In the context of cryptography, serration is a minor transposition cipher used to split up common letter pairs 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.

Ser*ra"tion (?), n.

1.

Condition of being serrate; formation in the shape of a saw.

2.

One of the teeth in a serrate or serrulate margin.

 

© Webster 1913.

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