Vigenere Cipher - Decryption is tricky

[menquin]

Hello,
For my current project I must create a vigenere cipher in an excel spreadsheet. I have successfully encrypted my plain text, but I have had some trouble decrypting it.

The vigenere cipher works like this:
You have two text strings; a message(plain text), and a keyword.
To encrypt each letter, you use the vigenere square.
The first letter of the message is 'E' - therefore we go to the 'E' column (cell F1)
The first letter of the keyword is 'M' - so we go to the 'M' row (cell A14).
We then use the square to find where the column and row intersect - this is in cell F14 - so the first letter of our encrypted message is 'Q'.


I have used the functions 'MATCH' and 'INDEX' to encrypt, but am having problems reversing them for the decryption.

Please see the document attached.

Many thanks.