C IPHER systems may, or may not, permit of variation by means of supplementary keys. But there are a number of excellent reasons why ciphers of the former class are superior to those of the latter.
In the first place, the value of a fixed-key cipher depends on how well it is kept secret. Should such a cipher become compromised by exposure to unauthorized persons, or by the interception and decipherment of message material, it would be necessary to prepare a new cipher for use in its stead.
On the other hand, if the secrecy of a changeable key cipher should become endangered, it would only be required to select a new key for use with the same cipher. This would be very much like losing the combination to your safe. You would not need to get a new lock. Just change the combination.
Obviously a changeable key cipher providing only a few variations would be of little value. To prevent the guessing of the key, as well as for other reasons here given, the number of changes should be practically unlimited.
For example, a frequent change of key might be necessary to prevent an accumulation of material in any one key against unauthorized decipherment. Or provisions may even be made for individually keying each message. Further, a cipher capable of many changes is best suited for use independently by a large number of persons employing different keys.
Cipher keys may consist of letters, words, phrases, numbers, dates, et cetera; and examples of most of these have been given, from time to time, in these pages. Word keys are generally preferred, however, becmise they are the easiest to remember, change, and communicate. And they do not have to be put into writing, thus preventing their loss by seizure.
Much could be said on this subject, but these few remarks would seem to justify No. 6 of the ten cipher essentials published in the May 28 issue, to the effect that a cipher should be capable of a practically unlimited number of changes, securable by keys which are easy to remember, change, and communicate. Essential No. 7, pertaining to the security of cipher systems, will be briefly considered next week.
Last week's cipher No. 33 used the following alphabet of only thirteen numbers, arranged in two "shifts," designated as "odd" and "even ":
1 2 3 4 5 6 7 8 9 10 11 12 13 (odd) A B C D E F G H I J K L M (even) N O P Q R S T U V W X Y Z
By prearrangement, any number of figures could be used in this cipher for each letter. In this case three were used, the first figure indicating the "shift" by being odd or even; and the sum of the first two, minus the third, indicating the place. In this way 379=A; 638—N; and so on. The message: "Agent arrived from New York this morning. Send me contract money in bank." This cipher can be further complicated by using nulls, or other processes besides addition and subtraction; and, if desired, each message can be keyed by its date.
The letters of cipher No. 34 were enciphered in groups of 6-4-4, the method otherwise being exactly the same as given for No. 32 last week, which was enciphered in groups of five. The message: "Escaped prisoners are hiding in a deserted house on Elm Street."
The Bazeries cipher. No. 35, is a substitution—transposition—null affair, using a simple alphabet derived from two key letters—ZF in this instance—prefixed to the cryptogram.
The alphabetic key is derived from the key letters by first changing them into a number, representing their places in the alphabet, and then into a word key. For example, Z=26 and F=6. The kev ZF thus becomes 266, or TWO HUNDRED SIXTY-SIX; which, by omitting the repeated letters, is boiled down to TWO-HUNDRESIXY.
These letters, combining I-J—Bazeries omits W—are then written into the first lines of a five by five square, which is completed with the remaining letters of the alphabet, as shown below. In the message alphabet the letters are placed in their natural order, downward by columns from left to right. Each letter in the message alphabet can now be represented by the letter occupying the same position in the cipher alphabet: A=T; B=N; C=I; and so on.
(Message) (Cipher) A F L Q V T W O H U B G M R W N D R E S C H N S X I X Y A B D IJ O T Y C F G K L E K P U Z M P Q V Z
To encipher a message, first arrange it in groups of three letters (a), substituting, at (h), the proper symbol for each letter. Next, reverse the groups, and prefix all groups beginning with vowels—as well as any other desired groups—with nulls, using the vowels.A-E-I-O-U-Y for the purpose. as shown in line (c). Finally, prefix the key, ZF, and regroup by fives (d) for transmission. In deciphering the above process is merely reversed.
(a) SEN DON EBA TTA LID (b) AMY CGY MNT KKT OFG (c) lYMA AYCG TNM ETKK GFG fd; ZFIYM AAYCG TNMET KKGFO...
Although it is of no great importance, Bazeries manages to have his illustrative message—of which No. 35 uses the translation—consist of complete groups of three letters; and to use enough nulls so that the final cipher group will contain five letters; and we have followed his example in both instances. Message No. 35: "Send one batalion of infantry to Le Creusot tonight by railroad."
Turning to this week's ciphers, those who favor the "crypt" type will find in No. 36 a delectable tidbit of medium difficulty.
No. 37 is a fixed-key transposition cipher in which the message has been transcribed in one order into a rectangle, and taken out in another order to form the cryptogram. The key here consists of knowing the dimensions of the rectangle, and the two orders of transcription. This cipher is hardly what you would call difficult. Yet we do not expect many answers.
No. 38 is another example of the Bazeries cipher, but varied, to make it interesting, in two small details. One of these changes lies in the method of prefixing the key to the cryptogram; and the other in the use of the letters in one of the diagonals of the cipher alphabet, instead of vowels, for nulls. The message is of a military nature. Can you read it?
CIPHER No. 36 (M. Walker, Akron, Ohio).
TXWDKJBA UKOZI PRYRAI MLKJUAS FRJWHR RGEBLITRN; ABQS, PRBAWLI YKCRI VRXWJKWLIAS BTTBJU XWSBA EWBXN, NBOJKOZ KONRJWXWLIAS.
CIPHER No. 37 (J. Lloyd Hood, Bastrop, Texas).
ATMETNES HTERTTTA FUHONTTT OHEPDBEU LRVOUTCR ESNIOCAE PSEOYVRE BLBHPENV DEARILAL CSC
CIPHER No. 38.
BNQAM QBQAP FMDCO RBQQZ OQAEK UQTPN BKOWT LLIQN OBQUK EKFTN QRMBL IBOVC KVQTK TOOMR BIOLM BOURM QWLFL LTTKB YBMAR