Some of you will have seen Sean and me going wild around Bletchley Park and will
deduce from that that we paid a visit to BP as they call it in the trade.
It's time now for me to move off from the Enigma story which I've done
two or three videos on already, and tell a story that in many ways is
even more relevant to the start of computer science than
even Enigma was. We've all heard of Alan Turing but
I would suspect that fewer of you have heard about Bill Tutte,
Max Newman, Tommy Flowers. But they're all worthy of hero status because they're all
connected with a type of
cipher traffic - mysterious cipher traffic -
that came to Bletchley Park's attention and
at Bletchley was always called Tunny traffic. They loved calling mysterious traffic by
the name of fish and there were things like
Sturgeon and Halibut as well as Tunny. But this one is going to be
Tunny traffic. Tunny is the original English word for what we nowadays call
Tuna - it's as simple as that.
Even before the war the Metropolitan Police
and the Foreign Office, jointly, used to listen
to the airwaves to see what was happening - you know, that shouldn't be happening
within the UK shores. Anything mysterious. Now an awful lot of course of what they were
listening to
was plain straightforward radio broadcasts or,
from the military point of view, it would be Morse code. Some of that Morse code
might well be
encrypted Enigma traffic, but it was in Morse. This one was weird
it wasn't in Morse it sounded almost like
a touch-tone telephone on steroids or something,
It was "meeeooow", "meeeooow" all over the place
And so what they did was they equipped
the listening stations, all over the UK,
because they all knew that war was almost inevitable;
they had a whole load of joint listening stations and these listened for Enigma
traffic.
They also listened for anything unusual -
they were called Y stations - capital Y -
for reasons I don't understand, but there again Bletchley Park was called Station X
and I don't understand that either. So these Y stations started saying:
"Look there's some weird stuff here would you like to see what it looks like?"
They were able to make a recording of the trace of this,
as if it was an oscilloscope but actually on a piece of paper
If you look here it's like a waveform and that is exactly what it is
and they were able to record this as a black
trace and when you look at it, it's like a mixture between a sort of
sine wave and a square wave. But once they got the experts in looking at this --
you've got to remember teleprinters were very common
they were used in stock exchanges, they were used in telegraphy. A
lot of traffic went over land-line
that was actually teleprinter code. This was broadcast
because presumably the land line was too long between
the far end of Greece and Germany, let's say.
So it had to be broadcast and the experts said
"That's broadcast teleprinter traffic, that is!". Now you know why Sean and I
have set
the scene by doing a video already on 5-hole paper tape codes,
the teleprinter alphabet and even on exclusive-OR.
The rule at the Police and the Foreign Office was that anything mysterious like this
gets sent to GC&CS.
We remember from the previous videos GC&CS
(Government Code and Cipher School) - initially based in Saint James's Park in London
but the great majority of it moved off to Bletchley Park in the late Thirties.
So Bletchley Park was accumulating this stuff. Big debate:
"Is this a Vernam Cipher?"
In the late nineteen twenties an American
employee of AT&T had come up with the idea
of doing secure, enciphered
teleprinter transmissions by doing what we would now call
an exclusive-OR between two
five-bit patterns. Yeah?! Great idea
because it would disguise the character you wanted to send ...
There's only one problem that if you want to apply that to a whole message
how would you arrange
to have, as it were, cipher key characters - randomly chosen -
to mix in, via exclusive-OR, with your plaintext that you wanted to send
and to send out the result? Well, what
Vernam proposed - and to my amazement
he patented it. And you think: "This is madness!
This is madness! This is secure! they shouldn't be patenting this stuff - it gives the
game away !"
This is AT&T and we're not at war yet! We're not having the competition
coming up with this bright idea. This has got commercial potential, you know, as well as
military.
We're gonna patent it. So Vernam patented the idea.
It would use two five-hole tapes and
what you could do is have this one,
which is the plaintext, running alongside this one
which has somehow been generated and this is the key.
This is the thing that's going to be exclusive-OR'd,
character by character, with this to give a cipher text.
The problem is the enormous one - these things are driven by sprocket holes.
You start trying to run two tapes simultaneously
through a piece of bespoke electronics, which they invented, which would do the merging
but you must keep them in exact sync. You do not want differential stretching
between the two things. Worse still when your supervisor says "This thing isn't going
fast enough, we need more speed!" and you speed it up
you know what happens next - the sprocket holes tear. And then you've got slippage
like you never dreamed of in your worst nightmares.
So, as a result all this problem of tape
synchronization it never really got to market.
It was quietly abandoned,
but the idea was good. But the people at Bletchley Park started saying:
"Is this a Vernam cipher
that we were taught about at training school before we started here at BP?"
But maybe the Germans have been smart enough to say we're not going to use the
second tape
because you've got to get it synchronized. We will provide
the key characters from a machine. It'll be like an
add-on to the teletype - the teleprinter. You type in your plaintext character
this thing we'll invent and it's probably going to have
wheels - rotor wheels - every good cipher machine has rotor wheels on it -
will somehow generate a 5-bit
International Teletype letter and then it'll automatically
exclusive-OR it and then we'll get an output
and that'll be our cipher and the good thing is that it will be a lot less problem to
keep the machine
in sync with what we're doing when you've only got one tape
to worry about. So in the end they discovered that that's
exactly what was happening - looked at more and more of this Tunny traffic:
How on earth are we ever, ever, going to get into this stuff ?
At the start of this signal it goes up high
but the rule in these intercepted messages, that were put out on this
piece of kit called the Undulator, is that if it's high it's a zero
and if it goes low it's a one and the neutral point is somewhere in the middle.
