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The Shœstring Foundation Weblog, Miscellaneous Byproducts

Matthias Bauer
bauerm (at) shoestringfoundation · org
reop pubkey

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Tue, 15 Jan 2008

What is a random sequence?


In Cryptography papers there are lots of statements like
Alice choses a random number k
or
Bob choses a random element of F_p
Can one recognize a number or a sequence of numbers as random?
Which of the following sequences is random:
00000000000000000000000
01101010000010011110011
11011110011101011111011
Answer: all of them are equally likely outcomes of 23 coin-flips.
Sérgio B. Volchan tells the history of the concept of randomness in mathematics in an article for the American Mathematical Monthly. It is quite fascinating IMHO how seemingly resonable definitions of randomness were put forward and shot down later to be replaced with the next definition. The most recent definitions preclude meaningful checks for randomness by examining finite parts of a sequence, so the conundrum remains: Is 7 a random number?

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That's how to write manuals


The Jupiter ACE was a home computer produced in the UK in the 80ies. It had a FORTH interpreter instead the usual BASIC of the C64, BBC micro, etc.
Their Manual explains the inner workings of the machine in an accessable way. Compare that to the thousands of VBA books that keep the reader totally in the dark what goes on behind the funny icons.

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Surprising results with IPv6


Spamfilters add complexity, which in turn makes v6 transition harder.
Setup:
Host A (running OpenBSD) has dual stack v4/v6 with routable v4 address
Host B (running Plan9) has dual stack v4/v6 with a subnet-local v4 address
Both machines have a routeable v6 address and run an MTA.
So I assumed that it should be possible to send mail from A to B. Turns out to be not that simple. The Plan9's MTA uses various heuristics to find out if incoming mail is spam (as do other MTAs). One of the checks is to connect to the MTA listed in the MX record for the sender's address' domain. Host A's MX record is v4-only, so B cannot connect to the MTA, so it rejects the mail. Not only the sender and the receiver have to be v6-enabled, but also the sender's MX (and probably the blacklist providers, etc).

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