3 

4 

REMARKS 

FROM NAME OR mu 

NECESSARY 

ACTION 

ND'TEAND 

REl'URN 

SEE ME 

SIGNATURE 

~ -. 

ursuant to E .0. 1352 
• - ,._'a 

THE PNEXJMATIC AUTfmN'l'ICATOR OR DEM 498 

Beterence - lo Tentative Securit7 Evalvation of APSAM Dl+9S Bruea 2/,7 

2. The Securit7 ot the Pneuma.t.ic Authenticator (DJ!X A98) Brus& 616 

3o .Pneumatic Authenticator Brusa 167 

The description ot the Pnelmatic Authenticator as planned. can be found in the 

references given above. 

The authenticator aa set up nov has two features wbich place certain limitations 

on the possible va.riabilit7 ot the dnice o One ia the use ot a constant i."lput point 

and the other is the use or 5 groupings ot ' in the reveraero For e.xample suppose 

each of the 5 groupings ot 3 in the rneraer is traced to the input end-plate. OnI.r 

one of the trigraphs obtained mulcl contain the constant input point. This means 

that tor each alignment ~ one out ot 5 ot the trigrapb1c groupings in the 

'1"898rser is used. With a variable inpqt point 8111' one of the 5 groupings could 

be used. Thia means that onq 'l/5 ot the potent.iallt;r ot the device is being used. 

In the above references it las been shown tlat with plannecl int.arrogations 

certain t11J9s of solutions can be found. The poasib1e eolutions mentioned made 

use ot a constant input point aJJd the groupings ot 3 in the reverser. It was 

aasumad that the en-. would be able to obtain replies tor challenges in. which 

all the paired rotors stand except one designated pair. In one solution it was 

shown how it might be possible to recowr the settings ot the pa1Nd rotors 

Hi - 86 o The settings ot a, am l:J.o could be reccnered just as ••il1' using 

the same t1J)e of planned interrogation. The output ftl.ues tram a, would be a 

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monoalphabetic substitution ot the input values into 81.o• That is the output 

tram a

5 bas the same sattern as the input into Rio· The constant input point 

and tha groupings of 3 in the rnerser can be used effecti~ to reccner nch 

a pattem. In some instances it "snowl:alla• rather e.asil7 and in others it is 

quite a laborous task. 

Suppose we have given the following replies obta1Ded from consecutive 

settings of RS - a

10 (the settings of the other rotors reoain constant). 

KCEFKOCl'BJGHOIC 

GONOGIDLGGICIPFD 

One could asaWll8 the input posit.ions into a

5 ot the 3 moat frequent lsttera, 

one ot which would be the constant input point A and t.be other two could be G and 

P. The initial setting of a

5 would be ass1.1med. 'Dae input. '9al.ues tor the 3 letters 

would be miciphered through a, at the proper :relative settings ot S,· A partial 

patterr1 ot the outpu.t 'Values from 1

5 is thus obtained. The initial setting of R:J.o 

· is aasumad am trigraphs from the reverser are deciphered at the proper rel.ati ve 

settings of ~O so as to obt.ain a part.ial pattem which agrees 'Id.th the part.ial 

output pattem trom 1

5

.. Eacb partial pattern suggests-values tor the al;her ao 

that when all correct e~ssumptions are mde the pattema build. up ver;r quickl.J'. 

Contradictions show up 1n most instances with incorrect assumptions. In some 

instances it there are not enough repeats ,_n the first partial output pattern 

from a

5 so as to eliminate aom ot the posflible aa~iona troa the trigraphs 

in the :reverser. it requires several trials to recover the patterns by hand. With 

repeats, contradictiou show up quickJ.7 and eliminate aama of the possible trials. 

(The setting iden~itied tor a

5 could onJ¥ be a relative setting). 

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.,, ~ 

ssem 

'!'bare are 15 x 14 x l3 • Z130 possible combinations tor the three input 

positions of A, G and Finto a

5

• Altogether there are 15

1

Xx1t ! }' • 455 

different trigraphs. The 1.55 trigraphs can be listed according to their cyclic 

interval patterns. There would be 15 tr:l.graphs tor •ch pattern.. 'l'o illustrate -

Interval pattem 1-1-1;3 1-2-12 1-6-8 

A B c A B D A B R 

B c D B c E B c I 

c D B c D , c D J 

D E , D I G D B I 

etc .. etc. etc. 

There are altogether 31 difterent interval. patterns. 

Instead ot considering :!730 possible asBUIDptions tor the three input 

positions into a, tor A, G am F, aasunm the use of one of the 31 interval patterns. 

