Cryptography Reference
In-Depth Information
• When A and R are in the same row, A is
encrypted as R and R (reading the row cycli-
cally) as M.
• When I and S are in the same column, I is
encrypted as S and S as X.
• When a double letter occurs, a spurious sym-
bol, say Q, is introduced so that the MM in
SUMMER is encrypted as NL for MQ and CL
for ME.
• An X is appended to the end of the plaintext if
necessary to give the plaintext an even number
of letters.
Encrypting the familiar plaintext example using
Sayers's Playfair array yields:
If the frequency distribution information were
totally concealed in the encryption process, the cipher-
text plot of letter frequencies in Playfair ciphers would
be flat. It is not. The deviation from this ideal is a mea-
sure of the tendency of some letter pairs to occur more
frequently than others and of the Playfair's row-and-
column correlation of symbols in the ciphertext—the
essential structure exploited by a cryptanalyst in solv-
ing Playfair ciphers. The loss of a significant part of
the plaintext frequency distribution, however, makes a
Playfair cipher harder to cryptanalyze than a monoal-
phabetic cipher.
V
igenère
C
iPhers
The other approach to concealing plaintext struc-
ture in the ciphertext involves using several different
