Large Primes

[Owen Lewis]

> -----Original Message-----
> From: ukcrypto-admin@chiark.greenend.org.uk
> [mailto:ukcrypto-admin@chiark.greenend.org.uk]On Behalf Of Peter
> Fairbrother
> Sent: 17 August 2002 02:05
> To: ukcrypto@chiark.greenend.org.uk
> Subject: Re: Large Primes
>
> Forget assymetric cyphers, if QC's come in they're gone. This means not
> using them for anything but short-term signatures/authentication, not key
> exchange.

You might have added that, with the current design of PK cryptosystems, also
'gone' is any useful purpose for the symmetric  bulk cipher.

> An otp is probably good for 100 years, but impractical.

Why so? Assume perfect randomness of key stream modified by filters to
detect and remove the random occurrence of certain weak key streams that
could otherwise reduce OTP to a Caesar cipher or too close thereto. How then
is OTP good only for 100 years?

>
> Just my opinion, and Brian's point about something completely unknown
> turning up is very relevant. I for one can't predict the future! We're all
> guessing here.

That's the point, isn't it? What ever the unquantifiable risk in using any
cipher, the use of a current PK cryptosystem *increases* it.

> Another possibility is that lifetimes may be extended to well beyond 100
> years in 100 tears time....

The lifetime of some information most certainly does and it is information
and not human lifespan that is the proper matter for cipher and cryptosystem
design to address.

How long must a secret remain secret?

	-	As long as the information is not known to others and harm will result
from its publication.

	-	As long as control of the information provides commercial or political
value.

Clearly, these two yardsticks may often be related but they are not of
necessity so.

Owen