[ CONFIDENTIAL. ] LETTER J OF JAMES F. GREGORY. Liei;t. Coi.onki. ani> A. D. TO LIEUT. GENERAL P. H. SHERIDAN, COVCFUMING TELEGRAPHIC CODE. WASH INGTON: OoVFRNMKNl P HINTING OF KICK. 1 8 H I) . [CONFIDENTIAL.] LETTER OK f JAMES F. GREGORY, Lieut. Colonel ani> aThTITT, to LIEUT. GENERAL P. H. SHERIDAN, CONCIHNING TELEGRAPHIC CODE. WASHINGTON: s OOVKRNMENT PRINTING OFFICE. 1 8 8 G . [Confidential.] Headquarters Army of the United States. Washington, D. C., June 11, 1885. Lieut. Gen. P. H. Sheridan, Commanding Army of the U. S., Washington, />. C. General: 1 have the honor to transmit here- with the revised compilation of “Slater’s Tele- graphic Code,” prepared by your direction for the use of the War Department. In the introduction is given Slater’s description of some of the ways in which the code may be used, which is sufficient for publication with the vocabulary. It seems proper to add here, and not for publication with the work, something more about the code and its uses, which may be sepa- rately printed if deemed advisable for transmis- sion to those entitled to its use. In deciding upon this system as best of all those I have examined, where ease of enciphering and deciphering messages, coupled with security from interpretation by persons unacquainted with the key, are the required conditions, 1 have been strengthened in my opinion of its merits by that of Prof. E. S. Holden, director of the Washburne Observatory, who is an acknowledged expert 3 4 Cryptographer. On page 424 of his pamphlet, “ The Cipher Dispatches,” he says: ‘‘The question is often asked, “Is there no safe cipher?” The answer is: there are many such, if one means by a safe cipher one which is almost or quite impos- sible to translate. The question should be modi- fied so as to include convenience of use, and then the “safe ciphers” are few in number. Perhaps the best and safest cipher for general use is found in Slater’s Telegraphic Dictionary of twenty-five thousand words. In this each word is numbered from 00001 to 25,000 consecutively. Suppose the message to be sent was Rely upon plain English. The words Rely upon, &c., would be looked out in the dictionary, and the numbers opposite them set down. To these numbers the sender adds a previously agreed upon number, as 4,397, 21,171, &c. He thus obtains four new numbers. These numbers are looked out in the dictionary, and the words standing opposite to them are sent. The process of reading the telegram by tin1 receiver of the message is simple. It is not likely that mes- sages of this kind can ever be read by one not acquainted with the key, if this key is occasionally changed, say at the 7th, 10th, 10th, 21st, &c., words. If it is not so changed aud there are enough messages, even this code can be read. Professor Holden, in a letter to me on the sub- ject, has made the following suggestions concern- ing the uses of the code in correspondence: 1. For messages which are not very secret, but yet private, let us take a plan like this: Date message in plain English. For all messages sent 5 in January add 111 to the word number in vocab- ulary, thus: Return to Washington. 10374+111=19485 22501 + 111=22612 24780+111=24900 Rickety transaction Spokane (Ft.) The message sent would be: Rickety trans- action, Spokane (Ft.). For ill messages in February add 222, in March 333, &c. This key could be known to many people and yet be safe enough. Next year make January some other number or change at any time. A simple complication of this is to add 111 for January and also to add (or subtract) as many units as make the day of the month; thus for January 20, add 111 + 20 = 131; you have then 303 keys per year, and all simple. A simple message in ordinary English: “Add 1437 to all ciphers until further orders,” would complicate this (for the decipherer) and yet leave it all clear for those possessing the key. Whatever scheme be used be careful to have it easy, for it is very hard to decipher the very easiest one of this kind, and a simple one only is needed for general private correspondence. 2d. For more private messages, but which it is desirable not to make too complicated. An arbitrary code like the following might do for these: Add to the 1, 2, 3, 4, nth word of message 13, 23, 33,43 . ... (n4-3) or in gen- eral (10+a) (20-j-a) (30+a) (u+a). This is perfectly simple in use and absolutely impossible to decipher if it is changed occasion- 6 ally—that is, if a is made a7, a", &c., once a month or even a year. 3d. For the few messages which need be care- fully guarded and kept secret from all but a very few, the preceding plan might be used, modified by reversing 1, 2, 3, 4, of the digits either of the message proper, or of (n+a). There are endless simple ways of doing this, but it should always be remembered the deciphering of the easiest of these ciphers is very hard—to the man without the key. To denote to the recipient of a message which scheme is used, certain signs may be employed, such as placing date at beginning or end of a message, by inserting arbitrary numbers in a mes- sage, or what not. One of the many complications easy to use is this: Write the message and change the code at every, or every other, or every tenth, or every hundredth word, thus: Achievable 301 Keturn 19374 19075 rotundity. Addict 401 To 22501 22902 unbiased. Ado 501 Wasliington 24789 25290 accuse. The message is: Achievable, rotundity addict unbiased ado accuse. The above illustrations are deemed ample for a thorough understanding of the nses that may be made of the book. The special schemes to be de- vised for practical use should be privately made known to those entitled to use them by letter or by word of mouth. 7 The labor of compiling the vocabulary has been performed by Mr. W. G. Spottswood. I have the honor to request that when printed the proof-sheets of this work be submitted to me for revision. The size of page most convenient for use is six and a half inches by four inches. Very respectfully, your obedient servant, JAMES F. GREGORY, Lieut. Colonel and A. D. C.