There is another of these devices which apparently destroys periodicity and is aimed at throwing all of this onerous work upon the shoulders of the decryptor without at the same time punishing the legitimate decipherer. This consists in shortening the two alphabets of the key, so as to leave some extra letter, which will never be used in any cryptogram. Encipherment, in this case, is accomplished in the regular way, producing a periodic cryptogram. The extra letter may then be inserted at points throughout the cryptogram wherever it can do the most harm. The decipherer, knowing that this one letter is always null, need merely erase it. But if this device is to be really useful, the omitted letter must not be always the same, and this trouble can be overcome as follows: In the shortening of the plaintext alphabet, we omit always the unwanted letter, as J. But in shortening the cipher alphabet, we omit first one letter and then another, according to agreement, and insert J in its place. The decipherer, knowing what letter is null, erases it; but the decryptor, granting that he knows what the process is, will still have to experiment with various letters before he learns which one (or more) of the 26 is the null of the moment.

Shortened alphabets are not uncommon in ordinary use. We meet with 25-letter alphabets in European examples, the letter W having been omitted for telegraphic reasons. This case can usually be distinguished from the one which precedes by the fact that the letter W is never found in a frequency count, and it presents only the minor trouble that the ordinary 26-letter slide will not make the decipherments, so that it becomes necessary to prepare another on which the letter W is not present. This case can, of course, be simulated by making use of a 24-letter alphabet.

These devices, taken as a whole, have added little, if at all, to the security of the straight-alphabet ciphers, though, for the most part, they have succeeded admirably in rendering their ciphers totally unfit for general purposes. Considered as single examples, they can, of course, prove troublesome. We trust that this will not be the case with some one or two of the appended practice cryptograms, but if so, we recommend that the student postpone them for a later investigation. Concerning example No. 122, he may find that some of the material presented in [Chapter XVI] applies also to the “running key” encipherment; with others, a trigram-search may assist in developing the interrupted key-word; and in one case, a clever decryptor should find a way for applying his Kasiski method.

122. By SABIO. (Vigenère with Running Key. SENT, AGENT, STOP, IMPREGNATED).
A R U N N I N G K E Y S O Q M A V Q X K L U E R S Z S S R F A H A I V
X W E T N K Z Q N V R A G W V E T F W N L K A T A I B S Z U H P E X U
B W W A S P N F F C. (These are a trifle tedious, but not inhuman).
• • • • • •• •• ••
123. By NEON. (Porta, with key-interruption. Plenty of trigrams!)
A P V K W T P K P V Y G Q P G A K J Z W J N I X J U Q O U K P V W F U
R F X N K C K P R K Q K W F U R G J O V Z O K G X J V Q S W T F K D L
L Y Q L X Z E F L Y U J V Z C X G Q L J M T X W K K P V T V B Y K X P
F J Z Q X B V C O V V H X Z K J Z U Y.
124. By WHOSIT. (Beaufort, with key-interruption. THEY, WHEN, IN, ON, UP, etc).
M X Y F U H P M J B C X O C K A L Q E D B Q A E P R B Z L G L W M J B
Z Z C S A A L A O E K K C W L L J B P H U W B L F Q O R B Z L A O E M
A L O K F P V H Y U Y H Y J L X O L X Z.
125. By B. Natural. (Gronsfeld, with key-interruption).
S O W H Z G H O C V V W L F F F X O F H H X Q S I H S O Y P P H K T Q
H Z F Y J Q G Q H O B X V X O F L R J L F W E A E F H O G G V O F E T
Y M U X O F T H S N F B U A O B W H V C V V H V A O F Q M A G V N H S
S C F U X O F V H E L O A O J O E C V V E Y F A V S N I P L E O U P W
T A G P Q K E T.
126. By TRYIT. (Gronsfeld, with interruptors. MY, TO, THE, OF, IS, BE, WHICH).
R H X G A P A S R E C Z T R T W Z A J Z S G Q A Z M T P E A U X G K Y
Z F W Z S G Q Y O E Y F C T P W B G K O D P W N D X Z A W F O W H T Z
B M O H K Q P K V K S Q N D J Z S L Z X L C R T T N H S H W.
127. By B. NATURAL. (Vigenère. One letter reserved as interruptor. Look out!)
P N B Y C A N D V N P N F Y Z G V N W E J N S I T T T Z B L N O S L N
X R N I L Z H N H M D X D X B Z N B I K W Z H N D J N B M D T N O I K
N E I I H T W Q M F A T N P Q U N T J W D C X N G I C X P Z B L N O S
L N O I J N O S L G N H S C K T Q D N X W N R I I I L M J T R N U M D T.

CHAPTER XVI
Auto-Encipherment

The term autokey (autoclave; “the autokey cipher”), as commonly used, refers to the kind of encipherment shown in Fig. 116, in which a message becomes its own key for applying some one of the multiple-alphabet ciphers — usually the Vigenère. It will be noticed from the figure that the auto-encipherment must be “primed” with a conventional key; and whenever the words key-length, period, and so on, are used in connection with auto-enciphered cryptograms, their actual reference is to the short initial key. A more accurate term would seem to be group-length. But that a term is needed for referring to something akin to the period of the ordinary Vigenère cryptogram can be seen when we consider the mechanics of decipherment:

Our present initial key, COMET, key-length 5, serves to decipher only one group of that length. The five key-letters obtained from this first decipherment will serve to decipher only one more group; from this, another five key-letters are obtained, and will decipher a third group, and so on. But our group-length, sometimes