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REF ID:A65648 

CODING AND DECODING MA.CHINE: 

by A· M. Ske llett 

August 26, 1954 

It is well known in the A.rt of Crystography to use a machine for coding and 

decoding a message. the principle of which ··1s to scramble and unscramble the letters 

of the alphabet in a predetermined pattern which varies with time. The present 

invention is electronic and provides such a machine having no moving pa.rte to the end 

that it may operate at extreme speeds. This extreme speed of operation is especially 

useful in deciphering a ooded :message since it provides for the trial of lDl!U1J oomb:i.nationa or scrambling patterns in a relatively- short time. 

Figure l shows an embodiment of the present invention in schematic form. Keys 

at the le:rt are provided for the letters of the alphabet similar to teletypewriter 

keys. The original message is fed into the nachine here as one might type a message 

on a teletypewriter. These keys are each connected to an individual radio frequencyoscillator which is turned on when its key is depressed. Thus pressing key "A" will 

oause the ·"A" oscillator to send out radio •ves. Each oscillator operates at a different frequency. 

On the right hand side of the figure there is a corresponding number of receivers 

each turned to a different frequency. Each receiver is connected so as to operate a 

priDtin-g :mechanism for a letter of the alphabet. This printing mechanism nay be similar to that of the receiving ma.chine of a. teletypewriter. For simplicity, no attempt 

has been nade to show this whole mechanism but the solenoid arrangement shown at the 

output of each receiver represents the actuating mechanism for such a machine. There 

is one receiver tuned to each of the osoilla.tor frequencies so that waves from one 

oscilla.tor can be received on only one receiver. Now the tuning of the receivers is 

set according to the scrambling pattern. 

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REF ID:A65648 

In operation. when Key A is depressed. radio waves of frequency "A" are sent 

out. Only the second or "B" receiver is tlm.ed to receive the A waves. Thus• the 

secCllld receiver is actuated and the letter B is printed. Likewise. depression of --

the H Xey, for example. results in the printing of letter F. etc. 

Thus. it is seen that a message typed out on the input keys is printed in coded 

form with the letters scrambled in accordance with the pattern by which the receivers 

are tuned. Also. it is obvious that if the scrambling pattern is reversed1 i.e •• if the 

receivers are tuned to the reversed scrambling pa~tern. the coded message when typed 

on the input keys will be deciphered and the original message will be printed. 

A message coded as above is .fairly easy to decipher by well !mown methods. However. if the scrambling pattern is changed after each letter. it becomes extremely 

di.fficul~ to decipher the message. It remains to be shown how the pattern is varied 

with time. 

We consider the receivers. They operate on the superheterodyne principle. The 

:incoming frequency 1'1 is modulated by a beat frequency fb supplied by the "beat 

frequency oscillator" o This modulation gives a difference or intermediate frequency 

(i.£.) which goes through the i.f filter. The tuning of the receiver is provided 

by the tuning of the i.f. filter. By varying the beat frequency. the receiver is 

made receptive to correspondingly varying :incoming frequencies. 

Let us choose some values for the frequencies to better tm.derstand the operation. 

VVe will assume a beat frequency of one megacycle. We also assume that the oscillator 

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REF ID:A65648 

frequencies range from l.~ mo. to 1.760 me. This provides a 10 kilocycle separation 

between adjacent letter oscillators, or a total spread of 10 X 26 or 26_0 kilocycles. 

Now, for example, if the B frequency fb is 1.51 megacycles, then the i.f. of the 

C receiver (which is tuned to it) is tuned to 510 KC (1.51 ~ l.O mo.) and striking the 

B Key wil~ print the letter c. The signal from the A oscillator will also be present 

in the input of the C receiver but will give rise to an i.f. of 500 KC which cannot 

get throu~h the i_.f. filter (band width less than 10 KC). Nor can the signal from 

~he C oscillator get through since this would give an i.f. of 520 RC. 

With all the filters tuned to the proper frequencies according to the pattern 

the one beat frequency maintains all of the i.f. frequencies in proper reJa ti on for .. passing through the filters. Now, suppose we vary the beat frequency by 10 KC. This 

will shift the reception of each receiver by 10 KC. and thus change the key or scrambling 

pattern. 

Using the example given above, consider that the beat frequency is changed to 

l'-

0.99 me. Then.f~ - fb equals 520 KC. instead of 510 KC. and the D receiver will now 

respond to the B oscillator to print the letter D. This is a shift of one letter in 

this case but the next shift in beat frequency by 10 KC., i.e. to 0.980 mo., will match 

the output of the B oscillator to the tuning of the A receiver (i.f = 530 KC.) and the 

letter A will be written when B key is pressed. 

These examples are for the particular scrambling pattern shov.r?l in Figure 1 but 

they serve to illustrate the changing of this pattern as the beat frequency is changed. 

