Cryptography Reference
In-Depth Information
TABLE 1.3
Plaintext letter
A
B
C
D
......
W
X
Y
Z
Ciphertext letter
D
E
F
G
......
Z
A
B
C
would be enciphered as
ILUH PLVVLOH.
In practice, however, one usually groups these letters into blocks, say 5 letters each. A
cryptanalyst can easily guess certain mappings if the ciphertext words are the same size as
the plaintext words. Thus, we would probably send the previous message as
ILUHP
LVVLO H.
To decipher, one simply shifts each ciphertext letter 3 letters up the alphabet, again tak-
ing wrap-around into account.
Every cipher has at least one key, which may need to be kept secret. In the case of the
Caesar cipher, the key is the shift value, say
k
= 3. This key must certainly be protected
from unauthorized users, as knowing it allows decryption. In general, we can choose any
shift value we wish for a Caesar cipher.
1.3
FREQUENCY ANALYSIS ON CAESAR CIPHERS
Of course, the Caesar cipher is easily breakable, using what is called frequency analysis. We
can proceed in the following way:
1.
Suppose the message is English text. (The message may not be English text, but the prin-
ciple remains the same.)
2.
Note that the most common letter appearing in English text is “E.”
3.
Examine as much ciphertext as possible. The character appearing most often is proba-
bly the character “E” enciphered.
4.
The distance between “E” and the enciphered character is the shift value.
Of course this guess may be wrong, but it is a pretty fair guess with this simple cipher.
Frequency analysis exploits the fact that languages are biased in that some letters appear
much more frequently in text than others, and that some ciphers preserve this bias. Fre-
quency analysis is only useful for simple ciphers, however, such as this one.
E
XAMPLE
.
Take a look at the following ciphertext, which was produced using a Caesar
cipher:
WFIDZ JVORT KCPVD GKZEV JJVDG KZEVJ JVORT KCPWF IDJFZ KZJNZ KYJVE
JRKZF EGVIT VGKZF EDVEK RCIVR TKZFE REUTF EJTZF LJEVJ JRCCK YZEXJ
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