enigma security measures

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Military and intelligence history mostly dealing with WorldWar II.

Christos military andintelligence corner

Sunday, August 5, 2012

Enigma security measures

The plugboard Enigmawas used by the German armed forces prior and during WWII. It

was a modification of the commercial version. Its plugboard gave it a huge security boost

compared to the standard version.

The Germans used the Enigma extensively and they were always worried about the

security of their main cipher system. Their cryptologic security departments, scattered

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across their cryptologic agencies, researched ways to break the Enigma and based on

their observations changes were made in operating procedures.

Here I will only look into the main security measures they took prior and during the war.

From changes in keying and indicator procedures to the mechanical modifications.

Almost all the information comes from The H istory of H ut 6 vol1 which is a British

postwar report declassified in 2006. Of course the best source would be a German report

detailing their security measures and the reasoning behind them but I dont know if such

a document existed or if it survived the war.

The plugboard Enigma and the German armed forces

In 1934 the Army and Navy agreed to use the plugboard Enigma as their main cipher

system. In 1935 the Airforce followed. At the start of WWII Germany was the only

country in the world to use a cipher machine for all its mid and high level traffic.

During WWII tens of thousands of Enigma machines were used by the Germans. It

seems that no one knows for sure how many Enigma machines were built. A good

estimate seems to be that more than 40.000 were constructed.

Enigma rotors and key settings

Initially only three rotors were used (I,II,III ). In December 1938 rotors IV and V were

introduced. These were used by all three services throughout the war. All the standard

rotors had one notch.

The Navy introduced three more rotors for its own networks in the period 1938-39. These

had two notches.

Up to the end of 1935 the key (wheel order and stecker) changed every three months.

From January 1936 every month and from October 1936 every day.

Kenngruppen

In order for the receiving party to identify the specific key used it was necessary to send

a five letter group called Buchstabenkenngruppe (letter identification group) together with

the message. The letter identification group was composed of one of the available 3-letter

kenngruppen plus two random letters in order to create a 5-letter group (Enigma traffic

was sent in 5-letter groups).

The navy used a different system. The kenngruppen was taken from a book(Kenngruppenbuch) and then enciphered with a bigram substitution table, before being

sent with the message.

Indicator procedures

The Enigma was used in the following way. First the machine was set up according to

the daily key which specified the wheelorder, ring settings and plugboard settings. Then

the cipher clerk chose a different starting position of the wheels for each message.

In order to communicate this position to the receiving party he had to first encipher it. The

starting position was initially enciphered at a standard setting called Grundstellung. Up to

1940 the practice was to encipher the indicator twice (called throw-on by BP).

For example if the Grund setting is AAK and we enciphered our message with the wheels

at OFW then we encode OFW at position AAK and get VNZ, then press OFW one moretime and get BLD . The indicator at the beginning of the message will be VNZ BLD. The

receiving party will set their machine at AAK and enter VNZBLD which will give them

OFWOFW. They will then set the rotors at OFW and proceed to decipher the message.

From September 1938 the Grund setting was dropped and the cipher clerk chose a

random starting position for enciphering the message and enciphered it at another random

position. In our example it would mean that instead of AAK the cipher clerk chooses

three random letters say TGM and enciphers OFW at that setting thus giving XYU and

again HLS. The indicator with the new system will be TGM XYU HLS.

In May 1940 the double encipherment of the indicator was dropped.

The Navy initially followed the Army-Airforce procedure of random indicator selection.

However this changed in May 1937 when they started to use an indicator book

(kennbuch) to select the indicator which was then enciphered on the Grundstellung and

the output was used as the message key. This was then communicated to the other

party after first enciphering it with a substitution table . Both Kennbuch and substitution

tables were changed several times during the war.

This procedure avoided operator mistakes and non random indicators. However Naval

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keys continued to use a Grund setting throughout the war. For some reason some Naval

keys continued to use repeating indicators after 1940.

In April 1945 the Navy introduced a change in their procedure by having 228 Grund

settings valid for one month instead of a different one for each day.

Plugboard connections

Initially 6-8 stecker connections were used. From January 1940 10 were used and this

remained standard procedure till the end of the war. A few keys, for whatever reason,

continued to use fewer steckers.

