We've done one already I think called the ... mentioning the Tiltman break and it's
got to the stage, now, where somebody comes and says: "Remind us. What was the
Tiltman Break. Why was it so important?" and so on. The Tiltman Break exploited a
weakness in any cipher that's based on bitwise exclusive-OR, as indeed all the
Tunny traffic exactly was - just that - I suppose I could very briefly - and not
using too much notation to frighten computer scientists - but just as
a reminder. What happens with exclusive-OR, which I'll denote with a + in a
circle. And effectively what happens with it is that if you take a piece of
plaintext and you add on to it a key character, what you'll end up with is an
enciphered version of that plain character. And it's all done with bitwise
exclusive-OR. And if you say: "Well, in what code are these characters expressed?" then,
in what we're talking about in the middle 1940s, it was the International
Teleprinter Code, five-hole, which had been well known for quite a few years,
since the '20s I think. And it was used for telegrams, telex, transatlantic
communications and so on. So what happens, then, is if you take something and
exclusive-OR it with something else - a 5-hole tape character - you will get
encrypted text. If you look at that and then visit the ZigZag Decryption video,
where Sean and I've done an even simpler example, you'll find out how
zig-zag decryption works. You get a bit of Plaintext#1 one, it predicts Plaintext#2
The Plaintext#2 goes on a little bit further and then that predicts a bit
more of Plaintext#1. And you flip-flop between the two streams. That's why it's
called zig-zag decryption. So, using zig-zag decryption on all of
three thousand characters, very early on in this game,
the Allies really had got an absolute bonanza here. Because if you then go back
and say: "Well, look, we know the plaintext now, we know what ciphertext we received,
exclusive-OR tells us that 'plaintext' + 'ciphertext' gives 'key'. We've got
3,000 characters-worth of key, So why was it called the Tiltman break? Well, it
was called the Tiltman break because Colonel John Tiltman, who was a
mathematician as well as being a military man, knew about the properties
and the weaknesses of exclusive-OR. And he knew that under addition, like this,
the key would cancel out and you could use this zig-zag techniquea. He did it and it
took him 10 days and he was a German expert as well.
He had enough mathematics knowledge but was a serious military-German expert.
We do make it painfully easy - and we do wave our hands and greatly oversimplify it.
You've got to remember that, in real life, what happens is you start zig-zagging and
then, suddenly, your German expert says:"Oh dear! I can't see that that's the start
of a new German word! Oh dear!" And, in the end, you might get blocked. But not often
because what you can then do is look further along the message and see if you
can guess another "island" [of plaintext] in the middle. It might be 'geheim'. That was another
good word: 'secret'. Why not drag 'geheim' through the D-stream with exclusive-OR
and see if you get something looking like 'geheim' at the same place below
it, in the other one [e.g. plaintext #2]. So, you got islands of decrypted stuff. And your job was to
link the islands and get everything - including the less common German words
that had appeared. And in the end Tiltman did it. 10 days it took him. I think it
would have taken him a lot less if he had to do it a second time, obviously,
because you get the hang of the zigzag technique. So Tiltman had done all this
work. But I think, for a month or two in the Research Section, nobody could
figure out what on earth sort of machine it was that could be generating stuff
that looked like this. So new recruit Bill Tutte, from Cambridge - enrolled for
a Chemistry degree, not Mathematics, but very keen on
recreational mathematics. He was given the job and he decided that having learned
at cipher school that if you think that there could be a periodicity in the
cipher then you'd better start looking for what period, what repetition rate it
was on . And prior to being put to work on this Tiltman Break he'd worked on a
similar cogs-based ciphering machine - a Swedish one called the Hagelin machine.
And I think it's fair to say that there's enough similarity there that is:
"Oh yeah! maybe it's a cogs machine? Prime numbers of [teeth on the] cogs because we've discovered
the prime, or relative prime, of the numbers of [teeth on the] cogs is the magic formula.
