A History of Cryptography - Introduction to Cryptography with Java Applets - page 7

We must count the frequency of each letter in the ciphertext, and then compare these
frequencies with the relative frequency table. Here are the counts for each letter:

A: 6 B: 17 C: 17 D: 39 E: 17 F: 67

G: 13 H: 25 I: 0 J: 26 K: 3 L: 4

M: 29 N: 30 O: 15 P: 1 Q: 6 R: 3

S: 15 T: 21 U: 28 V: 0 W: 0 X: 26

Y: 26 Z: 7

F is by far the most common letter, and its plaintext partner is probably E. The next most
common letters are D, N, M, U, J, X, and Y, which are likely the mappings of A, I, N, O, R,
S, and T. The least frequent ciphertext letters are I, V, and W, which are likely the mappings
of Q, X, and Z. These guesses may of course be wrong, but once you start trying different
combinations words will start to appear in the plaintext. As you progress, you can start to
make educated guesses about the mappings; this process starts out slowly, but quickly speeds
up. Table 1.5 shows the mapping for this cipher.
Using this mapping, we see that the plaintext is:

THELO RDISM YSHEP HERDI SHALL NOTBE INWAN THEMA KESME LIEDO
WNING REENP ASTUR ESHEL EADSM EBESI DEQUI ETWAT ERSHE RESTO
RESMY SOULH EGUID ESMEI NPATH SOFRI GHTEO USNES SFORH ISNAM
ESSAK EEVEN THOUG HIWAL KTHRO UGHTH EVALL EYOFT HESHA DOWOF
DEATH IWILL FEARN OEVIL FORYO UAREW ITHME YOURR ODAND YOURS
TAFFT HEYCO MFORT MEYOU PREPA REATA BLEBE FOREM EINTH EPRES
ENCEO FMYEN EMIES YOUAN OINTM YHEAD WITHO ILMYC UPOVE RFLOW
SSURE LYGOO DNESS ANDLO VEWIL LFOLL OWMEA LLTHE DAYSO FMYLI
FEAND IWILL DWELL INTHE HOUSE OFTHE LORDF OREVE R

1.5

POLYALPHABETIC SUBSTITUTION CIPHERS
As one can readily see, monoalphabetic substitution ciphers are notoriously easy to break.
In the case of the Caesar cipher, the shift value can be uncovered rather easily. One way clas-
sical cryptographers dealt with this was to use different shift values for letters depending on
their position in the text. For example, one may do something like the following:

• Let a 1 , a 2 , . . . , a n be the letters in a plaintext message. Consider the letter a p :

• If p is divisible by 4, shift a p 7 letters down the alphabet.

• If p is of the form 4 k + 1 for some k , shift a p 5 letters down the alphabet.

• If p is of the form 4 k + 2 for some k , shift a p 13 letters down the alphabet.

• If a p 2 letters down the alphabet.

Using this scheme, we can encipher the message

p is of the form 4 k + 3 for some k , shift

DEFCON FOUR

as shown in Table 1.6.