Chapter XXIII: Investigating the Unknown Cipher

When the type of encipherment is unknown, the decryptor’s first problem may concern the probable language used in the plaintext, and this he is usually able to determine from the source and history of the cryptogram.

His second problem is the major classification, and this, too, is usually simple, since transposition, as a rule, can be recognized by its appearance. It must, however, respond to a group-test, and for cases in which this is needed, the approximate percentages for English can be taken as follows:

Vowels, with or without Y, about 40% (Variation limits: 35% to 45%)
Consonants L N R S T about 30% (Variation limits: 25% to 35%)
Consonants J K Q X Z about 2% (May be influenced by nulls).

The 5% variation is suggested in the Parker Hitt Manual. In this connection, it should be pointed out that an apparent transposition with exactly 40% of vowels and 100% evenness in their distribution is suspicious. Many of the checkerboard systems result in this way, and also some of the codes based on pronounceable five-letter groups. Then, too, it is easily possible to construct a simple substitution cipher alphabet in such a way that the resulting cryptograms will resemble transposition, and even respond satisfactorily to a group-test. It should be carefully ascertained that a supposed transposition cryptogram does not contain the many repeated sequences which belong to simple substitution. As to those transpositions which do show an appreciable number of repeated digrams, they will probably have undergone one of the route transpositions, especially one in which columns were taken off in alternating directions.

* * *

Concerning the characteristics of simple substitution, these have been seen throughout the text; we have normal frequencies attached to the wrong letters, and we have those numerous repetitions of various lengths, occurring at all kinds of intervals, which are never found in a transposition. Here, too, we may apply a group-test, based only on the relative frequencies of letters. The five most frequent are supposed to represent the letters _E T A O N_ or their equivalents, and should total about 45% of the text. The nine most frequent should total about 70%; the eleven most frequent well over 75%; the five of lowest frequency (which would include all of those totally absent) should correspond to the normal behavior of the group _J K Q X Z_.

* * *

If the simple substitution frequency count is present without the repeated sequences, then we probably have a combination of simple substitution with transposition. It becomes necessary to rewrite the cryptogram into various new arrangements until one is found which will bring back the repeated sequences. Ordinarily, the simplest kinds of transposition will have been used; sometimes the transposition will have taken place in a complete-unit block, and there will be a clue in the total number of letters present in the cryptogram.

* * *

When all letters are present in the frequency count (or all but one or two in the possible cases of 25-letter and 24-letter alphabets), a period-investigation is usually indicated. The case of periodics has been seen at considerable length, though a final hint might be added for the detection of a possible Porta encipherment. One of our many collaborators, F. R. Carter, suggests that any Porta cryptogram, periodic or otherwise, ought to show from 52% to 53% of letters _N_ to _Z_ — the opposite of normal.

The characteristics of digram-encipherment have been mentioned. Other polygram ciphers show corresponding characteristics, according to the polygram length, though the trail grows fainter as polygrams grow longer. A trigram-system, for instance, might be present when the cryptogram is evenly divisible into three-letter groups; it might suggest period 3, and might even show repeated sequences whose length is a multiple of 3 and which begin at serial positions such as 1, 4, 7, 10, which are the beginnings of trigrams. A great many of the trigram systems will show only repeated digrams beginning at these serial positions, or separated by intervals which are divisible by 3.

A 5 x 5 square is often suggested in the fact of a missing letter; but the fact of 26 letters does not deny one, since the careful encipherer may make use of his missing _J_ instead of using _I_ exclusively. Great evenness in frequencies may suggest one of the key-lengthening devices, such as autokey and progressing key; and the practical absence of repeated sequences will usually mean that a transposition has been added to a substitution. It is never a bad idea, in a puzzling example, to make the various digram-counts (in chart form): An actual digram count, in which every letter is considered the beginning of a digram; a pair-count on separated pairs, as in Playfair; the two counts which could be made with the cryptogram marked off into three-letter groups; and the kind of pair-count which could be made in Playfair if the first cryptogram-letter were omitted. Many devices, as mentioned, may be uncovered simply by “running down the alphabet.” And if the cryptogram has come from an amateur “inventor,” it may be a case of digging into one’s memory for previous “inventions.” With this last case, however, the “inventor” very often fails to submit material in proportion to the amount of complication he has introduced.

