How to remove 'cipher' error from YouTube video downloader and download video in mp3 format?

I am trying to create a YouTube video downloader in python using pytube and tkinter. Most of the videos are getting downloaded fine but in some videos I am getting the error as 'cipher'. Can anyone help while this is happening?

Also I am trying to download in mp3 format but setting the 'audioonly' attribute to True downloads my file in mp4 format without the video. But I want the format in mp3. How can I do so?

Here is my code

def startDownload(url):
    global file_size
    path_to_save = askdirectory()
    if path_to_save is None:
        return

    try:
            global MaxFileSize, fileSizeInBytes
            choice = youtubeChoicesLabel.get()
            url=urlField.get()
            yt = YouTube(url)
            nome = yt.title
            # video2 = urlField.get()

            if (choice == download_choices[1]):
                print("720p video is downloading")
                selectVideo = yt.streams.filter(progressive=True, file_extension='mp4').first()

            elif (choice == download_choices[2]):
                print("Audio file is downloading")
                
                # selectVideo =yt.streams.get_audio_only()
                # new_filename= nome + '*.mp3'
                # default_filename=  nome + '*.mp4'
                # ffmpeg = ('ffmpeg -i ' %path_to_save %default_filename + new_filename)
                # subprocess.run(ffmpeg, shell=True)
                selectVideo = yt.streams.filter(only_audio=True).first()
                
                
            elif (choice == download_choices[0]):
                return

            
                fileSizeInBytes = selectVideo.filesize
                MaxFileSize = fileSizeInBytes/1024000
                MB =str(MaxFileSize)+ "MB"
                print("File Size = : {:00.000f}".format (MaxFileSize))

            
            

            
            # yt = YouTube(url)
            # st=yt.streams.first()
            st= selectVideo
            yt.register_on_complete_callback(complete_download)
            yt.register_on_progress_callback(progress_download)
            file_size=st.filesize
            st.download(output_path=path_to_save)
        




    except Exception as e:
        print(e)

Solution

The fix is pretty simple, it's there in the GitHub repo, but since this question came up here, this is how I fixed it. It's not due to the way your file works, it's a problem that happened because YouTube changed their search patterns or something from cipher to signatureCipher.

First, say pip install pytube3 in the terminal.
Second, go to the path - C:\Users\<user>\AppData\Local\Programs\Python\Python37-32\Lib\site-packages\pytube. Keep in mind the path will vary so replace <user> with your Windows username and python37-32 with your Python version that you install pytube on.
Third, open extract.py from the above navigated directory and head onto line 301 or search for the line:

cipher_url = [
    parse_qs(formats[i]["cipher"]) for i, data in enumerate(formats)
]

Now replace it with:

cipher_url = [
     parse_qs(formats[i]["signatureCipher"]) for i, data in enumerate(formats)
]

In case you damage this file while making changes to it, just say pip uninstall pytube3 and then install it again, and repeat. Keep in mind your imports will still be import pytube.