Identifying the FLD codes
We can try to identify the cryptosystems used in the FLD radio network by looking at Huettehnain’s statements and various TICOM documents.
According to Army cryptanalyst dr Buggisch (7) the designations F90 and F110 referred to French Army ciphers, read during the period 1939-1940:
‘F90 and F110 were German designations for French Army cipher systems before and during the campaign in FRANCE. Both were based on a four figure code, in one case the recipher consisted of a periodic adder [or subtractor] of length 11; in the other it was ordinary transposition, the transposition key being obtained from a key word which itself was taken from the code and shown by an indicator group. Both systems were being read from the winter of 39/40 to the end of the French campaign. Solution was by methods generally known in cryptanalytic circles. One of the codes turned, up again for a short period in De Gaullist traffic.’
Note that Buggisch’s description of the systems is similar to Huettenhain’s from ‘Einzeldarstellungen aus dem Gebiet der Kryptologie‘.
In the TICOM collection of the German Foreign Ministry’s Political Archive there are documents that have more information on the ciphers F90 and F110:
1). The TICOM documents T3611 and T3612 (8) have information on cipher F90, however document T3611 is not available due to deterioration.
According to document T3612 the cipher F90 was a 4-figure code enciphered with short additive sequences of 5, 7 or 11 digit length. It seems that the codebook consisted of 20 pages, each with 100 entries, totaling 2.000 code groups.
This system was used by the network FLD (Paris) in communications with stations fla, flb, flc, flf, flg, fak, fam, flq and others and it was solved thanks to a major cipher clerk error committed in September 1937.
It seems that the same message was sent twice, first without encipherment (so only the code groups were transmitted) and then with additive encipherment. Clearly this gave the German codebreakers an opportunity to identify the basic code groups and then solve the additive sequence used for encipherment. This success allowed them to correct their own relative code findings (from previous decipherments) into the actual French code values.
The information in TICOM document T3612 matches Huettenhain’s statements about a high level code enciphered with short additive sequences being solved completely in the years prior to WWII.
2). The TICOM document T3684 (9) describes system F110 (F4ZCW110 - French 4-figure code with simple transposition) and it says than from February 1938 the radio network of the French 14th Army, with stations in Lyon, Grenoble, Modane, Briancon, Chambery, Jausiers and Beurg-Saint-Maurice, started using this transposed code. The indicator was 55555 and the transposition key was created from the plain meaning of one of the codegroups. The example given in the report was:
p e r m i s s i o n o n n a i r e
13 2 14 7 4 16 17 5 11 8 12 9 10 1 6 15 3
The details in the report match Huettenhain’s statements about a French military district adjacent to Italy using a transposed code, with the transposition keys being created from the codegroups of the codebook and the first breakthrough coming in 1938.
The successors to systems F90 and F110
From the available TICOM documents it seems that in September 1939 both systems were changed. Cipher F90 was replaced by a new 3-figure code plus additive, while cipher F110’s successor used the same underlying code but with a new encipherment procedure.
1). TICOM document T3661 (10) contains a report by the cryptanalyst dr Ludwig Föppl, dated 18 December 1939. In the report Föppl says that the code F90, which was used in the military command radio network FLD (Paris), was changed in September and replaced by a new system.
The new system was a 3-figure code enciphered by additive sequences. It seems the encipherment consisted of a 20-digit number that was composed of two 10-digits parts. A peculiarity of the encipherment was that each 10-digit number was composed of all the ten digits from 0 to 9 used only once (11). This system was solved and it seems that the German designation for it was F135.
2). In the notes of dr Huettenhain there is a report from November 1939 that describes the solution of the successor to system F110 (12):
‘Report on the attachment to Army Group C evaluation section
On 2 September 1939 the French Army Code F110 was replaced by a new code so that traffic could no longer be broken currently.
On 3 September 1939 I was seconded to FRANKFURT-ON-MAIN in order to take part in the task of breaking this new code. The task was accomplished at the beginning of October so that all the September material could be read retrospectively.
This success was made possible in such a short time by the fact that
1) the necessary data (Code etc) was obtained by months of work in peace time, chiefly by Herrn TRAPPE (Chi OKW) and SCHMIDT (Chi OKW)
2) a close co-operation between the above named gentlemen and me could be established.
It was therefore, still possible in October to work on the October material with success. In addition to the above named gentlemen Herr Professor Dr. FOPPL was of great assistance in the solution of this system.
As the system was not changed on 1 November 1939 this code could be read currently again from the date when the October key was broken. On 3 November 1939 at the finish of my attachment in FRANKFURT-0N-MAIN I was sent to BERLIN.’
From Huettenhain’s report it seems that the underlying code remained the same (as in system F110) but the encipherment procedure was changed. By having the code the German codebreakers only needed to attack the encipherment and this was quickly achieved according to Huettenhain.