The sample ASP.NET/ASP site with the custom Http handler is available for download at:

http://download.microsoft.com/download/9/C/D/9CD0A246-5F89-4854-B4B6-CAE0D3F386BE/WEB343.msi

The sample also depends on a hashing helper that signs and validates the header values. 

That code is not in the MSI - but the sample code is instead shown below. You can create a project that outputs a DLL called "HashingHelper.dll".  For it to be callable from Classic ASP, you will need to run "gacutil -i" to install the assembly in the GAC and you will also need to use "regasm" to register the class in the assembly with COM.

HashHelper.cs:
using System;
using System.Collections.Generic;
using System.Text;
using System.Security.Cryptography;

namespace HashingHelper
{
    public class HashHelper
    {
        private static string hashKey =
            "You need to supply a valid 128 character long hex key here";

        private static byte[] bKey;

        static HashHelper()
        {
            bKey = ConvertStringKeyToByteArray(hashKey);
        }

        public static byte[] ConvertStringKeyToByteArray(string stringizedKeyValue)
        {
            byte[] keyBuffer = new byte[64];
            for (int i = 0; i < hashKey.Length; i = i + 2)
            {
                keyBuffer[i / 2] = Byte.Parse(hashKey.Substring(i, 2), System.Globalization.NumberStyles.HexNumber);
            }

            return keyBuffer;
        }

        public static string ConvertByteArrayToString(byte[] value)
        {
            StringBuilder sb = new StringBuilder(128);
            foreach (byte b in value)
            {
                int i = (int)b;
                sb.Append(i.ToString("X2"));

            }

            return sb.ToString();
        }

        public static string HashStringValue(string valueToHash)
        {
            using (HMACSHA1 hms = new HMACSHA1(bKey))
            {
                return ConvertByteArrayToString(hms.ComputeHash(Encoding.Unicode.GetBytes(valueToHash)));
            }

        }

        public static bool ValidateHash(string value, string hash)
        {
            using (HMACSHA1 hms = new HMACSHA1(bKey))
            {
                if (HashStringValue(value) != hash)
                    return false;
                else
                    return true;
            }
        }

#region COM support
        public HashHelper() { }

        public bool ValidateHashCOM(string value, string hash)
        {
            return HashHelper.ValidateHash(value, hash);
        }

#endregion

    }
}

AssemblyInfo.cs:

using System.Reflection;
using System.Security;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;

// General Information about an assembly is controlled through the following
// set of attributes. Change these attribute values to modify the information
// associated with an assembly.
[assembly: AssemblyTitle("HashingHelper")]
[assembly: AssemblyDescription("")]
[assembly: AssemblyConfiguration("")]
[assembly: AssemblyCompany("Me")]
[assembly: AssemblyProduct("HashingHelper")]
[assembly: AssemblyCopyright("Copyright Me")]
[assembly: AssemblyTrademark("")]
[assembly: AssemblyCulture("")]

// Setting ComVisible to false makes the types in this assembly not visible
// to COM componenets.  If you need to access a type in this assembly from
// COM, set the ComVisible attribute to true on that type.
[assembly: ComVisible(true)]

// The following GUID is for the ID of the typelib if this project is exposed to COM
[assembly: Guid("09f8b521-ad1b-4545-94e6-51e94d89a2ac")]

// Version information for an assembly consists of the following four values:
//
//      Major Version
//      Minor Version
//      Build Number
//      Revision
//
// You can specify all the values or you can default the Revision and Build Numbers
// by using the '*' as shown below:
[assembly: AssemblyVersion("1.0.0.0")]
[assembly: AssemblyFileVersion("1.0.0.0")]

[assembly: AllowPartiallyTrustedCallers()]