Restrict permissions to execute a stored procedure - sql-server-2008

Question

Here is the context.
two databases, database1 and database2.
an asp.net web application that accesses database1 using user user1 (SQL Server authentication).
a stored procedure sp1 in database1 that needs to modify some sensitive data in database2 through a stored procedure sp2 defined in database2. In other words sp1 calls database2..sp2.
I created a SQL Server user2 login that is mapped to user2 in database1 and user2 database2. user2 in database2 has the permission to execute sp2.
I want user1 to be able to modify the database2 only through the sp1. I don't want user1 to be able to run: exec database2..sp2.
Here is some code that I tried:
USE [database2];
GO
SET ANSI_NULLS ON;
GO
SET QUOTED_IDENTIFIER ON;
GO
CREATE PROCEDURE [dbo].[sp2]
WITH
EXEC AS CALLER AS
SELECT current_user
GO
GRANT EXECUTE ON [dbo].[sp2] TO [user2]
GO
In database1 I ran:
use database1
go
drop procedure sp1
go
create procedure sp1
as
declare #m nvarchar(4000)
select current_user as usr
execute as login='user2'
begin try
execute database2.dbo.sp2
end try
begin catch
set #m = error_message()
print #m
revert
end catch
go
grant execute on dbo.sp1 to user1
go
execute as login='user1'
go
execute dbo.sp1
go
revert
go
Unfortunately this (i.e. exec dbo.sp1 in the context of login='user1') doesn't work, it returns:
Cannot execute as the server principal because the principal "user2" does not exist, this type of principal cannot be impersonated, or you do not have permission
I also tried to compile sp1 with with execute as 'user2', however, it doesn't work.
Is there another way I can make this work?
Thanks
Update:
If I run this:
GRANT IMPERSONATE ON User::user2 to [User1];
GO
then the script works but it means User1 can now execute Database2..sp2.
What I want in a way is something like this: grant impersonate on User::user2 to [sp1], i.e grant the impersonation permission to a procedure but I am not sure it's possible.
I found this thread which I think it is relevant to my case as well.

Answer 1

ok, based on the thread I mentioned it is possible.
Here are the scripts to achieve this:
use database1
go
CREATE CERTIFICATE [PermissionsCert]
AUTHORIZATION [dbo]
ENCRYPTION BY PASSWORD = 'WeakPassword'
WITH SUBJECT = 'Used to test granting permissions to code',
EXPIRY_DATE = '2099-12-31';
ADD SIGNATURE TO [dbo].[sp1]
BY CERTIFICATE [PermissionsCert]
WITH PASSWORD = 'WeakPassword';
BACKUP CERTIFICATE [PermissionsCert] TO FILE = 'k:\Sql_Backup\PermissionsCert.cer'
use database2
go
CREATE CERTIFICATE PermissionsCert
FROM FILE = 'k:\Sql_Backup\PermissionsCert.cer'
CREATE USER [PermissionsUser] FROM CERTIFICATE [PermissionsCert];
grant execute on dbo.sp2 to [PermissionsUser]
go
I modified sp1 as follows:
alter procedure sp1
as
declare #m nvarchar(4000)
select current_user as usr
--execute as user='user2'
begin try
execute Database2.dbo.sp2
end try
begin catch
set #m = error_message()
print #m
revert
end catch
go
This works now:
execute as login='user1'
go
execute dbo.sp1
go
revert
go
However, this fails, which is what I want:
execute as login='user1'
go
execute database2.dbo.sp2
go
revert
go