And he said: "OK the head of the section - Gerry Morgan - has told me that they have reason
to believe that one of the cogs in this machine has got 23 teeth on it". So, we've
covered in another video how he started looking for 23 but accidentally found 41 [teeth].
However there is something following on after that which is intended to
randomize it sufficiently well that it will disguise the 41. But it's not been
done well enough and occasionally the 41-icity of this is breaking through the murk
and convincing us that that's right. So, he announces this to the rest of the
Research Section and they go mad for several weeks trying to find out what
the [teeth numbers on the ] other streams must be. Because, don't forget, it's teleprinter code. Five
streams 1 2 3 4 5 but looks like some kind of two-stage mechanism. There's an
initial attempt to encrypt this but then there's a follow-up that's trying
to disguise the periodicity of all these wheels. Well after a lot of effort by all
the Research Section they cracked it. So here is what happens. You feed in an
ordinary teleprinter character into this adaptor. It goes through the first set of
wheels which Bill Tutte - in all the things I have read nobody explains why - he decided he
would call the first set of wheels the 'chi wheels' The Greek character
chi which is written out like that [draws character on paper] and the second set that follows on
afterwards, there they are look, here's the first set that is then passed on
into a second set of wheels which he called the psi wheels. And there's the
character psi. And those psi wheels were meant to give an extra element of
disguise as to exactly what it was the chi wheels were doing and to make
decryption that bit harder. Chi wheels put out a certain pattern of 1s and
0s which gets exclusive-OR'd with the incoming character. They then all turn on
All the chi wheels always move in synchrony, together. But that doesn't get boring
because there's different numbers of teeth on these wheels. I mean here on
stream 1 there's the famous 41. Next wheel along has got 31 teeth, then 29 26
- not prime but relatively prime to all the rest - [then] 23. That first stage encryption
then travels onto the psi wheels. But the designers the machine thought it
would make it even harder for the Allies if we make it so that the psi wheels don't
always move on with every character. Let's put in a 'stutter' mechanism - or at
least that's what the decryptors came to call it. Sometimes they move and sometimes
they don't. OK that will make it harder?
Actually, in the end, it made it easier, as several statisticians pointed out. But that was
the way it was and the reason that Bill Tutte was able to prevail, and to see 41,
was that the psi-wheel patterns were really bad. And by 'really bad' I mean they
left the second set of wheels not rotating for 5. 6, 7 characters
on end. Y'know they didn't change the pattern at all! So, you've got this great
thing where very clearly it was been exclusive-OR'd in the second stage with
the same thing over and over and over again. And if that gets up to 6 or
7 times, it's basically saying [that] it's only a single-stage machine again
because the second stage isn't varying fast enough. Bill Tutte
said: "Thank heavens for this. We were so lucky in the war. Number one the
Tiltman Break: that chap [the machine operator] should not have sent that message again and [thereby] given us
3200 characters of key, potentially. But what really finished
them was that they failed to disguise the periodicity of the chi wheels
because they had inadequate psi wheel patterns that let great long unaltered stretches
come through. And that's what gave us just enough evidence that we could work
out that the psi wheels had got 43 47 51 53 59 and so on. So we won't go into
the details of precisely how that calamity occurred. Let's just say that
whoever was in charge of psi-wheel movements really goofed in the early
stages and as Tutte again says, it didn't matter that when we came back
later they [had] put their error right and made it better in future. By that time we
knew the periodicity of the wheels. So the only worry then was - and another of
Bill Tutte's colleagues, and later his PhD supervisor actually, a guy called
Shaun Wylie, who was down the hill in Hut 8, with Alan Turing and various other
worthies was brought up the hill, along with Jack Good and Donald Michie. People
were brought from Hut 8, and Naval Enigma, to help out with Tunny. And Sean Wylie, in
one of his writings about this, says: "Do you know the thing that panicked us
straight after finding out this wonderful layout of the machine [was] let's
hope these wheels were not interchangeable!" Could you imagine if
instead of having 41 31, so on, down here, it was possible to pick them out
and put them in different slots and permute them, like on Enigma, and he said:
'Thank heavens after analyzing several weeks of traffic we were convinced these
things were fixed; it was always going to be 41 31 whatever. So that's one big worry
out of the way. So that was more or less the situation that was left then. You'd
got this fabulous break - the key subtracted itself out; there was
enough indications in there to work out what the periodicity of the wheels was -
and they're all relatively prime to each other - but where do we go from here?
