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Consider the following scenario:
On a Windows Vista-based computer that has Microsoft Office Outlook 2007 installed, you create an S/MIME message.
You send this message as a clear-text signed message.
The recipient tries to open the message in Outlook 2007 on a Windows XP-based computer.
In this scenario, the message cannot be opened. Additionally, the recipient may receive the following error message:
Cannot open this item
The necessary security algorithm is not supported
Note You expect the recipient to be able to open the message. Additionally, the recipient should receive a warning message that states that Outlook 2007 cannot verify the signature.
Windows Vista uses the Cryptography Next Generation (CNG) algorithm to sign an e-mail message. This algorithm is unavailable in Windows XP. When Outlook 2007 calls the Crypt32 function to decode a signed e-mail message, the Crypt32 function fails immediately because it does not recognize the hash algorithm. Therefore, Outlook 2007 cannot decode the message.
Service pack information for Windows XP
To resolve this problem, obtain the latest service pack for Windows XP. For more information, click the following article number to view the article in the Microsoft Knowledge Base:
322389 How to obtain the latest Windows XP service pack
A hotfix is available to resolve this issue. After you apply this hotfix to the Windows XP-based client computer, a clear-text signed message can be opened, and Outlook will warn the recipient that it cannot verify the signature.
A supported hotfix is available from Microsoft. However, this hotfix is intended to correct only the problem that is described in this article. Apply this hotfix only to systems that are experiencing this specific problem.
If the hotfix is available for download, there is a "Hotfix download available" section at the top of this Knowledge Base article. If this section does not appear, submit a request to Microsoft Customer Service and Support to obtain the hotfix.
Note If additional issues occur or if any troubleshooting is required, you might have to create a separate service request. The usual support costs will apply to additional support questions and issues that do not qualify for this specific hotfix. For a complete list of Microsoft Customer Service and Support telephone numbers or to create a separate service request, visit the following Microsoft Web site:
Note The "Hotfix download available" form displays the languages for which the hotfix is available. If you do not see your language, it is because a hotfix is not available for that language.
To apply this hotfix, you must have Windows XP Service Pack 2 (SP2) or Windows XP Service Pack 3 (SP3) installed.
You must restart the computer after you apply this hotfix.
Hotfix replacement information
This hotfix does not replace any other previously released hotfixes.
To use this hotfix, you do not have to make any changes to the registry.
The English version of this hotfix has the file attributes (or later file attributes) that are listed in the following table. The dates and times for these files are listed in Coordinated Universal Time (UTC). When you view the file information, it is converted to local time. To find the difference between UTC and local time, use the Time Zone tab in the Date and Time item in Control Panel.
Windows XP SP2, x86-based versions
Windows XP SP3, x86-based versions
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
For more information, click the following article number to view the article in the Microsoft Knowledge Base:
824684 Description of the standard terminology that is used to describe Microsoft software updates