<system.diagnostics>
        <sources>
            <source name="System.Security.Cryptography.Xml.SignedXml"
                    switchName="XmlDsigLogSwitch">
                <listeners>
                    <add name="logFile" />
                </listeners>
            </source>
        </sources>
        <switches>
            <add name="XmlDsigLogSwitch" value="Verbose" />
        </switches>
        <sharedListeners>
            <add name="logFile"
                 type="System.Diagnostics.TextWriterTraceListener"
                 initializeData="c:\temp\NETXmlDsigLog.txt"/>
        </sharedListeners>
        <trace autoflush="true">
            <listeners>
                <add name="logFile" />
            </listeners>
        </trace>
    </system.diagnostics>

public class CustomXmlDsigExcC14NTransform : XmlDsigExcC14NTransform
{
    public override byte[] GetDigestedOutput(HashAlgorithm hash)
    {
        using (var s = new BinaryReader((Stream)this.GetOutput()))
        {
            var xmlBytes = s.ReadBytes((int) s.BaseStream.Length);
            var xmlString = Encoding.UTF8.GetString(xmlBytes);
            xmlBytes = Encoding.UTF8.GetBytes(xmlString);
            hash.ComputeHash(xmlBytes);
            return hash.Hash;
        }
    }
}