Problem Runnind SSIS Package Protected With Password From MSDB

Hello,
I need to run SSIS packages with the 32 bit dtexec version, and I am storing these files into the MSDB package store, in SQL Server 2005. I have chosen to encrypt sensitive data with password, and then to execute the packages with xp_cmdshell.
The problem is that I receive the following message when I try to run it from management studio (when running this, I am logged in with the sa user): exec master..xp_cmdshell 'dtexec /Ser ServerName /SQL "TestFolderPackage" /De "testPass"'
€œMicrosoft (R) SQL Server Execute Package Utility
Version 9.00.1399.06 for 32-bit
Copyright (C) Microsoft Corp 1984-2005. All rights reserved.
NULL
Started: 9:41:29 AM
Error: 2008-05-22 09:41:29.70
Code: 0xC0016016
Source:
Description: Failed to decrypt protected XML node "DTSassword" with error 0x8009000B "Key not valid for use in specified state.". You may not be authorized to access this information. This error occurs when there is a cryptographic error. Verify that the correct key is available.
End Error€?
If I run the same package with the same command specified above, but without sensitive data in it (a simpler version with no passwords and connection strings, it works fine)
exec master..xp_cmdshell 'dtexec /Ser ServerName /SQL "TestFolderPackage"'
If I run the firs package version (with sensitive data and password protected) from the command line, everything works well:
DTExec.exe /ServerName OLAP /SQL "TestFolderPackage" /De "testPass"
I know in ASP.NET when encrypting web.config section, the system stores the decryption keys in a app data folder, and in order to be able to read from the encrypted web.config, the user under which runs the ASP.NET must be granted access to that folder, by running asp_regiis.exe with some parameters. I believe here a have a similar problem, with users not being granted access to encryption keys.
Thanks in advance