Whenever somebody uses the same initial settings
which they shouldn't do - they're ordered not to do it - if ever this happens again and
gives what Bletchley Park calls a 'depth' - - and you remember the famous,
absolutely famous, indicator setting of the Tiltman Break was HQIBPEXEZMUG,
12 characters. Yeah, well, that was how they knew it was worthwhile exclusive
or-ing them because the operator, the next time round when he repeated it, sent
out this thing [again] saying: 'my settings are HQIBPEXEZMUG'. So, you know it's
worthwhile exclusive or-ing them together seeing what you can find out.
So, they relied absolutely on the fact that in the early days - and up until mid to late
1942 - the operators were told to send out the Indicator so the other end knew how
to set the wheels relative to one another.
What they didn't do except very infrequently, thank heavens, was to change
what's called the 'patterns' on the wheels. It's probably about time we said a
little bit more about that. What I've drawn out for you here on that diagram
is the simplest wheel of all with the fewest teeth that was 23. Here is my
notional starting point and you can see I've numbered them 01, 02, 03, 04.
So, just imagine, that after every exclusive-OR character that
contributes, this wheel is moving clockwise around from 6 to 7. And then it
moves to position 8 and so on. The dot means this is contributing a 1 when it
comes around to the start position. And otherwise it contributes a 0. So these
things, the the dots, are called the 'patterns' on the wheels.
Now, again, a huge stroke of luck for the Allies. It was such a pain
setting up all of these wheels, all 12 of them, with different patterns, and you had
a little slider to slide a ratchet up and down, as to whether it was 1 or 0, that
the Germans only changed the patterns every month. Phew! that meant that you had
one month to try and get depths - you know things where the person had
used the same Indicator twice. It may not be as a repeat of the same,
exactly same, message. That really was an absolute gift from on high.
Typically, though, an operator would say: "I've used HQIBPEXEZMUG, you know, that one
[that message] was about 'Parachute dispositions in Salonica' Oh! there's another one here I've got
to send which is about when General Katzenzinger next has his leave break.
Oh! what Indicator should I use there? Oh well! let's use this one again. They'll never decrypt
this it doesn't matter. So, very often, your zigzag was not between two similar
messages it was between two very different messages. But at least they'd
use the same settings. That, again, explains why you needed German [language] experts around to
say: "Oh! y'know, yes, that is the start of something or other [significant] down here".
So, that was the name of the game. Hope that the lazy operators give you depths by using
the same indicators. Then Bill Tutte came up with an extra method in late '41,
I think and said: "I know. Let's try and look for 'near depths' because a slightly less
lazy operator would say HQIBPEXEZMUG, we're not going to use
the same one twice. I'll only change one of them [Indicator letters]. It's such a faff.
Let me just change one of them. Now if only one of those settings was
changed and if it happened to be on a first stage chi wheel, Bill Tutte pointed
out that from the other four [chi settings] that you knew were the same as the last time, and
only one has changed, you can, with some mind-bending
attention to complexity and the help of German experts sitting next to you, say
it's not ZMUG at the end it's RMUG. So have we got any other messages, from
earlier on, that we've part-broken which had R in that position? Who knows?
