In 2013 we started a security study on one of the most widespread SSO protocols: OpenID. As described in previous posts, OpenID is a decentralized protocol, which provides a way to prove that a user controls an Identifier – URL.IDC. Additionally, OpenID is designed to support the usage of arbitrary IdPs: “An end user can freely choose which OpenID Provider to use ...“. Considering the properties of OpenID, we came up with the idea to study the relation between the IdP, generating the authentication token, and the Identifier URL.Idc contained in the token. In other words – is this relation critical regarding the security of OpenID implementations deployed on the SPs.