How To Encrypt And Decypt As A IUSR With Read And Write Only Rights

How to decrypt or encrypt without making user a db_owner. It is for a web
application and I do not want make the web user a db_owner. Is there a
way to make this work without making the user a db_owner. Currently the
user is a db_datareader and db_datawriter.

I created an asymmetric key
for encryption by password. I am not using a master key because I want
to keep the password seperately on the web server, so a hacker cannot
get access to both if database gets hacked.

These are the steps I took when I logged in to SQL server management studio using windows authentication:

CREATE ASYMMETRIC KEY ccnumber WITH ALGORITHM = RSA_512
ENCRYPTION BY PASSWORD = 'password';

INSERT INTO Payments (CreditCardNumber,enc_CreditCardNumber)
values( '458724124',
EncryptByAsymKey(AsymKey_ID('ccnumber'), '458724124') )

SELECT CONVERT(varchar(50), DecryptByAsymKey( AsymKey_Id('ccnumber'), enc_CreditCardNumber, N'password' ))
AS Creditcardnumber , Creditcardnumber
FROM payments where Creditcardnumber = '458724124'

When
I use the above select statement it works if I make the user a db_owner
but I get null if the user is just db_reader and db_writer.

Is there a way to do encryption without making the user a db_owner?