xp_repl_encrypt is a one way encrytion tool. This is how you would use it:

create proc sp_TestCCNum 
@cardNumber1 varchar(180) 
as
declare @cardNumber2 varchar(180) 
--here you would fetch the encrypted field value and forego the 
--encryption of this value 
set @cardNumber2 = '4111111111111111'
exec master..xp_repl_encrypt @cardNumber1 output 
exec master..xp_repl_encrypt @cardNumber2 output 
-- @cardNumber1 and @cardNumber2 are now encrypted
if(@cardNumber1 = @cardNumber2) 
begin 
print 'Numbers are the same' 
end 
else 
begin 
print 'Numbers are different' 
end
go
exec sp_TestCCNum '4111111111111111'
--http://www.devarticles.com/c/a/SQL-Server/Extended-Stored-Procedures-Intro-And-10-Cool-Examples/2/

The whole point of one way encryption is to not being able to decrypt it, just like passwords - nobody else should ever (yeah, right! perhaps brute force... Uhmmmm, that is an excercise left for others.) be able to decrypt the info, only verify that the entered info matches what is stored.