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## Ancient cryptography

Current time:0:00Total duration:2:09

# Frequency stability property short film

## Video transcript

[TYPING] Consider the following. Imagine two rooms. [DOOR CLOSING] [DOOR CLOSING] Inside each room is a switch. [CLICK] [CLICK] In one room, there is a man
who flips his switch according to a coin flip. If he lands heads,
the switch is on. If he lands tails,
the switch is off. In the other room, a
woman switches her light based on a blind guess. She tries to simulate
randomness without a coin. Then we start a clock, and they
make their switches in unison. [CLICK] [CLICK] [CLICK] [CLICK] Can you determine
which light bulb is being switched
by a coin flip? [CLICK] [CLICK] [CLICK] [CLICK] The answer is yes, but how? [CLICK] [CLICK] [CLICK] And the trick is to think about
properties of each sequence rather than looking for
any specific patterns. For example, first,
we may try to count the number of 1's and 0's
which occur in each sequence. This is close, but
not enough since they will both seem fairly even. The answer is to count sequences
of numbers, such as runs of three consecutive switches. A true random sequence
will be equally likely to contain every
sequence of any length. This is called the
frequency stability property and is demonstrated
by this uniform graph. The forgery is now obvious. Humans favor certain sequences
when they make guesses, resulting in uneven patterns
such as we see here. One reason this
happens is because we make the mistake of
thinking certain outcomes are less random than others. But realize, there is no
such thing as a lucky number. There is no such thing
as a lucky sequence. If we flip a coin
10 times, it is equally likely to come
up all heads, all tails, or any other sequence
you can think of. [CLICK] [CRICKETS CHIRPING]