[Mp4-tech] MPEG format test

[Rob Koenen (MPEGIF)]

> 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  <mailto:ssingh neomagic.com> Singh 
To: Dave Singer <mailto:singer apple.com>  
Cc: Anil Gupte <mailto:anil-list maxnetwork.com>  
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  <mailto:singer apple.com> Singer
To: Anil  <mailto:anil-list maxnetwork.com> 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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Apple Computer/QuickTime
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Apple Computer/QuickTime
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NOTE: Please use clear subject lines for your posts. Include [audio,
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