Making an app that changes device fonts for galaxy phones

TL;DR: To create a FlipFont you need a valid font file and XML file in assets. The APK needs to be signed with Monotype's private keystore, which isn't possible.

Samsung and other manufacturers use Monotype's FlipFont to change the system fonts without root. Samsung and Monotype Imaging Inc. want you to pay to change fonts and have made concerted efforts to block free fonts from being installed.

You don't need any code to create a font for FlipFont, so you can delete FontContentProvider.java.

To create an APK that works with FlipFont you need the following 3 things:

1) A valid font file in assets/fonts/ReplaceWithFontName.ttf

2) An XML in assets/xml/ReplaceWithFontName.xml

3) Build and sign the app with Monotype's private keystore.

Example XML for assets:

xml

<?xml version="1.0" encoding="utf-8"?>
<font displayname="ReplaceWithFontName">
  <sans>
    <file>
      <filename>ReplaceWithFontName.ttf</filename>
      <droidname>DroidSans.ttf</droidname>
    </file>
    <file>
      <filename>ReplaceWithFontName.ttf</filename>
      <droidname>DroidSans-Bold.ttf</droidname>
    </file>
  </sans>
</font>

Now, look at #3 again. You need to sign the APK with Monotype's keystore. You don't have access to this. Therefore, your APK will not work on Samsung devices.

The following method is decompiled from the Settings application on a Samsung device and is used to check if the FlipFont APK is signed with the correct key:

java

protected boolean checkFont(String apkname) {
  if (DEBUG) {
    Log.secD("FlipFont", "checkFont - checking apkname" + apkname);
  }
  if ("com.monotype.android.font.foundation".equals(apkname)) {
    return false;
  }
  PackageManager pm = this.mContext.getPackageManager();
  for (int i = 0; i < apkNameList.length; i++) {
    if (apkname != null) {
      if (apkname.equals(apkNameList[i])) {
        this.isCheckPlatformSignatures = pm.checkSignatures("android", apkNameList[i]) == 0;
        this.isCheckReleaseSignatures = Utils.isSignatureMatch(this.mContext, apkNameList[i]);
        Log.i("FontPreviewTablet", "apkname : " + apkname + ", isCheckPlatformSignatures : " + this.isCheckPlatformSignatures + ", isCheckReleaseSignatures : " + this.isCheckReleaseSignatures);
        if (!(this.isCheckPlatformSignatures || this.isCheckReleaseSignatures)) {
          if (apkname.equals("")) {
          }
        }
        return false;
      }
      continue;
    }
  }
  if (DEBUG) {
    Log.secD("FlipFont", "checkFont - check if valid certificate");
  }
  PackageInfo packageInfo = null;
  try {
    packageInfo = this.mFontListAdapter.mPackageManager.getPackageInfo(apkname, 64);
  } catch (Exception e) {
  }
  if (packageInfo != null) {
    Signature[] signatures = packageInfo.signatures;
    byte[] cert = signatures[0].toByteArray();
    try {
      MessageDigest md = MessageDigest.getInstance("SHA");
      md.update(signatures[0].toByteArray());
      if ("T84drf8v3ZMOLvt2SFG/K7ODXgI=".equals(Base64.encodeToString(md.digest(), 0).trim())) {
        if (DEBUG) {
          Log.v("FlipFont", "**Signature is correct**");
        }
        return false;
      }
      if (DEBUG) {
        Log.v("FlipFont", "**Signature is incorrect**");
      }
      return true;
    } catch (Exception e2) {
      e2.printStackTrace();
      InputStream input = new ByteArrayInputStream(cert);
      CertificateFactory cf = null;
      try {
        cf = CertificateFactory.getInstance("X509");
      } catch (CertificateException e3) {
        e3.printStackTrace();
      }
      X509Certificate c = null;
      try {
        c = (X509Certificate) cf.generateCertificate(input);
      } catch (CertificateException e32) {
        e32.printStackTrace();
      }
      if (DEBUG) {
        Log.secD("Example", "APK name: " + apkname);
        if (c != null) {
          Log.secD("Example", "Certificate for: " + c.getSubjectDN());
          Log.secD("Example", "Certificate issued by: " + c.getIssuerDN());
          Log.secD("Example", "The certificate is valid from " + c.getNotBefore() + " to " + c.getNotAfter());
          Log.secD("Example", "Certificate SN# " + c.getSerialNumber());
          Log.secD("Example", "Generated with " + c.getSigAlgName());
        }
      }
      String certIssuedByString = "CN=Ed Platz, OU=Display Imaging, O=Monotype Imanging Inc., L=Woburn, ST=MA, C=US";
      if (c != null && certIssuedByString.equals(c.getIssuerDN().toString())) {
        if (DEBUG) {
          Log.secD("FlipFont", "**Certificate data is correct**");
        }
        return false;
      }
    }
  }
  return true;
}

If you look at the above method you will notice that the APK signature isn't checked if the APK has a package name of "com.monotype.android.font.foundation".