REF ID: A~~~E6ftE'l' ULTRA 

In considering the reliability of cipher machines. we find 

that modern engineering and cryptographic techniques have accomp~ished this to a satisfactory degree. Although the machines are 

... 

complex. their reliability can be assured by having properly trained 

personnel to operate and maintain them. Accuracy is also one of 

the elements of reliability and a good cipher machine can yield a ~ 

high~ degl'ee of accUl'&cy~ompleteness of textL"'6!> '""ft a code ~ -------" ----------~+«a. ~ 1 o.. c.tdA.r\r 

17a.tem.<A mistake in one or two code groups~ obscure. alter. 

or render unintelligible the meaning of the whole message. but in 

cipher systems,~w:rong letters ma~~~rected, or missing 

J. 

letters supplied. The use of the w:rong key ;_;ong •setting" j 

may. of course. prevent the dec:!.pherment of a"'messagea--1c.. ~ H "-" .- 0...AA Aw. '-.Dt8ct~ ~ '1b~N~ ~l~~~d~~"~ Let's now consider the security angle. If reliability were 

the only or the moat important factor. code would be preferable 

to cipher for all echelons of command because the simplicity of 

a code book is to be preferred to the complexity of a large cipher 

machine. However. unenciphered code is not sufficiently secure for 

the communications of the highest echelon and headquarters. and 

when a second step of encipherment is added. it practically destroys 

the simplicity features of a code. In a properly designed cipher 

machine. embodying sound cryptographic principles. the si~le 

Declassified and approved for release by NSA on 05-28-2014 pursuantto E.O. 1352a 

fl. 

- Tr--
REF ID:A71137 

TCF BEICRB'! tiL'fltA 

step encipherment process can yield cryptograms or very great 

security. In a good code system, however, the solution or one or 

even or several messages does not entail the immediate breakdown 

or the entire system, with the consequent ability to read all messages, as is usually the case in a cipher system. A good cipher 

system may be compared to a library housed in a large structure 

ot many rooms with all doors and all windows securely locked. If 

an intruder can torce an entry into the structure, he will find a 

master key which will open all the locks and give him access to 

all the books in the library. A good code system (especially 

a two-part code) may be compared to a library housed in a similar 

structure, but no two locks are alike and no master key is available or can be made. Therefore, the lock on each door must be 

worked at patiently as a separate problem. Thus, although the 

intruder may force his way into one room, this gives him access 

to only a small part of the library; in order to read all the books, 

he must force his vay into each room, which takes much time, since 

each lock presents a separate and special problem. Code books, 

of course, can be rendered useless by compromise. Actual possession 

for a long period of time is not necessary; methods or rapid photography may be applied and a book of several hundred pages copied 

in a few minutes. 
REF ID : A 7113 7 ~OP BEORB~ T:JI/f.RA 

As regards rapidity, the speed of operation of a cipher 

ma.chine leaves even simple unenciphered code far behind in the 

matter of rapidity. 

In the matter of flexibility. we find that a cipher machine 

is much more flexible than a code and can be used for all sorts of 

messages. In a code containing words. phrases, and sentences prepared for a specific type of communication, rewording the original 

text as written by the originator is often necessary, it the words,' 

phrases, and sentences in the code book are to be used; otherwise, 

the original wording must be encoded word by word, or even syllable 

by syllable. 

Finally, with respect to econo!DJ, whether expressed in terms 

of money or manpower, cipher systems are more economical than code 

systems for high and intermediate echelon communications. A single 

clerk operating a rapid cipher machine can turn out 10 to 15 times 

as much work as one operating a code system; furthermore, codes 

must be pitepared, printed and distributed--steps which take much 

time and labor to effect. 

It's clear that high-grade and medium-grade systems should 

include all or as many as possible or the five factors discussed 

- 101-