How to overwrite EncryptedFile on Android?

Question

Using Android's EncryptedFile (androidx.security:security-crypto:1.1.0-alpha01), I can successfully write a file using the following code

File file = new File(context.getFilesDir() + File.separator + filename);
KeyGenParameterSpec keyGenParameterSpec = MasterKeys.AES256_GCM_SPEC;
EncryptedFile encryptedFile = null;
try {
    String masterKeyAlias = MasterKeys.getOrCreate(keyGenParameterSpec);
    encryptedFile = new EncryptedFile.Builder(
            file,
            context,
            masterKeyAlias,
            EncryptedFile.FileEncryptionScheme.AES256_GCM_HKDF_4KB
    ).build();
} catch (Exception exception) {
    // log error
}

// write file
try {
    BufferedWriter bufferedWriter = new BufferedWriter(
            new OutputStreamWriter(encryptedFile.openFileOutput()));
    bufferedWriter.write(string);
    bufferedWriter.close();
} catch (Exception exception) {
    // log error
}

However, when attempting to overwrite the same file, the write operation fails and the following is thrown

java.io.IOException: output file already exists, please use a new file

I found this to be an explicit check in EncryptedFile's openFileOutput()

if (mFile.exists()) {
    throw new IOException("output file already exists, please use a new file: "
            + mFile.getName());
}

To fix this, I was able to successfully overwrite by deleting the file if it existed before using it to build the EncryptedFile

File file = new File(context.getFilesDir() + File.separator + filename);
if (file.exists()) { file.delete(); }

... remaining code from top snippet above

This seems like a hack, but I also don't understand the choice to throw an exception for mFile.exists() in openFileOutput(). Is there a correct/better way to overwrite an EncryptedFile?

How to overwrite EncryptedFile on Android?

0 Answers