* * *

Of the examples to follow, there is none in which the system may not be learned through analysis, unless perhaps the final unnumbered cryptogram, and the material, in every case, should be suffcient for solution.

No. 163 follows Mr. Berkley’s encipherment plan, illustrated just above it.

No. 164 is said to have been taken from a German spy serving in the American army in France. This applies, however, to the first fifty groups only; the remainder was added to increase the length and to emphasize the plan followed by the spy.

No. 166 was accompanied by a plot:

“Supposed to have been found on the body of a man floating in San Diego Bay. Autopsy shows death by drowning. Victim was a local banker who had disappeared a few days earlier. Wife says no financial worries. No money missing. Banker had prospered during depression. Was yachting enthusiast. Our hero solved the cipher with the unconscious assistance of a radio crooner. Tragedy occurred in _August, 1932_.” The date was doubly underscored, but those who have read the message have found no reason for this and no explanation for the “crooner.”

157. By PICCOLA.

C S R Z V Y P Q Z J K H K V Q U U C V M R T W Z N G H Q S A K O X P M H D R W A J D F Q D F S R Z Z C G X P A J J T Z U L H T G S A H X J J L T R N N Z P B Z G R E B N F Y G E J N M T N J J Q H P J X M O B J A L X I A I C P F J O O F R N H.

158. By PICCOLA.

O C E E A T T I T K S N D T D S T H O O Y E A O E E P E B O T Y T A O A D S E O E T F T T T H R V W C T H O Y L T O O H L R B T T U H R R V R A W O B R U A O Y E H H L A B N E R L R K V C R I O N S E I D R U E R I P.

159. By TITOGI.

A H Y N U H C E S T I T N D O R F E H R W E A T F N R F P A O T M A T L H R E I O T N R L O D R H E E A T E S C T D I N W T S T O E A T S I T E C D U T M S O T R L D O N G N I I S O F A E T L I T A S.

160. By PICCOLA. (Veiled reference to crypt No. 166?)

(a) H Z M Q L D N N D Z S P R F S K L L L L. (b) L I L V M S T Z U G D H Z U Q X L L L L. (c) T V I U M F R U O Y U Q Y P S F W X L L. (d) L I L V M F P O E Y Z K F D V U E L L L. (e) G K P V D Z T A Y T B F Y Y C F I U L L. (f) Q B F P W Y C L U D V P Z Z O S W Y N C. (g) Q B R T F F G V T U E N S Z H B Q E R L.

161. By PICCOLA.

E G W G W G E G T U C L C U O X G K Z T E G O B G B Y L W M I Q N K Q Y E N F S C L H M N Y B X S E T N I W O C E G C B F C T C S Z T V G B E A E G T U R K F K B E G K X B C T G Z Y L X C H Y E G C U O X Y T Q F A D Q T T C U N B O G C O H X C E W E C U V E G C O C X Y X G B E A Y T K X F Q C O T B X N E G T U C O N T O P E L E K U V U N T O C N G N G B K W C E E C S Z K W N H E I K C C R E G C T E G T U R K F K T B R G M W X C F G Q N I C E B P E E W E N B K I Y F K F D O F E G N U C B G M T Z T F X C E W E C F V D T T U Z T E N E G W G L F M C T O V L.

162. By PICCOLA.

S P P A S T A S E F U N M T E H S O O A E S L E I C T R C H V U G S E L Y R E M E N E E R O S N E H I R A E T O R N S H M O D R O P E A O R P O S R Y P D O I N O C K G T.

The "NICODEMUS" Cipher (Harold Berkley) SPECIMEN ENCIPHERMENT

Key: M E T H O D I C M E T H O D I C M E T...
6 3 8 4 7 2 5 1 6 3 8 4 7 2 5 1 6 3 8...
Cryptogram:
T H I S I S E N A N S P O S E D E T C...
C I P H E R E D U S I N G T H E PFGVT VUHDG LMRIV
I V V I G E N E S A M E K E Y F ZOPUH MMVNB FOUDQ WSURF
R E A N D A F T O R B O T H O P BIOTP FGHRU VWHKR RWEVV
E R W A R D T R E R A T I O N S WULVA MPGWV MGEAQ CUYHW LBFUT.

