In the context of cryptography
, serration is a minor
used to split up common letter pair
s in simpler
ciphers. With the playfair cipher
and four square
cipher, it is
possible to look at the frequency of letter pairs.
Consider the following (common letter pairs have been underlined):
this is the clear text not encrypted
If the cipher uses letter pairs (as the playfair), the letter pairs will
be held together, and thus 'visible' to the codebreaker. The following is
the encrypted text of using the playfair cipher:
SLRT RTSL KDDB YSIB ZSOP IBMD YGQS DN
Note the pairs 'TH' becomes 'SL' and 'IS' becomes 'RT'.
Using serration upon this, the clear text gets paired up. Instead of
the pairs being:
th is is th ec le ar te xt no te nc ry pt ed
the pairs are now:
t h i s i s t h e c l e a r t
e x t n o t e n c r y p t e d
Once encrypted, it is:
BISP RFPI NRTF BIPD DKFG BGNA SBYI LI
After the serration has happened, the letter pair frequencies get all thrown
out. Even though 'BI' occurs multiple times, it does not correspond to
any frequent letter pair in a word.
Serration also helps foil the probable word attack.