-·~ . • EX!iit:UTIVE DIVISION 

,, MILITARY INTELLIGENCE BRANCH 

In replyln& refer to • REF ID:A414BB7'# · 

WAR DEPARTMENT 

OFFICE OF THE CHIEF OF STAFF 

WASHINGTON. 

April 11, 1919. 

Lt. Col. Joseph o. Mauborgne, 

Office of Chief Signal Officer, 

Virginia & 18th Streets, 

WashiDgton, D. c. 

Dear Colonel Mauborgne: 

My time has been often and seriously interrupted 

since you oalled my attention to the T. and T. oipher and Colonel 

George Fabyan's claim that he had shown its inseourity, but I am 

now able to send you the followiDg: 

. a) A deoipherment of a large portion of message No. 4, 

whioh Colonel Fabyan, as I understand, has not yet deciphered. 

b) ~draft of a reply to Colonel, Fabyan's.presentation 

of his results to us through Captain ~owell. 

o) A draft of a: letter from the Seore-ta.ry of war in reply to Fabyan's letter to him, transmitting his decipherment of 

message No. 2. 

I did not wish to reply until I had deciphered message No. 4; partly beoause 1 was interested in the problem, and partly beoause·- having ha.d no previous experience with the T. and T. 

cipher - I wished to see whether in practice it developed any weakness 

which did not appear in ~heory. 

My wolit merely confirms the conclusions Which you and 

Captain Yardley reached and expressed last year, namely, that two messages enciphered wi,th the same key a.re easily decipherable by modern 

,_ methods of attaok and that a single message is aosolutely indecipherable, both theoretically and practically. 

The problem of deciphering message No. 4 was very interesting in two or three particulars. As_ you will recall, .Mr. Gherardi 

originally infonned Fabyan that "Message No. 4 utilizes a portion of the 

enciphering tape employed with messages Nos. 2 and 3 but extends to a 

Character in the enciphering tape beyond the end of either of these two 

messages." ~he first problem was therefore to discover at what point in 

lios. 2 and 3 No. 4 began to run-parallel. As you will readily see, this 

@'pp roved for Release by NSA on 11-18-2014 pursuantto E .0. 1352a 
, I . 

-?;-.- ' . EXECUTIVE DIVISION ' ., REF ID:A414BB. MILITARY INTELLIGENCE BRANCH 

la replylog refer to 

( 

WAR DEPARTMENT 

OFFICE OF THE CHIEF OF STAFF 

WASHINGTON. 

April 11, 1919. 

Colonel Ma.uborgne - 2 

was not very diffiault. The process is mechanical and inevitable 

and oa.n be oarried'on by peveral. clerks working at the same time; 

so that al though it actually took me, working alone, about five 

hours to solve the problem, if there had been need of haste fifty 

or a hundred clerks could have got a solution in less than ten minutes. 

I am especially glad that I worked on this problem, 

because in the course of work I developed a considerable simplification of the method of attack on two messages with the same irrational running.key. lt is so simple that others have no doubt discovered it before, but I .bad overlooked it until now. l will expound 

1 t when I see yr:m.. 

The other matter-of special interest was the discovery 

of the extent to which. familiarity with the operations of the ma.chine 

and a visualization of the form of the message when deciphered aided in 

deciphering; but this of course is not new to you. Fabyan apparently 

took these operation-signs as :zmlls. 

encls. 

bbm 

• 

Sincerely yours, 

~~\Q.W..~ John M. Manly, 

Captain, o. s. Army.