Four Square Cipher

Description

A four square tables are prepared. The squares are filled with the letters of the alphabet, two of them with mixed alphabets. For example, the key word KEY applied to a 25 letter alphabet gives the following table:
1st plain text 1st cipher text
A B C D E K E Y A B
F G H I K C D F G H
L M N O P I L M N O
Q R S T U P Q R S T
V W X Y Z U V W X Y
2nd cipher text 2nd plain text
W O R D A A B C D E
B C E F G F G H I K
H I K L M L M N O P
N P Q S T Q R S T U
U V X Y Z V W X Y Z
To encipher the text, is is split into groups of two letters. If the number of the letters in the text is odd, a random letter must be appended (CipherClerk's Applet appends the first letter of the message): MESSAGE becomes ME SS AG EM. The two letters are located in the plain text squares and replaced by the letters found in the same row of the cipher text squares above or below the other letter: ME is thus replaced by OO, AG by EB.

CipherClerk's Applet's implementation requires two keyword, but doesn't check that they are different. You may choose from alphabets with 25 and 36 letters:


To proceed, you may