Outlook Add-In: How to read the writing text while creating new email in Outlook 2010?

Outlook Add-In: How to read the writing text while creating new email in
Outlook 2010?

I am developing an Outlook 2010 Add-In to encrypt and decrypt text in
email using RSA algorithm. I am using Visual C# 2010. I have finished
creating a program for RSA algorithm. I also created a new ribbon, and
three buttons (key generation, encrypt and decrypt) in the ribbon like the
image below.

What I want to ask is how to read or catch the blocked text above to my
program? The blocked text will be encrypted using RSA, after the cipher
obtained, the blocked message will be replaced by the cipher. The last
question, how to replace the blocked text with the cipher?
I have googling for the answer but can't find the right one. Any help is
greatly appreciated. Many Thanks.