That might help. So, he said, typically you might end up
with 20 or 30 possibilities, once you'd propagated this 'R not Z'
through all the morass of stuff that you've got. And he said but, yeah, with the
help of some really good German [linguist] guys that could work. And that was more
evidence, you see, because that could be backtracked into working out what the
wheel patterns were a bit more. So I think the feeling in the Research
Section was: 'This is OK so long as they keep sending out the Indicator settings'
But we need a lot more techniques to get us out of trouble if that isn't the case.
Well, everybody knows, of course, that Turing did everything at Bletchley Park.
Nobody else was of any importance at all :-) And I hope that one of the things that
this set of videos may be doing for you is to emphasize that, very important
though he was, he didn't do absolutely everything. One of the problems with the
secrecy, particularly in the late 70s and early 80s when this
news about Bletchley Park began to trickle out - but a lot was still under
the Official Secrets Act - is that some computer scientists, historians and
writers very incautiously started saying: "Oh well! If Turing took what the Poles
did for Enigma and developed it on, and then did Naval Enigma and got that
sorted out, he's such a genius it stands to reason it must have been him running
the whole Tunny / Lorentz / Colossus show". No - not true! I think our friend and
colleague, Jack Copeland, whose book you see up there, 'Colossus', would be the first
to want me to say: "No - Alan Turing had nothing to do with Colossus! Colossus was
purely Tommy Flowers." He [Turing] did make a contribution towards the decrypting Tunny effort
and it was a very very helpful one.
Here we are, in early '42, there's been the Tiltman break. Bill Tutte has worked back
from that, and the Research Section helped him to find out the disposition
of the wheels on this Tunny ciphering machine. What was the problem? Because
every message sent out on that machine had an indicator, showing the start
positions of all of the cogwheels. The bigger problem was the worry that the
patterns on the wheels - the patterns of 0s and 1s - a lot of people spent a
lot of brainpower working back through depths, where people had used the same, you
know, Indicator twice, trying to backtrack through the key text into saying: "What
does this mean about what the patterns are?" And, in the end, if you gathered
enough evidence - and certainly with the Tiltman break there was so much evidence
there they really did get the whole thing sorted out. They managed to work
out, for that month, exactly what all the wheel patterns must have been. [It] took a lot
of people a lot of effort, by hand. But it was done. But the worry was we're gonna
more and more be saying: "We want to get the patterns". At the moment we're saved
by the fact that we have a month to build up, and analyze, evidence before it
changes again. So, you know, let's say for a typical example, start of August, you
start collecting evidence as to what this month's patterns must be. You look
for depths. You've got all the indicators and by maybe August 20th, or
mid-August if you're very lucky, you've got enough evidence to work out
what the patterns on the wheels are for this month. Then you go back to all the
stuff that was transmitted earlier in the month, that you didn't understand. But
now you've got the patterns you can go back and see what the messages were. And
so long as the developments in the war are not happening at a breakneck pace
then the fact that this intelligence was two weeks old didn't matter too much.
Because it was high-quality intelligence. Remember this is Hitler's High Command
talking about strategic things, so it was still valuable. Suddenly I think it was
late '42 the moment they'd all dreaded. Calamity. Somebody in the German High
Command said: "... although it's inconceivable that this is being broken (!) we are idiots
if we start sending out stuff we don't need to send out, [i.e.] these Indicator
settings. Why not distribute a codebook to everybody and say I'm using wheel
patterns [correction: 'wheel settings'] no. 356 today?" That's exactly what they did on Enigma, remember.
[The] same argument came out. "Oh they don't understand it. It'll work.
It'll be all right". No! no, one step, y'know, belt and braces approach to this,
and don't send information that you don't need to. So, there they were then.
All of a sudden they didn't know what the Indicator was any more. But they did
know when things were being sent in depth because a lazy operator would say,
to his opposite number: "I'm using entry no. 356 in the codebook".
