How the Germans broke US Code B7 – Part One

well_chuffed, Going Postal

The TICOM teams brought back lots of documents from Germany , one of them described how the Germans broke the US letter code “B7”. This is a précis of that document.

Firstly , a description of the type of code it was , followed by Dr Hans-Kurt Müller’s description of how they figured it out. There were two papers , the first dealt with how they removed the encipherment from the code , the second , how they deduced what the code meant. This deals with the second part which is probably just as well , the first is probably mathematical which rules me out , the second is logic and I can follow it ; if I can , so can the rest of you.

The process is called bookbreaking whereby you figure out one bit which leads to another bit and so. It all looks a bit too easy to me , there may have been other clues which are not forthcoming.

A book code has a code which corresponds to a word or phrase. Here is a small part of a one part or alphabetical code.

ABABD A
ABACF Abaft
ABAHK Abandon
ABAJL ………….. it
ABALN Abandoned
ABAWZ ……………… by

ZYZYZ Zero

Such a code serves for encoding and decoding. An alphabetically ordered book code has weaknesses , the alternative is …

The two part or randomised code where the code is not in sequence and consequently , you need an encoding list and a decoding listen

Encoding Section
GAJVY A
TOGTY Abaft
FEHIL Abandon
BAYLT ………….. it
ZYZYZ Abandoned
NYSYZ ……………… by

ZYZYZ Zero

Decoding Section
ABABD Obstructed
ABACF Term
ABAHK Zero
ABAJL if it has not
ABALN To be sent by
ABAWZ Acceding

ZYZYZ Abandoned

With the two part code , discovery of the meaning of one the codes does not help with any adjacent codes.

The materials used by the Germans were basically the same as those used by other bookbreakers then and now , although data processing equipment has replaced the highly tedious copying processes of the Germans in 1940. The modern bookbreaker uses the following materials

1 The Original message
2 A Message Print (called later on Material I and II) a copy of the original messages with encipherment removed and reduced to its basic code groups.
3 An Index – an organised listing of all occurrences of every code group showing the message it occurred in , its position in that message and several code groups preceding and following each occurrence.
4 An Inverse Frequency List – a list of the code groups in order of frequency of use starting with the most frequently used group.
5 A Lane Log – called the “circuit catalog” later where its usefulness is well illustrated.

As code meanings are identified , a decode and encode are built up as further working tools.

The methods used by the German bookbreakers are like those of backbreakers everywhere. Every possible clue is used : frequency of a code group , its position in the message , its relationship to other groups , repetitions , patterns in beginnings and endings , plaintext preambles , known codes used on the same circuit or , as here , in the same message. There are always weaknesses and it is the purpose of the bookbreaker to find and exploit those weaknesses to establish his bridgehead.

The most effective method , unsporting though it may be , is discovery of a crib . the same text sent in plain text or in a known system. Apart from this , perhaps the most useful single aid is acquaintance with the past cryptologic practice of the senders. This is well illustrated in this case history which follows in Part 2.
 

© well_chuffed 2018
 

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