163. By PICCOLA.

T Y D Q V W P A Z O M B W B I R K F I O O G W C O G E F L T Q M S R F X T C J C M A W P P Q M E X V O Q C O C Z F S F W V F E V E R S A B E C V J J W S I P P H M M K O X V Y I D B D B C I S Y N L J C Y F K C W E N Z E I T J V L Z M I L I I R W K R O O S Z A W E K J V J G F M Q K G F N C K H P B R D L V I A P E S L V M D J Z Z V F Z F F R D B A D P Q W E N L A L O E K M F M F W X O K D W D G C K K K C Q R V.

164. By VULPUS.

P E N A R C P F T I Q E V A T E N B L A T K Q F O A R E N E U I P E P F U K X I L C N F Q E P C V B T A W A O B N C O E T I N D W B N A R D Q F O F N B V C P E P G V G P A V A P B P F O A O B S C L B V B T F W A N E W B T C S D N F M A N A O E V A R A R C T K Q E N B M B Q F V E V B X K O A P E T B U I P F O F Q E L E O B R D R B Q F U A W A S C U K L F P E W B O C O D N A M E L G V F V A N C N D M F N B V D T D L E P F V I T I Q E Q F O C O A U C L F L A O B M E P E N A S D L B T K L H N E P D. ..... U I L A L B O B M A V K M G U K R F P F U B U D M F W E T A T I Q E V B R C M B W A N F Z I L E N A Q F W B T C R D T B T K O E P E U A V A O F N B S C Z K V B W C U B O A L F O B M E X I T D Q C Q D W A P F Q E N A L A.

165. By PICCOLA. (Again that No. 166?)

R O V L L A B T L D L B C Q M P X L B A F B T C T A T C O R L T O L C R H P D T X L Y O A E L B X P H L X B T X X Q L D R G L T K X R L G D B K L D P P L O H L Y O A E L K O M X B L H O E L V C R R C R J L T K D T L R C I N X P L L L T K X L R C I N X P L V D B L V O R L P O R J L D J O L F Y L I O P O R X P L M D E N X E L K C T T L V K O L O H H X E X G L T O L I O Q M E O Q C B X L H O E L T V O L I X R T B L B C R I X L K X L V D B L D F P X L T O L B X R G L T K X L B O P A T C O R L F Y L E X T A E R L Q D C P L L B T C P P L C T L V O A P G L F X L V O E T K L D R O T K X E L R C I N X P L T O L H C R G L O A T L T K X L N X Y L L T K C B L Q A B T L F X L T K X L X W M P D R D T C O R L O H L T K X L E X H X E X R I X L T O L D L I E O O R X E L D R G L T K X L X Q M K D B C B L O R L D L G D T X L L M L B L T K X L T V O L I X R T B L K D B L R O T L Y X T L F X X R L M D C G L.

166. By CACHE. (Contributor, C. H. Price, died without explaining his key).

03 65 12 45 58 28 06 41 72 14 22 03 02 17 36 88 25 20 55 77 74 51 23 45 41 42 30 24 36 61 96 09 07 78 05 44 08 06 55 92 16 93 02 15 36 37 40 87 41 01 33 77 06 36 27 54 48 29 16 78 92 66 03 10 38 17 45 23 72 96 73 01 49 25 72 38 92 72 24 55 48 08 40 92 28 01 72 96 02 04 74 61 06 99 30 45 72 69 74 93 77 23 55 36 24 93 47 84 76 35 32 89 87 76 77 64 51 96 58 43 76 02 81 38 87 69 89 55 99 23 79 55 51 06 99 71 74 69 89 84 27 25 22 39 42 53 19 93 41 66 09 75 87 37 91 87 90 91 43 19 40 30 38 16 96 22 69 38 78 02 74 92 47 25 77 91 15 40 24 45 07 07 96 48 44 15 12 06 99 44 93 19 25 23 55 30 45 87 96 18 01 78 44 29 45 86 47 69 48 30 66 44 03 41 66 37 38 22 06 42 41. 59.

Here is one which nobody has ever been able to decrypt:

V Q B U P P V S P G G F P N U E D O K D X H E W T I Y C L K X R Z A P V U F S A W E M U X G P N I V Q J M N J J N I Z Y K B P N F R R H T B W W N U Q J A J G J F H A D Q L Q M F L X R G G W U G W V Z G K F B C M P X K E K Q C Q Q L B O D O Q J V E L.