And then, a little later on: "I'm still using 356". So you didn't know
what [Indicator setting] 356 was, but you knew it was a depth with the same start point. So,
the business about exclusive-OR addition and being able, maybe, to do a bit of
zig-zagging still applied. But it was [still] a pain not knowing the settings! So this
was the time then that by this stage Max Newman - who was one of Turing's tutors
remember, at Cambridge, had joined the Research Section and his brief - that he
set for himself - was to mechanize wherever possible,.
just like they done for Enigma. Bill Tutte came to him, very shyly, he says, one
morning and he [Newman] was sitting with the other head of section, Gerry Morgan, I think.
So, Max Newman was in charge of mechanization of any sort and he
formed a subset called the 'Newmanry' out of the Research Section and then they needed
another, much more linguistics-based, section run by a chap called Major Ralph Tester.
And that, of course, became the 'Testery'. So, the Testery had a few
mathematicians and a lot of linguists. The Newmanry had people who were
really concerned with: 'how do we get machines to be able to help us with this?'.
Any new technique - any variant on a technique was going to be very helpful.
So, Alan Turing, in early 1942, although he wasn't part of the Research Section then,
was down in Hut 8 doing Naval Enigma and they we're getting to be very successful, eventually.
So he said: "OK, I'm going to take a six-week sabbatical with the
Research Section [to] see what I can bring to the party". He did contribute, did Turing, two
things which were of enormous strategic help actually. The first, and in the end
less useful one, was that he said: "What I'd like to do is to have a rock-solid
method that - so long as you've got lots of depths, which we tend to get every
month - that you can, by dead-reckoning work out (and we could train people to
work out) what the patterns were. It was one that he could get to work. Again you
need expert German speakers with you and Bill Tutte said: "Y'know it was
wonderful but I could never get it to work. It was a branching explosion
of possibilities. And when I said to Turing: 'Well, which one of those do uou take?'
"Oh! you take the one that you know in your bones is the right one! "
And Tutte said: 'My bones were never good enough! I could never get his method to work.'
But he could and his collaborators [could] ! The idea, essentially, was that every
wheel has got a different repeat cycle. So, what you're saying is: 'Let's presume
that at the start position that tooth and the one next one - let's presume they
were 0 and 1 - they were contributing. Then let's say that on the next wheel
along and each start position it was 0 or 1, so you can see the binary
explosion beginning to take place but then let's go back and say: "No, no that
one was 1 1 and that one was 0 1. And then work out what happens 23
rotations later on the number 5 wheel, or 41 rotations
later on the number 1 wheel. You see it's all perfectly straightforward. You'd
have to be aware of the possibilities of they were both the same or they were
both different on different periodicities for different wheels. And you combine it
all together and everybody's sort of putting cold compresses on their head.
But eventually, if you're really persistent and your brain works that way,
they did actually succeed. But you had to have enough depths that you knew what
the key was. But also I think what came from Turing, and Jack Copeland's book
assures me that it was Turing, was the general observation, in all of this work
with the Tunny traffic that it was a good idea not just to consider the
characters themselves - of plaintext or ciphertext - but how about exclusive-ORing
them with the [character] one ahead? Now, just imagine a stream of 5-hole
characters, like this, coming down so, you know, it was h a p p y. Now that's your
[conceptual] paper tape. Do another one, exactly the same, alongside. But this time just slip
it back by one [character]. What would the net effect of that be? You are exclusive-ORing
every 5-bit letter with the one ahead of it, to see what happens. Why would you
do that? "Ah!" said Turing "because it will make all
of these doubled-letter occurrences stand out like a sore thumb". We have
referred to these in a previous video. How did they ever get into this traffic?
And the answer was: its language structure and the nature of exclusive-OR.