General, Officers and Staff keys

Naval keys had three settings: General, Officers, Staff. The General key (wheelorder,

ring settings, stecker) was not changed every day but was paired for two consecutive

days. That meant that the second day had the same wheelorder and ring settings but

differed in the stecker.

The Officers messages were enciphered first on the Officers key that had identical

settings with the General key but differed in the stecker. Then it was enciphered once

more on the General. Officers settings were changed every 10 days. The Officers key

was much harder to break than the General key.

Staff keys had their own settings (wheelorder, ring settings, stecker) and were

enciphered once more on the General key. With the technology of that time they were

more or less unbreakable.

Army keys changed every day. C ertain types of Army keys (Armee, Heeres, Wehrmacht)

had several versions. The General M/S (Maschinenschlssel) key for Geheim (secret)

traffic, the Stab M/S key for Geheimkommandosache (top secret) and OKH or Officers

for the highest level messages.

Airforce keys also changed every day. There was a general administrative key Red that

was used by all units on all fronts, a high level key Pink plus of course different keys for

different geographic areas and different levels of command.

Overview of security measures

Proliferation of keys

In 1942 all three services introduced more keys to cover different geographic areas

(Eastern front, Med, West etc) and operational commands (Fliegerkorps, Luftflotte, etc)

The army broke its Eastern key into several. The Luftwaffe introduced separate keys for

higher levels of command and the Navy introduced a separate key for the Atlantic U-

boats.

From then on the number of keys continued to increase.

Effects on breaking:

More keys meant more work for BP or more realistically that many keys were not

attacked at all. On the other hand traffic between different networks often led to

reencodements as the same message passed through different keys. This allowed BP touse reencodements from easy keys (like Red) in order to break into harder ones.

Also it seems that the need to construct more keys led the German cipher offic ers in

charge of constructing Enigma keys to take a shortcut

Parts of older keys were reused in order to create new ones. For example the wheel order

from 22 May 1941 would be coupled with the stecker settings for Ferbruary 2nd1939 in

order to produce a new key.

This German trick mostly occurred in 1942 and in Luftwaffe networks. The Brits called it

Parkerismus after the person who had discovered it.

Enigma Uhr

In 1944 the Luftwaffe introduced, on some nets, the Uhr device which changed the

stecker combinations automatically by turning a knob.

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One major effect of the device was that it got rid of the reciprocal encipherment of the

plugboard (although the reciprocity of the Enigma as a whole was not affected).

A description of the Uhr device is given in TICOM report I -20Interrogation ofSonderfuehrer Dr. Fricke of the Signal Intelligence agency of the Supreme command

Armed, Forces (OKW/Chi), p3

(a) Stecker Uhr. This was a small device to change the plugging. It gave 40 variations.They knew that the strength of the machine lay in the stecker and therefore wished to

divide the traffic load per stecker by 40. The machine was used only by the Luftwaffe,

which had only 1.000 or so machines for higher echelons.

Operational use:

It was first used in July 1944. Originally it was used on two keys, later it was extended to

cover 15 Luftwaffe keys.

Some keys used Uhr and UKW-D.


The History of Hut 6 says Uhr alone had on the whole little influence on our breaking of

keys though it did mean that there was sometimes a lot of technical work to be done

after the basic key was broken. Serious complications arose only when it was necessary

to break a key on an Uhr message or when Uhr was combined with D .

Umkehrwalze D

UKW-Dwas a new rewirable reflector used on the military Enigma. The people of

Bletchley Park called it Uncle D.
http://2.bp.blogspot.com/-JKseAAFbVeg/UB9mTkgbOTI/AAAAAAAAAVY/fal0YexB-Qk/s1600/UKW-D-a1.jpghttp://www.cryptomuseum.com/crypto/enigma/ukwd/index.htmhttp://2.bp.blogspot.com/-r40LwxIGupI/UB9mL1DfjsI/AAAAAAAAAVQ/47J7PTiDhx4/s1600/uhr-a1.jpg


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A w eek before it was introduced BP intercepted messages mentioning it. As the offic ial

history puts it the stoutest hearts might have quailed in those last days of 1943 had it

been possible for us to realize the hydraheadead nature of our veiled antagonist.

