DownloadActivity.kt

fun encryptDownloadedFile() {
    try {
        val filePath = filesDir.absolutePath + "/filename"
        val fileData = readFile(filePath)

        //get secret key
        val secretKey = getSecretKey(sharedPref)
        //encrypt file
        val encodedData = encrypt(secretKey, fileData)

        saveFile(encodedData, filePath)
           
    } catch (e: Exception) {
        Log.d(mTag, e.message)
    }
}