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The Playfair Cipher
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In 1854, Sir Charles Wheatstone invented the cipher known as "Playfair," named for
his friend Lyon Playfair, first Baron Playfair of St. Andrews, who popularized
and promoted the cipher. Its simplicity and its cryptographic strength compared
to simple substitution and Vigènere (a polyalphabetic substitution cipher) made it an immediate success as a field
cipher, used by the British in the Boer War and the First World War, and by
several armed forces as an emergency back-up cipher in the Second World War.
When Lt. John F. Kennedy's PT-109 was sunk by a Japanese cruiser in the Solomon
Islands, for instance, he made it to shore on Japanese-controlled Plum Pudding
Island and was able to send an emergency message in Playfair from an A
llied coast-watcher's hut to arrange the rescue of the survivors from his crew.
To encipher a message in Playfair, pick a keyword and write it into a
five-by-five square, omitting repeated letters and combining I and J in one
cell. In this example, we use the keyword MANCHESTER and write it into the
square by rows. It may be written in any other pattern; other popular choices
include writing it by columns or writing it in a spiral starting at one corner
and ending in the center. Follow the keyword with the rest of the alphabet's
letters in alphabetical order.
M A N C H
E S T R B
D F G I/J K
L O P Q U
V W X Y Z
First we need to prepare the plaintext message for encryption. To encrypt "THIS
SECRET MESSAGE IS ENCRYPTED," break it up into two-letter groups. If both
letters in a pair are the same, insert an X between them. If there is only one
letter in the last group, add an X to it.
TH IS SE CR ET ME SX SA GE IS EN CR YP TE DX
Now we encrypt each two-letter group. Find the T and H in the
square and locate the letters at opposite corners of the rectangle
they form:
. . N . H
. . T . B
. . . . .
. . . . .
. . . . .
Replace TH with those letters, starting with the letter on the
same row as the first letter of the pair: TH becomes BN. Continue this process
with each pair of letters:
TH IS SE CR ET ME SX SA GE IS EN CR YP TE DX
BN FR
Notice that S and E are in the same row. In this case we take
. . . . .
E S T . .
. . . . .
. . . . .
. . . . .
the letter immediately to the right of each letter of the pair,
so that SE becomes TS.
TH IS SE CR ET ME SX SA GE IS EN CR YP TE DX
BN FR TS
Now we see that C and R are in the same column. Use the letter
. . . C .
. . . R .
. . . I/J .
. . . . .
. . . . .
immediately below each of these letters, so that CR becomes RI. This is the
last special case, and the encryption proceeds without further incident.
TH IS SE CR ET ME SX SA GE IS EN CR YP TE DX
BN FR TS RI SR ED TW FS DT FR TM RI XQ RS GV
To decrypt the message, simply reverse the process: If the two letters are in
different rows and columns, take the letters in the opposite corners of their
rectangle. If they are in the same row, take the letters to the left. If they
are in the same column, take the letters above each of them.
Playfair Hint #1
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© | Updated November 2000
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