Hello Krish,
You achieve this by configuring SPNEGO in Portal , Kindly check the below links
http://help.sap.com/saphelp_nw70ehp1/helpdata/en/45/40a0de773a7527e10000000a114a6b/content.htm
http://wiki.sdn.sap.com/wiki/display/Security/Single+Sign-On+with+SPNego+%28NWAS+Java%29
Best regards,
Vijay Kalluri