Is it safe to put a "decrypt" routine in a public shared class?

I have a public shared class that my asp.net project uses.  It is in the APP_CODE folder.  
My asp.net project was created as a "website" and not a "web-project", which means I don't compile it at once, I make changes to each page, and then ftp them over to my server.  The first time someone accesses a page, the whole 500 page project recompiles. 
Anyway, in my public shared class, I have a "encrypt/decrypt" routine.  The idea is that certain user-entered information, such as email addresses, would be stored more safely if encrypted first.
But could those routines be accessed by a hacker who 'referenced' the public class?
As a side question - sql server has its own way of encrypting and decrypting database fields.  I could use that instead, and the advantage is that all the encryption and decryption would be invisible to my code.  The disadvantage is that the key would be presumably ...