The reflector D was used principally by the Luftwaffe and in 2 or 3 army keys. A navalversion was built but not used.

Operational use:

From January 1944 the Red key started to use reflector D for some of the traffic. The

wiring was changed every 10 days. From August 44 UKW-D was greatly expanded and

some keys used it exclusively.

Western Europe

August 1st1944 marked the first use of reflector D on Luftwaffe keys used by units in

Germany. According to the History of Hut 6 this radically affected their exploitation.

During the second half of 44 the German air keys went over to reflector D.

The German armys Home Administration (Wehrkreis) network Greenshank used reflector

D since 1943 (date of introduction not known but according to the BP history probably in

January 1943). This key used a different UKW-D wiring each day. Greenshank proved to

be BPs nemesis as it resisted attack for years. The final score shows only 13 days read

during the war. There were two more Wehrkreis keys identified by BP that used reflector

D and thus proved secure.

Italian front

In November 1944 Puma (army-airforce liaison key Italy) changed over to UKW-D and

the period of regular sustained breaking was at an end. Puma also used the Uhr device.

Leopard (Luftwaffe-Italy) used UKW-D in 1945.

Eastern front

From September 1944 reflector D was used in some Luftwaffe networks. By January

1945 UKW-D use was extensive.


Several D keys were solved because the Luftwaffe often used reflectors B and D on the

same key (called nearly D by BP). New cryptanalytic equipment had to be developed

and produced. These were the American Duenna and Autoscritcher and the British Giant.

These devices were introduced in late 1944.

Even this effort did not prevent a drop in the success rate. For example the Puma keywent from 100% to 35% read.

Uncle D proved to be Bletchley Parks toughest opponent.

Lckenfllerwalze
http://3.bp.blogspot.com/-zQA8-nFi3qs/UB9mWVax0cI/AAAAAAAAAVg/vsGzQtPxP18/s1600/UKW-D-a5.jpg


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The Germans understood that one of the greatest weaknesses of the Enigma was the

uniform movement of the rotors. In order to counter this they built a new rotor that had 26

notches that could be set in active or inactive position.

Details about the Lckenfllerwalzeare given in TICOM I-20 Interrogation of

Sonderfuehrer Dr. Fricke of the Signal Intelligence agency of the Supreme command

Armed, Forces (OKW/Chi), p4

(b) Because of the uniform motion of the enigma, they considered that if messages of

600 or 700 letters were sent, they could be broken. If the instructions on maximum

message length were followed, they knew everything would be all right, but they felt sure

that their instructions were not followed. So they developed a new wheel with 26 notches

which could be filled in as desired. These were called Lckenfllerwalze. They wished to

avoid certain numbers of notches per wheel, and particularly consecutive notches, for

with the latter it was difficult to predict the cycle except in special cases. Consequently

they ordered that wheels should be used with one, five, seven, or nine notches only andnever with consecutive notches. Some of those wheels were actually built in Berlin by

HEIMSOETH & RINCKE, who built the enigma. They were to be produced in numbers

by this firm and by Siemens Halske, and were expected to be ready on 1 May 1945.

They were not ready, however.

Operational use:

It was not introduced during the war. It would have probably defeated BP as it attacked

the greatest weakness of the Enigma, the uniform movement of the rotors.

Enigma M4

The 4-rotor M4 machinewas known to have been distributed in the Home Waters area in

1941. The Atlantic U-boats got the M4 in February 1942. Other commands took longer tointroduce it .
http://www.cryptomuseum.com/crypto/enigma/m4/index.htmhttp://www.cryptomuseum.com/crypto/enigma/lf/index.htmhttp://1.bp.blogspot.com/-SS9QfwW6-Xw/UB9mmnp4X7I/AAAAAAAAAVw/WgIbGPaoIGw/s1600/Lf-a6.jpghttp://2.bp.blogspot.com/-_RfeqQRUMZ0/UB9meCjCQwI/AAAAAAAAAVo/3Rb3441eI2s/s1600/lf-a1.jpg


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The M4 was an order of magnitude more secure than the 3-rotor version.

