Decrypt Sproc Returning NULL To Non DBO.

I'm still having issues with this despite my attempts to resolve. I even
have "with exec as dbo" in my sproc, and and "exec as dbo" in my execution,
but still the encrypted data returns nulls when I exec as a user other than
DBO. Below is precisely what I have done. All ideas are welcomed.

TIA, ChrisR


--If there is no master key, create one now

IF NOT EXISTS
(SELECT * FROM sys.symmetric_keys WHERE symmetric_key_id = 101)
CREATE MASTER KEY ENCRYPTION BY
PASSWORD =
'23987hxJKL95QYV4369#ghf0%94467GRdkjuw54ie5y01478d Dkjdahflkujaslekjg5k3fd117
r$$#1946kcj$n44ncjhdlj'
GO

CREATE CERTIFICATE HumanResources037
WITH SUBJECT = 'Employee Social Security Numbers';
GO

CREATE SYMMETRIC KEY SSN_Key_01
WITH ALGORITHM = DES
ENCRYPTION BY CERTIFICATE HumanResources037;
GO

USE [AdventureWorks];
GO

-- Create a column in which to store the encrypted data
ALTER TABLE HumanResources.Employee
ADD EncryptedNationalIDNumber varbinary(128);
GO

-- Open the symmetric key with which to encrypt the data
OPEN SYMMETRIC KEY SSN_Key_01
DECRYPTION BY CERTIFICATE HumanResources037;

-- Encrypt the value in column NationalIDNumber with symmetric
-- key SSN_Key_01. Save the result in column EncryptedNationalIDNumber.
UPDATE HumanResources.Employee
SET EncryptedNationalIDNumber = EncryptByKey(Key_GUID('SSN_Key_01'),
NationalIDNumber);
GO

-- Verify the encryption.
-- First, open the symmetric key with which to decrypt the data
OPEN SYMMETRIC KEY SSN_Key_01
DECRYPTION BY CERTIFICATE HumanResources037;
GO

-- Now list the original ID, the encrypted ID, and the
-- decrypted ciphertext. If the decryption worked, the original
-- and the decrypted ID will match.

alter procedure getDecryptedIDNumber
with exec as owner
as
SELECT NationalIDNumber, EncryptedNationalIDNumber
AS "Encrypted ID Number",
CONVERT(nvarchar, DecryptByKey(EncryptedNationalIDNumber))
AS "Decrypted ID Number"
FROM HumanResources.Employee;
GO

/*works for me, shows the decrypted data*/

exec getDecryptedIDNumber

USE [master]
GO

CREATE LOGIN [test] WITH PASSWORD=N'test',
DEFAULT_DATABASE=[AdventureWorks], CHECK_EXPIRATION=OFF, CHECK_POLICY=OFF
GO

USE [AdventureWorks]
GO

CREATE USER [test] FOR LOGIN [test]
GO

use [AdventureWorks]
GO

GRANT EXECUTE ON [dbo].[getDecryptedIDNumber] TO [test]
GO

GRANT IMPERSONATE ON USER:: dbo TO test;
GO

/*Now, open up a "file/new/DB Engine Query" and login with the test login*/
exec as user = 'dbo'
exec getDecryptedIDNumber

/*This returns NULL values where it should show the decrypted data*/