· ... ,.-; 

,. 

OP-20-GY 

MEMORANDUM. 

Mechanical Decoding. 

2. The apparatus required consists of the following items of 

standard International Business Machine equipment: 

Preparation. 

Punch, 

Sorter, 

Reproducer, 

Collator, 

Tabulator. 

3. Messages to be decoded are filed in any desired order, 

-preferably chronologically. They are then considered, for purposes of 

punching, one single message. The messages, including headings and other 

desired data, are now punched on cards. Only one code group is punched 

on a card. As the cards are punched, each successive card is assigned 

a .successive number in a single series. In other words, if there are 

100,000 cards -the first card of the first message will be 000001 and t._he 

last ca~d of the last message will be 100000. Cards used ~or headings 

and other data will also be numbered ace ord ing to their posit ion in the 

series. 

4. All the cards containing code groups are now sorted by code 

groups using the lower zone punches only. Sorting in this manner is 

equivalent to considering the code groups as numbers and sorting thereby. 

·It is done thus to simplify sorting and to permit using the collator 

later on. This set of cards is called the Message File. 

5. A card is now made up for each code g·roup whose value has been 

recovered. This card cont a ins the code group, the recovered value, and 

an "X" punch.· The code group should occupy the same field on the card 

as the groups on the message cards described above. These cards with 

recovered values are next sorted in the same manner as the message cards 

above, i.e., by code groups using the lower field only. They will then 

be in the same code group order as those of the message file. This set 

is called the Decoding File. 

Declassified and approved for release by NSA on 10-20-2014 pursuantto E.O. 1352a 
·. 

•. • REF ID:A4146!lal .._, 

Decodi:ng. 

6. To decode the traffic, the message file is placed in one 

hopper of the collator and the decoding file in the other. The machine 

is wired so that as the two sets of cards pass through the machine 

every message card for which there is. a corresponding decoding card 

will be selected out of tbs message file. At the same time every 

decode card for· vJh i ch there is a message card wi 11 also be selected 

out of the decode file. ·The decode cards and message cards thus 

selected are then plac~ed in the reproducer. Using the "X" punch for 

control, the recovered value on each decode card can now be repr-.oduced 

on all corresponding message cards. 

7. Having all recovered values entered on all coITesponding 

message cards, the latter, together with heading and other message 

data· cards, may now be sorted back into message form by means of . their serial numbers. This done, the cards are ready for printing 

on the Tabula tor • 

Procedure During· Cryptanalysis. 

8. The same procedure as above is followed except that it 

must be repeated periodically as solution progresses. Under ordinary 

circumstances it is believed that fresh \'\Ork sheets with all new 

values· entered to date could be prepared at least weekly. 

(Shown to· Friedman on 10 

May, 1938) 

J.N.W. 

/s/ J. N. Wenger