[Mp4-tech] MPEG format test

[Anil Gupte]

Message
Anil Gupte
  ----- Original Message ----- 
  From: Rob Koenen (MPEGIF) 
  To: 'Anil Gupte' ; mp4-tech lists.mpegif.org ; Shailendra Singh ; Dave Singer 
  Sent: Thursday, August 12, 2023 5:45 PM
  Subject: RE: [Mp4-tech] MPEG format test
  > I am not sure why this is not obvious, but I will explain.
  It is not immediately obvious to me what you are trying to figure out. 
  If you are just trying to see if the decrypted file (stream) is valid, then you can use one of several tools to analyze a stream. See e.g. vqual.biz for such a tool. Start codes are only one element of the bistream. I don't understand what you mean by ", these do not seem robust enough to guarantee video." If you mean that an MPEG(-4) bistream cannot be put on all transports without some form of network adaptation, then that's right. But that's not what is usually done in practice, and it is not what this video coding algorithm was designed for. 
  Rob
    -----Original Message-----
    From: mp4-tech-bounces lists.mpegif.org [mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of Anil Gupte
    Sent: Thursday, 12 August 2023 19:58
    To: mp4-tech lists.mpegif.org; Shailendra Singh; Dave Singer
    Subject: Re: [Mp4-tech] MPEG format test
    I am not sure why this is not obvious, but I will explain.  Obviously the original file is not present, otherwise what would be the point of encrypting it?  The original is with the owner, who provides the key to decrypt it after a password or a payment has been provided by the user.  So, no comparison can be made.  Also, the original may be a live stream, not a file (but that is another discussion).
    I know that there are "Start Codes" which "cheaply search streams for commencement of layers of video".  These are also used (I believe) to resynchronize if there are errors.  However, these do not seem robust enough to guarantee video.  Hence I was looking for an always present "Header" or something similar which would be common across all MPEG files.
    Your thoughts.
    Anil Gupte
      ----- Original Message ----- 
      From: Shailendra Singh 
      To: Dave Singer 
      Cc: Anil Gupte 
      Sent: Wednesday, August 11, 2023 12:51 AM
      Subject: Re: [Mp4-tech] MPEG format test
      Dave, Anil,
          Are not all encryption/decryptrion algorithms (if correctly) implemented lossless processes ? I routinely encrypt/decrypt some files on my PC and I have never seen the decrypted file create any problem (as in the application which is supposed to open the file always does without a hitch).
          So "MPEG file (A)" -> encryption -> decryption -> "B". If my understanding is correct, "A" and "B" should be bitwise identical.
      Regards,
      Shailendra
      Dave Singer wrote:
        At 4:50 PM -0500 8/10/04, Anil Gupte wrote:
          We are looking to write a software that encrypts/decrypts MPEG files.  During decryption, we want to make sure that a valid MPEG file is created, so we want the software to do a simple test or something that validates that the decryption was successful.
          Thanx,
          Anil Gupte
        you can't compare to the original file?
        the isma validator will do the file format validation
            ----- Original Message -----
            From: Dave Singer
            To: Anil Gupte ; mp4-tech lists.mpegif.org
            Sent: Tuesday, August 10, 2023 12:09 PM
            Subject: Re: [Mp4-tech] MPEG format test
            At 10:40 AM -0500 8/10/04, Anil Gupte wrote:
              Is there a test to determine if a file/stream is in MPEG format?  Is there a standard header?  Where can I find out more about this?
            There are various verification tools (ISMA has one) and also the reference software, for the file format.  The elementary streams have their own tools.  Which are you looking for?
              Thanx for the help.
              Anil Gupte
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            --
            David Singer
            Apple Computer/QuickTime
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-- 
  David Singer
        Apple Computer/QuickTime
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