Inherit Security settings through other portal sign on for cognos Connection

[Sak]

Hi All,

Below mentioned is a requirement provided by my client. Please help me out on how to achieve this if any one knows.

Users are logging in to Cognos connection through a webportal. Right now there is no security applied in cognos connection.
web portal is maintained by other vendor. Ther eis a link in that web page to cognos. If they click that link, the connection window opens.
If the user wants to login to that web page, they are logged in through a saperate sign on. The users can view the conetents according to their user security rights. Where as they can able to see all the cognos connection reports if they click on the cognos link.
In need to inherit the same sign on credentials to cognos connection too.. How can i achieve this. Kindly suggest me how to achieve this.