Vignare square, is an excellent cryptostystem.

Plain alphabet
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

Cipher alphabet
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
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
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
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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
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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

Ciphering
Message : i love you (selected for simplicity)

Keyword : u want win (it should be similar to the message in alphabet count)

Cipher text : c haix uwh

Here the message's 'i' is coded using keyword's 'u' with the help of Vignare square. Consider the last line's alpahbet 'i' and the line starting with keyword 'u'. The intercept gives 'c'. Similarly other alphabets are coded.

Try the above method of coding using another keyword plane plane. Decoding is difficult without knowing the keyword.