In order to solve the M4 keys a new 4-rotor bombe was needed.

Due to constant delays in the production of a British 4-rotor bombe the Americans

decided to build their own version and this was operational in September 1943. By late

1943 95 bombes were used and in 1944 160.

Thanks to the new bombe the U-boat key was broken usually within 24-48 hours.

However during the period February 1942 to September 1943 solving the U-boat key with

existing methods was a huge problem.

Keys of naval commands that used the M4 with repeating indicators were broken

without the need for a 4-rotor bombe.

Important M4 keys like those used by surface ships (Aegir, Neptun) resisted BP attack.

Sonderschlssel

In the summer of 1944 U-boats started carrying individual Enigma keys.

Operational use:

They were first used in November 1944 and by February 1945 they carried practically all

the operational traffic of U-boat Command.


They were practically unbreakable. As the Cryptographic history of work on the German

Naval Enigma puts it: The "Sonders" are about the only type of key which make one

feel the need for a statistical attack on Enigma instead of the normal method of cribbing.

Notschluessel

Notschluessel were emergency keys used when there was no way to issue new keys to

a specific unit or when compromise was suspected. A new Enigma key was generated

from a keyword (schlsselwort) and a discriminant (kenngruppe) from another word

(kennwort).

Operational use:

The Luftwaffe first used NOT keys in August 1944.


NOTs were broken through knowledge of the cipher instructions, through reencodements

and cryptanalytically.

The History of Hut 6 says Those systems were only intended by the Germans for use
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in emergency. However, NOT-keys have actually certain advantages over keys made up

in the normal way owing to their freedom from rules of keys which may help the enemy

cryptographer. They would have however, the fatal objection for regular use that if the

key is generated from one word the number of possible keys is limited so drastically that

some kind of key index becomes possible - for instance, on the second system the

number of keys is determined by the number of German words at least 12 letters long -

which must surely be much less than 100,000.

Random indicators

After abolishing the Grundstellung system the Army and Airforce allowed the Enigma

operator to choose both a random message key and a random setting with which toencipher it.

However the keys chosen by operators were not always random. This effect (called

cillies) was used by BP on its attack on the Enigma traffic.

The Germans introduced a new procedure to counter this. This procedure was as follows:

1). The cipher clerk had to choose a random text either from a book, a poem, a song etc

2). Set his Enigma at wheel order I, II, III, ring settings 01, 01, 01 and connect 10

steckers at random.

3). Choose a random Grund setting.

4). Enter the text in the Enigma and write down the output, then divide it into 6 letter

groups

Each 6 letter group comprises a Grund setting and message key. The operator then used

the new keys by setting the Enigma at the first three letters and enciphering (on the daily

key) the last three letters. The output would be used as a message key.

Operational use:

Hard to establish by Bletchley Park as there was no external indication of this new

procedure (apart from a fall in cillies). It may have been used extensively by Army and

SS keys.

According to t he report nderungen beim Schlsseln mit Maschinenschlssel the new

indicator procedure was to become effective in August 44.
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As the offic ial history puts it : It is impossible to interpret "Random Indicators" as

anything but an anti-cilli device - a far more radical one than CY. It does indeed kill cillies

and it is clear that the Germans had at last become conscious of this possible danger.The answer they now found to the danger of cillies was as effective as anything that

could have been devised - short of a complete change of the indicating system - and it

did lose us Orange. The only possible criticism we can make of the German action is that

(as so often) it was too late: cillies were dying when they were killed. The history of Hut

6 would have been different had the Germans in the full flush of their 1940 triumphs been

able to spare a thought for the suppression of cillies.

Wahlworts (nonsense words)

Instead of stereotyped beginnings random words were inserted at the beginning and end

of the message as an anti-crib device. Wahlworts were from 4-14 letters long.

Operational use:

This was ordered in the N.Africa army keys in December 1942. Nonsense words were

used extensively by N.African, Balkan and Eastern keys but not to such a degree in the

West.

Luftwaffe units inside Germany also used wahlworts.
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Cribing on addresses was no longer possible as the official history admits breaking on

straight addresses was now out of the question. The general effect on N.African keys

was limited since at that time the Brits relied on reencodements.

Against Luftwaffe keys the problem was more serious but again reencodements w ere

used.

As the offic ial history puts it It cannot be denied that in wahlworts the Germans hit on a

simple and effective method of making cribbing more difficult. It would have been still

more effective but for the eternal German blunder of "too little and too late", introduced in

1940 on a wholesale scale, wahlworts might have knocked out the infant Crib Room

before it had got properly on its feet: but in fact the Germans did not use the system at all

till halfway through the war and not until the last few months used it on anything

approaching a universal scale.

Mosse code

The Mosse code was a commercial code (5-letter) adopted with modifications by the

Luftwaffe.

Operational use:

Used by the Luftwaffe since early 1944.


It became a problem as an anti-crib device in 1945. When long addresses were changed

into a 5-letter code this stopped cribbing. However in cases were several regularly

occurring phrases were replaced by a codeword this provided a useful crib.

Overall the Brits rated it as a sound security measure.

Double encoding

On the Raven and Gadfly keys some messages were enciphered twice. First with the

standard procedure and then once again by using the settings of the Enigma at the end of

the message without resetting the rotors.


As the BP history says Double Encoding was used on too small a scale to haw any

effect worth mentioning on Hut 6 breaking. On the scale on which it was used it must

have been no less a nuisance to the German cipher clerks than it was to Hut 6.

Burying

Stereotyped beginnings were a security risk. In order to counter this burying was used.

Burying meant that the cipher clerk inserted the beginning and end of the message in the

middle of the text before encoding it.

Operational use:

This precaution was introduced in the Army in December 1942.


It was an effective anti-crib method.

CY procedure

The movement of the standard Enigma rotors was predictable due to their having only

one notch. The fast rotor moved with every character, the middle rotor moved once every

26 key depressions and the slow rotor (the left one) moved only once every 676 key

strokes (26x26). Since messages were limited to 250 characters this meant that the slow

rotor was stationary during encodement. In order to counter this effect the Germans hadthe cipher clerk move the rotor by hand in the middle of the message. After 70 to 130

letters the clerk chose a random letter and moved the slow rotor by hand to that position

(the new position must be at least 5 stages removed from the original one). Then he

wrote in the message CY followed by the letter that represented the new slow rotor

position and the letter following it in the alphabet. For example if he chose J then he

would type CYJK and continue the rest of the message with the slow rotor in the new


11/14

position J.

Operational use:

Introduced in September 1944 in some Luftwaffe keys. By October used on all Army

keys and the SS ones.

The report nderungen beim Schlsseln mit Maschinenschlssel says that the new

procedure for resetting wheels within messages would become effective on 15

September 44.


CY stopped cillying and affected cribs and reencodements. On the other hand it

occasionally provided a shortcut to the ring setting.

It did not stop BP success but it was a serious effort to counter the predictability of the

Enigma rotor movement.

Zusatz stecker

In May 1944 the Luftwaffe introduced a change in the plugboard connections every 8

hours. The Germans did not introduce three different stecker keys. Only a few of the

connections were changed each time.

This procedure created many problems and it was discontinued in mid June.
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Practically none. The official history calls it the most silly and trivial of their security

devices

Wheel-order permutations

From October 1941 Luftwaffe keys reversed their wheel-order at 12 noon.

The Army used a similar method from July 1942. Every 8 hours the rotors were moved

forward one step. For example if the daily key specified rotors I,IV and III in that order

then after 8 hours it would be III,I,IV and finally IV,III,I.

From September 1942 the Luftwaffe adopted the three wheel-order system.

Some Army keys (like Greenshank) used 6 permutations.


This was an effective anti-depth measure but the Brits did not use depths in any large

degree so they were not affected on this front. However the unintended consequence

was that Banburismus was no longer practical.

Conclusion

Despite all the talk about the Germans having complete faith in the security of the

Enigma the reality is that they introduced many new procedures in order to improve its

security.

New rotors were introduced prewar.

The indicator procedure was changed by dropping the Grundstellung and stopping the

double encipherment of the message key.

Wheel permutations were used to counter depths.

Random indicators were used as an anti-cilli device.

Nonsense words and burying were used against cribbing.

The uniform movement of the wheels was countered with the CY procedure.

Traffic was split into many different keys.

Even mechanical modifications were employed to upgrade the Enigma like the Uhr

device, the UKW-D reflector and the 4-rotor M4.

These measures show that the Germans understood the main problems of the Enigma.

Had they been taken together and on all networks they could have defeated Bletchley

Park.

There were two main reasons why the Germans did not defeat BP through their security

measures.

The first one was that due to the huge number of Enigma machines in use it was not

logistically possible to take simple measures such as rewire the rotors or introduce new

ones.

The other problem was the fragmentation of their cryptologic security departments. The

Germans had 6 main cryptologic agencies.

Out of these 4 were military (Army, Navy, Airforce and OKW/Chi). Each had a crypto

security department. It was the job of that department to prepare codes and ciphers for

the service and make sure that these were secure. There were good cryptanalysts in

these departments and thanks to their efforts the security of the Enigma was upgraded

during the war. However they did not have close contact with their counterparts in other

agencies and so could not share on their research and findings.

This led to each service having a different standard of security. For example the Navy

continued to use the Grund setting even after the Army and Airforce had abandoned itand the same thing happened with repeating indicators. The Uhr device was used only by

the Luftwaffe, the UKW-D on the same keys as the reflector B and so on and so forth

It should also be mentioned that the security departments were hampered by the

resistance of the services to changes in operational procedures. Their reasoning was that

the fighting troops should not be overloaded with new codes and procedures that were of
http://en.wikipedia.org/wiki/Banburismus


13/14

questionable value.

A unified cryptologic security department would have ensured that all three services

followed the same procedures and instituted security changes at the same time.

Still that does not necessarily mean that a centralized department would have defeated

BP. After all BPs centralization did not save British high level codes in the period 1939-

43.

Timelines

Year

Month Measure

1936 January wheel order and stecker changed every month

October wheel order and stecker changed every day

1937 May N avy int roduces Kennbuch-bigram system

1938 Naval rotors VI and VII introduced

September New army -airforce indicator procedure

December Rotors IV and V introduced for Army-Airforce

1939 Naval rotor VIII introduced

1940 May Double encipherment of the indicator stopped

1941 October wheel order reversed by midday on Luftwaffe keys

1942 New keys for all three services

February Enigma M4 used by Atlantic U-boats

July 3 daily wheel order changes

December nonsense words and burying introduced

1943 UKW-D on Greenshank

September Army drops use of Kenngruppen

November Luftwaffe drops use of Kenngruppen

1944 Random indicators used by army and SS

Mosse code used by the Luftwaffe

January UKW-D introduced on RED key

May 3 dai ly stecker changes

July Uhr device

August Notschluessel

September CY procedure

November Sonderschlssel used by U-boats

1945 April new indicator procedure by N avy


14/14

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Posted by Christos T. at 11:46 PM

Labels: Bletchley Park vs Berlin, Enigma, German codebreakers

Sources: HW 43/70 The History of Hut 6, Volume I, British intelligence in the Second

World War vol2 and vol3 part2, Decrypted secrets: methods and maxims of cryptology,

HW 25/1 Cryptographic history of work on German Naval ENIGMA by C H O'D

Alexander, HW 25/2 The History of Hut 8 by A P MahonCryptologia article: Enigma

Message Procedures Used by the Heer, Luftwaffe and Kriegsmarine, Cryptologia article:

ENIGMA-UHR, cryptomuseum(for the pics), Wikipedia, European Axis Signal

Intelligence in World War II vol2, TICOM I-20 Interrogation of Sonderfuehrer Dr. Fricke of

the Signal Intelligence agency of the Supreme command Armed, Forces (OKW/Chi)

Acknowledgments: I have to thank Frode Weierud and Ralph Erskine for answering my

questions on aspects of the Enigma history.

For those of you who want to actually use the Enigma check this simulator

Recommend this on Google

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Anonymous 8/24/13, 9:15 AM

Brilliant!

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