Article ID: 216829 - Last Review: June 23, 2005 - Revision: 3.2

How to Write a Client Authentication Certificate to a File in IIS 4.0

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This article was previously published under Q216829
We strongly recommend that all users upgrade to Microsoft Internet Information Services (IIS) version 6.0 running on Microsoft Windows Server 2003. IIS 6.0 significantly increases Web infrastructure security. For more information about IIS security-related topics, visit the following Microsoft Web site:
http://www.microsoft.com/technet/security/prodtech/IIS.mspx (http://www.microsoft.com/technet/security/prodtech/IIS.mspx)
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SUMMARY

Sometimes it may be useful to store a public key certificate for future use. The following ASP code takes the client authentication certificate provided by the client and writes it to a file called c:\Cert.txt.

For this to work, you must first configure a virtual directory to use SSL and require a client authentication certificate. You can analyze the certificate using the CERTUTIL tool that ships with the Windows NT Option Pack. For more information on how to do this, please see the following article in the Microsoft Knowledge Base:
216830  (http://support.microsoft.com/kb/216830/EN-US/ ) IIS: How to View the Contents of a Certificate
The following is the ASP code needed for this to work:

<% @Language = VBScript %>
<% Response.Buffer = True %>
<HTML>
<BODY> 
<% 
	'Instantiate the ASP FileSystemObject in order
	'to create a text file
	Set fs = Server.CreateObject("Scripting.FileSystemObject")

	'Create text file using append mode
         Set outStream = fs.OpenTextFile( "C:\cert.txt", 8, True )

	'Save certificate issuer information to text file
         outStream.WriteLine( "# Issuer: " & Request.ClientCertificate("Issuer") )

         'Extract certificate subject (user) and account information
	'from certificate
         su = Request.ClientCertificate( "Subject" )
         mx = len(su)
         for x = 1 to mx
            if mid(su,x,1)=chr(10) or mid(su,x,1)=chr(13) then
               su=left(su,x-1)+";"+right(su,mx-x)
	    end if
         next
         outStream.WriteLine( "# Subject: " & su )
	 outStream.WriteLine( "# Account: " & Request.ServerVariables("REMOTE_USER") )

	'Extract encrypted certificate text from certificate; encode text as 64-bit data 
         uue = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"

         outStream.WriteLine( "-----BEGIN CERTIFICATE-----" )
         cer = Request.ClientCertificate( "Certificate" )
         lcer = len(cer)
         l = 0

         for x = 1 to lcer step 3
            a1 = asc(mid(cer,x,1))
            if x+1 <= lcer then
                a2 = asc(mid(cer,x+1,1))
                if x+2 <=lcer then
                    a3 = asc(mid(cer,x+2,1))
                else
                    a3 = 0
                end if
            else
                a2 = 0
                a3 = 0
            end if
            outStream.Write mid(uue, (a1 and 252)/4 +1 ,1)
            outStream.Write mid(uue, (a1 and 3)*16 + (a2 and 240)/16 +1 ,1)
            if x+1 <= lcer then
                outStream.Write mid(uue, (a2 and 15)*4 + (a3 and 192)/64 +1 ,1)
                if x+2 <= lcer then
                    outStream.Write mid(uue, (a3 and 63) +1 ,1)
                else
                    outStream.Write "="
                end if
            else
                outStream.Write "=="
            end if
            l = l +4
            if l = 64 then
                outStream.WriteLine("")
                l = 0
            end if
         next
         if l > 0 then
	     outStream.WriteLine( "" )
         end if
         outStream.WriteLine( "-----END CERTIFICATE-----" )
         
         Response.Write "Client certificate information has been received and logged successfully<br>"
 %>
</BODY>
APPLIES TO
  • Microsoft Internet Information Server 4.0
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Keywords: 
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