Thus, we have a simple means of varying the scrambling pattern. It is equivalent to 

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REF ID:A65648 

changing the arrangement of 26 different connections by a single control. 

Figure 2 and 3 show the different patterns that would be obtained with this machine 

if we had only 10 inputs. The top row of numbers represents the 10 oscillators and 

the bottom row the 10 receivers. The connecting lines show the effective paths of 

the radio waves or the equivalent connections. Starting with the arbitrary pattern 

labelled l on Figure 2 the scrambling patterns are shown that would result from changing 

the beat frequency by. 10 steps of 10 KC e~ch. 

Going back to Figure 1, we find that the oscillators have frequencies ranging from 

1.51 mo. to 1.760 mo. and of course the receivers must be tuned through this same range 

so that there is a receiver tuned to each oscillator. Now, when we decrease the beat 

frequency by 10 KC. (or one step) one receiver will be tuned 10 KC above this range and 

will thus not receive a signal. It is obvious that we must have two i.f. filters for 

this receiver tuned 260 Kc. apart so that it will receive the correct signal. Similar 

reasoning shows that all receivers must have double i.f. filters to provide continuous 

operation through the cycle of patterns. It is equivalent to repeating the alphabet 

with 26 more receivers since the frequency of the last letter does not come out adjacent 

to the first. 

The cycle of operation for the beat frequency oscillator follows a saw tooth pattern 

in which the frequency decreases by 26 steps and then flies back to the original pattern 

to start the cycle again. A rotating condenser plate stepped arou..TJ.d by a pawl and 

ratchet, one step for each letter typed, provides a simple means of varying the beat 

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REF ID:A65648 

frequency. There is an all elec~ronic equivalent of this which would not have the 

limitation in speed imposed by mechanical action. It consists of a reactanoe tube 

connected to change the reactance of the tuned circuit of the beat frequency oscillator 

which tube is operated by an electronic cotmting stepping circuit. a number of £orm.s 

of which are well known. 

To set up the original pattern for the highest beat frequency one must set the 

tuning of the 2 X 26 i.f. filters. These filters may be of very simple form consisting of' only a coil and condenser so that varying either the induota.nce or capacity 

serves to tlm.e the filter. :Mechanical coupling between the two components of a pair 

of filters is readily provided. 

The oscillators would be very simple and could oo~tain only one-half a vacuum 

tube each. That is. only 13 double triodes would be needed. Alternately, 26 transistor oscillators would suffice. For the receivers it might be possible to get by 

without vacuum. tubes and to use two semiconductor diodes per receiver. one for modulation and one for detection. Alternately, one could use a s:ingle converter-diode 

vacuum tube for eaah receiver. The beat frequency oscillator would require only one 

or two vacuum tubes so that the total number of tubes or transistors would not be very 

large for the whole machine. 

No provision would be needed for the paths of' the radio waves. The coils of the 

tuned circuits could act as antennae and all of' them could be enclosed in the same 

small shielded space for proper transmission between oscillators and receivers. 

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REF ID:A65648 

To further increase the secrecy of the coded message, several of these machines 

could be used in tandem with different scrambling patterns in each. For this type of 

operation the signals out of the receivers of the first ma.chine would operate the 

oscillators of the next machine and only the receivers of the last machine would 

operate the printing type. The stepping of the beat frequency oscillators of all of 

the machines would be in synchronism. 

For deciphering a message, the scrambling pattern of vmich was unknown. it 'rould 

be of advantage to change the initial scrambling pattern at the start of each stepping 

cycle in a time that is short with respect to the time of one step or shift in beating 

oscillator frequency. This appears to be possible by providing the receivers vrith 

individual beat frequac.cy oscillators which may be set to a pattern by 26 separate 

inputs and which are connected together so as to step together. The description 

of this machine requires another memorandum. 

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REF ID:A65648 

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REF ID:A65648 

NATIONAL UNI E 

350 SCOTLAND ROAD 

ORANGE. NEW JERSEY 

CORPORATION 

A. MELVIN SKELLETT 

VICE PRESIDENT 

September 20. 1954 

7 

National Security Agency 

Department of Defense 

Washington 25. D. c. 

Attentions Dr. Howard H. Cam:i.signe 

Dear Dr. Campaign: 

\if«./ 

I am enclosing a desoription, in duplicate. of the 

machine that I talked to you about when I visited you last. 

I never got around to tel ling you what a cordial 

reception I received at Corona and how profitable that I felt 

the trip was. Thank you for setting it up for me. 

There was a booklet that I requested from you which 

you will recall was not available for sending to me. I do 

not believe that I need that booklet now and suggest you cancel 

the request. 

I will drop in and see you on my next trip to Washington. 

With best personal regards 

ama/eml 

enCbsures - 2 

A. U. Skellett