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The Polyalphabetic Cipher

The basic idea is to exchange each of the letters of a message with letters from a multiple ciphertext alphabets, based on a keyword.
The letter of the plaintext is substituted with a letter from the ciphertext alphabet that begins with each letter of the keyword in turn.
If the keyword is 'MADCOW', then you would use the cypher text alphabet that begin with 'M' for the first letter, 'A' for the next,
and so on, 'D' 'C' 'O' 'W', and then repeating the sequence again with the keyword until the entire message has been encrypted.
This can be shown by using a Vigenere table:

ABCDEFGHIJKLMNOPQRSTUVWXYZ Plain Text Alphabet

ABCDEFGHIJKLMNOPQRSTUVWXYZ Cipher Text Alphabets
BCDEFGHIJKLMNOPQRSTUVWXYZA
CDEFGHIJKLMNOPQRSTUVWXYZAB
DEFGHIJKLMNOPQRSTUVWXYZABC
EFGHIJKLMNOPQRSTUVWXYZABCD
FGHIJKLMNOPQRSTUVWXYZABCDE
GHIJKLMNOPQRSTUVWXYZABCDEF
HIJKLMNOPQRSTUVWXYZABCDEFG
IJKLMNOPQRSTUVWXYZABCDEFGH
JKLMNOPQRSTUVWXYZABCDEFGHI
KLMNOPQRSTUVWXYZABCDEFGHIJ
LMNOPQRSTUVWXYZABCDEFGHIJK
MNOPQRSTUVWXYZABCDEFGHIJKL
NOPQRSTUVWXYZABCDEFGHIJKLM
OPQRSTUVWXYZABCDEFGHIJKLMN
PQRSTUVWXYZABCDEFGHIJKLMNO
QRSTUVWXYZABCDEFGHIJKLMNOP
RSTUVWXYZABCDEFGHIJKLMNOPQ
STUVWXYZABCDEFGHIJKLMNOPQR
TUVWXYZABCDEFGHIJKLMNOPQRS
UVWXYZABCDEFGHIJKLMNOPQRST
VWXYZABCDEFGHIJKLMNOPQRSTU
WXYZABCDEFGHIJKLMNOPQRSTUV
XYZABCDEFGHIJKLMNOPQRSTUVW
YZABCDEFGHIJKLMNOPQRSTUVWX
ZABCDEFGHIJKLMNOPQRSTUVWXY

Using the keyword 'MADCOW', and the message 'FLEA BAG', would be encrypted by substituting the 'F' for an 'R', from the cypher
text alphabet that begins with 'M', and the 'L' would be substituted with a 'L' from the cypher text alphabet that begins with 'A'
The process would continue, cycling through the keyword, the resultant ciphertext would be 'RLHC PWS'

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