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Autokey Cipher
 
The Autokey Cipher is a constantly rotating cipher using the Vigenère tableau, but unlike a Vigenère cipher, which uses a word as the key and encrypts the text just once, the autokey cipher uses a single letter as the key and encypts the text for each character.

The entire original text is first rotated by the key letter chosen. The first letter of the result becomes the first letter of the final encrypted text and the key for the next rotation. The new text is rotated again using this new key. The second letter of the newly encrypted text becomes the next key, and the second letter of the final encrypted text. This process continues using the third, fourth, etc. letter until all characters of the text have been encoded.

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
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 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 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
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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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
  Example:

Using the simple word HELLO and the key letter B:

Find the key letter in the left row-header and the intersecting character of the text from the top column-header, continuing this method for each letter of the text, to get the first rotation "IFMMP". The new key letter is I and we then rotate IFMMP, continuing for every letter - see the table below:

Key Letter
Rotated text
Letter
Placement
Encrypted
Letter
B
IFMMP
1st
I
I
QNUUX
2nd
N
N
DAHHK
3rd
H
H
KHOOR
4th
O
O
YVCCF
5th
F

We can see from this that the word HELLO encrypted using the key letter B becomes INHOF.

To save you doing it all manually a converter is provided below.

 
 

Autokey Converter
Original Text:
Key Letter
Encrypted/Decrypted Text: