video as a source of public randomness

[JMKELSEY@delphi.com]

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>Date: Thu, 02 Nov 1995 00:39:29 -0800
>From: tcmay@got.net (Timothy C. May)
>Subject: Re: Video as a source of randomness

>Digitized video input, especially from something like a noisy channel (t.v.
>channel, for example), is quite likely to produce a lot more entropy bits
>per second than nearly any of us ever need. (One's PGP key could be seeded
>in a fraction of a millisecond, for example.)

This seems like a potential source of a stream of public random
bits.  If these can be authenticated and matched, this kind of thing
can be useful in a lot of protocols.  For example, if there is some
packet structure in the digital video transmission which has at
least 160 bits of entropy, then we can take the SHA1(packet[i]) to be
public random bit block i.  (If we're worried about entropy, we can
collect arbitrarily large numbers of packets to hash per 160-bit
public random block.)

For large-scale, above ground protocols, these packets would need
some kind of signature or other authentication.  However, for
protocols that could handle having the public random string checked
offline later, this idea provides a reasonably good public random
string that can be used without any knowledge or consent of the
broadcaster/cable system/satellite system/whatever.  If the
transmission is sent under encryption, so much the better.  This
does still leave the possibility that an attacker could control the
broadcaster's transmissions for a few seconds, but this seems
unlikely in practice.

An alternative might be the encrypted transmissions from any
communications satellite.  How many telephone calls are your
opponents able to reroute?

>--Tim May, who has both audio and video digitizers built into his Power
>Macintosh 7100av, but notes that none of the crypto programs he uses has
>any provision for using them, and so he doesn't use them for crypto
>purposes.

Note:  Please respond via e-mail as well as or instead of posting,
as I get CP-LITE instead of the whole list.

   --John Kelsey, jmkelsey@delphi.com
 PGP 2.6 fingerprint = 4FE2 F421 100F BB0A 03D1 FE06 A435 7E36


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