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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 -----BEGIN PGP SIGNATURE----- Version: 2.6.2 iQCVAwUBMJmz3kHx57Ag8goBAQGYiwQAkx3dxUUcGjsEKqRAlc0CTq6EaTzBrLRz ifrKgZPxfyD9F+sBTJGGPpHWc3mtfwJwV8HeIa/MY1Z1hpssN1ZY6ELlEn+4FFIA 3A+BNjB1PKrHebVZ0WHBJ3DI3h4/olF37bYNP261Uqd8CNig+fQ3VMtE0L0frBdr I+izpEMOwiw= =y7To -----END PGP SIGNATURE-----