Migrating SSRS custom security from 2008 R2 to SQL Server 2012

Hi,

We have built custom security in SSRS 2008 R2 and now we are migrating it to SSRS 2012. We are facing an issue, it always throws Security exception below and when we are changingweb.config file to below line we are getting "500 Internal server error". Tried everything, no luck... can someone please assist here whether we need to rewrite entire custom security code and then migrate it to SSRS 2012 custom security. Any help here much appreciated

<authentication mode="Forms">
      <forms loginUrl="logon.aspx" name="sqlAuthCookie" timeout="60" path="/"></forms>
    </authentication>   

    <identity impersonate="false" />

Regards,

Harish