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ke. 98 ,___ ____ _ Date: 5th March 1955 

Copy No.: #</ 

PL.86-36/50 USC 3605 EO 3 . 3 ( h) ( 2 ) 

Sl!DURITY OF J~fW4 qi .. 

BYI 

SIDllMi.'.i.RY 

This paper briefly describes the :..FSIJ! 4/i. and 

disousses its security. It is conc.luded that 

subject to certain qualifications about procedures for avoiding depth, the ma.chine is 

secure for al.l classifications ot tra.:ftio f~r 

at least the next thrcQ years. 

Distribution 

Standard • 

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Declassified and approved for release by NSA on 06-11-2014 pursuant to E .0. 1352\3 
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1. Introduction 

J....FStJ! qi .. is an on- or off-line additive-producing t·rd.oe-through rotor 

machine which has recently been introducec! far use in U .s. D/F and administrative net'vl'f'rks. It is a conversion from a once-thrnugh version call lil-294 

or SIGNIN (~ and P.ANDORA or.yptosystems). The machine and the associated 

cr,yptosystems (CENT .. h.Ull and IXION) are described and assessed in detail in 

BRUS: .. C/S 306 dated October 19th• 1953. It is there concluded that the naahiDa, 

wen used ilith these procedures, is secure for all classifications of trattic 

for at least the next 5 years. This paper 

(a) briefly describes the ma.chine and the associated procedures, 

(h) discusses its security. 

2. Brief Description 

(a) The machine 

123485678 

11 inputs 

5 + 1 = 6 outputs 

There are 8 26-point non...reversiblo rotors selected frm a 10-rotor 

set. Each rotor has a fixed alphabet anc1 an identical fixed 5-notch 

notchring. The rotors are pluggable but ;.~ll not> be replugged in the 

field. R1234 and R876 form cyclor.i.ctric cascades '\'d.th R1 and 8 t:Q.e fast 

rotors. :as is a bump rotor clrivon by one of the 6 outputs as in J\SAK 2-1 • 

s, between R4 and 5 is a pen:ianent sc.ra.oblc-r.dred separator. The reflector 

is w.U-ed '\"Ii.th 10 pairs and 2 triplets, as in l.1.St.ui 2-1, anC!. the key-producing arrangements aro likeiid.se the sane - 11 live inputs a.nd. 5 key outputs. The key is added level by level to 5-unit plai11 text, and is never 

transmitted in clear. The flatness of the key is discussed in Note .: .. , 

·.;here it is also shmm that there is a slight correlation bet•-:een the key 

and the stepping of the bur.Ip rotor. 

(b) 0.EN111JJR 

(i) Ea.ch pad consists of 120 pages which arc assigned in blocks to 

stations on a net. Each po.ge contains: (A) 4 rotor arra.ngecents 

each of Tlhich has 20 tetrano..r.i..e C.Csigna.tors froo a sories '1hich 

runs in order :fra.l 0001 to 9600 throughout the pad.. (B) 176 

randaJ. 4-l.etter groui..;>s, each closignated by a digraph; tho digraphs 

range from il to GT on each :;_:>age and are arranged alphabetically. 

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J .. n indicator consists of a tetra:nom giving the rott"r arrangement, 

and 2 digraphs giving the aligrment (as shown on the same page as 

the tetra:n.a:i.o). Both tetra.na."les and digraphs are creased. off a.:f'ter 

use. 

( iil) For off-lino use &lY unused tetranane can be used as an indicator. 

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Mossa.gos are lmitec1 to 6,ooo characters. For on-line use a rotor 

arrangenent can be used onl.3 once, but up to 20 necessary restarts 

throughout the12-hour cryptoperiod r:ay be cad.e on the saoe arrangeamt. A restart "r:By consist either of e. no".7 indicator or of' a 

counter reset as in (iv) belm-1'. 

(iv) Counter reset. 

(.t1.) 

(B) 

Half' duplex. ii. receive opera.tor can cut off' transr!lission if' 

he is getting garble. The lmole :net g•es Cfl/er to pl.a.in. The 

send opera.tor stops on a rand.en number of' positions, not loss 

than 201 ancl transmits his character counter reading. 