Suppose the interval rattem 2 - 2 - ll is assumed. The three input positions , 

would then be A ·- C - D.. There would be S va19 -:Jf selecting the inpit position 

ot the constant input A and 2 tor 0 am F. There muld be 6 orders to be considered. --- --------------------------- --------- -- - --------- -

Altogether there would. be 6 x ..31 • 186 assumpt.ions tor the thr• input positions 

~~ead ot Z/30a 

SupPose there were 31 people who were assigned to recover the setting 

(or relative setting) ot a

5 and the actual setting of ~o· Each individual 

tmuld have to assume the order ot t.tie input positions and the settings of RS and 

R:w· There would be a total of 6 x 15 z. 15 • 1350 possible assumptions tor each 

person.. In most instarices if there are two letters ot which each appears 5 or 

more t.imes in the replies tbe patterns bu1ld up more easiq am the number ot 

assumptions in inost instances are reduced considerabl.J'. 'lbis solution by no means 

co.mpl"Or.d.ses the device but might lead to other solutions which could COlllprOJllise the 

device. 

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Wiring or the reverser -

If' tw of the trigraphs in the reverser have the same }Bttemed interval 

and the rotors are moved as e block against the rneraer it would be possible 

to produce a 3 - way lobster ettect. For example suppose the two groupings 

C J N and D H 0 are used in the rnerser. The c;rclic interval pattems are 

1 - Z. - 4 and 4 - 7 - Z.. respectively i.e. they ha.Ye the sam c70lic pattern. 

tis means that an enclpherment of' the constant input point which comes into 

the reverser back through the a.se bJ' W8.J' ot C J N at setting s1, s2, s

3

, S~, s5 • 

A second. encipherment ot the constant. input point at settinc (s1+5),,(s2+5),(s3+5), 

(s4+5),(S5+5) would go thro~ the mze and the rnerser b7 WB1' of D Ho. The 

lobster ef'tect could b3 recognized in the t.:rigrapha o'Qtained from the two letter 

replies and the constant input point. If' the two replies K II and C Pare 

obtained trom. the two cballenges the lobster ef'tect is detected by the tact 

that C F A is f'ive positions trca M A K as - (!! N 0 A B £) ;(! B C D E !) ; 

(JiLMNO!) 

'ftle lobster ef'f'ect. camot be eliminat'3d complet~ by careful wiribg ot the 

reverser. It is true there are S patterned intervals Which haft no repeats aa 

1 - 1 - 13, 2 - 2 - 1111 3 - 3 - 9; 4 - 4 .. 1 and S - 5 - 5. But it is impossible 

to wire a reverser using all lift JBttemed intervals in the same :reverser. 

Coneequentl.T all reversers haft to include repeats in at least one of the . int.ervals for SODl9 of' the reverser tr1grap~1a. This means that two-way lobster 

effects could be produaed at the proper otf'eet tor the trigraphs which haft the 

repeats. It the offset tor t1IR> challenges (plus the constant input point) agrees 

with the otrset. tor two trigraphs which mve repeat intervals then it would be 

possible to associate the replies with the rewraer tr.I.graphs that were used in 

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each enciphe:rmant. an aample ot 5 reverser trigra.phs aid the patterned intervals 

are 

1-AEJ -4-5-6 

2-BDIC -2-7-6 

3-CLN -9-2-4 

4-FIO -3-6-6 

5-GHK -1-5-9 

A two WB.J' lobster would ahow up it trigraphs 1 and 2 are used at an ottaet ot one 

(A ia offset one rrom B, and J one 1'rom K) • 

stationary rotors interspersed among the stepping rotors could e:ind.nate the 

possible occurrence ot a lobster during a day's traffic. Uthe atational'J' rotors Co\(., 

placed onl7 nut to the rneraer it is possible to incrase the occurrences ot the 

lobster effect. This is because the combin.ed ettect of the reverser and stationa1"7 

rotors could produce two or more groupings of three with the same cyclic interval 

pa.ttem .. 

To avoid the carey oTer of the lobst•Ut effect from one da7's traffic to the 

next, the relative al.1grmm.t of the rotors with1D at least om paired set mu.st be 

offset trom an_y ot the previous day's relative alignments. 

How could the use ot plumed interrogation be eliminated? Or vbat changes 

could be m.d• in the use of the device in order to eliminate the possible use of 

planned interrogation? What could be substituted tor the uaa or the constant input 

point and the groupings of 3 in the rever-aer. 

Adding rotors will not eJim1mte the possibilit7 of so• ot the solutions. 

Som ot the vealmasaes or the device na:ight be of'!set b,r 

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1111/ Ill A .. 

mmla Double anciphement 

2. Varying the position ot the input point. 

3. Scrambled end-plate; but produce each letter of the reJ>l1' one at. tiM 

and read ae produced. 

z.. E.Uminate the use ot trigraphic rneraers. 

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Am.r lfol'IBD 

NSA-314 

June 195"-

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