would enable passcodes, secrets, et cetera to be stored in the Android KeyStore, an example would be for automatic decryption of an rclone config file without storing the password in a text file (e.g. encrypted by gpg) using rclone's --password-command, and would enable easy integration with the FingerprintAPI/Biometric Authentication, which would resolve #246 and would also be more convenient than a passphrase or using pass (possibly more secure)

Reference implementation

1. Using the KeyGenParameterSpec.Builder with PURPOSE_ENCRYPT | PURPOSE_DECRYPT, as well as examples of encrypting and decrypting a text with Cipher (this example would need to store the IV)
2. LokileCrypt is an implemented example of Android KeyStore supporting encryption/decryption, it merges the encrypted data and a random IV header, as already supported with cipher.getIV() which may be preferable so the IV is not stored separately. termux-keystore can also set a constant IV using IVParameterSpec but not preferable or derived from the alias, secret, such as what rclone does
3. How to get key from keystore on successful fingerprint auth
4. Android Fingerprint API Encryption and Decryption

• Implement Cipher Encrypt/Decrypt
• Support Encrypt/Decrypt purposes, Termux currently uses 12 for sign+verify
• Update to API level 30 by replacing setUserAuthenticationValidityDurationSeconds with setUserAuthenticationParameters
• Support non-Biometric AUTH_DEVICE_CREDENTIAL (see above)
• Credential/Biometric Prompt if key locked
• Option to keep key validated if Biometric enrollment changed using setInvalidatedByBiometricEnrollment
• Random IV header constructor and reader

[EduardDurech]

I will contribute to this as much as I can; however, I am not an Android Engineer nor do I usually use Java, @agnostic-apollo would you be able to assist/refactor my code? As well so it fits nicely into KeystoreAPI Thanks

[agnostic-apollo]

I am not an Android Engineer nor do I usually use Java

Directly working on encryption seems like a great way to start learning! :D

It's been a while since I looked/used keystore apis, so it will require me to research things as well. Encryption stuff needs to be handled with care and proper knowledge. It would also be good to have an external review as well from a good encryption apps dev as well, since its not my domain currently.

But yes, I can try to assist. Any refactoring will have to be done after next termux-app release, since that's a priority right now.

You can open a pull request and we can see.

Thanks

[EduardDurech]

Directly working on encryption seems like a great way to start learning! :D

Exactly what I've been up to ;)

@agnostic-apollo would you happen to remember if you had issues with using the "AndroidKeyStore" PROVIDER such as in the key .getInstance calls

termux-api/app/src/main/java/com/termux/api/apis/KeystoreAPI.java

Line 227 in 6e80b07

KeyPairGenerator generator = KeyPairGenerator.getInstance(algorithm, PROVIDER);

I keep getting java.security.NoSuchProviderException: no such provider: AndroidKeyStore apparently this happens with unit tests (regardless of using Roboletric, such as in my case using termux with ecj and dx).

I have tried so many different ways of circumventing it, that has been the better part of 2 days, and I cannot get a security provider implemented via Termux and ECJ, I've tried using BouncyCastle, custom providers, using GCMParameterSpec or IVParameterSpec directly, and all I can find for solutions to these online is to use the provider as "AndroidKeyStore", looping back to the same problem. I cannot run the KeystoreAPI.java generateKey either (removing Termux calls) for the same reason.

For now, I am just using anonymous keys without an alias using Javax Crypto, as Android KeyStore uses as a backend (analogues to Java Security for symmetric encryption it seems), just to test the encryption/decryption ciphers, but in order to use Android's Keystore API for Termux, Biometric Authentication, et cetera for integration with Android, I need to resolve this Provider issue

What is/was your development environment? If I can get the provider working I'm quite confident in getting this implemented

[agnostic-apollo]

termux-api app doesn't have any unit tests so that can't be the reason.

Apps should be built with gradle. Are you building on mobile? Don't have access to pc? I use Android Studio mainly.

For building in termux, check termux/termux-packages#7227 (comment). Some github release urls are inactive now.

Some reference implementation I used many years ago is at

https://github.com/agnostic-apollo/FTP/blob/master/app/src/main/java/com/allonsy/android/ftp/KeyHelper.java

https://github.com/agnostic-apollo/FTP/blob/master/app/src/main/java/com/allonsy/android/ftp/FTPLab.java

[EduardDurech]

I have a PC, just wanted access to my phone's hardware-backed keystore for testing. Again, not familiar with Android development so not sure if it's possible to do this in Android Studio over ADB, although this isn't mandatory, I just thought it would be easier to test e.g. Biometric Authentication, I don't know how these are handled in Studio. Same reason I've not used gradle as I rarely use it and just used ECJ in Termux, but I assume there's a reason Android developers use the tools they do :-) So will migrate the project over

Your KeyHelper code is similar to the other implementations of e.g. AES that I've been seeing, and in the Documentation, I just have a problem getting the AndroidKeyStore provider working as you have here

KeyGenerator keyGenerator = KeyGenerator.getInstance(KeyProperties.KEY_ALGORITHM_AES, AndroidKeyStore);

[agnostic-apollo]

Play button in android studio will build and install the app on your phone, then you can test however you like.

Code will be similar since it would have been copied and modified a bit.

[EduardDurech]

termux-api app doesn't have any unit tests so that can't be the reason.

