The Vigenere cypher is a transposition cypher that uses all the permutations of the alphabet (in-order) and a supplementary key to encrypt with.
The table of permutations of the alphabet is:
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 A 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 B 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 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 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 E 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 F 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 G 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 H 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 I 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 J 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 K 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 L 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 M 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 N 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 O 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 P 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 Q 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 R 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 S 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 T 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 U 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 V 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 W 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 X 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 Y 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 Z 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
In order to encrypt a plaintext, for example it is a good day with the key was, write the key repeatedly as many times as necessary above the plaintext:
Key: w a s w a s w a s w a s Plaintext: i t i s a g o o d d a y
and then look-up the letter of the key in the row alphabet, the plain-text letter in the column alphabet and write down the intersection:
Key: w a s w a s w a s w a s Plaintext: i t i s a g o o d d a y Cyphertext: e t a o a y k o v z a q
the cypher-text is thus: et ao a ykov zaq.
Decryption is similarly performed by writing the key repeatedly above the cypher-text:
Key: w a s w a s w a s w a s Cyphertext: e t a o a y k o v z a q
We then take (in-sequence) a letter of the key and go to the row corresponding to that letter, slide all the way across the row to find the cypher-text letter and then go up to find the column letter in order to reveal the plain-text letter:
Key: w a s w a s w a s w a s Cyphertext: e t a o a y k o v z a q Plaintext: i t i s a g o o d d a y
which results in the plain-text: it is a good day.