Finding the daily key (the first methods)

The first methods

• Introduction (This Page).
• Find the message keys (cycles)
• Find the right rotor (clock method)
• Find the Steckers (Grill method)
• The whole Walzenlage (F-catalog)
• Find the Ringstellung (AN method)
• --- My Software (tar)

Introduction

In late 1932, Rejewski cracked Enigma. In early 1933, the Poles built a machine that emulated Enigma. They were now able to decrypt German traffic... provided they knew the daily key !

The catalog method, invented by Rejewski, made it easy to find this daily key. However, this method was only implemented later (likely between 1935 and 1936). It required the prior construction of the Cyclometer. This machine enabled the creation of the actual catalog that lies at the heart of the method.

But before that (between 1933 and 1935), how did the Poles crack the daily keys? In fact, they did not rely on a single specific method, but rather on a combination of methods exploiting the weakness of the double encryption of the message keys at a specific rotor position known as the Grundstellung. As a reminder, it was this same indicator method that allowed Rejewski to determine the Enigma rotor wiring.

Methods for finding the daily key

1. Finding the message key
   As will become apparent when examining each method in detail, the starting point is knowledge of the message keys. Fortunately, German operators never used truly random keys, then they remained guessable.
2. Identifying the right-hand rotor (fast rotor)
   When the message keys are known, the right-hand rotor can be determined using the Clock method.
3. Determining the Steckers using the Grill method.
4. Identifying the other two rotors using the F-catalog
   Using data derived from the Grill method, the other rotors can be identified with the help of the F catalog.
5. Determining the Ringstellung and the Grundstellung.
   A significant number of messages began with "AN" (meaning "to"). The Poles exploited this fact to deduce the Ringstellung.

Recap: the indicator method used from 1930 to 1937 (and from 1937 to 1938)

A message key (Spruchschluessel) composed of three numbers (from 01 to 26) is chosen by the cipher clerk for each message. The rotors are positioned as part of the daily key (Grundstellung), and the message key is encrypted twice. Here is an example of the beginning of a message:

	1035-96-341 PKPJX IGCDS EAHUG WTQGR ...

• The time is represented by 1035 (10h35), 96 is the length of the message,
• The number 341 is the encryption network identifier.
• The first six letters of the message, PKPJXI, correspond to the encryption of ABLABL at Grundstellung FOL (or 06-15-12).
• The message (GCDSEAHUG …) is encrypted or decrypted by positioning the rotors at the ABL key (or 01-02-12)

• This method is described in one of the manuals provided by Asche. Bertrand transmitted this notice to the English and the Poles in 1932.
• In 1937, the number identifying the network was replaced by a Kenngruppe consisting of three letters. Multiple Kenngruppen could identify the same network; the Kenngruppe's placement varied according to the daily key. The manual describing this new method was photographed by Asche, and Bertrand passed it on to the Poles.
• In 1938, the Germans changed their procedure once again. The Catalog became obsolete. The Poles devised a new method to find the daily key: Zygalski's sheets.

References

• Rejewski's report from the French secret service archives (in French). SHD (Service des archives de l’armée française à Vincenes) – DE 2016 ZB 25/6. 1949. Fond Bertrand – dossiers 280 à 285. The German version, file 281, and the French version, file 282.
  This report describes the Polish methods, particularly the first methods for finding the daily keys.