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Vigenère
An improved code was developed by Vigenère, where a different row is used for
each character encryption, and is polyalphabetic cipher as it uses a
number of cipher alphabets.hen the way that the user
moves between the rows must be agreed before encryption. This can be achieved
with a code word, which defines the sequence of the rows. For example the
codeword GREEN could
be used which defined that the rows used were: Row 6 (G), Row 17 (R), Row 4 (E),
Row 4 (E), Row 13 (N), Row 6 (G), Row 17 (R), and so on (see Table). Thus
the message is converted as:
Keyword GREENGREENGREE
Plaintext hellohowareyou
Ciphertext NVPPBNFAEEKPSY
The great advantage of this type of code is that the same plaintext
character will be encrypted with
different values, depending on the position of the keyword. For example, if the
keyword is GREEN, ‘e’ can be encrypted as ‘K’ (for G), ‘V’ (for R), ‘I’ (for E)
and ‘R’ (for N). To improve the security, the greater the size of the code word,
the more the rows that can be included in the encryption
process. Also, it is not possible to
decipher the code by a frequency
analysis,
as letters will change their coding depending on the current position of the
keyword. It is also safe from analysis of common two- and three-letter
occurrences, if the keysize is relatively long. For example ‘ee’ could be
encrypted with ‘KV’ (for GR), ‘VI’ (for RE), ‘II’ (for EE), ‘IR’ (for EN) and
‘RK’ (for NG) [Example].
Plain a b c d e f g h i j k l m n o p q r s t u v w x y z
1 b c d e f g h i j k l m n o p q r s t u v w x y z a
2 c d e f g h i j k l m n o p q r s t u v w x y z a b
3 d e f g h i j k l m n o p q r s t u v w x y z a b c
4 e f g h i j k l m n o p q r s t u v w x y z a b c d
5 f g h i j k l m n o p q r s t u v w x y z a b c d e
6 g h i j k l m n o p q r s t u v w x y z a b c d e f
7 h i j k l m n o p q r s t u v w x y z a b c d e f g
8 i j k l m n o p q r s t u v w x y z a b c d e f g h
9 j k l m n o p q r s t u v w x y z a b c d e f g h i
10 k l m n o p q r s t u v w x y z a b c d e f g h i j
11 l m n o p q r s t u v w x y z a b c d e f g h i j k
12 m n o p q r s t u v w x y z a b c d e f g h i j k l
13 n o p q r s t u v w x y z a b c d e f g h i j k l m
14 o p q r s t u v w x y z a b c d e f g h i j k l m n
15 p q r s t u v w x y z a b c d e f g h i j k l m n o
16 q r s t u v w x y z a b c d e f g h i j k l m n o p
17 r s t u v w x y z a b c d e f g h i j k l m n o p q
18 s t u v w x y z a b c d e f g h i j k l m n o p q r
19 t u v w x y z a b c d e f g h i j k l m n o p q r s
20 u v w x y z a b c d e f g h i j k l m n o p q r s t
21 v w x y z a b c d e f g h i j k l m n o p q r s t u
22 w x y z a b c d e f g h i j k l m n o p q r s t u v
23 x y z a b c d e f g h i j k l m n o p q r s t u v w
24 y z a b c d e f g h i j k l m n o p q r s t u v w x
25 z a b c d e f g h i j k l m n o p q r s t u v w x y For a keyword of KING, determine the following Vigenère codes:
If you are struggling, here the mapping for KING:
Plain a b c d e f g h i j k l m n o p q r s t u v w x y z
6 g h i j k l m n o p q r s t u v w x y z a b c d e f
8 i j k l m n o p q r s t u v w x y z a b c d e f g h
10 k l m n o p q r s t u v w x y z a b c d e f g h i j
13 n o p q r s t u v w x y z a b c d e f g h i j k l m For example
phase becomes zpnyo, as p (read row 10 for K) gives Z, h (read
row 8 for I) gives P, a (read row 13 for N) gives n), and so on.
If you are struggling here is a Vigenère Calculator [Link].
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