How can I change the first two bits of an mp4 file to 00, 00 in swift?

Question

I am trying to change the first two bits of an mp4 file to 00, 00 to change it back to normal / working state. After downloading the mp4 file from an API, I discovered it would not play and acted weird, so on the internet I found out that some people said it was encoded and changing the two first bits to 0 makes it work! (It does, but) I don't know how to do that in swift any help would be appreciated!

Answer 1

I found out how to do it! This is the code:

import UIKit
import AVFoundation



// URL to a mp4 file called videoFile located in the project's folder:
let videoURL = URL(string: Bundle.main.path(forResource: "videoFile", ofType: "mp4")!)



// Func for "decoding" (changing it's 2 first bytes to 0):
func changeFirstTwoBytesOfFile(fileURL: URL) {

    var fileData = fileURL.dataRepresentation

    // Printing first 4 bytes of the unchanged file for checking:
    print("Bytes before change:")

    print(fileData[0])
    print(fileData[1])
    print(fileData[2])
    print(fileData[3])

    print("")
    print("")



    // Changing it's 2 first bytes to 0
    fileData[0] = 0
    fileData[1] = 0



    // Printing the changed file's bytes to check if the change took place:
    print("Bytes after change:")

    print(fileData[0])
    print(fileData[1])
    print(fileData[2])
    print(fileData[3])
}


changeFirstTwoBytesOfFile(fileURL: videoURL!)