If you have h a p p y, but you slide the second p up against the first one, on a
separate tape, and exclusive-OR them ... anything exclusive-ORd with itself gives
you 5 dots [5 zeroes]. Five dots is a very unlikely thing to occur by chance but if
you do the 'delta technique', as it was called, it doesn't matter what repeats
itself: it could be double 'p'; could be double 'z'; it could be - because I gather
that German typewriter operators, like [i.e. 'in common with'] my Dad [who was an English Cipher Clerk]
were taught to put in a double space after every full-stop. So a lot of the
traffic on Tunny was double spaces and those count as
well. So can you see that by delta-ing, as it was called, you are producing a
cascade of 0s just because of the way exclusive-OR works. And that message was
not lost on Bill Tutte. He said: "Look, the Germans are upping their game all the
time. They've really scuppered us, for the moment, by not sending the Indicator. [We've] got
to be able to work out what the relative settings of these wheels were, even
without the Indicator being given. How do we do that?"
And he went in to see Max Newman, the head of his section at that time, and
said: "I've got this bright idea - this could work". And I think Newman and Gerry
Morgan said: "Well yeah but do you need a depth Bill? >> Tutte: "No, no, this
would work on anything, so long as it's [a] sufficiently long message full of
ciphertext". I can use the delta method, and statistics, to work out what [the] relative
positioning must have been of, shall we say, the first two wheels looking left to right".
>> Newman: "Oh!, well how are you going to do that?" >> Tutte: "Well, you rely on the fact that when you
delta things together on a per character level you produce far more than probable
numbers of 5 dots, which is where two things that are identical have been
exclusive-OR'd together. So, on every single stream you're looking at, there
will be more dots [for 0s] than crosses [for 1s] and that will be particularly magnified
if you do the delta-ing first". And they said: "Well, have you any idea what the
skew is between ... you know ..." He said: "About 55 to 45,
sometimes, maybe creeping up to 60:40 or whatever". And, I mean, they were all good
statisticians. You can imagine. He was bombarded with "what ifs".
>> Newman: "Well this is all very well, Bill, but as we all know this could show up and look
plausible but actually fade away if you ... " >> Tutte: "Yes! you're going to need LOTS of ciphertext".
>> Newman: "How much?" And I think the answer, when you do the analysis, is [that] - to be pretty sure
that this isn't a freak result that you're getting - this ratio of ...
whatever it's showing up as ... 54 to 46 [say] you need about 2,300 characters to be sure.
So you do need long messages. But if you get one that is sufficiently long
what it will then ensure for you is that, at the right setting relatively, it will
show the 55:45 split. If you presume the wrong setting then randomness takes over
it will be closer to 50:50 then to 55;45 or 60:40, or whatever. But, in order to
make sure that that distinction between nearly 50:50 and nearly 55:45 is really
showing up correctly the Bletchley Park rule was don't rely on anything less
than 2,000 characters long, to show this up. But if it does show up then it is
odds-on correct that that is the relative positioning of those two [chi wheels].
But just imagine: you've got ... if you take wheel #1 and wheel #2 in the chi set, 41 X 31 = 1271
relative positions. And, for each one of those relative positions you've got to
squirt through at least 2,000 characters of ciphertext. [In total] several million dots and
crosses to be analyzed for even one setting of the wheels. Now you realize why
mechanization was absolutely essential to start finding the wheel settings
correctly. So y'know, Newman was delighted because he more or less said:
" ... this just proves what I've been saying, we must mechanize everything we can".
[Firstly by] electromechanical Heath Robinson and of course later on [Colossus]. This really made a huge
difference that chewing through 3 million possibilities for every pair of
streams would be pretty slow on an electromechanical machine but it
would be a heck of a lot faster [electronically]. And, of course, that is where it ties into
Colossus. Colossus could put these patterns to be searched for on a plug
board at the back [of Colossus] and basically just look for them. The thing that
Tutte then asked himself is: " ... well it's all about doing these deltas but suppose,
just suppose, that the Germans really get their chi wheel patterns so good that
actually stream #1 looks like 50:50 stream #2 looks like 50:50. They might be
able to disguise each individual stream quite well but would ... ah! think about it though!