PL 86-36/50 USC 3605 

Duplex. The receive operator \7ho is getting garble sends BREAK 

on his sontl. l.Ja.Chine. He resets and sends his nev coun:ter 

roe.ding on his send r.iachino. 

( v) There is no letter check procedure. 

(c) IXION 

IXION is like CENTJUJR except that (i) there are about 200 pages to a pad 

and (ii) the tetra.nar.ies are replaced by trigraphs "rlhich are arranged 

alphabetica.ll.y onl;y within a single rotor a.n-anger..ent. 

(a) CENTJ.,.'OR is the nomal proccC.ure. IXIO.N -r.r.i.11 onl.¥ be used \men T/A considerations warrant it. 

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Te 8! HANDL!D mt ACCOltDANC! hlTH lltSIC! 

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Clr'\CD 

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No. 98 
(b) 

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CENTJ .. UR and IXION arc ad.equate far all classifications up to Secret. 

There is a very s..:ia.11 chance of' readable depth ha.-revor (sec pa.ra.3(a) 

(1 ), (ii) am (iii)); if' CEN.1'/i.UR and IXION are used f'or Top Secrat 

traffic this risk must be accepted by the uaers. 

( c) Tho procedure should be exacin.ed to ensure that there is no danger 

of' depth as a result of 3(d)(v) (B) and (C) above. 

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TO B! t'IA:NDLl!D IN ACCOltBllNC! Wlfl I llt516 

Key properties 

5. (a.) In each stroa.i:i the probability of cross is 

20 • 11 + 6 (1 - 14.13) = 649 ~ 25 ~ ( 25.24) 1,300 

(b) Counted character.-dse the key is a bit rougher than this, but not 

seriously so. The calculation is lengthy and only the results aro 

quoted here. 

NUi,ber of Nuober of 1 

orosses in key charact~r characters Probability probability P.B. 

0 1 .0324- 30.84- .0378 

1 5 .0315 31.75 .ooao 

2 10 .0311 32.14 -.004-2 

3 10 .0311 32.15 -.0043 

4 5 .0313 31.93 .0022 

5 1 .0314 31.85 .004-3 

(c) l .. 11-d.ot is th~ the 1.10st popul.Ar character. Harrever approxi::atcly 

86,800 key c..11.o.racters arc needed before it can be expected to exceed 

its e:x;pected frequency in randor.1 r.iateria1 by 2 standard deviations. 

6. Buc;p rotor stepPing 

The probabilities ha.ve only been calculated for the tw extreme oases: 

Key 

1 .. 11 dot 

; ... 11 cross 

Probability th.a.t bu..lJ? rotor steps 

.490 

.499 

;,. correlation of this ore.er of size has no effect upon the security of 

the r:iachino. In the case of the other 30 key characters the correlation 

w.l.11 be s1:.Ja.llcr still. 

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7. Key Sap.ple 

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Te BE 1 IANBl:EB IN AGG91Uil)ANCE W'll'M IMICii 

______ JNo. 98 

The results of a count of 20, 000 cho.ractcrs of 1..S:lii/l. 2-1 key arc attached. 

1 .. lthough the figures are consistent with the theoretical ones the sample 

size is too s:Ja.11 to giva a significantly close correlation .• 

• • • • • 655 • •• • x 632 

x •• • • 639 x • • • x 640 

• x • • • 562 • x • • x 652 

xx. • • 668 x x • • x 630 

•• x •• 629 •• x • x 590 

x. x • • 625 x. x. x 686 

-. xx •• 636 • xx. x 636 

xx x •• 618 xxx.x 614 

••• x • 616 • • • xx 582 

x •• x • 607 x • • xx 599 

• x • x • 592 • x • x x 604 

x x • x • 643 xx • xx 684 

•• xx • 622 •• xx x 600 

x • x x • 598 x .. xx x 640 

• x x x • 5n .xxxx 685 

xxxx. 590 xxxxx 649 

20,000 

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