BTW by "unit testing" I meant a stand-alone Java code that I built in ECJ on Termux, I took the generateKey method, removed the Termux references, et cetera, just to see if I could use the same Java and Android Keystore API calls, as termux-keystore works, but referencing those libraries in a stand-alone code seems to be an issue

Play button in android studio will build and install the app on your phone, then you can test however you like.

Sounds good, but I should rebuild Termux-API and Termux, right? So they are signed by the same source?

[agnostic-apollo]

Without more info on code content and design and how you building and running, can't comment. Any ECJ issues is not something I have time to deal with anyway.

If you are using Github action builds or releases, all apps will have same key as local debug builds sourced from dev_keystore.jks.

[EduardDurech]

I was using the FDroid builds, but I've switched over and got everything running via Github builds in Android Studio, tested and working

Just a quick question, I'd prefer the "Apply Code Changes" in Android Studio to rebuilding every time, but I get
Error Changes were not applied. Class not found: com.termux.api.apis.KeystoreAPI$1 Reinstall and restart app

From what I read online, this is a problem with older Gradle versions, but updating to 7.2.2 does not resolve it. Any ideas off the top of your head?

Thank you so much for your help, that should be the last of my questions and now I can implement this

[agnostic-apollo]

Click app drop down next to play button -> Edit Configuration -> Enable Always install with package manager

Welcome.

[EduardDurech]

Have to exit Termux and have Termux:API open while doing it for it to work, assuming it's a conflict

[EduardDurech]

@agnostic-apollo this is nearly done, I am just trying to support an option to store the encrypted data in SharedPreferences but am not familiar with the whole Context part of Android, for example in FTPLab it seems that it is passed to the constructor, but from where?

Let's say given this snippet

public class KeystoreAPI {
    public static void onReceive(TermuxApiReceiver apiReceiver, Intent intent) {
        switch (intent.getStringExtra("command")) {
            ....
            case "encrypt":
                encryptData(apiReceiver, intent);
                break;
        }
    }
    
    ...
    
    public static void encryptData(TermuxApiReceiver apiReceiver, final Intent intent) {
        ResultReturner.returnData(apiReceiver, intent, new WithInput() {
            @Override
            public void writeResult(PrintWriter out) throws ...{
                ...
                encryptedData = ...
            }
        }
    }
}

Would it make the most sense to:

• Call some version of import com.termux.shared.termux.settings.preferences.*; as TermuxAPIApplication.java does

or

• Include context in onReceive call as UsbAPI.java does

  termux-api/app/src/main/java/com/termux/api/apis/UsbAPI.java

  Line 33 in ba2836b

  public static void onReceive(final TermuxApiReceiver apiReceiver, final Context context, final Intent intent) {

  send it to encryptData(apiReceiver, context, intent);
  and received as public static void encryptData(TermuxApiReceiver apiReceiver, final Context context, final Intent intent) {, do I also need to add it into ResultReturner.returnData(...)?

Then I should be able to ??

                encryptedData = ...
                SharedPreferences sharedPref = context.getPreferences(Context.MODE_PRIVATE);
                SharedPreferences.Editor editor = sharedPref.edit();
                editor.putString("FooPrefName", encryptedData);
                editor.apply();
                ...

[agnostic-apollo]

for example in FTPLab it seems that it is passed to the constructor, but from where?

Different places like https://github.com/agnostic-apollo/FTP/blob/068f9d57247cc70609d612bd6272fcc916b5ffd4/app/src/main/java/com/allonsy/android/ftp/FTPListFragment.java#L170

• Call some version of import com.termux.shared.termux.settings.preferences.*; as TermuxAPIApplication.java does

Yes, you must use it. Rebase against master branch for 9e72088 and then you should be able to do something like following. The last true is for commitToFile immediately, some phones have issues with saving shared preferences so it must be done, otherwise will lose data. The key name would likely vary if clients want to store different data under different keys, but should prefix them with keystore_api__encrypted_data__<client_key>. Make sure client key is not empty, and possibly matches ^[a-zA-Z0-9_-]+$

TermuxAPIAppSharedPreferences preferences = TermuxAPIAppSharedPreferences.build(context);
if (preferences == null) return; // log and return error
SharedPreferenceUtils.setString(preferences.getSharedPreferences(), key, data, true);

Include context in onReceive

Yeah, do that.

[EduardDurech]

What I did was add context in the onReceive API call

termux-api/app/src/main/java/com/termux/api/TermuxApiReceiver.java

Lines 148 to 149 in b296a0c

	case "Keystore":
	KeystoreAPI.onReceive(this, intent);

 case "Keystore": 
     KeystoreAPI.onReceive(this, **context**, intent); 

then

public class KeystoreAPI {
    public static void onReceive(TermuxApiReceiver apiReceiver, **Context context**, Intent intent) {
        switch (intent.getStringExtra("command")) {
            ....
            case "encrypt":
                encryptData(apiReceiver, **context**, intent);
                break;
        }
    }
    
    ...
    
    public static void encryptData(TermuxApiReceiver apiReceiver, **final Context context**, final Intent intent) {
        ResultReturner.returnData(apiReceiver, intent, new WithInput() {
            @Override
            public void writeResult(PrintWriter out) throws ...{
                ...
                encryptedData = ...
                SharedPreferences.Editor editor = context.getSharedPreferences("foo", Context.MODE_PRIVATE).edit();
                editor.putString("foobar", encryptedData);
                editor.apply()
            }
        }
    }
}

Is this the best way? This is one thing I am not familiar with so feedback would be appreciated

Edit: You responded while I was typing, will check haha