They're not independent! Because of the fact that we get five dots a huge
amount of time - in a delta stream occuring all time, every time there's a double letter,
what it means is that the dot-dot combination between #1 and #2 will be
more common than dot-cross, cross-dot or even cross-cross. It should be the most
common one of the four and they won't be able to disguise that because, you know,
there is a correlation between these things and it's particularly so that five
dots would be an incredibly [a rather] uncommon thing to occur by chance but it's
actually occurring a heck of a lot, because of the nature of exclusive-OR.
So, if we do this delta-ing, on whatever, the Germans might be fiendishly clever
enough, on any one stream, to be able to hide it, but the chances of them being
able to do it across pairs of streams ... it's not gonna happen. So, basically, what you're
looking for on stream #1 on stream #2, together, is the occurrence of dot-dot,
two dots. Really it ought to be 25% but it won't be. It'll be
55/45 times more common than 25% so it's these little things you can search
for. And if you think you've got something looking good after 2300
characters of ciphertext, you then say: "Right! Let's correlate stream #2 with stream #3".
And then we can find the relative settings of 2 to 3; 3 to 4 etc. So, after
only five runs, preferably helped by Colossus - about 12 minutes per run that took -
you could zizz through all of these millions of combinations, per relative
setting, and say: "That's it! the start settings of the chi wheels are as follows".
Of course, that's only the start of the process. So you've got the chi
part of the encryption. By the laws of exclusive-OR, if you add that [chi contribution] back
into the cipher stream, what will be left over is the psi-wheel contribution. Now,
as we found in the early days the psi-wheel settings were dreadful and tended to
just let things repeat over and over again.
Well, again the Germans had learned from a lot of experience - Oh! and by the way, of
course, the number of teeth on the psi wheels was known. So, actually, as a
by-hand method to follow on from the de-chi-ing which Colossus will do, then in
the first instance a lot of hand effort might go into getting the psi
contribution taken away, and revealing the absolute classic plaintext.
Now, what's going to happen if you think about it, is that by the time you look
at the psi contribution you might see little bits of plaintext showing through.
So, you need a German expert again and you need lots and lots and lots of
by-hand efforts to say what must the psi wheel settings have been, which when put
correctly will cause this to look like plaintext? If the first
set of wheels was taken away - the chi contribution; de-chi-ing - what you're now
doing is de-psi-ing but of course if it doesn't work it's called "deep sighing" [Joke]
>> Sean: [groans loudly] >> DFB: Thank you! Yes, but very largely it was possible to make that final adjustment
and get it all backed out and sorted and showing up very clearly. But this really
highlights again how immensely lucky the Allies were that the wheel patterns
didn't change very often. So, by lots of hard effort - aided by Colossus - we've managed
to discover the start settings, even without the Indicator. But you'll have
got the impression very clear there's still a lot of by-hand work had to be
done to get the full story to appear. And this gave rise to the well-known
Bletchley phrase which staggered me when I first saw it ... I couldn't understand it.
It said: "Just remember - Colossus time is far more valuable than computer time'
In those days a 'computer' was a person! A person who did computation. So, the
'computer time' was the by-hand effort needed to tidy up the story. And you
didn't worry that there was hours and hours and hours of that, because the wheel
patterns only changed every month. Why it was vital to use Colossus for finding
the start settings was that that changed [for] every single message. So, there we go then.
That 'Colossus time' is invaluable for finding the start settings of the wheels
and is far more precious to you than mere 'human computer' time. It wasn't,
believed me, until the 50s and 60s that the use of 'computer', as a person, began to
fade away. Even in the late '50s, in the space program, [see the movie 'Hidden Figures'], people using
electromechanical calculators were called 'computers'. You had to carefully
say: "No, no I mean an electronic computer; I mean a digital computer". It wasn't until
the '60s that 'computer' without any prefix came to mean:
'A piece of electronic machinery'.