SP2013 SSRS 2012 repor service add-in always uses NT authority\anonymous instead of windows authentication

Question

We are building a SP2013 farm with SQL2012 and the SSRS 2012 add in , looking at the rsreportserver.config we have added the parameter RSWindowsNegotiate as listed below. 
<Authentication>
    <AuthenticationTypes>
      <RSWindowsNegotiate/>     
< /AuthenticationTypes>
We have followed this article (http://technet.microsoft.com/en-us/library/cc281253.aspx) where the following is listed:
ASP.NET must be configured for Windows Authentication. By default, the Web.config files for the Report Server Web service and Report Manager include the <authentication mode="Windows"> .
However looking at the web.config of the report server the following parameters * (see below)are present but
not the authentication mode='windows', only <windowsAuthentication enabled="true" /> is listed. 
* <security>
      <authentication>
        <anonymousAuthentication enabled="false" />
        <windowsAuthentication enabled="true" />
      </authentication>
    </security>
The report server is operational as long as we use datasources  located on the same farm but as soon as try to utilize a datasource outside the farm the report server uses anonymous to access the datasource. We have configure the c2wt services and have
the proper spn's in place on the domain , so we should not be suffering from the double hop issue.
How do we configure the reportserver to accept windows authentication?
Any help is welcome.
Regards,
Jim

Answer 1

R0ber7o,
Thanks for your reply, The web application is configured to use Kerberos and the spn has been created for the web application account.Yes I do receive a Kerberos ticket when I access the web application:
Client: myuser @ DOMAIN.COM
Server: HTTP/webapp @ DOMAIN.COM
KerbTicket Encryption Type: RSADSI RC4-HMAC(NT)
Ticket Flags 0x40a40000 -> forwardable renewable pre_authent ok_as_delegate
Start Time: 1/3/2014 13:08:26 (local)
End Time:   1/3/2014 23:03:42 (local)
Renew Time: 1/13/2014 13:03:42 (local)
Session Key Type: RSADSI RC4-HMAC(N
What we do not understand is why the reporting service application will only work when it uses :
identity impersonate="false" & anonymousAuthentication enabled="true"
when we change the identity impersonation to true we cannot access the reporting services system settings in the CA anymore and receive an error message:
"The report server cannot decrypt the symmetric key that is used to access sensitive or encrypted data in a report server database"
Regards,
Jim