Nope, because the key stream doesn't repeat over any useful period, and
certainly not on a period that would be a multiple of the sync.  Once you
mix them to create the enciphered data stream, it all looks random.  The key
stream length can be on the order of 10^100 bits in length before repeating,
which looks to me like something on the order of once every 80 centuries at
video rates.

The 'keys' that you are probably thinking of, 64, 128 bits, etc., are used
to produce the pseudo-random key stream of great length.

This looks like a good page on the subject:
http://www.rsasecurity.com/rsalabs/faq/2-1-5.html


> -----Original Message-----
> From: Robert Rolf [mailto:Robert.Rolf@UALBERTA.CA]
> Sent: Tuesday, August 07, 2001 12:25 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [OT]: Jokers Play - was [EE]: Tilting SLA's
>
>
> Yes, but IF the stream repeats, particularly if the reptetion is at
> a common video rate (60Hz, 50Hz, 59.97Hz (NTSC color), then it's
> spoofable.
>
> An uncrackable system would have to have random keys, with random
> timing as well. A brute force attack could easily determin a fixed
> key since the data of a static image is unchanging.
>
> gacrowell wrote:
> >
> > Transmission Security is what it was called IIRC.  In the
> military crypto
> > systems I worked on (long long ago), the encrypted data
> stream was constant,
> > even during non-message periods, because even the knowledge
> of message
> > frequency is sensitive information.  Likewise I would
> expect an encrypted
> > video stream to have the sync encrypted as well,
> indistinguishable from the
> > frame data.
> >
> > GC
> >
> > >
> > > Only if the encryption keys change periodically (like
> every frame).
> > > If the scene is static, what's to prevent me from just
> capturing the
> > > ENCODED scene, and replaying the encrypted data, looping at the
> > > repetition point (vertical sync)?
> > >
> > > --
> > > http://www.piclist.com hint: To leave the PICList
> > > mailto:piclist-unsubscribe-request@mitvma.mit.edu
> > >
> > >
> >
> > --
> > http://www.piclist.com hint: The list server can filter out
> subtopics
> > (like ads or off topics) for you. See http://www.piclist.com/#topics
>
> --
> http://www.piclist.com hint: The PICList is archived three different
> ways.  See http://www.piclist.com/#archives for details.
>
>

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.