[Cryptography] the Herival tip - hut 6

[ianG]

While researching something else, I came across David Rees, 1918 - 2013.



The German operators of the Enigma machines were told which three rotors 
and which settings to use each day, but they had to choose the starting 
positions of the rotors and indicate their choices via the first three 
letters of their first messages. John Herivel, another of Welchman's 
recruits from Sidney Sussex College, predicted in February 1940 that 
some German operators might use short cuts that could be exploited by 
the Bletchley Park codebreakers.

For three months, this insight produced no result; but in May 1940 some 
of the German operators began to make the predicted mistakes, and David 
and his fellow codebreakers were able to use the technique known as the 
"Herivel tip" to break Enigma ciphers for some critical months from May 
1940. The initial breakthrough generated roars of excitement in Hut 6, 
but it had to be kept secret for at least 40 years. David remarked in 
2000 that "the Herivel tip was one of the seminal discoveries of the 
second world war". He thought that Herivel should have received more credit.

http://www.theguardian.com/education/2013/aug/29/david-rees



Herivel's great insight came to him one evening in February 1940 whilst 
he was dozing in front of his landlady's fire. It was that stressed or 
lazy operators who had set the rings when the rotors were in the 
machine, may then have left ring setting at the top, or near the top, 
and used those three letters for the first message of the day.[13]

For each transmitted message, the sending operator would follow a 
standard procedure. From September 1938 he would use an initial position 
to encrypt the indicator, and send it in clear, followed by the message 
key that had been enciphered at that setting. Suppose the initial 
position, the ground setting (German: Grundstellung) was GKX for 
example, he would then use Enigma with the rotors set to GKX to encrypt 
the message setting, which he might choose to be RTQ; which might 
encrypt to LLP. (Before May 1940 the encrypted message setting was 
repeated, but this makes no difference to Herivel's insight.) The 
operator would then turn his rotors to RTQ and encrypt the actual 
message. Thus the preamble to the message would be the unencrypted 
ground setting (GKX) followed by the encrypted message setting (LLP). A 
receiving Enigma operator could use this information to recover the 
message setting and then decrypt the message.

The ground setting (GKX in the above example) should have been chosen at 
random, but Herivel reasoned that if an operator were lazy, or in a 
hurry, or otherwise under pressure, he might simply use whatever rotor 
setting was currently showing on the machine.[12] If this was the first 
message of the day, and the operator had set the ring settings with the 
rotors already inside the machine, then the rotor position currently 
showing on the machine could well be the ring setting itself, or else be 
very close to it. (If this situation occurred in the above example, then 
GKX would be the ring setting, or close to it).

Polish cryptographers used the idea at PC Bruno during the Phoney War.[14]

https://en.wikipedia.org/wiki/John_Herivel#The_Herivel_tip

The effect predicted by Herivel did not immediately show up in the 
Enigma traffic,[13] however, and Bletchley Park had to continue to rely 
on a different technique to get into Enigma: the method of "perforated 
sheets", which had been passed on by Polish cryptologists. The situation 
changed on 1 May 1940, when the Germans changed their indicating 
procedure, rendering the perforated sheet method obsolete. Hut 6 was 
suddenly unable to decrypt Enigma.

Fortunately for the codebreakers, the pattern predicted by the Herivel 
tip began to manifest itself soon after on 10 May, when the Germans 
invaded the Netherlands and Belgium. David Rees spotted a cluster in the 
indicators,[12] and on 22 May an Air Force message sent on 20 May was 
decoded, the first since the change in procedure.[16] The Herivel tip 
was used in combination with another class of operator mistake, known as 
"cillies", to solve the settings and decipher the messages.[13][17] This 
method was used for several months until specialised codebreaking 
machines designed by Alan Turing, the so-called "bombes", were ready for 
use.[18]

...