Let's just do a straightforward addition of two 5-bit
codes to see what it would look like. The letter A - capital A -
in the International Teleprinter code - the Baudot-Murray code -
is one one zero
zero
zero and let me remind you one of the approved signals for
doing an exclusive-OR operation is a plus in a circle.
I'm going to add that on via exclusive- OR
to the letter X. Letter X is
one zero one one one
You all know how this works now exclusive-OR
gives a 0 if they're the same
and a 1 if they are different. Look that up
in your table - what character is that? - its the letter V.
So there we are look. The plaintext shall we say was A, the ciphering character given by
the randomizing device was X
and this particular character is V. What we're going to have to do
on this, surely, is to get enough evidence
about that random key stream before we can start working out how the heck it
works.
Mathematicians were around, were upon the scene,
arguably not as many as Bletchley Park really needed in the early days,
to say: "Look, this is an example of what we
in the Mathematics trade call an Additive Ciphering system.
It's doing arithmetic modulo a certain number.
This as I've said is arithmetic modulo two
it's binary addition, but without the carries.
And some wise soul said; "You know there are weaknesses in all additive ciphers.
Basically, at the end of the 19th century,
a man called Kerckhoffs had published a paper
about cryptography via additive ciphers and said the problem with them
all is
if you accumulate enough captured
ciphertext, even when you don't know the key, but
lots of different messages, if they're all done with the same
key you can eventually accumulate so much that via a
clever trick you can work out what the messages are
without even
knowing the key, because it kind of cancels
out. Oh! that could be helpful!
So all we've got to look out for then
is are they using the same key for any of this stuff?
Handy! Every single one of
these transmissions, before it came through on the Undulator -
they all came with an Indicator.
Now this really puzzled them at first. I've
copied one down, from my reference books.
Those 12 characters would be sent out and then there'd be a pause.
And then this stuff, the Undulator traffic would start.
However there was sufficient of this sent out, to train the operators, that
again the game was given away.
The cryptanalysts said: "I wonder if that's in code or cipher itself - oh! i dunno -
but, you see, could this be like Enigma?
They sent out blocks of three letters to show
the settings - the initial settings. Could it be like that ? "
Then somebody else would say: " Yeah but there's twelve of them - that doesn't make
sense;
ten would make sense - two layers of encipherment one after the other
First row of five, y'know, five bits
and then push it on again - do another one. But these could be settings of
cipher wheels just like Enigma. But how can we be sure that they aren't
in a code of their own. Do they really mean what they say? As part of the training
scenario
obvious to the poor operators trying to work this machine
there were some of the transmissions that said `in clear'
"M ist Michael"
"G ist Gerhard", Right?! So by the time you got
"O ist Otto", "K ist Karl"
you can see this is training the operators: "Yes, it's K for Karl,
you see the K and you wind the cipher wheel round until you see
letter K for Karl and by the fact that they
had elaborated, in the training, all of these as
boys' first names made
the listeners sure that these were what they seemed.
So the answer now is very clear, we have to sit and wait patiently;
we have to ask our Y stations to rigorously send us
exactly what the Indicator was, and if we wait long enough
somebody will use the same Indicator twice.
Absolutely against standing orders but it was known from Enigma this was a great
temptation in the heat of battle,
with people shouting at you, to use the same settings
twice. "It's alright - nobody will be listening. Hmmm! Oh yes they are ... "
This did happen, I guess you could say,
sooner rather than later. I think it was August 1941
they finally got a message which had got the same
indicator and they knew perfectly well that with that and
with a lot of hard, patient, work - because there were no computers to help you -
nevertheless we can probably disentangle this message.
Now, another thing, these messages seemed to be going between headquarters
of the German High Command. These were not between weather stations
they were not even between Luftwaffe stations, this was
serious stuff. This was going from Athens HQ to Berlin HQ,
shall we say. Clearly this stuff - and there wasn't much of it initially -
was going to be used by the high-ups - the German High Command.
As we now know it really was Hitler's favourite cipher machine.
It's great advantage, unlike Enigma,
was that it was user-friendly. You know what Enigma's like: you need a person to
punch the thing
a person to read the lamps, a person to send the Morse code.
Here you could connect two encrypted-type teletypes back-to-back, across a
land-line
or even across the airwaves, although that would be a bit
dicey to do with interference. But you could in principle
have your operators never see the ciphertext It goes in one
end, ... enciphered ... across ... deciphered out it comes. Therefore
because it's easy to use, and user-friendly, people would send
longer messages and that's music to the ears of the cryptanalyst brigade
they loved that! By this stage the listeners had decided that we needed
a new Y station, separately for this
Tunny traffic - remember the traffic was called Tunny
the machine that was generating it was called German Tunny - or just
Tunny. The machines that eventually they built at Bletchley Park, once they understood how
it worked,
to do the same job of deciphering, was called British Tunny -
but it was all Tunny. So here was this Tunny traffic
all stacking up, all waiting to be decrypted,
all waiting for two or more
to be sent out with the same key. Now at Bletchley, technically, that's called
a "depth". Getting a depth means you've got two or more messages
all with the same key. Absolutely against standing orders,
because the German cryptographers had warned the military
that this one - this kind of XOR additive cipher -
is even more sensitive to depths
probably even than Enigma. With Enigma you only got a short message out anyway
because
life's too short to send much more than about 50 characters.
This thing - in the end - they got 4000 characters-worth
and then, when the chap
phoned back "Sorry! didn't quite get that, could you send it again ?!"
My first thought was surely the sender guy would have had the
sense to make a tape of it and send it off-line in the first place?
No, he hadn't. He'd typed it in by hand without making a copy.
So when it was sent again you can imagine the sender
muttering and cursing. He used abbreviations; he made it shorter
and as we'll see, next time, that
made the deciphering job even easier