hello everyone there's a story going
around at the moment which seems to have
caught people's imaginations and it's
been reported in the press and some of
you have asked me to comment on it so
the story is a British couple we're
renovating their chimney and inside they
found what turned out to be a skeleton
of a world war two carrier pigeon and
attached to its leg was a little red
canister which curled up inside was a
secret coded message now this is quite
unusual for these carrier pigeons to
carry coded messages so it was sent to
GCHQ which is the code breaking arm of
mi5 and mi6 to see if they can decode
this message and find out what that
message says to read that message for
the first time in 70 years now they came
back and they concluded that it's going
to be very difficult to break if not
impossible now I'm going to leave the
actual message at the end of the video
if you want to try it but I want to try
and explain why it's going to be a very
difficult code to break they think the
type of code used was something called a
one-time pad cipher now you may have
tried codes and secret messages yourself
as a kid well you might have tried is to
ship the alphabet one place across so a
becomes B and B become C and C becomes D
and so on now you could have shifted the
alphabet 2/5 across or 3 places across
in fact you could shift the alphabet any
number from 0 to 25 now that's a very
easy code to break
there are 26 ways through it and you
just check them one at a time and the
one that makes sense is working now the
one time pad cipher is a very similar
idea but you shift each letter of the
message a different amount so if I want
to send the message hello what I do is I
make a key that is the same length as
the message itself so that would be just
five numbers I would then add those
numbers do each letter of the message
which would shift the letter a different
amount and I would make my code now if I
want to decode that the other person
would have the same key and then they
would subtract that from each
letter of the code to get the original
message
back again now why is it called a
one-time pad cipher
the idea is you would have to have these
pads of random numbers and you're only
supposed to use these ones in fact
instead of having random numbers they
would have pads of random letters it's
the same idea but a is a shift of 0 and
B is a shift of 1 and Z's a shift of 25
now this is what they think was used by
this carrier Pidgeon code why is this so
hard to break so each letter of the
message has been shifted a different
amount now this carrier Pidgeon code was
135 letters long put some numbers at the
end which identifies the pigeon itself
now each letter can be shifted any
amount from 0 to 25 so the total number
of keys and the total number of ways to
decode the message is vast in fact is a
1 followed by a hundred and ninety-one
zeroes it's somewhere in the region of a
Google squared even if you had a
computer checking every possible key it
would take something like thousands of
years to check them all but it's even
worse than that because if you did check
them all all you end up doing is simply
generating every possible message of a
hundred and thirty-five letters long
some of them will make sense a lot of
them will be gibberish but all of them
will be equally likely in fact if you
pick the right key you can generate any
message you want to so this is what I
did I took the first 36 letters of the
code this is the actual carrier pigeon
code I took the first 36 letters I made
two keys to decode it in two different
ways the first key decodes the message
so it reads singingbanana is better than
number file the second key decodes the
message so it reads number file is
better than singingbanana
and both those are equally likely in
fact in the war you might expect
messages like attack tomorrow or maybe
nothing to report and you can make keys
that generate both those messages
and they will both be equally likely
it's impossible to tell them apart you
don't know which one is the right answer
so the one time pad cipher is impossible
to break and I mean that that's 100%
mathematically proven to be impossible
if you don't have the key you'll just
end up generating every possible message
and they'll all be equally likely but
it's not perfect though so first of all
this random string of letters does need
to be random and just mashing your
keyboard isn't random enough so you can
make these yourself if you want but if
you want to make it you can pull out
tiles from a scramble bag or you use
some sort of naturally occurring random
event like radioactive decay to generate
your random string of letters it's also
very difficult to distribute these keys
can you imagine in a war running around
the battlefield with these pads of paper
so it's very impractical to use as well
but in the future they hope to use
something called quantum key exchange
where they'll be able to send the key
using particles of light once you've got
the key you can then just use the one
time pad cipher in the traditional way
now I'm going to leave you with this
code carried by this World War two
carrier pigeon but do remember it's
going to be very difficult to break if
not impossible but that's exactly what
it was meant to do it is doing its job
and if you have been thanks for watching
you