Re-encrypt segment error: get full samples: offset in mdata beyond size

[xzerth]

Hello! I have a usecase when I need to reencrypt segments: so I need to decrypt it and then encrypt again (with different key probably).
In general it combines the code from two examples: decrypt and encrypt.

Something like this:

func main() {
	key := []byte{
		0x69, 0x4F, 0x3E, 0xAA,
		0xBD, 0x56, 0xA2, 0x58,
		0x28, 0x2E, 0xE3, 0x0E,
		0x99, 0x6A, 0x32, 0x18,
	}
	keyIV := make([]byte, 16)

	initFile, err := os.ReadFile("init.m4i")
	encryptedVideoFile, err := os.ReadFile("video.m4v")
	initMp4, err := mp4.DecodeFile(bytes.NewReader(initFile))
	decryptInfo, err := mp4.DecryptInit(initMp4.Init)

	mp4File, err := mp4.DecodeFile(bytes.NewReader(encryptedVideoFile))
	for _, seg := range mp4File.Segments {
		err := mp4.DecryptSegment(seg, decryptInfo, key)
		if err != nil {
			fmt.Printf("Error decrypt segment: %v\n", err)
		}
	}

	// Uncomment this and error is gone
	// var buf bytes.Buffer
	// mp4File.Encode(&buf)
	// mp4File, err = mp4.DecodeFile(bytes.NewReader(buf.Bytes()))

	protectData, err := mp4.InitProtect(initFile.Init, key, keyIV, "cenc", key, decryptInfo.Psshs)
	for _, s := range mp4File.Segments {
		for _, f := range s.Fragments {
			err := mp4.EncryptFragment(f, key, keyIV, protectData)
			if err != nil {
				// HERE WE'VE GOT AN ERROR
				fmt.Printf("Error encrypting fragment: %v\n", err)
			}
		}
	}
}

And then it returns an error: get full samples: offset in mdata beyond size.
If I decrypt, then Encode() to buffer, then decode again and encrypt - it works. Can't attach init and video files here for some reason. Let me know if you need them, but I don't think it is the particular stream that causes this issue.

[tobbee]

@xzerth Sorry for the slow response.

I haven't looked into this use case, but it seems that the mp4File that you get after decryption still has some traces of the encryption that cause the error.

You should be able to output details about the mp4File you have after decryption by using the same mechanism as in mp4ff-info, which is

        err = mp4FIle.Info(os.Stdout, "all:1", "", "  ")

You can then compare that with what you get after the extra encode/decode step.

A potential issue is that you use initFile.init for the encoding. That init segment is from the encrypted input, but it should be an init segment for a clear file, so it should be better to use the decrypted initMp4 segment.

I hope this helps. Personally, a next step would be to use the debugger to find out where the problem happens and why.

[tobbee]

I found the problem. There was an offset in mdat that was not changed as the encryption boxes were removed. The fix will be part of a PR soon.